Experimental Study on the Influence of Drillhole Roughness on the Pullout Resistance of Model Soil Nails

Abstract

Pullout resistance of cement-grouted soil nails is a key factor affecting the safety conditions of retaining walls, slopes, and excavations. The mobilized frictional resistance at the interface between soil nails and surrounding soils depends on various parameters such as saturation ratio, water content, grouting under gravity or pressure, etc. The interface roughness condition between soil nail and soil is a critical factor but difficult to control in both laboratory and field owing to technical difficulties in creating a rough drillhole surface. The present study focuses on the pullout behavior of model soil nails with different interface roughness conditions in the laboratory to establish quantified correlations between frictional resistance and roughness angles of internal drillhole surface. Cross-sectional shapes of internal drillhole surfaces were created using four plastic rods with various shapes of external threads (four different values of roughness angles). Measured peak values of pullout resistance of soil nails with rough drillhole surface were compared with the soil nails with smooth drillhole surface. The present test results were verified with a previous analytical model, which considered the soil dilation to be an important factor influencing the pullout resistance of soil nails. Test results also indicate that the peak pullout resistance increases almost linearly with an increase in roughness angles. In addition, the pullout resistance values decrease approximately linearly with an increase in pullout displacement after peak pullout resistance is approached. The soil-nail diameter values of the soil nails with rough drillhole surfaces expanded substantially after being pulled out of the ground. This leads to a substantial increase in pullout resistance of soil nails.