site characterisation study - phases 1 and 2 - HKU Libraries - The ...

- 153 -(c) GSC present pseudo-dispersion curves and interpretedgeological models for all the traverses. The interpretation isbased on the pseudo-dispersion curves as no inversion orforward modelling has been made. They claim to havesuccessfully measured the thickness of the retainingstructures and located voids below the shotcrete surfaceprotection along TA02 and behind the masonry wall alongTB03.(d) MW present contoured shear wave velocity versus depthsections for the longer traverses. They were not able tomeasure the thickness of the retaining walls but were able todetermine shear wave velocity depth information for fill andnatural materials.(e) GA present selected dispersion curves and shear wave depthprofiles for all of the sites. They were unable to measure theretaining wall thickness but obtained consistent data formost of the horizontal traverses at the top of walls or at theslope sites.D.2.3 Assessment of the MethodAn experienced team of one geophysicist and a labourer can complete the field work ofshooting one traverse in 1 to 2 hours. None of the contractors reported problems withvibration noise at any of the sites. The depth of penetration of the method is dependent ongeophone spacing and is therefore constrained by site conditions. Where voids exist below arigid surface cover, such as a concrete slab, de-coupling of the geophone from the soil belowis reported by most of the contractors.Depth of penetration and resolution is dependent on the wave length of the Rayleighwave produced and detected. As noted above, the physical limitation of space on site will alsoaffect the maximum penetration depth. MW and GA achieved resolution of between OJm to0.2m while GSC and IGGE achieved resolution to between 0.5m and 1.0m. At sites withlimited space, penetration depths of 3m to 4m were generally achieved. At Site D, themaximum depth of penetration was 12m. Depths to discrete objects were not reported by anyof the contractors.Both MW and GA were not able to interpret the SASW data obtained from themasonry walls at Sites B and C. They both report that reproducible dispersion curves couldnot be obtained for shallow depths into the walls. They suggest that this is probably due to thewall facing being made up of discrete blocks separated by cement mortar and therefore notbeing a continuum and producing very complex wave propagation patterns which cannot beinterpreted with existing algorithms. The horizontal tie beams also cause similar effects, asdemonstrated in the GA report (Golder Associates, 1996a).