# Laboratory Tests on Swelling Properties of Field-Coring Gypsum Rock in Tunnels
###### Abstract  
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 120 th 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.  
gypsum rock; swelling behavior; laboratory test; swelling model +
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in swelling pressure over time. During the monitoring period of more than one year, the swelling pressure did not reach 16.0 MPa, but the overall swelling law was similar to laboratory research. The swelling pressure increased rapidly in the early stage, and then gradually became slower [4]. In the long-term test of swelling characteristics in these laboratories and fields, the swelling pressure was different, but its overall swelling law was similar. A rapid growth of the swelling force in the early stage of the test was observed.  
In some tunnel engineering, due to groundwater or construction water providing enough water for gypsum-rock hydration reaction, the tunnel could be destroyed by the swelling of gypsum rock in a short time. For example, the Chienberg road tunnel suffered severe swelling and damage in a short period of time during the excavation [11]. Some scholars have studied the swelling characteristics of mudstone, shale and other rocks. Some strength tests were carried out on the swelling mudstone samples with different water content, and a complete empirical formula for predicting the mechanical properties of swelling mudstone was established [19]. Wetting tests were conducted using an environmental scanning electron microscope (ESEM), which addresses the swelling property of argillaceous rock under resaturated conditions [20]. A solid model with cohesive elements based on the finite element method was used to predict the generation of micro-cracks in shale rock [21].  
The swelling of the existing research on gypsum rock characteristics is of great significance. However, these studies have focused on the long-term expansion properties of gypsum rock and are very time-consuming, both for laboratory experiments and field tests. Additionally, there are few studies on the swelling characteristics of gypsum rock in the short term. In this study, an S-shaped swelling-time model was put forward through laboratory experiments to explore the early swelling characteristics of gypsum rocks. Studying the short-term swelling characteristics of gypsum rock can help to better understand the swelling characteristics of gypsum rock and can provide some reference for preventing early swelling disasters.