CHAPTER 4
PERFORMANCE AND OBSERVED BEHAVIOR OF FIELD INSTRUMENTATION

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The excavation for the Chicago and State subway renovation project consisted of two areas of excavation. The primary excavation area was west of the Warde School, along State Street. This portion of the project involved constructing a mezzanine section atop the subway tunnel and making the subway handicap accessible by adding escalators and elevators. Figure 4-1 presents the subway renovation structural plan and includes the numbering scheme for the secant piles. The figure also shows the locations of the inclinometers with respect to the excavation.

The excavation area west of the Warde School was approximately 40 m long and about 24 m wide. At the center of the west side excavation area, the excavation was advanced to the top of the subway tunnel. The top of the subway tunnel was about 6.2 m below the ground surface (elevation -1.9 m CCD). Along the sides of the west excavation area, on either side of the tunnel, the excavation was advanced to a depth of 12.2 m (elevation -7.9 m CCD) everywhere along the length of the tunnel except at the locations of the escalators. At the escalator locations, the excavation was advanced to a depth of 13.7 m (elevation -9.4 m CCD).

The secondary excavation was located on the north side of the Warde School, along Chicago Avenue. The purpose of this excavation activity was to add new stairs and an escalator to the existing station. The secondary excavation area was approximately 24 m long and 7 m wide. The excavation was advanced to a depth of 8.2 m (elevation -4 m CCD) at this location.

Although the excavation activities were relatively uniform as the project progressed from the south end to north end along State Street, different deformation behavior of the ground was noted between the east and west sides of the primary excavation area. To provide a complete picture of the observed responses, representative observations are presented for the east and west sides of the State Street excavation and the south side of the Chicago Avenue excavation.

4.1 CONSTRUCTION SEQUENCE AND PROCEDURES

Construction Day 0 corresponds to the beginning of the secant pile wall installation at the project site. The construction days correspond to calendar days and advance sequentially until the end of construction. The construction days include weekends and holidays.

The construction activities at the site are separated into three distinct stages; (i) Stage 1—Wall Installation, (ii) Stage 2—Support System Installation and Excavation, and (iii) Stage 3—Station Renovation and Backfill. The actual beginning and duration of each stage differs depending on the location within the excavation. However, general groupings of construction activities are made based on construction sequence along the (i) east side of the primary excavation area, (ii) west side of the primary excavation area, and (iii) the south side of the secondary excavation area. The time history of the significant excavation and construction activities that occurred at the site is presented in Table 4-1. The overall construction activity time history for the project and a structural plan sheet for the station renovations are given as Appendix A.

Table 4-1. Days of Significant Excavation and Construction Activity

4.1.1 Construction Along the East Side of State Street

The secant pile wall was installed along the entire east side of the Warde School between Day 0 and Day 30. However, secant piles directly adjacent to Inclinometer 1 and Inclinometer 2 were installed between Day 0 and Day 11. The locations of the inclinometers relative to the excavation are shown in Figure 4-1. The secant pile wall was approximately 18.3 m deep and was constructed by drilling overlapping 915-mm diameter shafts. Center-to-center spacing of each shaft was 750 mm. Construction of the wall began by first drilling primary shafts. These shafts were located 1.5 m apart and were constructed by first auguring down to the clay crust and installing temporary steel casing. The steel casing was slightly oversized and provided support for the sand fill during shaft construction. The remainder of each shaft was typically drilled uncased. Once the primary shafts were drilled, W24x55 sections were placed in the holes and a neat concrete grout was end dumped from concrete trucks. The grout typically was placed within 2 hours of completing the hole, but occasionally longer intervals resulted from delays in grout delivery. Secondary shafts were installed between the primary shafts by auguring through “green” grout to provide the 150-mm overlap. Green grout is the condition of the grout at which it has hardened enough to provide adequate bending stiffness, yet weak enough so that the edges of the primary shafts can be removed using conventional drilled shaft equipment. Typically, the secondary shafts were installed within 24 hours of installing the primary shafts. The design 28-day compressive strength of the grout was 6.9 MPa. Significant excavation in a given area did not begin until at least 28 days after the wall in that area had been installed.

The excavation activities along the east side of State Street occurred between Day 28 and Day 163. Figure 4-2 shows the profile of the east wall excavation face. The pipe struts were installed without preload at a depth of 0.6 m below ground surface. Pipe Struts 1 and 2 were installed on Day 56 and Day 58, respectively, when the maximum depth of the excavation was approximately 3.7 m beneath the struts.

The maximum excavated grade at the strut locations was an approximate depth of 4 m during the installation of Strut 3, Strut 4, and Strut 5. Strut 6 was installed when the excavated grade at that location reached approximately 4.5 m. Both levels of tiebacks were installed with excavated grade no more than 0.6 m below the tieback level. The maximum unsupported height of the wall was 4.5 m and occurred after the installation of Strut 6 (Day 79) and prior to stressing the upper level tiebacks (Day 81). The excavation was advanced to the bottom depth of 12.2 m at locations adjacent to Inclinometer 1 and Inclinometer 2 on Day 116. The entire east side was advanced to the bottom depth of 12.2 m on Day 149. The pit for Escalator #4 was excavated to a completed depth of 13.7 m on Day 163. The pit for Elevator #3 was also excavated to a final depth 13.7 m and was completed on Day 165.

The tiebacks were installed by coring through the secant pile wall and drilling down to the stiff and hard clay (elevation -10.7 m CCD). Each tieback was approximately 30.5 m long. The tieback holes were cased with temporary 178-mm diameter steel casing in areas where the tiebacks passed through the softer clays. The upper level tiebacks were cased for the first 10.7 m of the hole and the lower level tiebacks were cased for the first 4.6 m. The steel casing was advanced using an external water flush. The minimum bond length for the east side tiebacks was 9.1 m. Once installed, each tieback anchor was tested to 1.33 times the design load and the load was locked off at 80 percent of the design load. Along the east side, the design load for the upper and lower level tiebacks were 278 kN/m and 351 kN/m, respectively. Experience gained through performance and proof testing showed the required loads could be attained without regrouting, and consequently only about one-half the anchors were actually regrouted.

The base slab between Stair #7 and Escalator #4 was poured between Day 172 and Day 177. Other activities of note along the east side of State Street included pouring the remainder of the pit slab between Escalator #4 and Elevator #3, pouring the base slab for the mezzanine, and pouring the exterior walls of the mezzanine. These activities occurred between Day 178 and Day 224. A summary of the activities that occurred during this time can be found in Table 4-1. Backfilling along the east side of State Street began on Day 225. Struts 1 and 2 were removed on Day 247. Strut 3 was removed on Day 249 and Struts 4, 5, and 6 were removed on Day 258.

4.1.2 Construction Along the West Side of State Street

The secant pile wall along the west side of State Street was installed between Day 0 and Day 37. An abandoned drift tunnel was encountered on Day 9 during the installation of the south end of the west wall. Wall installation activities at location of the drift tunnel were stopped and were not resumed until Day 24. The portion of the west secant pile wall adjacent to Inclinometer 4 was installed between Day 15 and Day 18. The portion of the west secant pile wall involving the drift tunnel was installed between Day 24 and Day 37. The days and the sequences in which all the secant piles were installed along the west side are given in Table 4-1.

The construction procedures used for the secant pile wall along the west side of State Street were the same as those used for the east wall. However, the total installation time for a single shaft was typically longer for the west wall than for the east wall. It took as long as 5 hours after drilling a shaft for grout to be placed at some locations along the west wall.

Although some minor excavation activities began on the south end of the west wall on Day 19, the excavation along the west side did not begin in earnest until Day 59. The excavation activities along the west side of the primary excavation area were not as consistent as those observed along the east side. Figure 4-3 presents the profile of the west wall excavation face. The average depth of the excavated grade was about 2.6 m below ground surface at the time Struts 1, 2, and 3 were installed. Strut 4 was installed on Day 60. At this time, the depth of excavation was 2.6 m at the Strut 4 location, but the maximum depth of the excavation along half of the west excavation face was 5.8 m. From Day 60 to about Day 67, soil from various other excavation activities was stockpiled in the northwestern corner of the primary excavation area. About 1/3 of the west side was backfilled to a height of approximately 1.2 m above the original grade. The stockpile was removed on Day 74 and excavation activities along the west side were resumed. Strut 5 and Strut 6 were installed on Day 74 and Day 79, respectively. The unsupported heights of the west secant pile wall were limited to a maximum 4 m during excavation, which occurred at the Strut 5 location. The excavated grade was advanced to depths of approximately 6.1 m and 9.1 m during the installation of the upper and lower level tiebacks, respectively. The upper level tiebacks were located at a depth of 4.6 m and the lower level tiebacks were located at a depth of 8.5 m. The excavated grade was not more than 1.5 m below the tieback levels during the installation. The excavation was advanced to a depth of 11.9 m at the locations near Inclinometer 4 on Day 117. The excavation was advanced to the 11.9-m depth across the entire bottom of the west side of State Street on Day 137 and remained at that depth until Day 156.

The excavation was completed to a final depth of 12.2 m on Day 156. Also at this time, the pit for the west side escalator (Escalator #1) was excavated to a completed depth of 13.7 m.

The installation procedures used for the tiebacks along the west wall were very similar to those used for the east wall tiebacks. One difference was that the minimum bond length for the west side tiebacks was 10.7 m. Also, the design loads differed from the east side; the upper level tiebacks were designed for 397 kN/m and the lower level tiebacks were designed for 429 kN.

The base slab between Stair # 6 and Escalator #1 was poured between Day 156 and Day 163. All subway station renovations along the west side of State Street were completed on Day 224. Backfill activities along the west side commenced on Day 225.

4.1.3 Construction Along Chicago Avenue

The construction procedures along Chicago Avenue on the north side of the Warde School differed from those along State Street. There were no tiebacks used at this location and the secant pile wall system was only used for about half the south wall. Soldier piles and lagging were used for the rest of the wall at this location. A plan view of this configuration is shown in Figure 4-1. The soldier piles were HP10x42 sections placed in 510-mm diameter drilled holes. The center-to-center spacing of the soldier piles was 1.8 m. The combination of secant pile wall and soldier pile and lagging support systems were used because the excavation along the north side of the Warde School was not as deep nor as wide as the excavation along the west side of the school. The presence of the existing subway station provided some additional basal stability to the north side excavation.

Figure 4-4 shows the profile of the excavation along the north wall. The excavation at this location occurred between Day 125 and Day 196. Two levels of cross-lot struts were installed without preload at depths of 0.9 m and 4.3 m below ground surface. The upper level struts consisted of HP10x42 sections. The lower struts were HP12x53 sections. The upper level struts were installed on Day 128 and Day 133. The excavation was advanced to a maximum depth of 2.1 m on Day 130 and remained at that depth until Day 137. The excavation was advanced to approximately 8.2 m on Day 191 to accommodate the pit for the north side escalator (Escalator #3). The area west of the escalator pit was lowered to a final depth of 7 m and the area east of the pit was completed a final depth of 6.1 m between Day 191 and Day 196. The lower level struts were installed on Day 206.

4.1.4 Reduction in the Bending Stiffness of the Wall

The excavation along State Street was completed and demolition of portions of the existing subway tunnel walls began during the period between Day 110 and Day 140. Prior to backfilling, portions of the secant pile wall had to be chipped away so that the required clear span between the east and west mezzanine walls could be attained. The secant pile walls between Strut 1 and Strut 6 were chipped to the flange of the W24x55 sections for their entire exposed length. The east wall was chipped to the flange between Days 110 and 140. The west wall was chipped to the flange between Days 117 and 134. The majority of the chipping activities along the north wall occurred on Day 199. Removal of the hardened grout from the wall resulted in a reduction in bending stiffness of the wall.

The station renovations were completed from Day 172 to Day 255. After the major structural modifications to the station were completed, the excavation was backfilled. The backfill activities began on about Day 225.

4.2 OVERVIEW OF FIELD INSTRUMENTATION RESPONSES

Ground movement, building settlement, and pore water pressure observations are presented and related to the excavation and construction activities over the extent of the project. These observations provide summaries of the performance of the excavation support system and the response of the Warde School during the project. The instrumentation data collected during the course of the project are presented as Appendix B. The optical survey data is given in Table B-1. Table B-2 presents the pore water pressure data. The strain gauge data for the struts are given in Table B-3 and Table B-4 presents the load cell data for selected tiebacks. The inclinometer data is presented in Table B-5. It is noted that the only the A-axis data is presented for Inclinometers 1, 2, 4, and 5. The B-axis data for these inclinometers showed that the ground movement parallel to the plane of the excavation was relatively small at the inclinometer locations. Therefore, this data was not included. Also, the deformations recorded at Inclinometer 3 during the project were negligible and thus not used in any of the analyses. As a result, the Inclinometer 3 data are not included in Appendix B.

4.2.1 Ground and Building Movements

4.2.1.1 Responses along East Side of State Street

Lateral ground movements resulting from the excavation and construction activities along the east side of State Street are evaluated using the Inclinometer 1 and Inclinometer 2 responses. These lateral movements are compared to settlement data obtained from points on the Warde School adjacent to the inclinometer locations. Although Inclinometer 2 was also located along the east side of the State Street, its data were locally affected by installation of a tieback. As a result, the lateral movements observed for Inclinometer 2 near elevation -3.4 m CCD were not completely representative of the lateral response of the clay at that elevation.

Summaries of the movements that developed throughout construction at these locations are presented in Figure 4-5 and Figure 4-6. These figures show deformations plotted against time for the duration of the project. The horizontal movements were measured for both inclinometers at depths corresponding to the maximum lateral deformation: (i) within the soft clay (elevation -3.7 m CCD), (ii) at the bottom of the excavation (elevation -7.9 m CCD), and (iii) in the stiff clay below the bottom of the excavation (elevation –11.0 m CCD).

The Inclinometer 1 and 2 deformations in Figure 4-5 and Figure 4-6 are the lateral movements perpendicular to the secant pile wall in the east-west direction. Inclinometer 1 and Inclinometer 2 were located approximately 2.2 m and 1.7 m west of the school, respectively. The settlement shown in Figure 4-5 was obtained from optical survey data at settlement point W10 on the west exterior wall of the Warde School near Inclinometer 1 (see Figure 3-2). The settlement shown in Figure 4-6 was obtained from survey data at settlement point W8, near Inclinometer 2 (see Figure 3-2). Settlement point W10 was located approximately 2.2 m east of Inclinometer 1 and settlement point W8 was located about 1.7 m east of Inclinometer 2. Also plotted in Figures 4-5 and 4-6 are the major construction activities at the Inclinometer 1 and Inclinometer 2 locations.

It is apparent from both figures that the settlement of the building follows the development of lateral movements in the soil. The lateral movements within the soft clay (elevation -3.7 m CCD) and the settlements at W8 and W10 were virtually identical until Day 145 when excavation was complete and the secant pile wall had been chipped to the flange at both inclinometer locations. Thereafter, the settlements became slightly greater than the maximum lateral movements. Of the approximately 40 mm of lateral deformation measured at Inclinometer 1, 10 mm occurred during wall installation, 18 mm developed as the soil was excavated and the lateral support installed, and 12 mm occurred during tunnel demolition and station renovation as a result of creep and the reduction of wall stiffness.

The maximum amount of lateral deformation at Inclinometer 2 was about 10 mm less (30 mm total) than was observed at Inclinometer 1. Similar to what was observed at Inclinometer 1, 10 mm of deformation occurred during wall installation. About 12 mm of deformation at Inclinometer 2 was attributed to installation of the support system and excavating to the final grade. From Figure 4-6, it is noted that there is a sharp dip in the elevation -3.7 m CCD curve at Day 74. This dip is a result of installing the upper level tieback at that location. This response is presumably a very local effect and, hence if the tieback did not pass so close to Inclinometer 2, then more deformation would have been recorded in the soft clay. The last 8 mm of deformation occurred in response to the reduction of wall stiffness and creep.

Figures 4-5 and 4-6 show that lateral movements and settlement first occurred as the secant pile wall was installed. The movements increased very slightly during the installation of the struts and the upper level tiebacks. However, both figures indicate a rapid increase in movements once the excavation was advanced below the stiff clay crust at elevation -1.2 m CCD. Both figures show a dramatic reduction in the rate of deformation once the excavation is completed, between Day 114 and Day 116. However, both figures show an increase in the rate of movement between Day 150 and Day 175. This increase was caused by the reduction in wall stiffness caused by chipping away the face of the secant pile wall.

Placing the base slab between Day 172 and Day 177 at both the Inclinometer 1 and the Inclinometer 2 locations reduced the rate of lateral movements. This was because the slab acted as a diaphragm once it abutted the existing tunnel.

Note that the movements associated with stress relief from the excavation were less than one-half the total movement. This is significant in that the movements associated with stress relief during excavation and support placement are many times the only ones considered when predicting the movements associated with deep excavations.

4.2.1.2 Responses along West Side of State Street

Lateral ground movements resulting from the excavation and construction activities along the west side of the State Street are evaluated using the responses observed at Inclinometer 4. Summaries of the movements that developed throughout construction at Inclinometer 4 are presented in Figure 4-7. Similar to Figure 4-5 and 4-6, deformations are plotted versus time and the horizontal movements are measured at depths corresponding to the maximum lateral deformation: (i) within the soft clay (elevation -4.4 m CCD), (ii) at the bottom of the excavation (elevation -8.1 m CCD), and (iii) in the stiff clay below the bottom of the excavation (elevation -11.1 m CCD). The excavation and construction activities shown above the deformation curves in Figure 4-7 are applicable to the area adjacent to Inclinometer 4. The figure shows that the values between Day 109 and Day 123 were estimated. The original Inclinometer 4 was destroyed while installing a sanitary sewer line on the west side of State Street.

A new inclinometer was installed approximately 1 m north of the original location. Based on the deformation trends observed at Inclinometers 1 and 2 at this same time, it was assumed that the increase in lateral movements at Inclinometer 4 were relatively linear between Days 109 and 123. Consequently, the deformations shown in Figure 4-7 by the dashed lines were extended to Day 123 at the slope observed prior to the inclinometer being damaged. The deformations after Day 123 were obtained by adding measured deformations from the new inclinometer to the estimated cumulative value at the end of Day 123.

It is observed from Figure 4-7 that the general lateral deformation trends on the west side of the excavation are very similar to those on the east side. However, the magnitude of deformation on the west side of State Street was approximately 30 percent greater than on the east side.

An estimated 57 mm of lateral deformation was measured within the soft clay layer at Inclinometer 4. Of this total deformation, approximately 17 mm occurred during the wall installation. This deformation represents 34 percent of the total lateral deformation measured within the soft clay layer. An additional 36 mm (59 percent of the total) of lateral deformation was measured during the excavation and support installation stage and the remaining 4 mm (7 percent of the total) of deformation occurred during the station renovation. Thus, just a little more than half of the lateral deformations measured on the west side are in response to excavation-induced stress relief. The percentages of movements caused by excavation-induced stress relief are similar to those observed on the east side, where 45 percent of the total lateral deformation developed in the soft clay as a result of this factor.

From Figure 4-7, it is noted that the majority of lateral deformations resulting from stress relief did not occur until the excavation was advanced about midway into the soft clay layer at elevation -1.8 m CCD. This is apparent from observing that the lateral deformation measured at elevation -4.4 m CCD only increased from 17 mm to about 20 mm from Day 25 to Day 92. The corresponding change in the excavated depth during this time was from elevation +4 m CCD to elevation -1.8 m CCD. However, as the excavation was advanced from that point to the final elevation of -8.2 m CCD (Day 93 to Day 156), the lateral deformation increased from 20 mm to an estimated 53 mm. Along the east side, the significant increase in lateral movements occurred once the excavation was advanced below elevation -1.2 m CCD.

Figure 4-7 shows that the rate of deformation appeared to slow between Day 120 and Day 130, but then began to increase after Day 130 at the previous rate. The decrease in the deformation rate was in response to the excavation being stalled at elevation -7.6 m CCD. The increase in the rate of deformation corresponds to advancing the excavation to its deepest point (elevation -8.2 m CCD) beginning on Day 137 and chipping the secant pile wall between Day 117 and Day 134. The excavation was completed at Inclinometer 4 location on Day 156 and the pit slab for Escalator #1 was completed on Day 163. Only minor creep movements (approximately 4.4 mm) were observed after the pit slab was completed.

4.2.1.3 Responses along Chicago Avenue

Figure 4-8 presents the lateral deformations measured at Inclinometer 5 located between the school and the excavation along Chicago Avenue. These deformations represent the lateral movements at the north side of the Warde School. The figure shows deformations in both the east-west (B-axis) and north-south (A-axis) directions. The depths at which the lateral movements were measured corresponded to: (i) the depth of maximum lateral deformation for Inclinometer 5 (elevation -2.6 m CCD), this also corresponds to the bottom of the excavation on the north side of the Warde School; (ii) the bottom of the Blodgett layer (elevation -6.8 m CCD); and (iii) the bottom of the Deerfield layer (elevation -9.9 m CCD). Also shown in Figure 4-8 are the settlement data obtained from settlement point W13. This settlement point was adjacent to the Inclinometer 5 location. Two excavation profiles are given in Figure 4-8, one represents the construction record at Chicago Avenue at the Inclinometer 5 location, and the other represents the construction record at the northwest corner of the school.

Although the north-south direction was the primary direction of measurement, it is apparent that the east-west direction represented a significant component of the movement. The total lateral movement is represented by the resultant of the A- and B-axis movements and is also shown on the figure. Figure 4-8 shows that the lateral deformations in the east-west direction were consistently greater than those in north-south direction throughout the project. More importantly, the resultant of the two components of the lateral movement tracks the development of the surface settlement much better than either of the individual components.

The north secant pile wall was installed between Day 70 and Day 79. In response to this activity, the maximum lateral deformation perpendicular (north-south component) to the secant pile wall was about 4 mm, but the maximum lateral deformation parallel (east-west component) to the secant pile wall was about 10 mm.

Presumably, the increase in the east-west component on Day 79 was in response to the excavation at the northwest corner being lowered beneath the clay crust. The figure shows that the excavated grade at the northwest corner of the east secant pile wall was advanced to approximately elevation -1.2 m CCD on Day 79. The excavation activities on the north side of the Warde School occurred between Day 125 and Day 208. During this period, the north-south component of the maximum lateral deformation increased from about 4 mm to about 14 mm and the east-west component of the maximum lateral deformation increased from 10 mm to 20 mm. During this time, the excavated grade at the northwest corner was advance from elevation -6.4 m CCD to elevation -8 m CCD on Day 150. The excavated grade at the Inclinometer 5 location was not advanced below the clay crust until about Day 170. It appears that, at least until Day 170, the east-west component of lateral movement was primarily in response to the excavation along State Street. There were no additional lateral deformations observed after the pit slab was pour. The pit slab acted as a continuous support across the bottom of the excavation.

The optical survey data showed that from Day 70 to Day 79 the settlement at W13 increased from 6 mm to 11 mm. By comparing this settlement with the resultant lateral deformations at elevation -28 m CCD, which also increased to the 11 mm during this time, one can see that the settlements were caused by the combined effects of the installation of the north secant pile wall and the excavation along State Street. Figure 4-8 shows that the settlement rate increased after Day 170, as a result of excavating below the clay crust along Chicago Avenue. The settlement increased to a maximum of about 28 mm on Day 208. These settlement responses indicate that the deformational behavior of the northwest corner of the building was a function of the 3-D excavation geometry at that location.

4.2.2 Pore Water Pressure Response

Pneumatic piezometers were installed at elevations -5.8 m CCD and -8.8 m CCD adjacent to Inclinometer 1 and at elevations -6.4 m CCD and -9.45 m CCD adjacent to Inclinometer 4. These piezometer locations were designated Piezometer 1 and Piezometer 4 to correspond to the inclinometer locations Piezometer 1 was located 2 m from the secant pile wall whereas Piezometer 4 was located 3 m from the wall. Data were collected at both piezometer locations throughout the project. However, the piezometers at the Piezometer 1 location were destroyed during installation of the upper level tiebacks. Figure 4-9 presents the pore water pressure responses at the Piezometer 1 location, up to the day the piezometers were destroyed. The pore water pressure responses at the Piezometer 4 location are presented in Figure 4-10. The figure uses two colors for each piezometer curve. This is because the original piezometers at this location were destroyed as a result of the west side construction activities. New piezometers were installed at the same depths of the original piezometers, a few feet north of the original location. The first color in each curve represents the pore water pressure data from the original piezometers and the second color is the replacement piezometers. Note that there was some overlap for the piezometer at elevation -9.45 m CCD.

Similar responses were observed at both the Piezometer 1 and Piezometer 4 locations. The data obtained before construction began indicate that essentially hydrostatic conditions initially existed in the clays. The elevation of the piezometric surface at the Piezometer 4 location was approximately +0.75 m CCD and the piezometric surface at the Piezometer 1 location was approximately +0.6 m CCD. The piezometric surface is slightly above the top of the stiff clay crust layer and is apparently sloping towards Lake Michigan.

Figures 4-9 and 4-10 show that large drops in water pressure occurred as the secant pile wall was installed. The water heads dropped as much as 5.5 m at the Piezometer 1 location and about 4.5 m at the Piezometer 4 location. Essentially, the open shafts of the secant piles near the piezometer locations acted as temporary sinks. Because grout was placed within each of the holes soon after excavation, the water levels at Piezometer 4 recovered most of their initial values within about 38 days. The excavation had not proceeded below the clay crust at this time (Figure 4-10). The water levels at Piezometer 1 had recovered about 82 percent of their initial values before the excavation was advanced below the clay crust on Day 60. Afterwards, the water levels dropped in response to the excavation until the piezometers were destroyed on Day 87. From this point, the Piezometer 4 data are used to illustrate the pore pressure responses to the subsequent construction activities.

The pore pressures at Piezometer 4 were relatively steady from about Day 38 to Day 90. Thereafter, the excavation along the west side was lowered below elevation -1.8 m CCD, whereupon the pore pressures began to drop. This time corresponds to the time where the clay crust was excavated and significant movements began to develop during excavation. Another transient response was observed when the second level tiebacks were stressed. Water heads increased as much as 5 m, but dissipated within several days. Given that the anchors passed within 2 m of both piezometers and that the pressures rapidly dissipated, the pore pressure responses caused by stressing the anchors were localized. By Day 150, the water levels had dropped about 3 m from their initial values at both piezometers. This drop increased the effective stress in the soft clay by 30 kPa, apparently enough for small consolidation settlements to develop. If one assumes that the pore pressures on both sides of the excavation responded to the construction activities in a similar manner, the data beyond Day 150, shown in Figure 4-5, can be interpreted in light of these pore pressure changes. At settlement point W10, movements larger than observed in the inclinometer data at elevation -3.7 m CCD began to develop. These movements were presumably a result of the small consolidation settlement arising from the drop in water pressure. The maximum excavation-related drop in the water level was observed on Day 203, prior to backfilling. At this time, the heads in the piezometers were 7.1 m at elevation -9.45 m CCD and 4.5 m at elevation -6.4 m CCD. After Day 203, the pressures began to recover slightly. However, after backfill was completed on Day 354, water levels remained about 2.4 m below the initial values. The water levels should increase slowly over time depending on the effectiveness of the waterproofing for the renovated station.

4.3 DETAILED OBSERVATIONS OF LATERAL MOVEMENTS

Detailed observations of lateral movements are given for locations corresponding to the excavation activities along the east and west sides of State Street and along Chicago Avenue. Inclinometer 1 is representative of the east side of State Street, because the lateral responses at Inclinometers 1 and 2 were very similar (see Section 4.2.1.1). Inclinometer 4 was used to represent the west side of State Street and Inclinometer 5 is representative of the activities along Chicago Avenue.

The detailed observations include inclinometer responses to significant excavation and construction activity. Activities common to all three locations are given in Table 4-1. The table shows the activities and the days of occurrences at the three locations. Note that the activities are listed in groups of the major construction stages of the project. The subsequent sections discuss the lateral movements observed at the Inclinometer 1, 4, and 5 locations during each construction stage.

4.3.1 Stage 1 – Wall Installation

Table 4-1 indicates that the secant pile wall installation activities at the representative inclinometer locations along State Street (Inclinometer 1 and Inclinometer 4) were completed between Day 2 and Day 18. The table also shows that the secant pile wall was installed along Chicago Avenue between Day 70 and Day 79. However, these dates only include the installation of the secant piles that influenced the responses of the inclinometers. Using a trail-and-error method of comparing secant pile installation with inclinometer response, it was ascertained that the inclinometer responded to installation activities when secant piles were being installed at a center-to-center spacing of 3.8 m. Referring to Table A-1 in Appendix A, it is seen that the entire east secant pile wall was installed between Day 0 and Day 30. The complete west secant pile wall was installed between Day 9 and Day 32. The dates given in Table 4-1 for the Inclinometer 5 location include installation of all the secant piles along Chicago Avenue.

4.3.1.1 East Side of State Street

Figure 4-11 presents the lateral responses for Inclinometer 1 and the settlement response of the Warde School at a section perpendicular to the east secant pile wall through Inclinometer 1. The inclinometer was located 2.3 m from the centerline of the secant pile wall. This section is typical of the response in the middle of the east secant pile wall and shows the effects of construction on the Warde School. Lateral soil movements developed as the wall was installed with the maximum movement occurring within the soft clay. Note that the stability number (Peck, 1969) of the clay at the depth of maximum movement was 7.8. This high stability number would suggest that inward movements would occur due to stress relief as a shaft was drilled without support (i.e., with no drilling mud or casing). The lateral movements extend to the depth of the secant pile wall. The building settled as much as the soil displaced laterally, with the settlement extending as far behind the excavation as the secant pile extended below the bottom of the school’s foundation. These patterns of movement are expected because the movements into the shafts of the secant pile wall are the result of undrained deformations in saturated clay.

Figure 4-11 shows that the maximum lateral deformation at the end of the secant pile wall installation (Day 11) was approximately 10 mm. This initial 10 mm of deformation corresponded to about 26 percent of the total lateral deformation observed along the east secant pile wall. The figure also indicates that below the soft clay layer, the inflection points of the deformation curves correspond approximately to the layer interfaces. This trend is likely the result of the differences in shear strength and stiffness among the layers. Peck and Reed (1954) indicated that the undrained shear strength of individual strata in the downtown area of Chicago generally increases in steps from one stratum to the next.

4.3.1.2 West Side of State Street

Figure 4-12 presents the lateral soil movements during wall installation at the Inclinometer 4 location. The inclinometer was located approximately 3 m from the centerline of the west secant pile wall. The figure shows that the maximum deformation of 17 mm observed at the end of the secant pile wall installation was greater than what was observed at Inclinometer 1. The point of maximum deformation occurred in the medium clay layer at elevation -5 m CCD. The larger lateral movements at Inclinometer 4 reflect the shafts of the west wall secant piles being open for longer times prior to placing the concrete. Along the east secant pile wall, steel casing was advanced well into the soft clay when constructing shafts near the Inclinometer 1 location and concrete was typically placed within 2 hours of drilling the shafts. However, along the west secant pile wall the steel casing was only advanced to the stiff clay layer and, in some instances, the steel casing was not used at all. In addition, concrete placement for the west secant pile wall sometimes took as long as 5 hours after drilling the shaft.

Another factor contributing to the larger lateral movements at Inclinometer 4 are the higher insitu soil stresses on the west side of the excavation. Construction of the Warde School decreased the insitu stress state along the east side of the project site as a result of the net unloading associated with the excavation of the basement and the reloading associated with the construction of the building. Thus, for the same undrained shear strength, the stability number is higher on the west side of the excavation than on the east side. As a result, soil squeeze into the open, uncased shaft and the subsequent ground loss will occur faster and be greater on the west side of the excavation than on the east side. No settlement data were recorded on the west side of the excavation.

4.3.1.3 Chicago Avenue

Figure 4-13 gives the lateral soil response for Inclinometer 5. The inclinometer was located 700 mm south of the centerline of the north secant pile wall and represents the lateral soil response along the north side of the Warde School. The figure gives the lateral movements in the direction towards Chicago Avenue (4-13a) and the movements in the direction towards State Street (4-13b).

It should be noted that the inclinometer was damaged several times during the installation of the secant pile wall along Chicago Avenue (refer to Table A-1 in Appendix A). Consequently, the upper 3.5 m to 4 m of the data reflects movements caused by impacts against the inclinometer casing and not lateral deformations of the soil resulting from wall installation. More reliable lateral deformation data were observed below elevations corresponding to the middle of the stiff clay crust (approximately elevation +0.6 m CCD). As was the case in other inclinometers, it is apparent that maximum wall installation-related deformations occurred within the soft clay layer. However, the magnitude of maximum deformation in the direction perpendicular to face of the excavation was about 40 percent of that observed at the Inclinometer 1 location during wall installation along State Street. Figure 4-13a gives the maximum deformation in response to wall installation for Inclinometer 5 as approximately 4 mm, as compared to 10 mm at Inclinometer 1. Figure 4-13b shows that the maximum lateral deformation at Inclinometer 5 in the direction toward State Street was just slightly less than 10 mm on Day 79, of which 6 mm occurred as a result of installing the wall between Day 70 and 79. The decidedly 2-D movement in response to wall installation was caused by the fact that Inclinometer 5 was located within 0.8 m of the wall, and thus was affected locally in two directions by the drilling operations. As was noted for the other inclinometer locations, inflection points along the depth of the curves were observed at the approximate layer interfaces. The bottom of the secant pile wall served as the lower bound for the lateral movements.

4.3.2 Stage 2 – Excavation and Support System Installation

Stage 2 covers the period of excavation and support system installation. The struts along State Street were installed between Day 60 and Day 74. All upper level tiebacks for both the east and west secant pile walls were completed as of Day 106 and all lower level tiebacks were completed as of Day 114. The struts along Chicago Avenue were installed between Day 128 and Day 206. No tiebacks were used on the north side of the Warde School. Dates given in Table 4-1 for strut and tieback installation are for the struts and tiebacks in the vicinity of the inclinometers only. Refer to Appendix A for a complete record of the support system installation.

4.3.2.1 East Side of State Street

Figure 4-14 presents the Stage 2 lateral deformations observed along the east secant pile wall. The grade was excavated to elevation +2.7 m CCD on Day 59. Negligible movement occurred from the end of wall installation to when the excavation was lowered to elevation +2.7 m CCD. Significant lateral movements began to develop as the excavation was advanced through the clay crust. This large increase in movements was due to the reduction in the basal heave factor of safety. Clough et al. (1989) defined the factor of safety against basal heave for relatively wide excavations (H/B < 1) as

(4.1)

where H is the depth of the excavation, B is the effective width of the excavation, N_c is the bearing capacity factor, S_uu is the undrained shear strength above the bottom of the excavation, S_ub is the undrained shear strength below the bottom of the excavation, and γ is the total unit weight of the soil above the bottom of the excavation. Note, the presence of the tunnel restricts the width of the failure zone. Thus, to account for the limited failure zone, the top of the tunnel is used as a rigid base. Table 4-2 gives the basal heave parameters prior to the excavation advancing below the stiff clay crust and after the excavation has advanced into the soft clay. The undrained shear strength values above the bottom of the excavation were obtained from the weighted averages of the layers within the depth H. The undrained shear strength values below the bottom of the excavation are the average values within the failure zone. From Table 4-2, it can be seen that the factor of safety decreased by about 53 percent after the excavation was advanced below the clay crust

Table 4-2 Estimates of Factor of Safety Against Basal Heave

Figure 4-14 shows that the lateral movements extend to the depth of the secant pile wall. The building settled as much as the soil displaced laterally, with the settlement extending as far behind the excavation as the secant pile wall extended below the bottom of the school’s foundation. The equality of lateral movements and settlements are expected because the movements that occur are essentially undrained.

Figure 4-14 shows that the incremental deformations increased slightly more than 2 mm as the excavation was advanced below the first tieback level (elevation -1.4 m CCD) on Day 81, and an additional 3 mm before stressing the first level tieback on Day 87. The increment of movement between excavating below the first tieback level on Day 81 and excavating below the second tieback level (elevation -5.2 m CCD) on Day 102 was about 9 mm, resulting in a cumulative deformation of 21 mm. The upper portion of the wall moved about 2 mm toward the soil both times the tiebacks were stressed. As a result, the rate deformation was temporarily halted at each instance the tiebacks were stressed. This can be seen from the response of the soft clay layer in Figure 4-5. After the tiebacks were stressed, the secant pile wall moved incrementally toward the excavation in response to excavation-induced stress relief. The excavation was lowered to the final elevation of -7.9 m CCD on Day 116. When the excavation reached final grade, the maximum lateral movement increased to 28 mm. The school settled as the secant pile wall moved laterally.

The maximum settlement of the school at the end of excavation was equal to this maximum lateral movement of 28 mm. Note that the extent of the settlement trough did not increase during this time.

4.3.2.2 West Side of State Street

The observed and estimated Stage 2 lateral deformations for Inclinometer 4 are presented in Figure 4-15. As was discussed in Section 4.2.1.2, the original Inclinometer 4 was destroyed on Day 109. Values reported after that date were estimated based on incremental data from the new inclinometer and observed trends of increase in lateral movements at Inclinometers 1 and 2. The lateral movements at Inclinometer 4 during this stage were about twice those observed at Inclinometer 1. The excavation along west secant pile wall was advanced below the strut level (elevation +2.7 m CCD) on Day 59 with no significant lateral movements. Significant lateral movements again developed as the excavation was advanced through the clay crust. Two millimeters of movement occurred as a result of advancing the excavation below the first level tieback to elevation -1.8 m CCD on Day 81. Note that the shape of the deformation curves indicated cantilever movement of the wall until the first level tieback was installed on Day 81. This large cantilever-type movement is in contrast to that observed on the east side of the excavation, where very little cantilever movement was observed. Apparently, load from the west side of the excavation was being transferred to the east side through the struts, resulting in larger cantilever-type movements on the west side.

The increment of movement between excavating below the first tieback level on Day 81 and excavating below the second tieback level (elevation -4.9 m CCD) on Day 98 was about 12 mm. This is compared to the 9 mm of incremental movement at Inclinometer 1 for the same excavation activity, even though a greater height of the wall was unsupported along the east side than was observed on the west side prior to stressing the second level tieback. The maximum lateral deformation increased to 43 mm on Day 123, which corresponded to the excavation along the west side being advanced to elevation -7.6 m CCD. The stress relief-induced movements continued to increase until the excavation reached final grade at elevation -8.2 m CCD on Day 156. The maximum lateral deformation was 57 mm on Day 156. From the shape of the lateral response curves shown in Figure 4-15, it is observed that deep inward movement of the secant pile wall characterized the incremental lateral deformation when the excavation was lowered from the second tieback level to final grade.

The secant pile wall was chipped from elevation -4.26 m CCD to elevation -7.62 m CCD between Day 117 and Day 134. This process resulted in the wall becoming more flexible and ultimately contributed to the increase in lateral movements in the soft clay layer.

4.3.2.3 Chicago Avenue

The lateral deformation measured at Inclinometer 5 for the Stage 2 construction activities are presented in Figure 4-16. The lateral movements toward Chicago Avenue are given in Figure 4-16a while those toward State Street are shown in Figure 4-16b. The depth of the excavation along the north side of the Warde School was about half that along State Street. Consequently, the maximum lateral movements along north secant pile wall are smaller than those observed on either side of State Street.

Cumulative lateral movement at Inclinometer 5 was 3 mm in the north-south direction and 11 mm in the east-west direction on Day 109. At this time, the north side excavation was extended to below the upper strut level at elevation +2.1 m CCD. An additional 2 mm of movement was observed in the north-south direction between the initial excavation and installing the upper level strut on Day 128. No movement was observed in the east-west direction during this time. The excavation along the north side was first advanced into the soft clay layer (elevation -1.5 m CCD) on Day 170. This resulted in a maximum incremental movement toward Chicago Avenue of 2 mm and maximum incremental movement towards State Street of 4 mm, clearly showing the effects of the excavation on State Street.

Figure 4-16 shows that after Day 170 the incremental movements were primarily directed towards Chicago Avenue as a result of excavating from -1.5 m CCD to -4 m CCD on Day 208. The increment of lateral deformation in the soft clay between Day 170 and Day 208 was 7 mm toward Chicago Avenue and 1 mm toward State Street. There were no additional excavation activities along the east side of State Street after the pit for Escalator #3 was completed on Day 163. Thus, deformations observed after Day 163 were primarily caused by excavation activities along Chicago Avenue, although small amounts of creep movements may have developed as well. Note that at the Inclinometers 1 and 2 locations creep movements were observed after the excavation activities had been completed. It was observed that the rate of creep at these two inclinometers produced an increment of maximum movement in the soft clay layer of about 2 mm for a given 38-day period. This maximum increment of movement compares to the 1-mm increment toward State Street observed at Inclinometer 5 between Day 170 and Day 208.

4.3.3 Stage 3 – Renovation and Backfill

The last major stage in the Chicago Avenue and State Street subway renovation project involved completing the capital improvements and backfilling the excavation. Table 4-1 indicates that this stage began with chipping the secant pile wall adjacent to the inclinometer locations. However, chipping of the secant pile wall occurred prior to completing the excavation at the Inclinometer 4 and Inclinometer 5 locations.

4.3.3.1 East Side of State Street

The lateral deformations measured at Inclinometer 1 during this stage of the construction are presented in Figure 4-17. Very small creep movements developed after the excavation was advanced to final grade. This can be seen from the incremental movement between Day 116 and Day 137.

To accommodate the exterior walls of the new mezzanine section, the grout on the face of the secant pile wall was chipped to the face of the flange from elevations -4.3 m to -7.6 m CCD between Days 137 and 140. This chipping activity resulted in a reduction of the bending stiffness of the wall. Consequently, more movements were observed between Days 137 and 176, when the bottom slab for Escalator #4 was poured, than were observed during the remainder of this stage of construction. Reducing the stiffness of the wall resulted in approximately 5 mm of incremental movement within the softer clays. After Day 176, approximately 3 mm of small creep movements continued until the excavation was completely backfilled. Note that the tiebacks were not cut, and remain stressed in place. Removal of the top level of bracing on Day 258 resulted in small cantilever movements near the top of the wall, as is evident from the Day 266 curve in Figure 4-17.

4.3.3.2 West Side of State Street

The estimated lateral deformations measured at Inclinometer 4 for Stage 3 are presented in Figure 4-18. The base slab for Escalator #1 was completed on Day 163 and the backfill was placed between Day 225 and Day 310. A total of 3 mm of incremental creep movement was observed between the time when the escalator bottom slab was placed and backfill was completed. The west secant pile wall was also chipped to the flange along its exposed length However, unlike the east secant pile wall, the west secant pile wall was chipped prior to advancing the west side excavation to final grade. The chipping activities occurred between Day 117 and Day 134 and the excavation along the west side was completed on Day 156. Hence, the incremental lateral wall movements given in Figure 4-18 are a result of creep and strut removal.

4.3.3.3 Chicago Avenue

Figure 4-19 presents lateral deformations measured at Inclinometer 5 during Stage 3. It is apparent from the figure that little movement occurred along the north side of the Warde School after the excavation was advanced to final grade on Day 208. Very minor creep movements in the direction of State Street were observed after the base slab for Escalator #3 was poured (Day 210). There were no additional lateral movements observed in the direction of Chicago Avenue after the base slab was pour. The base slab effectively acted as an additional support level across the bottom of the excavation.

4.3.4 Summary of Lateral Deformations and Discussion

4.3.4.1 Incremental Lateral Soil Movements

Figure 4-20 shows a comparison of the lateral increments of movements that occurred adjacent to the excavation at Inclinometers 1, 2, 4, and 5. The incremental movements are shown for the three stage of construction: wall installation (20a), excavation to final grade (20b), and station renovation and backfill (20c).

Inclinometers 1 and 2 along the east side of the excavation exhibited very similar responses, with the exception of the kink in the data at elevation -3.4 m CCD at Inclinometer 2. This kink was the result of installing a first level tieback next to the inclinometer. Inclinometer 4 along the west side of the excavation showed incremental movements that were about 50 percent larger than those on the east side. Incremental movements along the north side at Inclinometer 5 were about 50 percent less than those on the east side.

During wall installation, 17 mm of lateral movement developed at the Inclinometer 4 location, which was about twice that at the other locations. The magnitude of this movement depended on the amount of time the shafts remained open after it was drilled and before it was filled with grout. Near the location of Inclinometer 4, the shafts remained open for as long as 5 hours, as compared to times of no more than 2 hours along the east side of the excavation. During excavation, the increment of movement was about twice as much along the west wall compared to that along the east wall and about four times as much as that along the north wall. The large difference in incremental movements between Inclinometer 4 and Inclinometer 5 was primarily due to the different depths of excavation and the geometry of the excavation adjacent to the inclinometer. However, along the east side the excavation the depth and geometry of the excavation and the construction sequences were similar to that along the west side. The larger movements observed during the wall installation at the Inclinometer 5 location apparently impacted the subsequent movements. As the soil deformed more into the open shaft, the soil mobilized its shear strength, leaving less available resistance for stresses imposed by subsequent excavation. Cunningham and Fernandez (1972) observed similar responses. They reported larger movements during excavation at locations where larger movements occurred as large diameter caissons were installed near a slurry wall before the excavation began. The presence of surcharge loads on the west side, in the form of construction equipment and materials, could have also contributed to the larger movements. No such loads were imposed on the east side of the excavation because of the lack of space between the secant pile wall and the school. Another contributing cause to the larger incremental movements during excavation could be the higher insitu stresses in the soil on that side of the excavation. There were no buildings on the west side, whereas the 3–story Warde School with its 3-m deep basement imposed less stress on the wall than the soil on the west side.

The smaller creep movements along the west secant pile wall can be attributed to the base slab being placed more quickly at that location than along the east wall. The creep movements along the north wall were negligible because the excavation was extended only partially into the soft clay layer.

4.3.4.2 Lateral Deformation Vectors

Figures 4-13, 4-16, and 4-19 indicated that although the north-south direction (towards Chicago Avenue) was the primary direction of measurement for Inclinometer 5, a significant component of the movement was in the east-west direction (towards State Street). This suggests that the lateral movements observed at Inclinometer 5 were heavily influenced by excavation activities performed along the west side of the school. Prior to excavating to final grade, the magnitudes of movements in the east-west direction were greater than those observed in the north-south direction. The implication is that the movements resulting from the excavation-induced stress relief at the corner were a function of the 3-D geometry of the excavation. Thus, in order to compare the lateral deformational response of Inclinometer 5 with those of Inclinometer 1 and Inclinometer 2, the deformations at Inclinometer 5 are presented in term of the deformation resultant. The movements towards Chicago Avenue serve as the northern component of the resultant and the movements towards State Street serve as the western component. Figure 4-21 presents a plan view of the lateral deformation vectors measured at Inclinometers 5, 1, and 2 to show the effects of opening the excavation along Chicago Avenue at the same time as the State Street excavation. The deformations are given at depths within the soft clay stratum corresponding to the maximum observed lateral movements in the inclinometers. For Inclinometer 1, the depth of maximum movement was 7.9 m below ground surface. The depth of maximum movement for Inclinometer 2 was 8.6 m and the depth for Inclinometer 5 was 6.9 m. The construction days used in the figure correspond to the days of significant construction activity presented in the previous sections.

Figure 4-21 shows that the ground movement was nearly perpendicular to the State Street excavation at the Inclinometer 1 and Inclinometer 2 locations, indicating that the movements were in response to the activities along State Street. The slight inclination observed in both inclinometers on Day 11 is in response to secant pile installation activities. Between Days 11 and 81, Inclinometers 1 and 2 respond to the excavation advancing towards the north. Note that movements between Day 137 and Day 176 were directed towards the south at Inclinometer 1 and towards the north at Inclinometer 2. These incremental movements were caused by excavating the escalator base slab pit located between the two inclinometers (refer to Figure 4-1). The direction of movement observed between Day 266 and Day 353 for both inclinometers was in response to the backfill activities, which proceeded from south to north. The lateral ground movements at Inclinometer 5 were primarily oriented towards State Street until day 109 indicating that the ground movements to this point developed mostly as a result of activities along the State Street excavation. The small component of northward movement between Day 0 and Day 79 developed as the secant pile wall was installed along Chicago Avenue. The excavation along State Street was completed at the Inclinometer 1 location on Day 116. At this time, the lateral movements began responding more to the north excavation activities. This resulted in the deformation vector moving towards the northwest.

4.3.4.3 Comparison of Settlements and Lateral Movements

Figure 4-22 shows a comparison of lateral movements measured at Inclinometers 1 and 2 and settlement data for sections taken perpendicular to the east secant pile wall through the inclinometer locations. Because the settlement data represents soil movements at elevation +0.6 m CCD, the inclinometer data were plotted horizontally by rotating its axis 90 degrees about this elevation, at the respective inclinometer locations. Good agreement is observed between the two data sets at both locations. Slight differences developed after backfill was completed on Day 355, as a result of small consolidation settlements arising from the lowered water table (see Figure 4-10). The agreement between the settlement behind the excavation support wall and the lateral movements shows that there was essentially no volume change in the saturated clays until after the excavation had reached final grade. The volume changes that occurred after Day 117 were a consequence of the excavation-induced drop in the groundwater level. The agreement between the inclinometer and settlement data suggests that settlement behind the support wall can be reliably estimated from inclinometer data when considering excavations through saturated clay.

This fact is extremely beneficial when designing an excavation support system to minimize excavation-related damage to adjacent structures. Current state-of-the-art procedures use finite element models to design excavation support systems. Finite element models used to predict wall deflections provide relatively good accuracy if a representative soil model is used and construction procedures are accurately represented in the analysis. However, prediction of the ground surface settlement induced by excavation is not as good as that of the wall deflection. The finite element models tend to underpredict the vertical movements at the wall and overpredict the vertical movements away from the wall (Finno and Harahap, 1991), and hence any direct predictions of distortions under a building will not be accurate. However, based on the observations of the agreement between inclinometer and settlement data, computed lateral deformations can be used to predict the settlement distribution behind the support wall, and hence to predict expected distortions beneath a building. This approach is valid only for cases involving no volume changes during excavation, as is typical for excavations through saturated soft to medium clay.

4.4 LOADS IN CROSS-LOT BRACES AND TIEBACKS

The loads in the struts and tiebacks located adjacent to the Inclinometer 1 location are given in Figure 4-23. These loads included strain gauge data from Strut 4 and Strut 5 and load cell data from the upper and lower tiebacks below Inclinometer 1. The loads in the struts and tiebacks that corresponded to the Inclinometer 2 location are given in Figure 4-24. These loads included strain gauge data from Strut 3 and Strut 4 and load cell data from the upper and lower tiebacks below Inclinometer 2. In the figure, the loads are given in units of force per unit width of the tributary area of the support. The loads presented for the tiebacks were obtained dividing the load recorded by the load cell by the center-to-center spacing of the tiebacks (1.5 m). The loads per width of wall given for the struts were obtained by dividing the axial load derived from the strain gauge readings by the center-to-center spacing of the struts (6.1 m). The axial loads were calculated by first plotting the observed strains with respect to their distances from the neutral axis, which yielded the strain distribution in the strut. To remove the effects of bending due to self-weight of the strut, the axial strain at the neutral axis was multiplied by the cross-sectional area and the modulus of elasticity of the strut to obtain axial load.

The struts were not preloaded prior to recording the strain measurements. Consequently, the strut loads begin at zero and gradually increased to between 115 kN per m and 130 kN per m in response to the excavation. The design load for all struts was 120 kN per m. After the excavation reached its full depth at both locations (Day 114 at Inclinometer 2 and Day 116 at Inclinometer 1), fluctuations of ± 25 kN per m were observed. The fluctuations observed in the strut loads are primarily due to fluctuations in the temperature. The upper and lower level tiebacks on the east side were locked-off at 220 kN per m and 280 kN per m, respectively. These lock-off loads were equivalent to 80 percent of the design loads.

Both Figure 4-23 and Figure 4-24 show that when the lower level of tiebacks were installed and prestressed, the loads in the upper level tiebacks decreased by about 20 kN per m. During the same time, the loads in the lower tiebacks increased by a similar amount. Thereafter, the tieback loads essentially remained constant with variations of no more than ± 10 kN per m. This response suggests that the anchors embedded in the stiff to hard clays did not creep significantly over the 170-day period when data were obtained.

4.5 SUMMARY

The construction at the site was separated into three stages; wall installation, support system installation and excavation, and station renovation and backfill. Lateral soil movements of the secant pile wall developed as the wall was installed with the maximum movement occurring within the soft clay. The lateral movements extended to the depth of the secant pile wall. The lateral movements associated with stress relief from the excavation were on average about one-half the total movement. The significance of this observation is that often only the movements associated with stress relief during excavation and support placement are considered when predicting the movements associated with supported excavations. Some of the lateral movements were attributed to a reduction in bending stiffness of the wall, which was the result of chipping the secant pile walls to the flange of the W24x55 sections for their entire exposed length.

It was observed that larger lateral movements occurred at the location of Inclinometer 4 on the west wall compared to the locations of Inclinometer 1 and 2 on the east wall. This was due to the presence of surcharge loads on the west side, in the form of construction equipment and materials and the higher insitu stresses in the soil because of the absence of an overcompensated structure on that side of the excavation. The larger movements on the west side were also a function of the large movements that occurred during wall installation.

At the Inclinometer 5 location, the lateral movements were divided into a north-south component (toward Chicago Avenue) and an east-west component (toward State Street). The resultant of the two components of the lateral movement tracks the development of the surface settlement much better than either of the individual components and suggests that the deformational behavior of the northwest corner was a function of the 3-D excavation geometry at that location.

It was observed that Warde School settled as much as the soil displaced laterally, with the settlement extending as far behind the excavation as the secant pile wall extended below the bottom of the school’s foundation. The adjacent settlements were virtually identical to the lateral movements within the soft clay until after the excavation was complete and the secant pile wall had been chipped to the flange. Thereafter, the settlements became slightly greater than the maximum lateral movements.

The struts were not preloaded prior to recording the strain measurements. Consequently, the strut loads begin at zero and gradually increased to between 115 kN per m and 130 kN per m in response to the excavation. The tieback anchors were embedded in the stiff to hard clays. Loads in these elements essentially held their lock-off loads and did not creep significantly over the 170-day period when data were obtained.

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