Normal contact forces in jacking of casings under conditions of their floating-up and sticking

Abstract

Trenchless technologies are commonly used in the collector tunnel construction in the world. The most advanced technology in this regard is microtunneling/casing jacking—when casing pipes are forced in ground using bentonite slurry to reduce friction and to make soil mass stable. Two phenomena are possible in this case: floating-up of the casing and its sticking under differential pressure. Tunneling in water-saturated ground necessitates prediction of jacking forces. Here, it is critical to determine resistance to the casing moving off from the rest. To this effect, tangential forces along the casing–soil contact are analyzed, as a rule. The main characteristics are usually the contact area in the roof of the underground opening and the normal force distribution along the casing. The estimation procedure uses three parameters: flexural stiffness of casing, soil hardness and construction clearance. This article offers a model representation of joint casing–soil deformation under conditions of floating-up and sticking of the casing in the roof of the opening under the action of differential pressure. The system of ordinary differential equations describing the casing behavior in the zones of contact and no-contact with soil is obtained and solved. The analytical solution is used to determine the contact zone size and to estimate the normal contact forces. The contact zone sizes versus reduced floating-up lengths of casing and soil hardness in the actual value range are plotted. It is found that the contact zone sizes grow when casing length is within the typical floating-up inflexion length and drop when casing length is 5–10 typical inflexion lengths with respect to soil hardness. In a general case, the contact zone size increases with decreasing flexural stiffness of casing and hardness of soil. The normal contact force distribution has a shape of a hip roof and features higher concentration at the end points of the contact zone in soil of higher hardness. The research results can be used to improve the model of casing breakaway in order to have more accurate estimates of jacking forces.