Geohazard Caused by Groundwater in Urban Underground Excavation

Table 2

Input parameters of soils and structures.

(a) Sandy soil (hardening soil with small strain)

Layer	Depth (m)	Drainage type	Unit weight (kN/m^3)	(^o)	(kPa)	K0	(kN/m^2)
2	2.0–6.5	Drained	20.9	32	26.22 × 10^3	0.4701	1.44 × 10^5
4	8.0–17.0	Drained	20.6	32	21.9 × 10^3	0.4701	1.41 × 10^5
5	17.0–23.5	Drained	18.6	32	24.06 × 10^3	0.4701	1.41 × 10^5
6	23.5–28.5	Drained	19.6	33	26.25 × 10^3	0.4554	1.6 × 10^5
8	30.5–42	Drained	19.6	34	29.58 × 10^3	0.4408	1.62 × 10^5
9	42–60	Drained	19.6	34	33.15 × 10^3	0.4408	1.77 × 10^5

Note:  = 0.3; γ_0.7 = 1 × 10⁻⁴; E_ur^ref = 3E₅₀^ref; E_oed^ref = E₅₀^ref; m = 0.5; permeability = 2 × 10⁻⁴ cm/sec; Rf = 0.9.

(b) Clay (Mohr-Coulomb)

Layer	Depth (m)	Soil type	γ (kN/m^3)	Su (kPa)	(kPa)		K0	Permeability (cm/sec)
1	0.0–2.0	CL	19.3	28	13,060	0.3947	1.0	1 × 10^−8
3	6.5–8.0	CL	19.7	21	9800	0.3953	1.0	1 × 10^−8
7	28.0–30.5	CL	18.6	84	39,200	0.3953	1.0	1 × 10^−8

(c) Improvement soils

Depth (m)	Drainage type	γ (kN/m^3)	(kN/m^3)	(°)	(kPa)		K0	Permeability (cm/sec)
10.0–16.0	Undrained	21	100	34	120,000	0.3	0.44	1 × 10^−6

^#Using the function of UC test implemented in PLAXIS, an equivalent undrained shear strength (Su) of routed sand is thus defined. Through an empirical approach [15], an assumption of Eu = 500 Su is adopted to determine the undrained Young’s modulus (Eu). Effective soil stiffness is required to be applied in the analyses so an equation of E′= 2 (1 + v’)/3 × Eu is used to determine (0.3 of is used). In engineering practice, standard requirement of improved soils outside the pit for launching of shield is in the range of 1 × 10⁻⁵ to 1 × 10⁻⁶ cm/sec but tends to be stricter (1 × 10⁻⁶ cm/sec) for highly permeable ground (such as sand) with high ground water level to reduce the risk [16].

(d) Retaining structure

Parameter	Name	Value	Unit
Compressive strength of concrete		28	MPa
Thickness	d	0.9	m
Young’s modulus	E	24.8 × 10^6	kPa
Young’s modulus x 70%	70%E	17.36 × 10^6	kPa
Unit weight	w	5.5	kN/m^3
Poisson’s ratio	ν	0.2

(e) Steel strut

Strut level	Strut level	Preload (kN)	Section area (m^2)	EA (kN)	60%EA (kN)
Level 1	1H400x400x13x21	450	0.0219	4.59 × 10^6	2.75 × 10^6
Level 2	2H400x400x13x21	1000	0.0437	9.18 × 10^6	5.50 × 10^6
Level 3	2H400x400x13x21	1400	0.0437	9.18 × 10^6	5.50 × 10^6
Level 4	2H400x400x13x21	1400	0.0437	9.18 × 10^6	5.50 × 10^6