Interaction between Tunnelling and Groundwater
Younghoon Lee 1,a, Sanghyo Lee2,b
1Department of Architectural Engineering, Hanynag University, 222,Wangsipri-ro, Sungdong-gu, Seoul, Korea
2Innovative Durable Building and Infrastructure Research Center, Hanyang University, 55 Hanyangdaehak-ro, Sangrok-gu, Ansan-si, Gyeonggi-do, Korea
,
Abstract
This paper addresses the interaction mechanism between tunnelling and groundwater. A 3D stress-pore pressure coupled finite-element model was adopted to perform a parametric study on the influencing factor such as the relative permeability between ground and lining. The results indicated that the ground and lining responses are significantly influenced by the relative permeability of the lining. Also highlighted is the importance of the stress-pore pressure coupled analysis in the numerical prediction of tunnel behavior. Implementations of the findings from this study are discussed in great detail.
KEYWORDS: Tunnelling and Groundwater
1. INTRODUCTION
Tunnelling beneath the groundwater table causes changes in the state of stress and the pore water pressure regime. In such tunnelling problems, there are three important issues that have to be addressed during design and construction stages including construction, stability, and environmental issues. First, water inflows during tunnelling significantly hamper the tunnelling works, thus resulting in a global increase in the construction costs. Second, as the stress-strain-strength characteristics of the surrounding ground are governed by the effective stress, the change in the pore water pressure regime during the tunnelling process can affect the short- and long-term tunnel stability. Third, the direct environmental consequence of water inflows during tunnelling is the drawdown of groundwater level in the surrounding aquifer. The short- and long-term drawdown can affect vegetation, groundwater supply and chemistry. The related groundsubsidence occurring as a result of the reduction in water pressures in the soil layers can damage structures.
2. PARAMETRIC STUDY
2.1 Problem considered
The problem considered in this study was a hypothetical tunnelling situation of which a 10-m-diameter horseshoe shaped tunnel is constructed 25 m below the ground level in the ground condition frequently encountered in Seoul, Korea. Figure 1(a) shows the ground profile considered in this study. As shown, the ground considered consists of 5.0 m of miscellaneous non-cohesive fill/alluvium material. Underlying the fill/alluvium layer is a 15-m-thick decomposed granite soil layer followed by a 20-m-thick moderately weathered granite rock layer through which the tunnel is excavated. Below the weathered rock layer is a solid rock layer. For simplicity, the groundwater table was assumed to be located at the ground surface level. In regard to the post-tunnelling groundwater flow regime, a drawdown condition with no recharge at the ground surface during the tunnelling process was assumed.
(a) ground profile (b) tunnel cross section
Figure 1. Ground profile and tunnel cross section
2.2 Cases analysed
In order to form a database that can help identify the underlying interaction mechanism between the tunnelling and the groundwater, a number of cases were constructed by varying the relative permeability of the lining to the ground through which the tunnel is excavated. For comparison, a total stress analysis with no consideration of the stress-pore pressure coupled effect was additionally conducted on the same tunnelling condition. Table 1 summaries the conditions analysed in this study.
Table 1. Summary of Cases Analyzed
Type of analysis / kL/ kS / kS (m/s)Coupled analysis / 0.0, 0.02, 1.0 / 6×10-7, 6×10-8
Total stress analysis
(TSA) / · Saturated unit weights are used for the soils and rocks.
· Shear strength properties are assumed to be the same as those used in the coupled analysis.
Note) kL=lining permeability, kS=permeability of ground surrounding lining
3. FINITE-ELEMENT ANALYSIS
3.1 Modelling
A 3D finite-element model was considered in this study using the commercially available code ABAQUS. The tunnel was assumed to be excavated in full face, and therefore, only one half of the domain was considered in the finite-element model by making use of the symmetry about the tunnel centerline. The finite-element mesh extends to a depth of two times the tunnel diameter (D) below the tunnel springline and laterally to a distance of 8D from the tunnel centerline. The vertical displacements were only allowed at the vertical boundaries whereas pin supports were applied to the bottom boundary. With regard to the hydraulic boundary conditions, a no-flow condition was assigned to the vertical boundary corresponding to the plane of symmetry. The pore water pressures on the right-hand vertical boundary and the far side boundary opposite to the front boundary were assumed to be constant throughout the analysis. Locations of these boundaries were selected in accordance with the results of preliminary analyses so that the presence of the artificial boundaries does not significantly influence the stress-strain-pore pressure field in the domain.
The Drucker-Prager model parameters can be converted from the Mohr-Coulomb model parameters using the following equations:
(1)
(2)
where is the effective internal friction angle; and is the effective cohesion of the soil defined in the Mohr-Coulomb yield criterion.
Table 2. Material Properties Examined in Finite Element Analyses (base condition)
Material
/ c’(kPa) / ’ (°) / (°) / n / k(m/sec) /
(kN/m3) /
E
(MPa) / KoFill/Alluvium / 5 / 30 / 20 / 0.45 / 210-6 / 18 / 30 / 0.43
Decomposed
granite soil / 50 / 38 / 15 / 0.4 / 110-6 / 20 / 70 / 0.5
Weathered granite rock / 100 / 40 / 15 / 0.3 / 610-7 / 22 / 100 / 0.5
Solid
Granite rock / 200 / 45 / 15 / 0.1 / 110-9 / 25 / 100 / 0.7
Shotcrete / - / - / - / 0.2 / 110-8 / 23 / 2000
Notes: c’=cohesion; ’=internal friction angle; =dilatancy angle; n=porosity; k=permeability; =total unit weight; E=deformation modulus; Ko=lateral earth pressure coefficient
4. PORE WATER PRESSURE
Figure3 presents the variation of pore water pressure regime of the section S11 with the distance to face (L) for cases having two different lining permeability ratios, i.e., and 1.0. Note that the section S11 is located 2.0D ahead of the front boundary. As can be seen for both cases, the pore water pressures start to significantly decrease well before the face reaches the section (see for L=-1.4D) and tend to continually decrease as the face advances with the decrease being greater for the permeable lining case (). The drawdown level and the pore water pressures around the tunnel after the passage of the face depend greatly on the lining permeability. For example, the drawdown level at the early stage of excavation does not appear to significantly change with the face advance for the case due to the watertightness of the lining while the drawdown continues with the face advance for the permeable lining case, i.e., . A significant pore water pressure recovery upon complete passage of the tunnel face is seen for the case . Although the two lining permeabilities represent ideal situations, the results presented above demonstrate that the drawdown of the groundwater takes place well before the tunnel face reaches a particular section (i.e., L<-1.4D), and that the post-tunnelling pore water pressure regime is significantly influenced by the lining permeability.
6. CONCLUSIONS
This paper examined the interaction mechanism between tunnelling and groundwater based on the results from a series of 3D stress-pore water coupled analyses on a hypothetical tunnelling condition frequently encountered in urban areas in Korea. Attention was paid to analyzing the results of analyses so that the effect of relative lining permeability on the pore water pressure regime as well as the ground and lining responses can be identified. On the basis of the results of the present study, the following conclusions can be drawn. The results indicated that the drawdown of groundwater during tunnelling commences well before the tunnel face approaches a given section, and that the post-tunnelling pore water pressure regime around the tunnel depends significantly on the lining permeability. In addition, the relative lining permeability with respect to the ground permeability has a more pronounced influence on the lining response than the post-tunnelling groundwater level. Also revealed was that soils around the tunnel experience stress paths that are quite different from those without the coupled behavior. Perhaps the most important conclusion is that results of analyses for tunnelling cases considered in this study can be far from realistic when the stress-pore pressure coupled behavior is not considered.
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