THE CASE HISTORY 1. Project overview

The case, situated in the Sinyi district of the Taipei city, is a 30-story structure with a seven-level basement. The excavation site was about 121.8 m long and 66.1 m wide.

Fig. 2 shows the plan view (Ou 2006, Ou 2011). No buildings existed near the

excavation. The diaphragm wall, designed with 27.5 MPa of the compressive strength (f’c), was 1.5 m in thickness, and extended from ground surface down to a depth of 57.5 m, about 6.5 m penetrating into the silty sand /gravel formation. The excavation depth was 32.5 m, which was completed using the top-down construction method with seven levels of concrete slab as shown in Fig. 3. The concrete slabs 1FL and B1FL used the beam-plate system, which had thicknesses of 25 cm and 20 cm, respectively; the concrete slabs B2FL~B6FL used the flat slab system, which had a thickness of 61 cm.

After excavating down to GL-29.4 m (GL refers to the ground surface level), 3-H400x400x13x21 mm steel struts, with a spacing of 6.4 m and slope of 4.6:6, were installed as temporary supports. The site was then excavated to GL-32.5 m.

Fig. 2. Plan view and the instrumentation of the case history.

To reduce the lateral wall deflection and ground settlement induced by excavation, 3 cross walls of 1.0 m in thickness and 26 m in intervals (L′ ) were constructed and their depths were between GL-1.5 m and GL-45m in the north-south direction. Ten buttress walls of 1.0 m thickness and 55 m depth, with lengths varying from 6 m to 15 m, were constructed where relatively large lateral wall deflection was expected (Fig. 2). The cross walls and buttress walls between GL+0 m to GL-1.5 m were backfilled with the in-situ soil, those between GL-1.5 m to GL-22 m were cast with 13.7 MPa concrete, and those below GL-22 m were cast with 24.0 MPa concrete. Both the cross wall and buttress wall were demolished with excavation process.

Many inclinometers were installed along the excavation sides. The inclinometers SI-8 and SO-1 and their corresponding settlement measurement sections, SET8 and SET1, were used to compare with analysis results (Fig. 2). The complete field observation data for this project are shown in Ou (2006).

2.2. Subsurface soil profile

The bedrock is located at a depth of 66.7 m, and its SPT-N value is greater than 50.

Above the bedrock are silty clay (CL) and silty sand with gravel (SM/GW). The groundwater table is located at GL-3 m. The porewater pressure in silty clay is generally hydrostatic, but the piezometric porewater pressure in the SM/GW was 10 m below the ground surface, i.e., at GL-10 m. The total unit weight (γt), effective internal friction angle (φ’), over-consolidation ratio (OCR), SPT-N value, undrained shear strength (su) and porewater pressure (uw) are shown in Fig. 4. As shown in the figure, the undrained shear strength of clay was obtained from the triaxial unconsolidated undrained (UU) test and the effective strength parameters for clay and sand/gravel were obtained from triaxial consolidated undrained test and the direct shear test, respectively.

Fig. 3. Locations of the slabs and steel strut for the case history.

0 25 50

Fig. 4. The subsurface soil layers for case history.

2.3. Monitoring results

The relationship between the maximum wall deflections and excavation depths for excavations in soft clay in Taipei, under the plane strain condition and without remedial measures like soil improvement, has been studied by Ou (1993), as shown in Fig. 5. As shown in this figure, the maximum wall deflection (δhm) increases with the excavation depth (H). The wall deflection in soft clay is generally greater than that in sand. The ratio of the maximum wall deflection to excavation depth (δhm/H) is around 0.2% to 0.5%, in which the upper limit is mostly for clay, the lower limit for sand and those for the alternating layers of sand and clay fall in between the two limits. This figure represents general trend of wall deflections for excavations in Taipei. Fig. 5 also presents the maximum wall deflections obtained from the 8 inclinometers in the north-south direction at the final stage in this case history. Because the case was situated in clayey layer, the relationship should be close to upper limit. However, the observed results are actually below and near the lower limit, much smaller than those of the excavations with similar depths in Taipei (Ou 1993), also much smaller than those of the cases compiled by Clough (1990).

hmδ

Fig. 5. Relationship between the maximum wall deflection and excavation depth.

Fig. 6 shows the lateral wall deflection and ground settlement at the final stage of excavation at SI-8 (SET8), which was located at the section where the cross wall was installed. As shown in this figure, the wall deflection was extremely small, and the maximum wall deflection reached barely 2.61cm. The wall displayed a concave type of displacement, in which the maximum wall deflection occurs near the excavation surface while the wall below the excavation surface showed a linear type of displacement

because of the restraining effect of the cross wall. The ground settlements were also comparatively small, with a maximum value reaching 1.2 cm. As shown in Fig. 6, the δhm/H was equal to 0.08%, much less the lower limit of the general trend for deep excavations in Taipei. This implies that the cross wall has a significant effect in reducing wall deflection and ground settlement.

Fig. 6 also shows the lateral wall deflections and ground settlement at the final stage of excavation at SO-1 (SET1) section, located at the midpoint between two cross walls.

As shown in this figure, the wall deflection and the ground settlement were of the larger

magnitude than those at the SI-8 (SET8) section. The δhm was 4.74 cm. The ground settlement was also very small, with a maximum value being 1.34 cm. As shown in Fig.

6, the δhm/H was equal to 0.14%, which is also less than the lower limit of the general trend.

Fig. 6. The observed wall deflections and ground surface settlements at the final excavation stage for SI-8 (SET8) and SO-1 (SET1) sections.

3 NUMERICAL ANALYSIS