Behavior of a pressure-grouted soil-cement interface in direct shear tests

An interface between compacted soil and a structure is commonly encountered in various geotechnical engineering projects, e.g., soil nails, retaining walls, shallow foundations, pile foundations, and so on. The interface strength depends on the way the soil-structure interface is formed. A cast-in-situ interface is very common in many geotechnical projects. This kind of interface is formed by placing concrete/cement grout over the prepared soil surface. The cement part can be formed over a prepared soil surface in two ways: (1) by normal gravity grouting and (2) by pressure grouting. In this study, a series of interface direct-shear tests was performed between compacted, completely decomposed granite (CDG) soil and cement grout under saturated conditions with different grouting pressures and normal stresses. The behaviors of the shear-stress- displacement curves of the soil-cement interface are similar to those of CDG soil. Grouting pressure and normal stress have influence on the behavior of the soil-cement interface. The failure envelopes for different grouting pressures are observed to be linear. The apparent effective interface friction angles are constant for different grouting pressures. On the other hand, apparent effective adhesion intercepts increase with grouting pressure.When the shear plane is fixed, the apparent effective interface friction angles for different grouting pressures are greater than the effective friction angle of CDG soil under the same normal stresses, which implies that a compacted CDG soil-cement grout interface behaves as a rough interface. The variation in interface shear strength with grouting pressure (grouting-pressure envelope) at the same shear-plane level is approximately linear, and declivities are constant for different normal stresses. A model is proposed for interface shear strength under saturated conditions that considers grouting pressure as an independent variable. The predicted interface shear strength of the proposed model agrees fairly well with the experimental data.