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Civil-Comp Proceedings
ISSN 1759-3433
CCP: 81
Edited by: B.H.V. Topping
Paper 266

Evaluation of the Soil Initial Tangent Moduli (E0,G0) by the Shear Wave Propagation Method

J. Bencat

Department of Structural Mechanics, Civil Engineering Faculty, University of Zilina, Slovakia

Full Bibliographic Reference for this paper
J. Bencat, "Evaluation of the Soil Initial Tangent Moduli (E0,G0) by the Shear Wave Propagation Method", in B.H.V. Topping, (Editor), "Proceedings of the Tenth International Conference on Civil, Structural and Environmental Engineering Computing", Civil-Comp Press, Stirlingshire, UK, Paper 266, 2005. doi:10.4203/ccp.81.266
Keywords: soil dynamics, nondestructive determination of soil parameters, wave propagation, elastic moduli.

Initial tangent moduli of soils can be measured in the field or in the laboratory. If only low-amplitude strains occur (strain less then 0.001 percent), the key soil parameters are the initial tangent moduli (G0,E0). The most direct field method and box test method for determining initial tangent moduli is seismic testing. Shear and constrained moduli are evaluated directly from shear (Vs) and compression (Vp) waves velocities, respectively, using the theory of visco-elasticity.

In the impact-seismic method (ISM), body wave velocities are commonly evaluated from visual determinations of times of arrival of the waves at receivers or by time-determination techniques based on correlation and spectral analysis.

It is common practice in the seismic methods (ISM) to use a linear source-receiver array with one or two receivers located at a distance 1 (m) from source, Bencat & Stehlíková [2].

Other techniques based on correlation and spectral analysis theories can be employed to determine body wave velocities in the ISM test (as well as other seismic tests such as the cross-hole and the down-hole method). This approach offers benefits in two areas. First, interval velocities have fever potential errors than direct velocities. Second, the techniques can be fully automated. Furthermore, the retrieval of additional information such as strain rate effects and material damping is possible evaluate, e.g. via the spectral analysis.

Records obtained from the shear wave velocity measurements by an ISM test at a box soil media are analysed as an application of the travel-time determinations explained above. Details of the box set up, and equipment used can be found in reference Bencat [1].

The time records of wave motion are obtained with two vertical acceleration transducers. The time interval obtained from first arrival of the shear wave is t = 29,29 msec. This corresponds to a shear wave velocity Vs = 94,23 ms-1 and modulus for subgrade material (MS: = 1735 kg/m3, w = 13.9%, v = 0.3) E0 = 40,05 MPa.

The maximum value of the cross-correlation function (CCF) of the two time records is occurs at time t* = 29,40 msec. The shear wave velocity calculated with this time is Vs = 93,87 ms-1 and modulus E0 = 39,75 MPa.

The phase of the cross-spectral function (CROSS-ESD PHASE) is discussed in the paper along with the coherence function and the energy cross-spectrum (CROSS-ESD MAGNITUDE). For a signal with no background noise, the coherence function (CF) should have a value of one at any frequency if the system is considered linear.

Comparison between mean value of the statistical value colection of all corresponding static moduli (Es), from the Static Loading Tests (SLT) and seismic resilient moduli (Es) at the layer of soil under investigation (MS, test T 11-3/6) gives ratio E/Es = 2.35.

There were evaluated ratios E0/Es for each layer of the soils creating a corresponding combination of the subballast and subgrade in the box using the procedure described above in Bencat [1].

The ratio between the dynamic moduli (ED),from the Dynamic Loading Tests (DLT) and the static moduli (ED/Es) and between the initial tangent and the static resilient moduli (E0/Es) for an individual of soils in the box test (layer) combination are given in the paper.

Techniques to determine travel times of the body waves in geotechnical seismic testing have received little attention. Travel times are required to calculate the shear wave velocities from which the initial tangent moduli (G0,E0) are determined. With the advent of portable signal processing equipment (e.g. Bruel-Kjaer Portable PULSE 3560 MA-System) or portable PC with relevant software (e.g. DISYS or DAS16) other techniques for travel-time determination, such as those based on correlation and spectral analysis, can also be implemented in the field.

Bencat, J., "Model and Theoretical Investigation of Wave Propagation through Bodies and Environment", Research Report III-7-3/9c, UTC Zilina (in Slovak), 1991.
Bencat, J., Stehlíková, M., "Evaluation of the Initial Tangent Moduli by Seismic tests", 10th European Conference on Earthquake Engineering, Balkema, Rotterdam, 1995.

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