Laboratory Tests on Swelling Properties of Field-Coring Gypsum Rock in Tunnels

The reaction between gypsum rock and water results in swelling deformation and swelling pressure. Swelling deformation and swelling pressure cause damage to underground engineering such as tunnels. It is of significance to study the swelling characteristics of gypsum rock. The variation in maximum radial free-swelling ratio, axial free-swelling ratio, lateral restricted-swelling ratio and lateral restricted-swelling pressure of gypsum rock with a water immersion time of 2880 min were investigated experimentally. The early swelling characteristics were further discussed and described by an S-shaped model. The results show that the swelling ratio and swelling pressure increase rapidly as the immersion time increases for the first 120 min. Subsequently, the swelling ratio and swelling pressure increase slowly and become stable as the immersion time further increases. At the 120th minute, the maximum radial free-swelling ratio, axial free-swelling ratio, lateral restricted-swelling ratio and lateral restricted-swelling pressure of gypsum rock reach more than 80% of their final values (2880 min in the present study). Based on the swelling characteristics of gypsum rock during the first 120 min, an S-shaped swelling-time model was introduced to describe the early swelling behavior of gypsum rock

Study on Identification of Construction Method for Ultra-Large-Span Tunnel

Although the determination of tunnel construction methods is extremely critical for the construction of ultra-large-span tunnels, the determination of construction methods is still at a qualitative level, which relies on the engineering experience of on-site technicians and lacks rigorous and systematic theoretical basis and technical standards. By means of orthogonal test method, the proper construction method was established for the deep-buried ultra-large-span tunnel where the tunnel excavation span, tunnel surrounding rock strength, and rock integrity coefficient were set as the main control factors. The stability of tunnel surrounding rock under various test conditions was quantified according to the plastic zone properties calculated by the three-factor and five-level orthogonal test model. Meanwhile, the macro form and quantitative method of test combinations under different levels of various factors were proposed to obtain the influence of each factor on the stability of tunnel surrounding rock, and thus the functional relations between various factors and tunnel stability were obtained. On this basis, the identification and the criterion of the ultra-large-span tunnel construction method were established, which can quantitatively reflect the contribution of excavation span of the tunnel, the number of lateral drifts in cross section, surrounding rock strength, and rock integrity coefficient to surrounding rock stability of the tunnel. The construction method calculation results of the Malin tunnel, a practical underground project, are obtained according to the orthogonal test model calculation. Based on the method, Malin tunnel can be constructed safely and efficiently. The research results could provide the theoretical basis for the identification and selection of construction method for ultra-large-span tunnel

Laboratory Tests on Swelling Properties of Field-Coring Gypsum Rock in Tunnels

The reaction between gypsum rock and water results in swelling deformation and swelling pressure. Swelling deformation and swelling pressure cause damage to underground engineering such as tunnels. It is of significance to study the swelling characteristics of gypsum rock. The variation in maximum radial free-swelling ratio, axial free-swelling ratio, lateral restricted-swelling ratio and lateral restricted-swelling pressure of gypsum rock with a water immersion time of 2880 min were investigated experimentally. The early swelling characteristics were further discussed and described by an S-shaped model. The results show that the swelling ratio and swelling pressure increase rapidly as the immersion time increases for the first 120 min. Subsequently, the swelling ratio and swelling pressure increase slowly and become stable as the immersion time further increases. At the 120th minute, the maximum radial free-swelling ratio, axial free-swelling ratio, lateral restricted-swelling ratio and lateral restricted-swelling pressure of gypsum rock reach more than 80% of their final values (2880 min in the present study). Based on the swelling characteristics of gypsum rock during the first 120 min, an S-shaped swelling-time model was introduced to describe the early swelling behavior of gypsum rock

Estimation of the Occurrence Time of Thaumasite Sulfate Attack on Tunnel Lining Concrete

The thaumasite sulfate attack (TSA) on tunnel concrete structure has been reported increasingly in the past decades. Previous investigations on the formation of thaumasite were focused on identifying the deterioration products and reaction mechanisms, while the occurrence time of TSA on tunnel concrete structures was not reported. A highway tunnel exposed to TSA was reported in the present study. The development of tunnel diseases and results of experimental tests conducted in the tunnel and in the laboratory were analyzed to investigate the occurrence time of TSA on concrete. Results revealed that the thaumasite was formed in a range of 18 to 36 months after the construction of Dugongling tunnel. The preconditions for the formation of thaumasite on tunnel concrete structures are available in Shanxi Province, China, due to the special conditions of stratum lithology and climate. The compositions of corrosion products of lining concrete under TSA varied for site studies and for laboratory tests. Site investigations on TSA on tunnel lining concrete should be paid more attention in further research

Constitutive Model of Swelling Gypsum Rock

Swelling of soft rock, such as gypsum rock, is one of the major threats in tunnel engineering, causing structure damages such as floor heave and inward movement of sidewalls during construction and operation. It is of practical significance to study the swelling mechanical behavior of such rocks by tests. Swelling strain tests and swelling stress tests were performed by swelling test apparatus to study the variation of swelling strain with time and the swelling stress-strain relationship for gypsum rock samples, respectively. Three stages of the swelling strain on the time-strain curve of gypsum rock samples were noticed, which are defined as rapid swelling stage, slow swelling stage, and steady stage. And it was further found that the swelling strain caused in the slow swelling stage is of 76% of the total swelling strain. A constitutive model is proposed to describe the stress-strain relationship in swelling considering the swelling deformation and swelling pressure. The proposed model was verified using test data, which shows good agreements in describing the relationship between swelling strain and swelling stress, also in the conditions of maximum swelling strain and maximum swelling stress under lateral restraint situations

Dilatancy Characteristics and Constitutive Modelling of the Unsaturated Soil Based on Changes in the Mass Water Content

Most soil mechanics theories are limited to strain hardening and shrinkage under high compressive stresses, and there are some shortcomings in the selection of suction or degree of saturation as the water content state varies in the constitutive models of unsaturated soil. Based on the triaxial shear tests of unsaturated compacted soil (a silt of high plasticity) with different water content and confining pressure (low-confining), a shear dilatancy model of unsaturated soil based on the mass water content is proposed in this paper. The influence of the water content on the shear deformation characteristics of the unsaturated soil is analysed. The stress–dilatancy relationship and the prediction equation of the minimum dilatancy rate of the unsaturated soil under different water content and different confining pressure are provided. Selecting the mass water content as the state variable, a constitutive model suitable for the dilatancy of unsaturated soil is established. The method of determining model parameters based on the mass water content is analysed. The applicability of the model is verified by comparisons between the predicted and experimental results

Experimental Investigation of Crystal Blocking in Drainage Pipes for Tunnels in the Karst Region

Crystal blockage of tunnel drainage pipes is one of the main causes of problems such as lining cracking and water leakage. The study of the crystal development rule is of great significance for the design of tunnel drainage systems and the long-term safety of tunnel support structures. In this paper, a series of experimental studies on the crystallization development law of drain pipes are conducted. The effects of the connection method of the drain, the diameter of the pipe, the spacing of the circular drain, and the material of the drain on the crystallization development pattern are investigated. The results show that the groundwater environment has a great influence on the crystallization development of the drain pipe. As the drain diameter and the spacing between two adjacent circular drains increased, the time for complete blockage of the drain is prolonged. The rate of crystallization on the drainage pipe can be effectively reduced by changing the material of the drainage pipe from polyamide (PA) to polypropylene (PP). The present study provides a reference for research work related to crystallization blockage in tunnel drainage pipes