Design and Performance of the Deep Excavation of a ... - ScienceDirect

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ScienceDirect Procedia Engineering 165 (2016) 682 – 694

15th International scientific conference “Underground Urbanisation as a Prerequisite for Sustainable Development”

Design and performance of the deep excavation of a substation constructed by top-down method in Shanghai soft soils a,b

a,b

a,b

Qiping Weng *, Zhonghua Xu , Zhihou Wu , Ruobiao Liu

a,b

a East China Architectural Design & Research Institute Co., Ltd., China Shanghai Engineering Research Center of Safety Control for Facilities Adjacent to Deep Excavations, China

b

Abstract The 500kV Hongyang Underground Transmission and Substation was situated in the downtown of Shanghai city in China. The project comprised a 5-storey building above ground and three levels basements. The excavation area and depth of this project were about 10800 m2 and 24.0 m, respectively. The ground soils were mainly thick soft soils comprising Quaternary alluvial and marine deposits. Surrounding environment of this project was quite complex. Considering the excavation area and depth, the soil condition, and the protection requirements of the adjacent facilities, top-down method was adopted to construct the foundation pit of this project. The excavation was supported by 1.2 m thick diaphragm walls. Three levels of underground structures and two additional temporary struts were used to support the diaphragm wall. Steel tubes erected in bored piles were used as vertical support system. An extensive instrumentation program was carried out to monitor the performance of the deep excavation and the adjacent facilities. Monitored data of the retaining structure and the surrounding facilities were analyzed in this paper. Observation results showed that top-down method was effective to control the deformation of the excavation. Design and construction of this project can be used as a reference for similar deep excavations. © 2016 2016Published The Authors. Published by Elsevier Ltd. © by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the scientific committee of the 15th International scientific conference “Underground Peer-review under scientific committee of the 15th International scientific conference “Underground Urbanisation as a Urbanisation as aresponsibility Prerequisite of forthe Sustainable Development. Prerequisite for Sustainable Development Keywords: soft soils; foundation pit, substation, top-down method, monitoring.

* Corresponding author. E-mail address: [email protected]

1877-7058 © 2016 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license

(http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the scientific committee of the 15th International scientific conference “Underground Urbanisation as a Prerequisite for Sustainable Development

doi:10.1016/j.proeng.2016.11.766

Qiping Weng et al. / Procedia Engineering 165 (2016) 682 – 694

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1. Project Description The 500kV Hongyang Underground Transmission and Substation (HUTS) located in the downtown area of Hongkou District in Shanghai, China. The project comprised a 5-story building above ground and three levels basements which would be used for accommodating large power transformers. Frame-shear wall structure and pile foundation were adopted for this project. The project was bounded by Yixian Road on the west side and Sanmen Road on the north side, as shown in Figure 1. The shape of the excavation site was quite regular (approximately 166 m × 68.4 m). The excavation area and depth of the project were 15916 m2 and 24 m, respectively. The environment was quite complex around the excavation. The Yixian Raod was about 40 m away from the excavation on the west side. The minimum distance between the foundation of the Elevated Yixian Road and the excavation was about 51.4 m. There was a railway between the Yixian Road and the excavation. The minimum distance between the railway and the excavation was only about 15.0 m. A large number of municipal pipelines such as power cables, water supply pipes, and gas pipelines were distributed on the west side of the excavation. The minimum distance between the pipelines and the excavation was about 12.25 m. The Sanmen Road was about 8.0 m away from the excavation on the north side. There were also several pipelines under the road and the minimum distance between the pipelines and the excavation was about 5.0 m. On the other side of Sanmen road, there was a 4storey building with brick and concrete structure. The building was about 33.2m away from the excavation. 2. Ground Condition The HUTS project site was situated at Yangtze River Delta alluvial plain. According to the geotechnical investigation report (ECEPDI, 2013), the ground soils at the construction site were mainly thick soft soils comprising Quaternary alluvial and marine deposits. As shown in Figure 2, from ground surface to a depth of about 100 m, the underground could be divided into 8 layers, among which Layer ĸ, Layer Ļ, Layer ľand Layer Ŀ could be subdivided into 2, 5, 2 and 2 sub-horizontal layers, respectively. The first layer (Layer ķ) was a less than 4.3-m-thick artificial fill in general. The second layer was divided into two sub-layers, namely, Layer ĸ1, and ĸ3. Layer ĸ1 was brownish yellow silty clay with medium-soft plastic and the thickness of this sub-layer ranged from 0.5m to 2.9m. Layer ĸ3 was gray sandy silt with an average thickness of 3.1m. The third and fourth layers were very soft silty clay (Layer Ĺ) and very soft clay (Layer ĺ). This two layers had large void ratio, low shear strength and high compressibility. Average thickness of Layer Ĺ and Layer ĺ were 3.3 m and 5.9 m, respectively. Mean value of the water content of Layer Ĺ and Layer ĺ was about 47% and undrained shear strength was about 34 kPa. Underlying was the fifth layer, which was divided into five sub-layers, namely, Layer Ļ1-1, LayerĻ1-2, LayerĻ2-1, LayerĻ2-2, and LayerĻ2-3.The fifth layer was mainly gray silty clay and sandy silt with medium palstic and medium to high compressibility. The physical and mechanical properties of this layer were much better than that of Layer ĺ. Sandy silt in this layer composed the first confined aquifer. The sixth layer was divided into two sub-layers, namely, Layer ľ2-1, and Layer ľ2-2 with an average thickness of 12.7 m and 11.7 m, respectively. The sixth layer was mainly gray silty clay and silty sand with softmedium palstic and high to medium compressibility. The next layer was divided into two sub-layers, namely, Layer Ŀ1 and Layer Ŀ2. Layer Ŀ1 was dense fine sand and Layer Ŀ2 was medium sand. The SPT N values of Layer Ŀ 1 and Layer Ŀ2 were about 60 and 72, respectively. Sand in this layer composed the second confined aquifer in Shanghai. Underlying was the silty clay (Layer ŀ) with palstic and medium compressibility. Table 1 shows the geotechnical parameters of the soils at the construction site.

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Fig. 1. Plane view of the foundation pit

Fig. 2. Profile of soils

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Table 1. Geotechnical parameters of the soils ω (%)

γ (kN/m3)

e

c (kPa)

φ (e)

Es0.1-0.2 (MPa)

kv (cm/s)

kh (cm/s)

N

ĸ1 Silty clay

36.8

18.8

1.0

11

24

5.3

6.3E-08

5.2E-08

4

ĸ3 Sandy silt

34.9

18.6

1.0

7

30

8.0

1.2E-05

3.2E-04

7

Ĺ Very soft silty clay

46.6

17.4

1.3

9

13

3.1

2.3E-07

3.0E-06

2

ĺ Very soft clay

47.6

17.2

1.3

9

10

2.3

2.3E-07

7.2E-07

2

Ļ1-1 Soft silty clay

42.3

17.8

1.2

10

15

3.1

2.5E-07

2.9E-06

4

Ļ1-2 Silty clay

34.0

18.4

1.0

12

26

4.2

1.2E-05

7.1E-05

/

Ļ2-1 Sandy silt

28.9

18.6

0.9

6

31

8.9

7.9E-05

2.1E-04

16

Ļ2-2 Clayey silt

29.3

18.5

0.9

10

22

7.1

6.5E-06

5.4E-05

20

Ļ2-3 Sandy silt

29.6

18.3

0.9

6

29

15.6

3.5E-05

9.6E-05

28

ľ2-1 Silty clay with silty sand

31.7

18.0

1.0

10

26

5.2

8.7E-06

3.0E-05

26

ľ2-2 Silty clay interbedded with silty sand

27.1

18.6

0.9

6

32

9.0

1.3E-04

2.5E-04

25

Ŀ1 Fine sand

25.3

19.1

0.8

6

35

14.9

1.0E-03

1.2E-03

60

Ŀ2 Medium sand

16.4

19.7

0.6

5

36

15.2

3.8E-04

3.0E-04

72

ŀ Silty clay

27.9

19.6

0.8

33

18

8.0

1.1E-07

2.1E-07

/

Soil layer

Note: wn=natural water content; J˙unit weight; e˙voids ratio; c˙cohesion obtained from direct shear test; φ˙angle of internal friction obtained from direct shear test; Es0.1~0.2˙compressibility modulus; kv˙vertical coefficient of permeability; kh˙horizontal coefficient of permeability; N˙blow counts of standard penetration test.

3. Design of the Supporting System of the Excavation 3.1 Overall design scheme Considering the excavation area, the excavation depth, the structure of the substation, and the protection requirements of the adjacent facilities, top-down method was adopted to construct the foundation pit of this project. The soil was designed to be retained by diaphragm walls, which in turn were supported by the three levels floors of the basement and two levels additional temporary struts. Steel tubes erected in bored piles were used to support the slabs and temporary struts. 3.2 Soil retaining structure The soil of the excavation was retained by 1.2 m thick dual-purpose RC diaphragm walls. At the excavation stage, the diaphragm walls were used as retaining and waterproofing structures. At the service stage, the diaphragm walls would be used as a part of permanent walls of the basement. It was decided to embed the retaining wall toe into the ľ2-1 layer (see Figure 3) to cut off the first confined aquifer in Layer ᬉ2-1 and Layer ᬉ2-3. The embedded length was 38.0 m, making the diaphragm wall as deep as 57.7m. As the lower part of the diaphragm wall (below the elevation of -43.300 m, see Figure 3) was only used to cut off the first confined aquifer, no rebar was used in this part of diaphragm wall. The widths of the wall panels were 6 m. The rigid steel I-beam joints which had effective water sealing performance were adopted to joint the panels of the diaphragm walls. Jet grout columns with diameter of 2.4 m were installed at the outside of the joints of the diaphragm walls to form a second barrier against water. The walls were made of Grade 35 reinforced concrete. Toe grouting was carried out on all panels. Reinforced concrete lining walls with thickness of 1.0 m were successively constructed in accordance with excavation progress to increase wall stiffness and also to form a third barrier against water.

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3.3 Lateral support system The diaphragm walls were braced at the floor levels by the three basement slabs. The roof slab was also used as platform for soil excavators, dump trucks and other construction machines. Eight big access openings (see Figure 4) were distributed in the slabs to facilitate the removal of the excavated materials and delivery of building materials. This would improve the working environment underneath the roof slab and increase the construction speed. Height of the second basement was 7.4 m and the vertical distance from the B2 slab to the formation level was 8.95 m. In order to reduce the spacing of the lateral support system and control wall movements, temporary RC struts were installed between the slabs in the second and third level basement (see Figure 3). Figure 4, Figure 5, and Figure 6 show the plane view of the B0, B1, and B2 slab, respectively. Figure 7 shows the plane view of the two levels temporary RC strut frame system. The section size of the temporary main struts was 1400 mm h 900 mm.

Fig. 3. Sectional view of the retaining system

3.4 Vertical support system Temporary steel columns at the same position of the permanent columns were erected in bored piles to support the underground structures that were constructed from the top level downward at the excavation stage. The design value of load acting on a single temporary steel column was 8000 kN. Steel tubes with diameter of 550 mm and thickness of 16 mm, filled with Grade 60 concrete, were used as vertical support columns. Perpendicularity of 1/500 was required for the installation precision of these columns. The steel tubes would finally be encased in concrete to be transferred into permanent columns. Figure 8 shows an elevation view of the steel tube showing its connection with slabs and temporary struts.


/=

/=

/=

/=

/=

/=

Figure 4. Plane view of the B0 slab



Figure 7. Plane view of the temporary struts

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688


Figure 8. Elevation view of the vertical support system

A total of 152 bored piles were installed to support the steel tubes at the excavation stage. After the construction of the bottom slab and the recovery of groundwater, these piles would be subjected to uplift load caused by groundwater pressure acting on the bottom slab. This meant that these piles would be compressed at the excavation stage and tensioned at the service stage. Design of these piles thus should meet the requirements at both the excavation stage and the service stage. The diameter of these piles was 1000 mm. The fine sand layer (Layer ླྀ1)

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was selected as bearing layer for these piles. Pile tips went down to a depth of about 80.1 m under the ground surface and the effective length of the piles was 56.1 m (see Figure 8). The design value of bearing capacity of a single pile was 8000 kN, which equaled to the design value of load acting on a single temporary steel column. These piles were made of Grade 35 reinforced concrete. Perpendicularity of 1/400 was required for the installation precision of these piles. 4. Construction and Monitoring Construction of the excavation involved diaphragm wall and vertical supports construction, soil cut, underground slabs and temporary horizontal struts construction. The installation of diaphragm walls and piles commenced in October 2013 and it took roughly 10 months to complete. Excavation started in September 2014. After finishing the excavation of the first layer soils, the B0 slab was constructed. Then, soil excavation, construction of underground slabs and temporary horizontal struts were conducted alternatively using top-down method. Construction of the whole underground structure lasted for about two years totally. Table 2 shows the construction sequence of the HUTS project. Figures 9 to Figure 12 show some photos of the construction progresses of the project. Table 2. Construction sequences of the HUTS project Stage Stage1 Stage2 Stage3 Stage4 Stage5 Stage6 Stage7

Construction activities Diaphragm walls and piles construction First level soil excavation and B0 slab construction Second level soil excavation and B1 slab construction Third level soil excavation and first level temporary struts construction Forth level soil excavation and B2 slab construction Fifth level soil excavation and second level temporary struts construction Final soil excavation and bottom slab construction

Figure 9. Construction of diaphragm wall and piles

Figure 11. Construction of B1 slab

Finishing Time 2014/7/10 2014/11/8 2015/2/3 2015/4/14 2015/7/15 2015/8/28 2015/10/25

Figure 10. Construction of B0 slab

Figure 12. Construction of bottom slab

Field monitoring is necessary to provide a means by which geotechnical engineers can verify the design assumptions and the contractors can execute the work with safety and economy (Ng 1998). Observed performance

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of deep excavations has been reported by many researchers (such as O’Rourke 1981; Clough and O’Rourke 1990; Ng 1998; Ou et al. 1998; Finno and Bryson 2002; Liu et al. 2005; Blackburn and Finno 2007; Tan and Li 2011; Tan and Wei 2012). These field data have contributed a lot to understand the performance of excavations and adjacent facilities. To monitor the performance of the excavation and the effects of the excavation on the surrounding facilities, various instruments were installed at the construction site in this project (see Figure 1 and Figure 4). Inclinometer tubes were installed in the diaphragm walls to measure the lateral displacement of walls. Displacement survey points were installed to monitor the vertical displacement of steel columns, adjacent railway, roads and pipelines. Wells were installed inside and outside the foundation pit to monitor the phreatic water table and confined ground water table (see Figure 1). 5. Monitored Results and Analysis 5.1 Lateral displacement of diaphragm wall Figure 13 depicts the lateral displacement in the diaphragm wall at stages 3, 4, 5, 6, and 7 (see Table 2) which were the main excavation stages. It can be seen that the lateral displacement of wall gradually developed into bulging proﬁles as the excavation proceeded. Generally speaking, the most obvious deflection increments were observed in Stage 3, Stage 4, and Stage 5. The maximum lateral displacement of wall was 51.8 mm at the depth of 21.0 m and it was occurred at inclinometer CX4 at stage 7. The ratio between the maximum lateral displacement of diaphragm wall and the excavated depth was 0.22%. As the length of west and east side of the foundation pit (166 m) was much larger than that of the north and south side (68.4 m), maximum lateral displacement observed at inclinometer CX4 (51.8 mm, in the west side) and CX14 (50.9 mm, in the east side) were much larger than that observed at inclinometer CX 9 (41.7mm, in the west side) and CX19 (44.6 mm, in the east side). This is consistent with the findings of Ou (1996). Lateral displacement at CX11 (40.4 mm) and CX18 (41.6 mm) were much smaller than that at CX4 and CX14 due to the corner effect (Lee et al., 1998).


Fig.. 13. Lateral displacement of diaphragm wall at different stages.

Wang et al. (2010) collected 32 case histories of deep excavations constructed by top-down method in Shanghai. They found that for walls constructed by top-down method (see Figure 14), values of maximum lateral displacement of wall δhm generally ranged from 0.1%H to 0.55%H with an average value of 0.27%H, where H is the excavation depth. Maximum lateral displacement of diaphragm wall in inclinometers CX4, CX9, CX14, and CX19 in this project are also shown in Figure 14. Average value of δhm/H for the four inclinometers CX4, CX9, CX14, and CX19 was 0.2%H. It can be seen that data points of this project fall below the average line. This shows that deformation control in this project was quite successful. 160 Values of other projects in Shanghai CX4, CX9, CX14, CX19 of this project

140

G hm

100 Ghm (mm)

H 5%

.5 =0

120 Average: Ghm=0.27%H

80

Ghm=0.2%H

60 40

H G hm=0.1%

20 0

0

2

4

6

8 10 12 14 16 18 20 22 24 26 28 H (m)

Figure 14. Maximum lateral displacement of wall versus excavation depth based on Wang et al. (2010)

691

692


5.2 Vertical Displacements of the central posts Figure 15 depicts the vertical displacement of central columns. It can be seem that the central columns were uplifted due to the rebounding caused by soil excavation. The maximum vertical displacement was 15.9 mm occurring at L35 at stage 7. The vertical displacement at L34 and L35 was much larger than that at L32 and L37. This is because that L34 and L35 located at the center of the foundation pit while L32 and L37 were near the diaphragm wall which restricted the rebounding of the soils around it. This is consistent with the findings of Xu et al. (2006).

Vertical displacement/mm

20 18 Stage4 Stage5 Stage6 Stage3 Stage7 16 14 12 10 L32 8 L33 6 L34 4 L35 2 L36 0 L37 -2 2014.11.8 2015.02.03 2015.04.14 2015.07.05 2015.08.28 2015.10.25 Date

Figure 15. Vertical displacements of central columns

5.3 Settlement of adjacent railway Figure 16 shows the settlement of the adjacent railway on the west side of the foundation pit. Small settlement (maximum value 3.2 mm) was observed during the period of construction the diaphragm wall and piles (Stage 1). Then the railway settled with the proceeding of the excavation. Maximum settlement occurred at point T14, which was near the center of the foundation pit. The maximum settlement was 22.9 mm at the final stage. It can also be seen that settlements at point T1, T2, T3, T4, T23, T24, and T25 were less affected by the excavation because these points were far away from the foundation pit. The maximum differential settlement was 4.7 mm and it occurred between T4 and T5. Though the railway settled, the normal operation of the railway was no affected. 0

Settlement/mm

5 10 15 20 25 30 35 40 T1 T2

Stage1 Stage2 Stage3 Stage4 Stage5 Stage5 Stage7 T3

Range of the excavation

T5 T4

T9

T7 T6

T8

T11 T10

T17

T15

T13 T12

T14

T16

T23

T21

T19 T18

T20

T22

T24 T25

Figure 16. Settlement of adjacent railway

5.4

Groundwater table variation Figure 17 shows the variation of confined groundwater table in Layer ᬉ2-3. Monitoring points Y1~Y3 located inside the foundation pit while Y4~Y11 located outside the foundation pit. It can be seen that when pumping confined groundwater inside the excavation, drawdown of groundwater table at the measured points Y1~Y3 inside the excavation was very obvious (with a maximum value of about 22.0 m). However, during the whole excavation stages, drawdown of groundwater head at the measured points Y4~Y11 was less than 1.7 m and within the range of

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local code. This also indicates that the construction quality of the diaphragm was quite good and it successfully cut off the confined ground water layers. 0

Confined groundwater table (m)

-5 -10 -15 -20 -25 -30 Y1 Y7

-35 -40 2014/9/1

2014/11/1

Y2 Y8

2015/1/1

Y3 Y9

Y4 Y10

2015/3/1

Y5 Y11

2015/5/1

Y6

2015/7/1

2015/9/1

2015/11

Time (d)

Figure 17. Variation of confined groundwater table inside and outside the excavation

6. Summary The 500kV Hongyang Underground Transmission and Substation (HUTS) located in the downtown area of Shanghai with quite poor geological condition and strict requirements of environmental protection. Top-down method was adopted in this project. The excavation was retained by 1.2 m thick diaphragm walls. Three levels underground slabs and two levels temporary struts were used to support the diaphragm walls. Steel tubes with diameter of 550 mm and thickness of 16 mm, filled with Grade 60 concrete, were used as vertical supporting columns. Monitored results show that maximum lateral displacement was only 51.8 mm and the ratio between the maximum lateral displacement of wall and the excavated depth was 0.22%. The central columns were uplifted due to the rebounding caused by soil excavation. And the maximum vertical displacement was 15.9 mm. Maximum settlement of the adjacent railway was 22.9 mm and the normal operation of the railway was no affected. Large drawdown of confined groundwater inside the excavation caused almost no influence on the outside confined groundwater layers as the diaphragm wall successfully cut off the confined ground water layers. Design and construction of this project was quite successful and it can be used as a reference for similar deep excavations.

References [1].Blackburn, J.T. and Finno, R. J. (2007). “Three-dimensional responses observed in an internally braced excavation in soft clay”. J. Geotechnical and Geoenvironmental Engrg. 133(11): 1364-1373. [2].Clough, G.W. and O’Rourke, T.D. (1990). “Construction induced movements of in situ walls”. Proc., ASCE Conf. on Des. and Perf. of Earth Retaining Struct., Geotech. Spec. Publ. No. 25, ASCE, Reston: 439–470. [3].East China Electric Power Design Institute (ECEPDI). (2013). “Geotechnical investigation report of the 500kV Hongyang Underground Transmission and Substation”. (in Chinese). [4].Finno, R.J. and Bryson, L.S. (2002). “Response of building adjacent to stiff excavation support system in soft clay”. J. Perf. Constr. Facil., ASCE, 16(1): 10-20. [5].Lee, F.H., Yong K.Y., and Quan K.C.N., et al. (1998). “Effect of corners in strutted excavations: field monitoring and case histories”. J. Geotechnical and Geoenvironmental Engrg. 124(4): 339-349. [6].Liu G.B., Ng C.W.W., and Wang Z.W. (2005). “Observed performance of a deep multistrutted excavation in shanghai soft clays”. J. Geotechnical and Geoenvironmental Engrg.131 (8): 1005-1013. [7].Ng, C.W.W. (1998). “Observed performance of multipropped excavation in stiff clay”. J. Geotechnical and Geoenvironmental Engrg. 124 (9): 889-905. [8].O’Rourke, T.D. (1981). “Ground movements caused by braced excavations”. J. of Geotechnical. Engrg. Div. 107(9): 1159-1178. [9].Ou, C.Y., Chiou, D.C., and Wu, T.S. (1996). “Three-dimensional finite element analysis of deep excavations”. J. Geotechnical and Geoenvironmental Engrg. 122(5): 337-345.

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[10].Ou, C.Y., Liao, J.T., and Lin, H.D. (1998). “Performance of diaphragm wall constructed using top-down method”. J. Geotechnical and Geoenvironmental Engrg. 124(9): 798-808. [11].Tan, Y. and Li, M.W. (2011). “Measured performance of a 26 m deep top-down excavation in downtown Shanghai”. Canadian. Geotech. J. 48(5): 704–719. [12].Tan, Y. and Wei, B. (2012). “Observed behaviors of a long and deep excavation constructed by cut-and-cover technique in shanghai soft clay”. J. Geotechnical and Geoenvironmental Engrg. 138 (1): 69-88. [13].Wang, J.H., Xu, Z.H., and Wang, W.D. (2010). “Wall and ground movements due to deep excavations in shanghai soft soils”. J. Geotechnical and Geoenvironmental Engrg. 136(7): 985-994. [14].Xu Z.H., Wang W.D., and Wang J.H. et al. (2005). “Performance of deep excavated retaining wall in Shanghai lowland area”. Lowland Tech. International. 7(2): 31-43