Investigation of Composite Compacted Ground Using Microtremors

This paper presents an interesting and unique case study of a composite compacted site where the upper section of dynamically compacted material achieved in the first stage of compaction was subsequently removed, reinstated, and recompacted in lifts in the second stage using conventional roller compaction. Dynamic compaction was employed initially in this area because of the need to densify deep fill materials. Although a number of mechanical methods are already available for assessing deep compaction, it has been particularly rare to find a cost-effective method that can be applied to a deep and extensive compacted site. Noninvasive techniques based on measurement of the horizontal-to-vertical spectral ratio (HVSR) of ambient vibrations (microtremors) are proposed in this paper to assist in a pilot appraisal of this area, which occupies a part of a deep and laterally extensive compacted site. First, the key features of the measured HVSR curves were interpreted to give a preliminary insight into the quality of compaction achieved. Second, a trial-and-error forward modeling procedure fitting the theoretical HVSR curve to the measured HVSR curve then allowed the shear-wave velocity ($V_s$) profile conveying the compaction quality of the compacted ground to be inferred. An initial calibration was carried out to match the inferred $V_s$ profile in a relative sense against the cone-penetration-test (CPT) data at a test location. Verification was further made by comparing the inferred $V_s$ profiles against independent CPT, dialatometer-test (DMT), and dry-density data at two other locations. The HVSR technique was then applied to appraise the consistency and quality of compaction at grid points not covered by the localized mechanical and other independent tests.