The Effects of Compaction Post Grouting of Model Shaft Tips in Fine Sand at Differing Relative Densities—Experimental Results

This paper describes a laboratory study conducted on the effects of compaction grouting of model shaft tips embedded in a dry cohesionless soil prepared to different target relative densities. These tests were conducted within a circular steel test chamber 2.4 m in diameter and 2.7 m high. The soil used in this series of tests was a dry sub-angular fine sand (SP) having a D₅₀ of 0.24 mm and a C_u of 1.64. The soil was prepared to relative densities of 50%, 70%, and an initial test on a variable but high relative density specimen. The chamber was filled with the sand in one foot lifts, and densified by vibration in the case of tests on soils at 50% and 70% relative densities, and by hand tamping for soil in the initial variable high density test. The model shaft was placed within the chamber as filling of the tank progressed. Densities were measured using a combination of nuclear density gage measurements and dynamic cone penetrometer (DCP) readings. Nuclear density gage readings were made at 300 mm height intervals as filling progressed while DCP recordings, when possible, were made subsequent to the completion of tank filling operations and application of overburden pressure. At both 50% and 70% relative densities two shafts were load tested, with one being tip post-grouted while the other was not. Additionally a lone ungrouted shaft was load tested within soil of the Initial (variable high density) Test. Grouting of the shaft tip involved pumping a low mobility compaction grout, into the said tip using a system of hoses and pipes as currently used in the industry. Tip pressures induced during the grouting stage together with volume of grout pumped and strains induced within the shaft and test chamber were continuously monitored and recorded in real-time. Based on analyzed load test results, a significant improvement in tip capacities is noted for the grouted shafts over the ungrouted shafts. This tip capacity increase was thereafter analyzed with a view to developing relationships between the grout volume pumped, tip pressures induced during grouting and the load — displacement response of the grouted model shafts. The behaviors of the tip and skin friction capacities during load testing were also studied.