Mechanical behavior of soil nail bar composites with laboratory tests

Abstract

Past studies on soil nailing have a common assumption that a soil nail could be simplified as a single material solid cylinder. However, a soil nail is a composite material and consisted of the following basic components: steel bars, cement grout, PVC centralizers, steel couplers, and a plastic hose, etc. They are combined and work together to keep the fundamental functions in soil reinforcement.

Since these components are made by various constituent materials, the physical and mechanical properties of them are greatly different, making the deformations through soil nail diverse. The different deformations could be incompatible, which has significant effects on the soil nail integrity and performance due to the intrinsically existing vulnerable areas.

One critical issue is the key component cement grout sleeve, functioning as the adhesive to transfer stress between the ground and steel bar, may encounter tensile failure on the out-layer surface or along the interface between steel bar and itself. The consequence would be cracking and detachment of cement grout and then malfunction of soil nails. Therefore, it is of great importance to examine the mechanical behavior of soil nails by taking into account the behavior of the different components.

In this research extensive laboratory tests have been carried out to examine and quantify the mechanical properties of soil nail’s each component as well as their simple combinations. Based on this, a series of physical modeling of soil nail in tension and bending tests are designed to simulate the deformation of soil nail under loading. The characteristics of soil nail’s overall behavior and cracking initiation/propagation in cement grout are observed and interpreted.

Besides, two soil nail’s component replacement trials based on the conventional design are conducted with attempt to enhance the overall soil nail strength and facilitate the installation procedure. A new concrete mix design with fine aggregates (sands) is adopted to substitute the neat cement grout for minimizing the local failure in cement column. Optimum sand ratio which is sound for postponing the cracking is evaluated as well. Another trial is the application of hollow core steel bar in place of the solid one. Compared to the traditional drill-and-grout soil nailing method, this new technology has advantages of fewer installation steps through a self-grout method. Similar laboratory tests are done to exam the impact on soil nail’s capacity from this change and preliminarily evaluate the feasibility of using of hollow core steel bar in practice.