Keywords

1 Introduction

Working analysis of a deep foundation pit constructed by top-down method was performed in this study. The selected project for analysis is the 27 Lang Ha building. The foundation pit of the building includes 2 basements, with a depth of 8.9 m. The plan size of the foundation pit is 30 × 40 m. The building consists of 27 floating floors. The construction method in the form of top-down. A typical design cross-section is shown in Fig. 1 [1].

Fig. 1
A close-up of a paper with the Soletanche Bachy logo. It displays a typical design section of a diaphragm wall. The soil layers and dimensions are mentioned.

Typical design section

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In order to consider the working of the above-mentioned foundation pit, a parametric study has been implemented in detail considering the influence of factors such as diaphragm wall depth, diaphragm wall stiffness, surface load, and initial groundwater level. The results of numerical simulation analysis, combined with field monitoring data, are the basis for evaluating effective design and construction solutions.

2 Modeling

The finite element method was used to study the problem. The software Plaxis 2D version 8.6 [2] was employed to simulate the problem according to the plane strain problem model.

2.1 Geometry

The analyzed cross section is shown in Fig. 2. A finite element mesh consists of 15 node plane strain elements. The total number of elements is greater than 1000 elements. The number of elements varies depending on the simulation case. Assuming the problem is symmetric, a half of the problem was considered.

Fig. 2
A software Plaxis 2-dimensional graph with meshes highlights the simulated sections.

The simulated section

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2.2 Soil Properties and Initial Groundwater Level

The ground foundation consists of 5 soil layers, from top to bottom, including the filling 1, sandy clay 2, mud 3a, sandy clay 3 and 4. The soil was modelled by Mohr–Coulomb model. The model parameter values are shown in Tables 1 and 2 for short-term and long-term working conditions, respectively. Note that due to incomplete geological data, some parameter values were assumed empirically.

Table 1 Parameters of Mohr–Coulomb model (short term condition)
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Table 2 Parameters of Mohr–Coulomb model (long term condition)
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The initial groundwater level is located 2 m below the natural ground (at an elevation of –2.0 m). During the construction of the foundation pit, to ensure that the foundation pit is dry, it is necessary to carry out a suction pump to lower the groundwater level to the bottom of the excavation at each stage.

2.3 Diaphragm Wall and Shoring System

The diaphragm wall is made of cast-in-place concrete, using bentonite solution to support the wall. The concrete has a strength of C30. The wall thickness is 0.6 m. The wall is modelled according to a linear elastic model with parameters given as follows: E = 2.57 × 107 kN/m2, EA = 1.540 × 107 kN/m, EI = 4.626 × 105 kNm2/m, w = 9 kN/m/m, ν = 0.2. Slabs and temporary struts can be simulated by single-ended anchor struts with stiffness EA = 3,500 × 107 kN.

2.4 Surface Load

The surface load is assumed to be evenly distributed on the ground with strength q = 10 kPa.

2.5 Construction Stages

The construction sequence is assumed as follows:

Stage 1: Construction of the diaphragm wall to a depth of −16.5 m.

Stage 2: Construction of the top slab, then digging the foundation pit to elevation –1.5 m, applying surface load q = 10 kPa.

Stage 3: Digging the foundation pit to the elevation –3.5 m, constructing the middle slab and installing a temporary strut system, pumping to lower the groundwater level to the level of the bottom of the foundation pit.

Stage 4: Digging the foundation pit to elevation –6.0 m, constructing the bottom slab, pumping to lower the groundwater level to the level of the bottom of the foundation pit.

Stage 5: Digging the foundation pit to elevation –8.9 m, pumping to lower the groundwater level to the level of the bottom of the foundation pit.

2.6 Parameter Studies

The influence factors such as diaphragm wall stiffness (C30, C40), diaphragm wall depth (H = 13.5 m, 16.5 m, and 19.5 m), surface load (q = 10,20, 50 kPa), and initial groundwater level (−2.0, −3.5, −6.0 m) were studied in detail. The ground soils were considered with two cases of short-term and long-term conditions, corresponding to undrained and drained geotechnical experimental data, respectively (see Tables 1 and 2). The total number of analyzed cases was 30 cases.

3 Simulation Results and Discussion

3.1 Ground Settlement

The results of calculation of ground settlement at the final excavation stage (stage 5) under short-term working condition are shown in Figs. 3a, b, c with values q = 10, 20 and 50 kPa, respectively. We considered the stiffness of the diaphragm wall by employing concrete grades C30 and C40. The results show that the differences in settlement values simulated with two concrete strengths C30 and C40 are not large.

Fig. 3
Three line graphs plot ground surface settlement versus distance from the edge of the excavation with different values of q such as 10, 20, and 50 kilopascals. The lines start from the origin as a concave-down decreasing curve and flow as a concave-up increasing curve.

Effect of wall stiffness on the surface settlement (short term)

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Similarly, under long-term working condition, the results of simulated ground settlement at the final excavation stage are shown in Fig. 4. The results show that the stiffness of the wall does not significantly affect the settlement of the ground surface. Compared with the simulated settlement values under the short-term condition (Fig. 3), those under the long-term condition have larger values.

Fig. 4
Three line graphs plot ground surface settlement versus distance from the edge of the excavation with different values of q such as 10, 20, and 50 kilopascals. The lines start from the origin as a concave-down decreasing curve and flow as a concave-up increasing curve.

Effect of wall stiffness on the surface settlement (long term)

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Figure 5a and b shows the effect of the depth of the diaphragm wall on the surface settlement at the final excavation stage under short-term and long-term working conditions, respectively. The calculation results reveal that the wall depth does not significantly affect ground surface settlement, especially under short-term conditions. The ground surface settlement under the long-term condition (Fig. 5b) is larger than that under the short-term condition (Fig. 5a).

Fig. 5
Two line graphs plot ground surface settlement versus distance from the edge of the excavation with different heights such as 13.5, 16.5, and 19.5 meters. The lines start from the origin as a concave-down decreasing curve and then flow as a concave-up increasing curve.

Effect of depth of diaphragm wall on the surface settlement

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The effect of surcharge load on the surface settlement (final stage) is shown in Figs. 6a and b, respectively, under short-term and long-term working conditions. The calculation results show that the surface load has a significant influence on the ground settlement, especially under short-term conditions (Fig. 6a). The larger the load, the greater the settlement of the ground surface. Under the long-term condition (Fig. 6b), the ground settlement has a larger value than that under the short-term one (Fig. 6a).

Fig. 6
Two line graphs plot ground surface settlement versus distance from the edge of the excavation with different values of q such as 50, 20, and 10 kilopascals. The lines start from the origin as a concave-down decreasing curve and then flow as a concave-up increasing curve.

Effect of surcharge load on the surface settlement

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Figure 7 shows that the initial groundwater level (GWL) has a significant influence on the ground surface settlement, especially under long-term conditions. The shallower the initial groundwater level, the greater the ground settlement value due to the large distance between the initial groundwater level and the foundation pit bottom elevation. In the long-term condition (Fig. 7b), the ground surface settlement has a larger value than that in the short-term condition (Fig. 7a).

Fig. 7
Two line graphs plot ground surface settlement versus distance from the edge of the excavation with different G W L values such as 6, 3.5, and 2 meters. The lines start from the origin as a concave-down decreasing and then flow as a concave-up increasing curve.

Effect of initial groundwater level on surface settlement

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3.2 Diaphragm Wall’s Horizontal Displacement

Figure 8 shows that in the comparison of diaphragm walls with different concrete strengths C30 and C40, the concrete strength stiffness has a small influence on the horizontal displacement of the diaphragm wall.

Fig. 8
A line graph plots depth from the ground surface versus the horizontal displacement of the wall. The lines start from the origin as a concave-up decreasing and then flow as a concave-down decreasing curve.

Effect of wall stiffness on the horizontal displacement of the diaphragm wall

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Figure 9 shows that the surface load has a significant effect on the horizontal displacement of the diaphragm wall. However, when the load value is small (q = 10, 20 kPa), there is not too much difference in the horizontal displacement caused by the surface load.

Fig. 9
A line graph plots depth from the ground surface versus the horizontal displacement of the wall. One of the lines starts from the origin as a concave-up decreasing curve and then flows as a concave-down decreasing curve. The other line flows almost vertically downward.

Effect of surface load on the horizontal displacement of the diaphragm wall

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The initial groundwater level has a significant influence on the wall displacement value (Fig. 10) under long-term working conditions. The shallower the initial groundwater level, the larger the horizontal displacement value, especially the wall segment below the elevation of the excavation bottom. It is noted that the horizontal displacement of the diaphragm wall could be reduced if the small strain soil stiffness and its stress state dependency could be taken into consideration.

Fig. 10
A line graph plots depth from the ground surface versus the horizontal displacement of the wall. Three lines start from the origin as a concave-up decreasing and then flow as a concave-down decreasing curve.

Effect of initial groundwater level on the horizontal displacement of the diaphragm wall

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3.3 Diaphragm Wall’s Internal Forces

The results of bending moment in the diaphragm wall at the final construction stage (excavating to the bottom elevation of the foundation pit) are shown in Figs. 11, 12, 13 for the walls with different concrete strengths of C30 and C40. Comparisons are also made considering different surface loads q = 10, 20 and 50 kPa, for both short-term and long-term working conditions. In general, in all comparison cases, the more rigid the wall, the larger the bending moment in the wall. However, the difference in bending moment values is not large for walls with the aforementioned stiffness. The effect of wall stiffness will be further investigated with employing a wider range of strength values.

Fig. 11
Two line graphs plot depth from the ground surface versus the bending moment. Values are estimated. Short term, C 30, (300, negative 10). C 40, (negative 500, negative 6), (100, negative 2). Long-term, C 30, (200, negative 10). C 40, (negative 500, negative 6). Both start and end at the origin.

Effect of the concrete strength of diaphragm wall on the bending moment (q = 10 kPa)

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Fig. 12
Two line graphs plot depth from the ground surface versus the bending moment. Values are estimated. Short term, C 30, (300, negative 10). C 40, (negative 500, negative 6), (100, negative 2). Long term, C 30, (0, negative 10). C 40, (negative 500, negative 6). Both lines start and end at the origin.

Effect of the concrete strength of diaphragm wall on the bending moment (q = 20 kPa)

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Fig. 13
Two line graphs plot depth from the ground surface versus the bending moment. Values are estimated. Short term, C 30, (300, negative 10). C 40, (negative 500, negative 6), (100, negative 2). Long term, C 30, (0, negative 10). C 40, (negative 2000, negative 6), (negative 250, negative 2).

Effect of the concrete strength of diaphragm wall on the bending moment (q = 50 kPa)

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As can be seen in Fig. 14a, the surface load has a negligible influence on the desired moment value in the wall (C30) under short-term conditions. However, under long-term conditions, a large surface load (q = 50 kPa) has a significant effect on the bending moment in the wall with the same stiffness (Fig. 14b). The above results are also suitable for walls with greater stiffness (C40).

Fig. 14
Two line graphs plot depth from the ground surface versus the bending moment. Values are estimated. Some of the trends are. Short term, 50 kilopascals, (0, negative 8), (negative 500, negative 6), (100, negative 2). Long term, 50 kilopascals, (0, negative 16), (negative 2100, negative 6).

Effect of surcharge load on the wall bending moment

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The influence of the initial groundwater level on the bending moment in the wall is shown in Fig. 15. In general, the shallower the groundwater level, the greater the value of the bending moment in the wall, especially for the part of the wall below the elevation of the bottom of the pit.

Fig. 15
A line graph plots depth from the ground surface versus the bending moment. Values are estimated. G W L 6 meters, (0, negative 8), (negative 500, negative 6). G W L 2 meters, (100, negative 10), (negative 500, negative 6), (50, negative 3). G W L 3.5 meters almost overlaps with G W L 2 meters.

Effect of initial groundwater level on the wall bending moment (long term)

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4 Conclusions

Study on the influence of the deep excavation of the 27 Lang Ha high-rise building by top-down construction method on the deformation of the adjacent ground surface and the diaphragm wall has been analyzed in detail.

The analysis was implemented by the finite element method, employing the plane strain problem. A parametric study has been carefully analyzed, by considering influencing factors such as wall stiffness, wall depth, ground surface loading, and initial groundwater level, with considering two cases of short-term and long-term ground working. The total number of analyzed cases was 30 cases. Following are the analysis results.

The settlement of the ground surface adjacent to the deep excavation depends significantly on the surface load and the initial groundwater level. In particular, the larger the surface load and/or the shallower the initial groundwater level, the larger the settlement of the ground surface. Under the long-term condition, the surface settlement has a larger value than that under the short-term condition. Within the scope of the present study, the depth and the concrete strength of the diaphragm wall did not largely affect the settlement of the adjacent ground surface.

The horizontal displacement and the bending moment values in the diaphragm wall depends significantly on the initial groundwater level and the large surface load. In general, the shallower the initial groundwater level, the greater the horizontal displacement and the greater bending moment values in the wall, especially the part of the wall located below the bottom elevation of the foundation pit.