A stress-displacement solution for a pressure tunnel with impermeable liner in elastic porous media

ARTICLES

A stress-displacement solution for a pressure tunnel with impermeable liner in elastic porous media

M.B. Wang^I,*; G. Wang^II

^ISchool of Civil Engineering, Ludong University, Yan'tai 264025 – P.R. China

^IIState Key Laboratory of Mine Disaster Prevention and Control, Shandong University of Science and Technology, Qingdao 266590 – P.R.China

ABSTRACT

Considering the effects of the changes of the pore water pressure around the opening, the construction sequence of the tunnel and the interaction between the liner and the surrounding geomaterial on the mechanical response of the tunnel and in conjunction with the analyses of the continuity and boundary conditions for the stress and displacement and hydraulic conditions, the elastic problem for a deep pressure tunnel with impermeable liner in a saturated elastic porous media that obeys Terzaghi's effective stress principle is investigated. The influences of the relative liner thickness and rigidity and the relative distance of the point under investigation to the tunnel axis on the stress-displacement fields for various combinations of the mechanical and geometric parameters are evaluated and discussed.

Keywords: tunnel, liner, porous media, relative liner thickness and rigidity, Terzaghi's effective stress principle.

1 INTRODUCTION

With the development and upgrade of infrastructures, the demand for tunnel construction is increasing all over the world. Therefore, an extensive amount work has been done on the geomaterial-liner interaction of the tunnel based on elastic theory [7, 15].

When a tunnel is excavated under water-bearing geomaterial, seepage toward the tunnel takes place and the hydraulic head distributions around the tunnel are changed. Water inflow and water pressure controls are needed in the design, construction and exploitation of tunnels. Uncontrolled water behaviour may cause additional loads on the liner, mechanical instability, discomfort and adverse environmental impacts. Consequently, the loads imposed by the surrounding geomaterial, the fluid pressure inside the tunnel, and the changes in pore pressure in the surrounding geomaterial due to any leaks through the liner should be adequately considered in the liner design of a pressure tunnel. In addition, it is well-known that the internal pressure in tunnel results in an expansion of the liner which then transfers part of the load to the surrounding geomatrial. The pore water pressure in the surrounding geomaterial has a double effect: first it counteracts the expansion of the liner caused by the inside pressure, and second it can change the stress field in the surrounding geomaterial around the tunnel decreasing effective stresses. Even though some of the effects of pore water pressure on tunnel support have been investigated [1, 3-6, 12-14, 16, 17], there are many aspects that require further scrutiny; in particular a criterion for tunnel support is needed where groundwater flow conditions are included.

One of the most important problems associated with tunneling is to determine the deformation produced by tunnel excavation. Even though this is a key aspect of tunneling, due to many complex factors affecting the solution, there are still a limited number of closed-form solutions than can be used to predict the deformation produced by tunnel excavation. Empirical or quasi-empirical methods are usually available. However, Chou and Bobet [8] note explicitly in their paper that empirical methods have significant shortcomings: (1) they have been developed or have been validated from a limited number of cases; (2) they should be applied only to tunnels that fall within the scope of the cases from which the method was developed; (3) only few soil and geometry parameters are taken into account; (4) they do not consider construction methods; and (5) they cannot give the complete solution of a tunnel with liner.

The present paper proposes an elastic stress-displacement solution for a deep pressure tunnel with impermeable liner excavated in a saturated elastic porous geomaterial that obeys Terzaghi's effective stress principle [2, 18], as shown in Fig. 1. In all the analyses the following assumptions are made:(1) the geomaterial and the liner are always elastic; (2) the permeability of the geomaterial is homogeneous and isotropic; (3) the groundwater field is a steady-flow seepage field; (3) the cross-section of the tunnel is circular; (4) plane strain conditions are applicable at any cross-section of the tunnel; (5) the tunnel is deep enough so that the stress distribution before excavation is homogeneous. Even though some assumptions may be too restrictive, the method has the following advantages: (1) it is simple to use; (2) it can properly model the tunnel construction sequence; (3) it can be used for preliminary design, which facilitates a more advanced or detailed modeling of the tunnel.

2 PROBLEM STATEMENT

Consider an infinite elastic plane oxy (geomaterial made in elastic porous material, or Region 1) that is homogeneous, except for the presence of a deep circular lined tunnel subjected to uniform internal pressure -q_o (with q_o a positive number and tensile stresses are considered as positive in the work), or Region 2 (see Fig. 1). The elastic constants of the geomaterial are denoted by G₁ (shear modulus) and γ₁ (Poisson's ratio), and the polar coordinates in the oxy plane by r and θ. It is assumed that the tunnel-axis is aligned with the direction of the third out-of-plane z-axis. The inner and outer radii of the liner are denoted by a and t, respectively, and its elastic constants by G₂ and γ₂. Axi-symmetry conditions for geometry and loading will be assumed (i.e., gravity will be disregarded), so the problem in Fig.1 is representative of the case of a deep tunnel excavated in elastic geomaterial subject to uniform initial stresses. Here, we define t/a as the relative thickness of the liner and the relative rigidity of the liner, which is defined as

In addition, as discussed in the main text, a dimensionless variable β is defined in terms of the outer radius of the liner t and the radial distance r to the tunnel axis, as follows

Also, the solution is expressed in terms of the two Lame's elastic parameters, λ and G respectively, that are related to the Young's modulus E and the Poisson's ratio γ of the liner and the geomaterial as follows

and

Prior to excavation, the total stresses and the pore water pressure in the geomaterial are uniform and equal to -σ_o and , respectively. The tunnel is then excavated and water is drained from inside the tunnel, leading to the non-uniform pore water pressure distribution shown in Fig.1, where q_w = at the outer boundary of the liner and q_w = at and beyond the radial distance r = r^*. The pore water pressure distribution is depicted by Regions I and II in Fig.1, respectively. By using Terzaghi's effective stress principle, the total stress can be decomposed into effective stresses and pore water pressure and the equation can be written in matrix notation as

where σ represents a component of total normal stress, σ' the corresponding component of effective normal stress, and q_w the pore water pressure.

3 THE CLOSED-FORM FULL FIELD ELASTIC SOLUTION

According to Eq.(2), the radial distance r can therefore be expressed as follows

In view of the relationship (6), the first derivation of an arbitrary function with respect to the variable r, can be equally expresssed in terms of the derivative of the same function with respect to the variable β, i.e.,

similarly, the second derivative of an arbitrary function with respect to the variable r results

Firstly, the solution for pore water pressure is discussed. The differential equation in a steady flow field can be expressed in terms of the pore water pressure q_w and the radial distance r as follows [10]

In view of the transformation (6), Eq.(9) can be equally expressed in terms of β as follows

The general solution of Eq.(10) is obtained as follows

where A₁ and A₂ are integration constants.

In reference to Fig.1, the values of the pore water pressure at the distances r = t (or β = 1) and r = r^* are as follows, respectively

and

By substituting Eqs.(12) and (13) into Eq.(11), the coefficients A₁ and A₂ in Eq.(11) are determined as

and

Consequently, the solutions for the pore water pressure in Regions I and II result in as follows, respectively

and

Next, we discuss the derivation of the solution for displacements and stresses. Considering Eqs.(6) and (7), the differential equation of equilibrium for the axisymmetric problem can be expressed as

In addition, the radial strain є_r and є_θ can be equally expressed in terms of the radial displacement u_r and the variable β as follows, respectively

and

According to Terzaghi's effective stress principle, only the effective components of normal stresses induce mechanical deformation, therefore, the elastic stress-strain relationships, as defined by the theory of elasticity, may be revised as

where, and denote the total strains including the components prior to excavation. Similarly, prior to excavation, between the strains, defined by and , and the stresses by and , the following relationships yield

Hence, the incremental strains ε_r, ε_θ and ε_z may be obtained as follows

In terms of Eqs.(27)-(29), for the special case of plane strain, i.e., ε_z= 0, the following expressions results in

wherein, σ_r - q_w, σ_θ - q_w and σ_z - q_w represent the effective components of radial, tangential and axial stresses, respectively, while σ_o represent the initial effective in situ stress.

By substituting Eqs.(30) and (31) together with Eqs.(19) and (20) into Eq.(18), the following differential equation representing the equilibrium condition in terms of the displacements is obtained

where, the second subscripts 1 is related to the components in the surrounding geomaterial. For region I, noting Eq.(16), the solution of Eq.(33) gives the following expression for the radial displacement

where, the superscript I is related to the components in the region I of the surrounding geomaterial. In addition, based on Eqs.(30) and (31), the radial and tangential stresses are obtained as, respectively

and

As for region II, noting Eq.(17), the following expressions for the displacement and stresses result in

where, the superscript II is related to the components in the region II of the surrounding geomaterial.

In reference to Fig.1, the boundary or continuity conditions at the distances r = t (or β = 1), r → ∞ (or β→ 0) and r = r^*(or β = β^*) are summarized as

where, q_s denotes the normal pressure along the interface between the liner and the surrounding geomaterial for the case that the liner is impermeable. Eqs.(40) through (43) - with and defined by Eqs.(35), (38), (34) and (37), respectively - conform a system of four algebraic equations to solve four unknowns, the variables and , as follows

In summary, substituting Eqs.(44) and (45) into Eqs.(34)-(36) and Eqs.(46) and (47) into Eqs.(37)-(39), respectively, the final expressions for the displacements and stresses for regions I and II are obtained as follows

The above expressions for radial and tangential stresses correspond to total stresses. According to Terzaghi's effective stress principle (Eq.(5)), the effective radial and tangential stresses for regions I and II, are computed as follows

By making = 0 in Eqs.(48)-(53), the classical Lamé's solution can be recovered. In this case, the total and effective stresses are the same and the solution for radial displacement, radial stress and tangential stress, result to be, respectively

For another special case that the pore water pressure distribution in the surrounding geomaterial is uniform, by considering in Eqs.(48)-(53), the following expressions for displacement and stresses are obtained

For this case there is no distinction between regions I and II, in terms of Eqs.(16) and (17), the solution for the pore water pressure is

Finally, the solution for the support pressure q_s is discussed below. It is assumed that the radii of the opening immediately after excavation, after the elastic deformation finishes and before the liner is installed, are t'", t" and t', respectively. The construction sequence of the tunnel is modeled with regard to Fig.2. Then, we define

as the relative radius misfit between the surrounding geomaterial and the liner. This misfit is assumed of the order of the admissible strains in linear elasticity.

Based on Kirsch's solution [11] and Eq.(48), t" is expressed as follows

The case of η = 0 means that the liner is installed after the elastic deformation finishes, and η = η_max means that the region of r < t'" in the surrounding geomaterial is replaced by the liner. Thus, η_max can be written in the following form

Substituting (65) into (64) yields the following relation

Next, the elastic deformation rate of the surrounding geomaterial is defined by

Taking advantage of Eq.(60), δ can be rewritten

Inserting Eqs.(65), (66) and (68) into Eq.(63) gives

In terms of the Lamé's solution and Eq.(6), the stresses and displacement in the liner may be equally written as follows (see Fig.3)

Furthermore, it is noted that the liner will be installed when the tunnel partially deforms. Hence, the following relationship results in

By substituting Eqs.(48) and (72) into Eq.(73), the expression of q_s is obtained

In terms of Eq.(48), the displacement along the interface between the liner and the surrounding geomaterial may also be written in a explicit form as follows

where and hereafter, (1) denotes the support displacement corresponding to the case that the liner is impermeable.

In terms of Eqs.(74) and (75), it may be found that the pore water pressure distribution in the surrounding geomaterial has significant influences on the support pressure q_s and displacement (1). The support pressure q_s and displacement (1) may decrease with increasing the pore water pressure along the outer boundary of the liner and the reverse trend may occur if to decrease the pore water pressure . As for the internal water pressure q_o, it may be found that the support pressure q_s increases with increasing the internal water pressure q_o. However, the support displacement (1) decreases with increasing the internal water pressure q_o.

4 NUMERICAL RESULTS AND DISCUSSIONS

In this Section, in order to illustrate the appllication of the obtained solution, we take γ₁ = γ₂ = 0.3, δ = 0, G₁/σ_o = 10³, = 0.5σ_o, = 0.025σ_o, q_o = 0.05σ_o and r^* = 5a²/t.

The variations of the normalized support pressure (q_s/σ_o) and displacement (G₁u^Ir1(1)/(σ_oa)) with relative liner rigidity for various relative liner thicknesses are depicted in Figs.4 and ⁵. It is seen that in general the varying trend of the support pressure with respect to relative liner rigidity for various liner thicknesses is similar to that of the support displacement. Namely, both of the support pressure and displacement monotonically increase with increasing liner rigidity and thickness. From Figs.4 and ⁵, it is further seen that in the range of 1 < Γ < 200, the influences of the liner rigidity and thickness on the support pressure and displacement are significant. However, in the range of 200 < Γ < 800, the variations of the support pressure and displacement with various values of Γ or t/a become insignificant. Finally, when 800 < Γ, the variations of the support pressure and displacement are almost independent of the values of Γ and t/a.

In Figs.6 and ⁷, we illustrate the variations of the normalized effective stresses in the liner and the surrounding geomaterial with relative liner thickness (t/a) and distance to the tunnel axis (r/a) when Γ = 10 [9, 15]. It is found from Fig.6 that at first the effective radial stress increases with increasing distance to the tunnel axis and then holds a maximum value when the distance to the tunnel axis reaches a certain magnitude for a specific liner thickness. Moreover, it is observed that with increasing relative liner thickness these aforementioned maximum values of increases and the corresponding position of the points holding a maximum value in the surrounding geomaterial gradually approaches to the outer boundary of the liner. When the distance to the tunnel is further increased, may also gain a minimum value when those points under investigation in the surrounding geomaterial reach the position of r^* (or β^*).

The variations of the normalized tangential stress /σ_o versus the relative distance to the tunnel axis with different relative liner thicknesses when Γ = 10 are shown in ^Fig.7. In contrast to the varying trend of the normalized radial stress, it is found that at first in the liner the normalized tangential stress decreases with increasing the relative distance to the tunnel axis and then along the interface between the liner and the surrounding geomaterial there is a jump or discontinuity in the tangential stress, i.e., the tangential stress in the liner is much higher than that in the surrounding geomaterial. And the jump values decrease with increasing relative liner thickness. As for the tangential stress in the surrounding geomaterial, it is found that /σ_o also decreases with increasing r/a and t/a. In addition, compared with the variation of the radial stress at r = r^*, it may be observed that the tangential stress does not exist extremum property at r = r^*.

By comparing Figs.8 and ⁹ with Figs.6 and ⁷, an interesting phenomenon is observed that the variations of the normalized effective stresses with relative liner rigidity are similar to those with relative liner thickness. Moreover, it may be found from Figs. 6 and 8 that around the position of r^* the fluctuation range of the minimum value of with relative liner rigidity compared with relative liner thickness is large, and that for a certain liner rigidity the distance to the tunnel axis of those points holding a minimum value of decreases with increasing the liner thickness. However, for a certain liner thickness the position of those points holding a minimum value of is almost fixed.

It is also seen from Figs.6-9 that in a range about 1 < r/a < r^*/a the influences of the liner thickness and rigidity and the distance to the tunnel axis on the stress fields are significant and when r^*/a < r/a < 8 the influences become trivial. As for r/a > 8, the variations of the stress fields are almost independent of the values of r/a, t/a and Γ.

The variations of the normalized radial displacement with relative liner thickness and distance to the tunnel axis when Γ = 10 are depicted in Fig.10. It is seen that in general the normalized radial displacement decreases monotonically with increasing relative liner thickness and distance to the tunnel axis. However, when the relative liner thickness is larger than a certain value, i.e., t/a > 1.2 and those points under investigation are in a range about 1 < r/a < 1.5 or 2 < r/a < 3.5, a maximum or minimum value of the radial displacement will occur when the distance to the tunnel axis reaches a certain magnitude for a given liner thickness (see Fig.10(a).

Fig.11 illustrates the variations of the normalized radial displacement with relative liner rigidity and distance to the tunnel axis when t/a = 1.10. It may be found that, when Γ < 20, y_r decreases monotonically with increasing r/a and t/a. However, if the relative liner rigidity Γ > 20 as shown in Fig.11, the varying trend of u_r is non-monotonic and u_r may obtain a negative maximum value when the distance to the tunnel axis reaches a certain value for a specific liner rigidity. It also be noted that, when Γ > 50, the radial displacement occurring at the inner boundary of the liner trends to 0.

Furthermore, it may be found from Figs.10 and 11 that in a range about 1 < r/a < 5 the influences of the liner thickness and rigidity and the distance to the tunnel axis on the radial displacement are significant and when 20 < r/a the influences become insignificant. As for r/a > 50, Figs.10 and 11 indicate that the variations of the displacement field are almost independent of the values of r/a, t/a and Γ.

In Section 1 in the paper, the pressure tunnel is assumed that it is deep enough. However, Figs.6-9 indicate that, when those points under investigation are in the range of r/a > 8, the influence of tunnel excavation upon them is almost trivial. Thus, a conclusion may be drawn that, when the ratio between the embedded depth of a pressure tunnel and the outer radius of the liner is larger than 8, the results about the stress field in the paper are applicable for the stress analysis of the present tunnel. In addition, by comparing Figs.6-9 with Figs.10 and 11, it may be found that the influence of the tunnel excavation upon the displacement field is significantly larger than upon the stress field and only when the ratio between the embedded depth of a tunnel under investigation and the outer radius of the liner is larger than 50 the results about the displacement field in the paper are applicable for the displacement analysis of the tunnel.

5 CONCLUSIONS

In the present paper, an elastic stress-displacement solution for a deep pressure tunnel with impermeable liner in a saturated elastic porous geomaterial that obeys Terzaghi's effective stress principle is derived. The proposed solution considers the effects of the changes of the pore water pressure around the opening, the construction sequence of the tunnel and the interaction between the liner and the surrounding geomaterial on the mechanical response of the tunnel. Finally, the influences of the relative liner thickness and rigidity and the relative distance of the point under investigation to the tunnel axis on the stress-displacement fields for various combinations of the mechanical and geometric parameters are demonstrated numerically.

Acknowledgements The authors would like to deeply appreciate the financial support by the Project of Shandong Province Higher Educational Science and Technology Program (Grant No. J11LE03), the Natural Science Foundation of China (Grant No. 51009086), the Project supported by the Doctoral Foundation of Ludong University (Grant No. LY2011013), the Research Award Fund for Outstanding Middle-aged and Young Scientist of Shandong Province China (Grant No.BS2010HZ015) and the Program for Changjiang Scholars and Innovative Research Team in University (Grant No. IRT0843).

Received 10 Jun 2011;

In revised form 04 Oct 2011

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Publication Dates

• Publication in this collection
  20 Apr 2012
• Date of issue
  2012

History

• Received
  10 June 2011
• Accepted
  04 Oct 2011