Soil compaction control by monitoring compacted states based on soil stiffness indices

A number of empirical equations, expressing the various types of soil stiffness indices (SSIs) of compacted soil as functions of dry density ρd and degree of saturation Sr, are summarized in this study. They are generalized in the normalized form of SSI= CSSI·FSSI(ΔSr)·GSSI([Dc]1Ec). CSSI is the coefficient, different for different SSI test conditions, FSSI(ΔSr) is a decreasing function of ΔSr= Sr – “the optimum, (Sr)opt” with FSSI(ΔSr = 0) = 1.0, and GSSI([Dc]1Ec) is an increasing function of the degree of compaction by the Standard Proctor test, [Dc]1Ec, with GSSI([Dc]1Ec = 100 %) = 1.0. Under the various conditions of the data, the GSSI functions are rather similar, whereas the FSSI functions are largely different. With drained SSIs evaluated at relatively low strains and relatively low confining pressures, FSSI is considerably large when ΔSr < 0 due to the fairly large effects of Sr on these SSIs. As a result, when Sr is in a certain range just below (Sr)opt, the SSIs become the effective indices of water content w, rather than ρd. The compacted water content can be estimated and controlled to remain within a narrow range centered at the target by keeping the frequently measured field SSIs between the defined upper and lower thresholds. The procedure for determining the target and the upper and lower bounds of the SSIs, based on field calibration compaction test data and relevant normalized empirical SSI equations, is explained. A simplified method is proposed, which suggests that, when field SSIs are kept between 0.5 and 2.0 times the target SSI, compacted w values are expected to be approximately maintained between 0.8 and 1.2 times the target w.