A Comparison Of Shear Wave Velocities Obtained From The Crosshole Seismic, Spectral Analysis Of Surface Waves And Multiple Impacts Of Surface Waves Methods

Abstract

Presented herein is the comparison of the shear wave velocity results, with respect to depth, obtained from three separate test methods at a soil test site. The Crosshole Seismic (CS) test was performed utilizing three cased boreholes nominally 10 feet apart. The direct shear wave velocity was obtained from the results of the CS test. The Spectral Analysis of Surface Waves (SASW) test method and Multiple Impact of Surface Waves (MISW) test method were also performed at the same test site for comparison purposes. Neither the SASW nor MISW test methods require the installation of boreholes, thus reducing the cost of the test. SASW and MISW differ slightly from one another in the equipment used, method of data collection, and differ significantly in data processing. This paper includes a comparison and discussion of the test results, as well as backgrounds of each test method. Introduction Construction of foundation systems for civil structures often requires detailed information of the site soil properties. Bore logs provide soil samples for soil type classification and laboratory testing to determine strength and consolidation parameters among other properties with respect to depth. A number of soil-boring related in-situ tests have also been correlated with soil strength (e.g. standard penetration test, cone penetration test), however in the interest of accuracy it is certainly advantageous to measure a in-situ soil property directly related to soil modulus. Shear wave velocity (Vs) has become the standard property from which in-situ soil modulus is determined due to its direct relationship with modulus via the soil mass density (which can be assumed with little error or easily measured from soil samples) as well as it’s relative ease of measurement, due to the advancement of seismic techniques. A number of in-situ test methods have been developed to measure Vs with respect to depth; within this paper, three methods, Crosshole Seismic (CS), Spectral Analysis of Surface Waves (SASW), and Multiple Impact of Surface Waves (MISW), will be described and compared. Traditionally, CS testing has been considered the most accurate method in determining Vs, because it is a direct measurement of the wave speed. SASW and MISW however, can be employed much more rapidly and economically because the methods are performed on the ground surface (unlike CS where at least two boreholes are required to perform the testing). Yet, no known research has been performed to directly compare the accuracy of the latter two methods to equivalent CS results. With this objective, all three tests were performed at the same soil site for direct data comparison.