Numerical studies on tunnel floor heave in swelling ground under humid conditions
Highlights
► A humidity diffusion based model is proposed to simulate the floor heave of tunnel. ► The floor heave is greatly depending on the degradation by humidity corrosion. ► The floor heave is sensitive to the lateral pressure and the swelling coefficient. ► The time-dependent deformation is caused by stress and environmental corrosion.
Introduction
In the swelling rock engineering, water is one of an important factor causing the floor heave of tunnel because of the water adsorption by the flakey structure of the clay minerals [1]. Most of studies in laboratory tests as well as in-situ observations show that considerable pressures develop when preventing the swelling strains. For example [2], the swelling behaviors in a large number of tunnels in Baden-Württemberg (Southwestern Germany) have caused high heave in unreinforced tunnel floors and strong swelling pressure against inverts of tunnels with resisting supports. In severe cases in which it was attempted to prevent or impede swelling strains by means of a stiff tunnel lining, swelling pressures often developed that were high enough to lead to the destruction of the tunnel lining, even a 30 cm thick concrete support was destroyed during construction along large tunnel sections by shear failure [3].
To avoid such failures methods for computing the swelling pressure and the deformation development history were proposed in the past several decades. The first analytical approach to the swelling problem was given in [4], where the stability of tunnel masonry-work in connection with the construction of the lower Hauenstein tunnel in Switzerland was studied. And then the simplified methods of Einstein et al. [5], Grob [6] and Kovári et al. [7] were based upon a priori assumptions concerning the stresses or strains along the vertical symmetry axis beneath the tunnel floor. However, they did not predict the complete deformation and stress fields. By treating the swelling process on a continuum-mechanical basis, other researchers [8], [9] proposed the complete stress–strain relations of swelling rocks. Wittke-Gattermann and Wittke [3] also proposed a constitutive law to describe the swelling phenomena in a realistic way and successfully applied to the conditions of the exploration gallery for the Freudenstein tunnel at the high-speed railway line Mannheim–Stuttgart. Furthermore, the movement of water and the chemical processes in the swelling rock should also be taken into consideration in the numerical model, in which the seepage flow equations must be considered simultaneously to the equations of stress analysis. Therefore, Anagnostou [10] proposed an improved computational model for tunnels in swelling rock that the swelling in the tunnel is analyzed as a thermo-hydro-chemo-mechanical (THCM) phenomenon using a theoretical framework founded on basic geochemical and thermodynamical principles, making it possible the formulation of consistent triggering swelling events as well as the identification of conditions conducing to either their evolution or exhaustion.
It is known that the main reason for the swelling of rock is the suction of water by the clay minerals. However, most of the priori studies focused on the influence of free water on mechanical behavior of swelling rock. Another type of water, i.e. water vapor, is seldom taken into consideration in the tunnel stability studies. In fact, numerous studies indicated that humidity is also an important factor effect on the mechanical behavior of rocks. The static fatigue tests conducted on granite and anorthosite have shown that in a humid environment the long-term strengths of these crystalline igneous rocks could be less than 60 percent of their dry instantaneous strengths [11]. Such reduction in strength has implications for the design and construction of deep tunnels, mines and other underground installations. As the same as free water, the influence of humidity on mechanical behavior of rock by two important ways: mechanical and physico-chemical effect [12], [13]: the lowering of the effective stress lowers the fracture strength, and the physico-chemical effect results in the mechanical properties of rock being time dependent which is a corrosive processes. For the swelling rock, in addition to the mechanical responses above mentioned, the humidity also causes the volume change. However, in the past several years, studies pay more attention on the effect of moisture or humidity on mechanical behavior of rock, hardly any of researches on the floor heave of tunnel under humid condition, especially the relationship between crack growth and the time-dependent mechanical behavior during the period of floor heave, even the crack growth processes at the humid condition have not been clarified yet. Therefore, it is important to further study the long-term behavior of floor heave under the humid condition to improve engineering construction. When the mechanism of floor heave is correctly understood can a mechanic model representing the actual state be established, and then effective numerical calculation can be performed. Therefore, this paper describes the development and implementation of a two-dimensional, plane strain finite element model to calculate both the stress and time-dependent deformation mechanical behavior of swelling rock under high humid condition, and then focus on the discussion of influence factors on floor heave of tunnel.
Section snippets
Introduction of numerical model
An important phenomenon of the mechanical behavior of swelling rock is time-dependent deformation [14]. The consequence of the time-dependent deformation of floor in swelling rock is the deterioration of rock strength and elastic modulus, resulting in the collapse of the tunnels. To study the time-dependent deformation of rock, the rheology theories are broadly used in rock engineering [15], [16], [17]. The classic models of creep deformation are usually described by using viscoelasticity or
Modeling strategy
Under the high humid condition of underground engineering, elastic modulus and strength will degrade with humidity increasing. The increased humidity also results in swelling of rock. Therefore, damage may occur during the humidity diffusion process. To model the floor heave and damage processes caused by humidity diffusion, the model shown in Fig. 3 is adopted as an example, where points A and B are at the center of floor and right sidewall, and are used to observe or measure the deformation
Effect of elastic modulus degradation by humidity on floor heave of tunnel
To study the effect of elastic modulus degradation by humidity on floor heave of tunnel, the parameter ω in Eq. (9) is set to 0.3, 0.6 and 0.8, respectively, but with other parameters remaining unchanged. Fig. 4 shows the relationships between time and the displacements at Point A and B shown in Fig. 3. It can be seen that the deformation at the beginning of excavation rapidly increase over time. If the tunnel maintaining the relatively high humidity condition, moisture will continuously
Conclusions
As the underground engineering exposed to high humid condition, the free surface subjected to the action of humidity, and the mechanical properties of surrounding rock degrade with the humidity increase. Based on the point of view of humidity diffusion, the effect of humidity on stability of the floor heave is discussed in this paper. When a tunnel is exposed to external environment with high humidity, humidity is the major factor for its time-dependent behaviors. The change in humidity not
Acknowledgment
This project was financially supported by the National Basic Research Program of China (973 Program, Grant No. 2011CB013503), the National Natural Science Foundation of China (51121005, 51004020), the China Postdoctoral Science Foundation funded project (201104563) and the Fundamental Research Funds for the Central Universities (DUT12ZD102).
References (47)
Crack growth and development during creep of Barre granite
J Rock Mech Min Sci Geomech Abstr
(1979)- et al.
Creep damage modeling for quasi-brittle materials
Eur J Mech A Solid
(2005) - et al.
Modeling of creep in rock materials in terms of material degradation
Comput Geotech
(2003) Subcritical crack propagation in rocks: theory, experimental results and applications
J Struct Geol
(1982)- et al.
The theory of subcritical crack growth with applications to minerals and rocks
- et al.
Creep and time-dependent damage in argillaceous rocks
Int J Rock Mech Min Sci
(2006) - et al.
Effect of relative humidity and temperature on subcritical crack growth in igneous rock
Int J Rock Mech Min Sci
(2010) - et al.
Effects of humidity and temperature on subcritical crack growth in sandstone
Int J Solids Struct
(2011) Numerical simulation of progressive rock failure and associated seismicity
Int J Rock Mech Min Sci
(1997)- et al.
Moisture diffusion of concrete considering self-desiccation at early ages
Cem Concr Res
(1999)
Effect of moisture on strata control in coal mines
Eng Geol
Simulation of progressive fracturing processes around underground excavations under biaxial compression
Tunneling Underground Space Technol
A review of expansive phenomena in Wagenburg North Tunnel
Rev Acad Col Cienc Rev Acad Colomb Cienc
Über die stabilität von tunnelmauerwerk unter berücksichtigung der erfahrungen beim bau des Hauenstein-Basistunnels, Schweizer
Bauzeitung
Design of tunnels in swelling rock
Rock Mech Rock Eng
A model for swelling rock in tunnelliing
Rock Mech Rock Eng
Delayed failure in rock loaded in uniaxial compression
Rock Mech Rock Eng
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