Control and Prevent Land Subsidence Caused by Foundation Pit Dewatering in a Coastal Lowland Mega City: Indicator Definition, Numerical Simulation and Regression Analysis

doi:10.21203/rs.3.rs-1221768/v1

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Research Article

Control and Prevent Land Subsidence Caused by Foundation Pit Dewatering in a Coastal Lowland Mega City: Indicator Definition, Numerical Simulation and Regression Analysis

https://doi.org/10.21203/rs.3.rs-1221768/v1

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published 18 Jan, 2023

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Coastal mega cities are often commercial centers because of convenient traffic. Safe elevation above sea level is vital for their sustainable development. Global climate change and sea level rising increase flood risk especially in the lowland subsidence area. Shanghai of China was selected as research background. Although groundwater exploitation had been strictly restrained to control land subsidence and reserve safe elevation, lowering groundwater level during underground excavation cannot be avoided. Foundation pit dewatering (FPD) was intensively performed in underground exploitation during urbanization and city renewal. The FPD settlement accelerated land subsidence. Controlling FPD subsidence was urgent. Normally, the maximum horizontal influence radius of foundation pit excavation was less than three times excavation depth (H), and the 3H settlement was only caused by the FPD. The 3H maximum settlement was defined as the evaluating indicator of FPD land subsidence, and the corresponding 3H drawdown was defined as the control indicator of land subsidence. The FPD conceptual models were established on the basis of estimation and investigation of foundation pit information, including pit area, pit shape, pit depth, and curtain depth. Numerical models were established and a total of 5650 FPD numerical simulations were performed to investigate the land subsidence and FPD drawdown. Multi-factor regression analysis was conducted to obtain relations between land subsidence and FPD drawdown. Regression models were established between the 3H drawdown and the shape, area, depth, and curtain depth of foundation pit on the basis of the numerical simulations. A typical example introduced to verify the regression models. The regression models were used to manage the FPD land subsidence by controlling the 3H FPD drawdown. The results can provide reference for the land subsidence control in a coastal lowland city.

costal lowland city

foundation pit dewatering (FPD)

3H subsidence and drawdown indicator

numerical simulation

regression analysis

Table 1. Typical engineering geological layers influence FPD land subsidence in Shanghai, China

Layer No	Layer name	Depth (m)
①	Topsoil layer	0.0
②	First sand layer	0.9-7.0
③	First hard soil layer	1.0-5.0
④	First soft soil layer	3.0
⑤	Second soft soil layer	18.0-20.0
⑥	Second hard soil layer	15.0-30.0
⑦	Second sand layer (the first confined aquifer)	27.0-30.0

Table 2. Principle of LSPC partition in Shanghai, China

Engineering geological area			Characteristics	Remarks
Partition	Sub partition
Ⅰ (Lakes and marshes plain area)	-		Division of two hard layers	First confined aquifer (⑦) is dominated by silty soil.
Ⅱ (Coastal plain area)	Normal sedimentary area	Ⅱ₁	Layers ⑥ and ⑧distribution area
		Ⅱ₂	Layer ⑥distribution area; Layer ⑧ absence
	Paleo channel area	Ⅱ₃	Layer ⑥ absence; Layer ⑧distribution area	First confined aquifer (⑦) is dominated by sand.
		Ⅱ₄	Layers ⑥ and ⑧ absence area
Ⅲ (River estuary sand island area)	-		Distribution area of non-hard soil layer	First confined aquifer (⑦) is most missing
Ⅳ (Tidal flat geomorphic area)	-		New reclaimed area	First confined aquifer (⑦) is dominated by sand

Table 3. Principle of SDDC partition in Shanghai, China

Table 4 Length-width corresponding to area of foundation pit (unit: m)

Area(m²) Ratio	1000	2000	3000	4000	5000
1:1	31.6×31.6	44.7×44.7	54.8×54.8	63.2×63.2	70×70
1.5:1	38.7×25.8	54.8×36.5	67.1×44.7	77.5×51.6	86.6×57.7
2:1	44.7×22.4	63.2×31.6	77.5×38.7	89.4×44.7	100×50
2.5:1	50×20	70.7×28.3	86.6×34.6	100×40	111.8×44.7
3:1	54.8×18.3	77.5×25.8	94.8×31.6	109.5×36.5	122.5×40.8
Area(m²) Ratio	6000	7000	8000	9000	10000
1:1	77.5×77.5	83.7×83.7	89.4×89.4	94.9×94.9	100×100
1.5:1	94.9×63.2	102.5×68.3	109.5×73	116.2×77.5	122.5×81.6
2:1	109.6×54.8	118.3×59.2	126.4×63.2	134.2×67.1	141.4×70.7
2.5:1	122.5×50	132.3×52.9	141.4×56.6	150×60	158.1×63.2
3:1	134.2×44.7	144.9×48.3	154.9×51.6	164.4×54.8	173.2×57.7

Table 5 Numerical simulation parameters

Soil sequence number	Soil property	Water content (%)	Gravity γ(KN/m³)	Void ratio e	Hydraulic conductivity (cm/s)		Compress coefficient D _0.1~0.2 (MPa^-1)	Compress modulus E_0.1~0.2 (MPa)
Soil sequence number	Soil property	Water content (%)	Gravity γ(KN/m³)	Void ratio e	K_V	K_H	Compress coefficient D _0.1~0.2 (MPa^-1)	Compress modulus E_0.1~0.2 (MPa)
②	Cohesive soil	40	18.0	0.95	3.00×10^-4	3.50×10^-7	0.33	34.9
③,④	Muddy clay and silty clay	45	17.3	1.25	2.17×10^-7	3.00×10^-4	1.04	9.2
⑤	Cohesive soil	35	18.4	1.035	1.16×10^-7	3.50×10^-6	0.29	19.3
⑤₂	Silty soil and silt	33	19.6	0.935	3.50×10^-4	3.50×10^-6	0.07	50.1
⑥	Cohesive soil	25	19.5	0.72	1.07×10^-7	1.07×10^-7	0.22	30.4
⑦	Silty soil and fine sand	30	18.1	0.76	6.15×10^-4	6.15×10^-4	0.15	115.7
⑧	Clay mix ed with silt	30	19.7	1.01	2.22×10^-7	2.22×10^-7	0.35	42.4
⑨	Silt and fine sand with medium and coarse sand	20	17.3	0.68	9.72×10^-4	9.75×10^-4	0.14	93.3

Table 6 Numerical simulation scenarios

SSoil sequence number	Partition	Features		Working conditions of controlled drawdown
SSoil sequence number	Partition	Stratigraphic characteristics	Aquifer floor depth B(m) (The roof depth D is used in the stratum)	Digging depth(m)	Insertion depth of diaphragm wall into target aquifer(m)	Aspect ratio	Area (m²)
⑤₂	⑤_2Ⅱ-1	Paleochannel area of coastal plain	B less than 30	The partition is small and remote, refer to ⑤_{II -2}		1:1 1.5:1 2:1 2:1.5 3:1	1000 2000 3000 4000 5000 6000 7000 8000 9000 10000
	⑤_2Ⅱ-2	Paleochannel area of coastal plain	30< B < 60	16/20/24	5/10
	⑤_2Ⅲ-2	Distribution area of non-hard soil layer	30< B < 60	Chongming Island is not counted for the time being
	⑤_2Ⅳ-2	New continental area	30< B < 60	Partition too small can refer to ⑤_{II -2}
⑦	⑦_Ⅰ-1	Division of two hard layers	30< B or less than 60	Aquifer with shallower depth and smaller thickness can be directly separated
	⑦_Ⅱ1-1	Average distribution partition of Layer ⑥ and Layer ⑧	30< B or less than 60	20/24/28	5
	⑦_Ⅱ1-2	Average distribution partition of Layer ⑥ and Layer ⑧	B > 60	20/24/28	5/10/15/20/25
	⑦_Ⅱ2-3	Layer ⑥ distribution and Layer ⑧lost areas	Communication of Confined Aquifers	20/24/28	5/10/15/20/25/30
	⑦_Ⅱ3-1	Layer ⑥ deletion and Layer ⑧distribution area	30< B or less than 60	16/20/24/28	5/10/15
	⑦_Ⅱ3-2	Layer ⑥ deletion and Layer ⑧distribution area	B > 60	28/32	5
	⑦_Ⅱ4-3	Layer ⑥ and Layer ⑧ missing areas	Communication of Confined Aquifers	20/24/28	5/10/15/20/25
	⑦_Ⅳ-1	New continental area	30< B or less than 60	The partition is too small to refer to ⑦_{IV -2}
	⑦_Ⅳ-2		B > 60	20/24/28	5/10/15
	⑦_Ⅳ-3		Communication of Confined Aquifers	24/28	5/10/15/20/25
⑨	⑨_Ⅰ-1	Division of two hard layers	D Hard Soil Layer	Partition too small can refer to ⑨_{II 1-2}
	⑨_Ⅰ-2	Division of two hard layers	D > 60	The depth of aquifer is so deep that no dewatering is needed in the e× cavation of foundation pit at present.
	⑨_Ⅱ1-1	Layer ⑧ distribution area	D less than 60	Partition too small can refer to ⑨_{II 1-2}
	⑨_Ⅱ1-2	Layer ⑧ distribution area	D > 60	32/36	5
	⑨_Ⅱ3-1	Layer ⑧ distribution area	D less than 60	Partition too small can refer to ⑨_{II 3-2}
	⑨_Ⅱ3-2	Layer ⑧ distribution area	D > 60	32/36	5
	⑨_Ⅲ-1	Distribution area of non-hard soil layer	D less than 60	Chongming Island is not counted for the time being
	⑨_Ⅲ-2	Distribution area of non-hard soil layer	D > 60	Chongming Island is not counted for the time being
	⑨_Ⅳ-1	New continental area	D less than 60	Partition too small can refer to ⑨_{II 3-2}
	⑨_Ⅳ-2	New continental area	D > 60	The depth of aquifer is so deep that no dewatering is needed in the excavation of foundation pit at present.

Table 7 Ratio of drawdown of Y to X (length-width direction)

Partition	Ratio of drawdown of Y to X
⑤₂_Ⅱ-2	0.849
⑦_Ⅱ_1-1	0.909
⑦_Ⅱ_1-2	0.908
⑦_Ⅱ_2-3	0.910
⑦_Ⅱ_3-1	0.906
⑦_Ⅱ_3-2	0.879
⑦_Ⅱ_4-3	0.908
⑦_Ⅳ₂	0.882
⑦_Ⅳ₃	0.931
⑨_Ⅱ1-2	0.967
⑨_Ⅱ3-2	0.962

Table 8 Ratio of 3H drawdown with different length–width ratio

Aspect ratio Partition	1:1	1.5:1	2:1	2.5:1	3:1
⑤_2Ⅱ-2	0.929	0.969	1.000	1.023	1.042
⑦_Ⅱ1-1	0.945	0.978	1.000	1.018	1.035
⑦_Ⅱ1-2	0.931	0.969	1.000	1.022	1.042
⑦_Ⅱ2-3	0.961	0.982	1.000	1.012	1.023
⑦_Ⅱ3-1	0.939	0.973	1.000	1.022	1.045
⑦_Ⅱ3-2	0.927	0.970	1.000	1.024	1.047
⑦_Ⅱ4-3	0.948	0.980	1.000	1.015	1.028
⑦_Ⅳ2	0.937	0.975	1.000	1.016	1.033
⑦_Ⅳ3	0.963	0.985	1.000	1.011	1.021
⑨_Ⅱ1-2	0.977	0.990	1.000	1.008	1.017
⑨_Ⅱ3-2	0.978	0.989	1.000	1.010	1.022

Table 9 Drawdown increment with 1 m increment for pit depth

Serial number	Partition	Drawdown increment (m)
1	⑤₂_Ⅱ-2	0.28~0.83
2	⑦_Ⅱ_1-1	0.77~1.15
3	⑦_Ⅱ_1-2	0.23~1.22
4	⑦_Ⅱ_2-3	0.13~0.99
5	⑦_Ⅱ_3-1	0.36~1.30
6	⑦_Ⅱ_3-2	0.49~0.76
7	⑦_Ⅱ_4-3	0.09~0.83
8	⑦_Ⅳ₂	0.29~1.04
9	⑦_Ⅳ₃	0.09~0.73
10	⑨_Ⅱ_1-2	0.87~1.20
11	⑨_Ⅱ_3-2	0.86~1.26

Table 10 Drawdown reduction with 1 m cutoff depth increment of when Foundation Pit Area, Depth and Length-to-Width Ratio were constant

Serial number	Partition	Reduction of 3H drawdown (m)
1	⑤_2Ⅱ-2	0.13~0.39
2	⑦_Ⅱ1-1	-
3	⑦_Ⅱ1-2	0.12~0.59
4	⑦_Ⅱ2-3	0.02~0.36
5	⑦_Ⅱ3-1	0.09~0.70
6	⑦_Ⅱ3-2	-
7	⑦_Ⅱ4-3	0.01~0.32
8	⑦_Ⅳ2	0.13~0.51
9	⑦_Ⅳ3	0.03~0.27
10	⑨_Ⅱ1-2	-
11	⑨_Ⅱ3-2	-

Table 11 3H drawdown increment with 1000 m² foundation pit area increment

Serial number	Partition	Increasing 3H drawdown(m)
1	⑤_2Ⅱ-2	0.02~1.16
2	⑦_Ⅱ1-1	0.05~1.19
3	⑦_Ⅱ1-2	0.06~2.30
4	⑦_Ⅱ2-3	0.07~1.52
5	⑦_Ⅱ3-1	0.02~2.11
6	⑦_Ⅱ3-2	0.03~0.71
7	⑦_Ⅱ4-3	0.06~1.28
8	⑦_Ⅳ2	0.05~1.60
9	⑦_Ⅳ3	0.14~1.08
10	⑨_Ⅱ1-2	0.03~0.74
11	⑨_Ⅱ3-2	0.03~0.78

Table12 3H drawdown fit goodness coefficient

Table 13 3H drawdown and fitting optimality coefficient for normal foundation pit

Serial number	Partition	a	b	c	f	d	e	k	Goodness of fit coefficient
1	⑤_2Ⅱ-2	2.939×10^-5	0.075	1.720	2.609	-0.027	0.511	0.101	0.956
2	⑦_Ⅱ1-1	2.307×10^-5	0.088	0.000	2.765	0.005	0.500	0.082	0.980
3	⑦_Ⅱ1-2	3.842×10^-5	0.065	2.252	2.990	0.036	0.477	0.107	0.880
4	⑦_Ⅱ2-3	2.922×10^-5	0.044	1.065	4.510	1.017	-0.008	-3.919	0.933
5	⑦_Ⅱ3-1	2.945×10^-5	0.072	1.911	2.973	0.299	0.350	0.145	0.942
6	⑦_Ⅱ3-2	1.932×10^-5	0.054	0.000	3.374	0.021	0.491	0.113	0.975
7	⑦_Ⅱ4-3	2.746×10^-5	0.041	0.957	4.665	-0.217	0.610	0.057	0.937
8	⑦_Ⅳ2	2.689×10^-5	0.060	1.524	3.219	-0.226	0.608	0.068	0.946
9	⑦_Ⅳ3	3.460×10^-5	0.041	1.230	3.916	1.014	-0.006	-4.395	0.928
10	⑨_Ⅱ1-2	4.791×10^-6	0.035	0.000	7.688	0.354	0.322	0.059	0.992
11	⑨_Ⅱ3-2	4.114×10^-6	0.033	0.000	9.860	0.974	0.013	1.716	0.993
Fitting formula		Ds - Deep drawdown value of target aquifer water level at 3H control point outside foundation pit (m) D-Depth of diaphragm wall insertion into target aquifer roof (m) W-Depth of Foundation Pit Excavation(m) M - Foundation Pit Area (m²) P - Length to Width Ratio of Foundation Pit (Length/Width)

Table 14 3H drawdown and fitting optimality coefficient for narrow and long foundation pit

Partition	Coefficient			Goodness of fit coefficient
Partition	a	b	f	Goodness of fit coefficient
⑦_Ⅱ_1-1	0.000	255.581	0.284	0.999
⑦_Ⅱ_1-2	0.083	0.935	2.834	0.851
⑦_Ⅱ_2-3	0.000	30.549	0.999	0.972
⑦_Ⅱ_3-1	0.117	0.486	2.205	0.987
⑦_Ⅱ_4-3	0.048	3.941	1.606	0.879
⑦_Ⅳ₂	0.000	1564.174	0.388	0.942
⑦_Ⅳ₃	0.000	50.082	0.597	0.982
Fitting formula	D_s—Deep drawdown value of target aquifer water level at 3H control point outside foundation pit (m) D—Depth of diaphragm wall insertion into target aquifer roof (m) W—Depth of Foundation Pit Excavation(m)

Table 15 Drawdown affected distance and fitting optimality coefficient

Serial number	Name		a	b	c	f	Goodness of fit coefficient
1	⑤_2Ⅱ-2		0.017	0.280	2.953	3.055	0.994
2	⑦_Ⅱ1-1		0.994	17.616	0.000	1.152	0.924
3	⑦_Ⅱ1-2		0.192	1.325	37.326	1.762	0.813
4	⑦_Ⅱ2-3		0.018	0.152	2.212	4.073	0.838
5	⑦_Ⅱ3-1		0.109	1.250	23.296	1.847	0.904
6	⑦_Ⅱ3-2		0.079	1.299	0.000	1.731	0.980
7	⑦_Ⅱ4-3		0.015	0.138	2.131	4.117	0.902
8	⑦_Ⅳ2		0.094	1.161	21.316	1.767	0.924
9	⑦_Ⅳ3		0.051	0.243	5.631	2.788	0.886
10	⑨_Ⅱ1-2		0.473	9.139	0.000	1.253	0.940
11	⑨_Ⅱ3-2		0.003	0.148	0.000	4.325	0.993
Fitting formula		Dy-Influence distance of water level drawdown of target aquifer for dewatering outside foundation pit (m) D-Depth of diaphragm wall insertion into aquifer roof (m) W-Depth of foundation pit excavation(m) M - Foundation pit area (m) P - Length to width ratio of foundation pit (Length/Width)

Table 16 3H drawdown under different area, depth and diaphragm wall depth for layer ⑤₂

Partition	Foundation pit depth(m)	Insert depth of diaphragm wall (m)	Foundation pit area (m²)
Partition	Foundation pit depth(m)	Insert depth of diaphragm wall (m)	1000	2000	3000	4000	5000	6000	7000	8000	9000	10000
⑤₂_Ⅱ-2	16	5	2.83	3.23	3.47	3.60	3.73	3.81	3.90	3.96	3.99	4.01
	16	10	2.05	2.48	2.75	2.91	3.01	3.13	3.22	3.29	3.33	3.36
	20	5	5.04	5.91	6.34	6.59	6.78	6.91	7.06	7.16	7.24	7.31
	20	10	3.63	4.44	4.92	5.20	5.43	5.62	5.78	5.89	5.98	6.06
	24	5	6.61	7.77	8.36	8.72	8.96	9.20	9.38	9.56	9.70	9.80
	24	10	4.76	5.83	6.46	6.88	7.21	7.43	7.65	7.84	8.00	8.08

Table 17 Drawdown of 3H under different area, depth and diaphragm wall depth for layer ⑦

Partition	Foundation pit depth(m)	Insert depth of diaphragm wall (m)	Foundation pit area (m²)
Partition	Foundation pit depth(m)	Insert depth of diaphragm wall (m)	1000	2000	3000	4000	5000	6000	7000	8000	9000	10000
⑦_Ⅱ1-1	20	5	4.51	4.95	5.25	5.45	5.58	5.7	5.80	5.88	5.94	5.99
	24	5	8.11	8.94	9.48	9.85	10.07	10.25	10.38	10.48	10.54	10.59
	28	5	11.17	12.36	13.15	13.68	14.05	14.37	14.64	14.87	15.04	15.15
⑦_Ⅱ1-2	20	5	4.94	5.75	6.21	6.51	6.73	6.92	7.11	7.21	7.34	7.40
		10	3.60	4.63	5.22	5.61	5.87	6.13	6.28	6.44	6.57	6.70
		15	2.87	3.89	4.50	4.91	5.21	5.47	5.64	5.81	5.96	6.10
		20	2.28	3.20	3.77	4.16	4.45	4.71	4.89	5.06	5.21	5.35
		25	1.60	2.24	2.70	2.96	3.19	3.45	3.62	3.79	3.93	4.04
	24	5	8.11	9.46	10.23	10.75	11.13	11.45	11.78	11.96	12.18	12.29
		10	6.42	7.62	8.61	9.26	9.82	10.14	10.40	10.67	10.90	11.12
		15	4.70	6.40	7.42	8.10	8.61	9.05	9.34	9.63	9.88	10.12
		20	3.70	5.26	6.20	6.86	7.35	7.79	8.09	8.38	8.64	8.88
		25	2.62	3.68	4.39	4.88	5.26	5.65	5.98	6.27	6.52	6.70
	28	5	10.87	12.71	13.78	14.51	15.04	15.65	15.97	16.22	16.54	16.70
		10	8.53	10.24	11.59	12.48	13.30	13.71	14.08	14.46	14.78	15.09
		15	6.30	8.60	9.98	10.92	11.62	12.23	12.64	13.05	13.39	13.73
		20	5.02	7.07	8.35	9.24	9.93	10.53	10.94	11.35	11.71	12.04
		25	3.52	4.94	5.90	6.58	7.20	7.72	8.09	8.50	8.84	9.08
⑦_Ⅱ2-3	20	5	1.27	1.67	1.92	2.10	2.23	2.34	2.44	2.52	2.61	2.68
		10	0.80	1.17	1.44	1.65	1.80	1.94	2.05	2.16	2.25	2.34
		15	0.60	0.94	1.19	1.39	1.55	1.69	1.81	1.92	2.01	2.10
		20	0.47	0.78	1.01	1.20	1.36	1.49	1.61	1.71	1.82	1.91
		25	0.39	0.66	0.87	1.05	1.20	1.33	1.45	1.55	1.65	1.74
		30	0.33	0.57	0.76	0.93	1.07	1.21	1.32	1.42	1.52	1.61
	24	5	3.08	4.07	4.70	5.15	5.48	5.77	6.02	6.23	6.46	6.64
		10	1.94	2.87	3.54	4.04	4.44	4.79	5.07	5.34	5.57	5.79
		15	1.46	2.30	2.93	3.43	3.83	4.19	4.48	4.75	5.00	5.23
		20	1.16	1.91	2.49	2.97	3.36	3.71	4.00	4.27	4.45	4.75
		25	0.96	1.62	2.16	2.61	2.98	3.32	3.61	3.88	4.12	4.36
		30	0.81	1.40	1.90	2.32	2.68	3.00	3.29	3.55	3.79	4.02
	28	5	4.64	6.16	7.13	7.85	8.37	8.83	9.22	9.56	9.94	10.23
		10	2.92	4.36	5.38	6.17	6.80	7.34	7.78	8.20	8.57	8.92
		15	2.21	3.50	4.47	5.25	5.88	6.43	6.89	7.32	7.70	8.07
		20	1.76	2.91	3.82	4.55	5.17	5.71	6.17	6.60	6.99	7.35
		25	1.46	2.49	3.32	4.01	4.60	5.13	5.58	6.01	6.39	6.76
		30	1.24	2.16	2.93	3.59	4.15	4.66	5.10	5.51	5.89	6.25
⑦_Ⅱ3-1	16	5	2.68	2.97	3.17	3.30	3.38	3.46	3.51	3.55	3.57	3.60
		10	1.93	2.36	2.61	2.77	2.88	2.98	3.04	3.10	3.13	3.16
		15	1.38	1.76	2.00	2.16	2.27	2.37	2.43	2.51	2.55	2.59
	20	5	6.46	7.20	7.68	8.01	8.21	8.42	8.57	8.66	8.71	8.81
		10	4.67	5.71	6.32	6.71	6.99	7.23	7.39	7.56	7.64	7.69
		15	3.35	4.26	4.84	5.24	5.51	5.76	5.91	6.12	6.22	6.30
	24	5	9.75	10.90	11.66	12.17	12.50	12.84	13.08	13.24	13.33	13.42
		10	7.04	8.65	9.58	10.20	10.63	11.02	11.27	11.54	11.67	11.75
		15	5.05	6.45	7.34	7.95	8.38	8.77	9.01	9.33	9.50	9.63
	28	5	12.55	14.09	15.10	15.79	16.24	16.71	17.05	17.27	17.41	17.44
		10	9.06	11.17	12.40	13.21	13.80	14.32	14.67	15.03	15.23	15.35
		15	6.50	8.33	9.50	10.30	10.87	11.40	11.72	12.16	12.39	12.56
⑦_Ⅱ3-2	28	5	3.96	4.41	4.73	4.97	5.18	5.35	5.50	5.58	5.66	5.69
⑦_Ⅱ3-2	32	5	5.93	6.64	7.14	7.54	7.88	8.15	8.39	8.58	8.68	8.73
⑦_Ⅱ4-3	20	5	0.91	1.19	1.39	1.54	1.67	1.78	1.86	1.95	2.02	2.08
		10	0.56	0.84	1.03	1.18	1.32	1.45	1.54	1.63	1.70	1.77
		15	0.42	0.66	0.84	0.99	1.13	1.25	1.34	1.43	1.50	1.58
		20	0.33	0.54	0.71	0.85	0.98	1.09	1.18	1.27	1.34	1.41
		25	0.26	0.45	0.60	0.73	0.86	0.96	1.04	1.13	1.20	1.27
	24	5	2.35	3.08	3.61	4.01	4.41	4.69	4.92	5.11	5.26	5.41
		10	1.45	2.18	2.69	3.10	3.49	3.82	4.05	4.28	4.47	4.63
		15	1.09	1.73	2.21	2.60	2.93	3.29	3.53	3.77	3.96	4.13
		20	0.86	1.42	1.86	2.23	2.55	2.89	3.12	3.36	3.55	3.72
		25	0.69	1.18	1.58	1.93	2.22	2.54	2.77	3.00	3.19	3.36
	28	5	3.55	4.83	5.70	6.31	6.74	7.15	7.49	7.87	8.12	8.36
		10	2.20	3.32	4.12	4.86	5.39	5.86	6.24	6.61	6.91	7.16
		15	1.65	2.65	3.40	4.06	4.62	5.08	5.46	5.84	6.15	6.41
		20	1.31	2.18	2.87	3.45	3.95	4.47	4.84	5.22	5.52	5.79
		25	1.06	1.83	2.45	2.99	3.46	3.95	4.31	4.68	4.98	5.25
⑦_Ⅳ2	20	5	3.42	3.89	4.19	4.41	4.57	4.70	4.78	4.84	4.89	4.94
		10	2.39	3.02	3.38	3.63	3.81	3.96	4.05	4.16	4.24	4.30
		15	1.76	2.32	2.66	2.90	3.08	3.23	3.32	3.44	3.52	3.57
	24	5	6.20	7.09	7.64	8.05	8.35	8.61	8.77	8.89	8.99	9.09
		10	4.33	5.49	6.17	6.63	6.97	7.26	7.43	7.64	7.80	7.90
		15	3.19	4.21	4.84	5.29	5.62	5.92	6.09	6.31	6.46	6.57
	28	5	8.46	9.69	10.45	11.03	11.46	11.83	12.05	12.24	12.39	12.53
		10	5.92	7.52	8.46	9.10	9.57	9.97	10.22	10.51	10.74	10.89
		15	4.36	5.77	6.65	7.28	7.75	8.16	8.40	8.70	8.92	9.09
⑦_Ⅳ3	24	5	2.25	2.91	3.40	3.77	4.05	4.31	4.51	4.72	4.87	5.01
		10	1.35	2.04	2.54	2.91	3.21	3.51	3.72	3.94	4.12	4.27
		15	1.01	1.61	2.07	2.44	2.73	3.02	3.24	3.46	3.65	3.80
		20	0.79	1.32	1.74	2.08	2.36	2.64	2.86	3.08	3.26	3.42
		25	0.63	1.09	1.47	1.79	2.05	2.32	2.53	2.74	2.92	3.08
	28	5	3.47	4.52	5.29	5.90	6.35	6.76	7.09	7.44	7.68	7.91
		10	2.10	3.18	3.97	4.58	5.11	5.53	5.88	6.20	6.52	6.76
		15	1.57	2.52	3.26	3.84	4.36	4.78	5.14	5.50	5.78	6.04
		20	1.24	2.07	2.74	3.29	3.78	4.19	4.55	4.91	5.20	5.45
		25	1.00	1.69	2.32	2.84	3.32	3.70	4.04	4.39	4.68	4.94

Table 18 Drawdown of 3H under different area, depth and diaphragm wall depth for layer ⑨

Partition	Foundation pit depth(m)	Insert depth of diaphragm wall (m)	Foundation pit area (m²)
Partition	Foundation pit depth(m)	Insert depth of diaphragm wall (m)	1000	2000	3000	4000	5000	6000	7000	8000	9000	10000
⑨_Ⅱ1-2	32	5	2.47	2.77	2.94	3.05	3.13	3.21	3.25	3.30	3.33	3.36
⑨_Ⅱ1-2	36	5	5.93	6.67	7.09	7.37	7.57	7.76	7.89	8.02	8.12	8.17
⑨_Ⅱ3-2	32	5	1.63	1.88	2.01	2.11	2.18	2.23	2.28	2.32	2.35	2.40
⑨_Ⅱ3-2	36	5	5.08	5.86	6.27	6.57	6.78	6.95	7.10	7.24	7.33

Table 19 Site stratigraphic characteristic of Qilianshan South Road Station of Subway 11 Shanghai

Soil sequence number	Soil property	Floor elevation (average)(m)	Water content%	Gravity KN/m³	Strength parameter		Indoor permeability test
Soil sequence number	Soil property	Floor elevation (average)(m)	Water content%	Gravity KN/m³	Cohesion c(kPa)	Internal friction angle phi friction angle	K_V (cm/s)	K_H (cm/s)
②₁	Sandy silt with silty clay	0.59	31.3	18.5	4	30.5	7.18×10^-5	1.29×10^-4
②_3-1	Sandy silt	-5.88	31.1	18.3	3	33.5	2.01×10^-4	4.40×10^-4
④	muddy clay	-9.81	48.3	16.8	14	11.5	2.17×10^-7	6.95×10^-7
⑤_1-1	clay	-17.91	39.8	17.5	16	13.0	8.36E-08	1.16×10^-7
⑤_1-2	Silty clay	-23.71	34.7	17.9	16	18.0	5.74×10^-7	1.04×10^-6
⑥	Silty clay	-26.73	24.2	19.4	47	16.0	1.02×10^-7	1.07×10^-7
⑦₁	Silty sand	-30.82	29.3	18.4	0	32.5	3.19×10^-4	6.05×10^-4
⑦₂	Fine sand	-37.68	27.0	18.9	0	32.5	3.34×10^-4	6.24×10^-4
⑧₁	Silty clay with silty sand	-46.03	32.7	18.4	20	19.5	2.77×10^-6	5.43×10^-6
⑧₂	Silty Clay-silt interbedding	-63.70	32.5	18.3	21	22.0	1.45×10^-6	3.32×10^-6

Table 20 Comparing calculation and observation subsidence

Observation section	Monitored 3H subsidence (mm)	Calculation 3H subsidence (mm)
North side	7.00	7.24
East side	5.00	6.58

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Control and Prevent Land Subsidence Caused by Foundation Pit Dewatering in a Coastal Lowland Mega City: Indicator Definition, Numerical Simulation and Regression Analysis

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