The measurement of reinstatement backfill properties

Abstract

The Home Committee Report, published in 1985, identified the need for the reinstatement of service trench backfill to be performed to a higher and more reliable standard. This end was perceived as desirable, not only to improve road quality and safety, but also to increase the protection afforded to utility apparatus, thereby reducing the number of excavations required for its repair and replacement. The replacement of existing method specifications with a performance specification, based on the testing of an appropriate property of the reinstated material, is one possible means of improving the quality of reinstatement works. The Clegg meter is finding increasing usage for the purpose of compaction measurement. This device is simple to use and measures the decceleration response of a mass in collision with the ground surface. The decelartaion response is characterised by the Impact Value. The sampled volume is, however, very small in relation to the volume of material compacted and, for full evaluation, each compacted layer must be individually tested prior to the placing of the subsequent layer. The ideal test would be non-intrusive and operable from the final surface of the granular backfill; wave transmission velocity measurements are thus a potential technique. The propagation of sinusoidally excited Rayleigh-type surface waves is identified as the most promising method of measurement for reinstatement backfill quality. Experimental measurements of particle displacement at depth lend credence to the classical assumption that the depth of propagation of Rayleigh-type surface waves is equal to one-half the wavelength (z = γ/2).Included in this thesis is a review of the salient aspects of reinstatement works and a presentation of the necessary theory of wave propagation. The experimental results presented relate to tests on two sand materials compacted using different levels of compactive effort in all above-ground laboratory based trench and also to tests conducted on a crushed limestone aggregate under field conditions. Results from a short programme of field-based case studies are also presented. In addition to the surface wave velocity and Impact Value results, values for dry density and CBR are also presented. The data generally show a good correlation with compactive effort, whether defined simply as the number of passes/layer (N(_p)) or by a new parameter, the ratio of N(_p) to the mean layer thickness (N(_p)/h). Sensitivity analyses indicate that the Impact Value is a more sensitive measure of the degree of compaction than are either the surface wave velocity or the dry density. Conversion of the surface wave velocity results to shear modulus values, using measured density values, gives a sensitivity to compactive effort that is broadly comparable to that of the Impact Value. A novel technique for the analysis of Rayleigh-type surface wave attenuation data is presented. This allows the calculation of the material attenuation coefficient while obviating the need for potentially erroneous estimates as to the state of the attenuation curve, close to the source of vibration, to be made.