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1 This document is downloaded from DR-NTU, Nanyang Technological University Library, Singapore. Title Stiffness profiles of residual soil sites using continuous surface wave method Author(s) Aung, Aung Myo Win; Leong, Eng Choon Citation Date 2012 URL Rights 2011 Kasetsart University. This paper was published in Asia-Pacific Conference on Unsaturated Soils: Theory and Practice and is made available as an electronic reprint (preprint) with permission of Kasetsart University. The paper can be found at the following URL: [ One print or electronic copy may be made for personal use only. Systematic or multiple reproduction, distribution to multiple locations via electronic or other means, duplication of any material in this paper for a fee or for commercial purposes, or modification of the content of the paper is prohibited and is subject to penalties under law.

2 Unsaturated Soils: Theory and Practice 2011 Jotisankasa, Sawangsuriya, Soralump and Mairaing (Editors) Kasetsart University, Thailand, ISBN Stiffness profiles of residual soil sites using continuous surface wave method A.M.W. Aung & E.C. Leong School of Civil & Environmental Engineering, Nanyang Technological University, Singapore, ABSTRACT: In this study, continuous surface wave (CSW) tests were conducted to study the dispersion characteristics and stiffness profiles of two unsaturated residual soil sites in Singapore. Rayleigh wave dispersion curves were obtained by plotting measured Rayleigh wave phase velocities (V ) against frequency. Dispersion characteristics of both sites were found to be normally dispersive. Stiffness profiles were then determined through inversion analyses. The stiffness profiles were found to be in good agreement with stiffness estimated from standard penetration test (SPT) and cone penetration test (CPT) data. The degree of weathering with depth at the two sites was also estimated from stiffness profiles. It was found that both sites have been heavily to a substantial depth. KEYWORDS: continuous surface waves; Rayleigh waves; dispersion curves; inversion; shear-wave velocity 1 INTRODUCTION Surface wave methods have been used to obtain the shear wave velocity (V s ) or stiffness profile with depth. Surface wave methods mainly consist of (1) observation of Rayleigh or Love surface waves, (2) determination of dispersion characteristics; and (3) estimation of ground stiffness profiles based on inversion analyses (Tokimatsu 1995). The methods can be broadly classified into active and passive methods. Active methods involve the use of a vibration source (e.g., Nazarian 1984, Tokimatsu et al. 1991, Park et al 1999), while passive methods utilize passive sources such as background microtremor wave fields (e.g., Aki 1957, Capon 1969, Asten and Henstridge 1984, Okada 2003). Surface wave methods offer an advantage of noninvasive mapping of the stiffness profile with depth over a large area compared with conventional site investigation methods which are usually invasive. In this study, continuous surface wave (CSW) tests were conducted to obtain the stiffness profiles of two residual soil sites in Singapore. The stiffness profiles were then compared with stiffness estimated from standard penetration test (SPT) and cone penetration test (CPT) data. In addition, a method to determine the weathering grade of a residual soil site using stiffness profile from CSW test is proposed. Weathering profiles of test sites were then inferred from the stiffness profiles obtained from CSW tests. 2 CONTINUOUS SURFACE WAVE METHOD A CSW method is an active surface wave method. A typical CSW test set-up is schematically shown in Figure 1. Figure 1. CSW test set-up The vibration source generates continuous sinusoidal waves at various controlled frequencies typically from 5 Hz to 100 Hz; receivers (geophones) positioned in a co-linear array with the vibration source detect the ground motion data which are recorded by the data-acquisition system. The ground motion data can be further processed to obtain the Rayleigh wave phase velocity (V ) at each frequency of vibration to obtain the dispersion curve. Detailed procedure for determination of a dispersion 895

3 curve from CSW test can be found elsewhere (e.g., Matthews et al. 1996). The dispersion curve can then be inverted to estimate the stiffness or shear wave velocity (V s ) profile of the site using various inversion algorithms (e.g., Yuan and Nazarian 1993, Lai and Rix 1998, Socco and Boiero 2008). 3 TEST LOCATIONS Field tests were conducted at two sites, site-a and site-b, in the Nanyang Technological University (NTU) campus, which is located in the western part of Singapore belonging to the Jurong Formation. Upper portions of Jurong Formation have been subjected to excessive weathering and have transformed into residual soils of silty clays (Leong et al. 2003). In addition, the water tables in these residual soil sites are known to exist at great depths. the increase in the stiffness of each underlying layer. As can be seen in Figures 3 and 4, the stiffness profile of site-a is found to be normally dispersive, while that of site-b is inversely dispersive. Hence, site-a exhibits typical characteristics of a residual soil site. For site-b, the borehole log shown in Figure 4 suggests that the surface layer up to the depth of about 1.6m is made up of fill, which was found to be stiffer than the underlying residual soils, and hence resulting in an inversely dispersive profile. Considering only the residual soils below the fill layer, the stiffness is found to increase with depth. 4 RESULTS AND DISCUSSIONS 4.1 Dispersion curves and stiffness profiles Experimental dispersion curves were inverted using the inversion algorithm developed by Lai and Rix (1998). Inversion process consists of (1) forward modelling determination of a theoretical dispersion curve based on an estimated stiffness profile; and (2) minimisation iterative matching between the theoretical and experimental dispersion curve by varying the estimated stiffness profile. Studies showed that V measured at each frequency may correspond to different modes of Rayleigh wave propagation depending on the stiffness profile (e.g., Gucunski and Woods 1992, Tokimatsu et al. 1992). A stiffness profile may be called normally dispersive where the stiffness of each underlying layer increases with depth. In normally dispersive profiles, V mainly correspond to the fundamental mode of propagation at all frequencies. It may be called inversely dispersive if softer layers are overlain by stiffer layers or if a relatively soft layer is sandwiched between stiffer layers. In inversely dispersive profiles, V measured over different frequency range may correspond to different modes of propagation. Experimental and theoretical dispersion curves of site-a and site-b are shown in Figure 2. Borehole logs and the inverted stiffness profiles for site-a and site-b are shown in Figure 3 and Figure 4, respectively. It can be seen from Figure 2 that fundamental mode prevails in the dispersion curve of site-a. However, higher mode of propagation is involved over the high frequency range (>45 Hz) in the dispersion curve of site-b. In qualitative terms, a typical residual soil site is expected to exhibit normally dispersive characteristics because the degree of weathering typically decreases with depth resulting in (a) Site-A (b) Site-B Figure 2. Dispersion curves of site-a and site-b 4.2 Comparison of Stiffness Profiles with SPT and CPT Data There are various equations in the literature (e.g., Maheswari et al 2010) to estimate shear wave velocity (V s ) from SPT N-values for sands, clays and all soils. These empirical equations can be written in a general form as below: 896

4 Figure 3. Borehole log, weathering profile and stiffness profile of site-a Figure 4. Borehole log, weathering profile and stiffness profile of site-b 897

5 V s = N (1) where (with the unit of V s ) and are fitting parameters typically accounting for soil types and overburden pressure. Fitting parameters for soils and clays are used in this paper and summarised in Table 1. Knowing the density ( ), small-strain stiffness (G max ) can be obtained from V s using Eq. (2). G max = V s Table 1. Summary of fitting parameters Authors (m/s) Remarks Imai and Tonouchi (1982) 97 Uncorrected SPT-N Yoshida et al. (1998) 55P 0.14 a ( z/p a ) 0.14 z = overburden pressure, P a = atmospheric pressure; energy corrected SPT- N (2) Stiffnesses estimated from SPT data (Eq. 1) and CPT data (Eq. 3 and Eq. 6b) correlate well with CSW test stiffness profiles as can be seen in Figures 3 and Degree of Weathering Weathering classification uses a variety of approaches for different situations, for instance, weathering processes, original state of fresh rock, etc. In this paper, an alternative approach to estimate the degree of weathering at a residual soil site from its stiffness profile obtained from CSW tests is proposed. Variation of soil stiffness from soft, near surface layer, to hard, near parent rock, may be related to the weathering classification by Little (1969) shown in Figure 5. Maheswari et al. (2010) Energy corrected SPT-N Relationships between V s (or G max ) and cone resistance (q c ) of CPT test are not as readily available as those between V s and SPT-N. In this paper, the G max q c relationship for clays proposed by Mayne and Rix (1993) as shown in Eq. (3) is used. G max = P a (q c /P a e (kpa) (3) where P a = atmospheric pressure, e = void ratio, and = 100 for clays. For the tested residual soil site, e = 0.8 and = 44 were used. In addition, another G max q c relationship is suggested (Eq. 6b) by modifying existing empirical correlations (Eq. 4 and Eq. 5) summarised by Look (2007). E (large-strain) = 3.5 P a (q c /P a ) (MPa) (4) E (small-strain) = E (large-strain) / 0.1 = 35 P a (q c /P a ) (MPa) (5) where E = Young s modulus By elastic theory, G max = E (small-strain) / 2(1+ ) (6a) G max = [35 P a (q c /P a )] / [2(1+ )] (MPa) (6b) where = Poisson s ratio Figure 5. Typical weathering profile of residual soil (from Little 1969) The classification in Figure 5 suggests that the soil matrix is made up of 50-90% rock for Grade III weathering. To approximate rock and soil fractions in other grades and relate them to stiffness, the relationship shown in Eq. (7) is proposed using Voigt model, assuming a two-media soil matrix at different weathering grades. G i = n G rock + (1-n) G soil (7) where i = weathering grades I to VI; and G I and G VI are taken as G rock and G soil, respectively. In Eq. (7), n represents rock fractions with n = 1 (100% rock) for Grade I (fresh rock) and n = 0 (0% rock) for Grade VI (soil). Stiffness properties in relation to weathering grades are not available for Jurong Formation. However, stiffness properties of Singapore s Bukit Timah Granite residual soils have been reported by Sharma et al. (1999) in terms of E (small-strain) in relation to weathering grades. Therefore, a range of n-values for different weathering grades were determined from stiffness properties of Singapore s Bukit Timah Granite residual soils. For 898

6 each weathering grade, stiffness (G i ) were determined from E (small-strain) using Eq. (6a), assuming = 0.2 for Grade I and II and = 0.3 for Grade III to VI. Knowing G i, G rock and G soil, n-values were estimated from Eq. (7). Sharma et al. (1999) reported identical stiffness (= 2,080 MPa) for both Grade IV and Grade V, hence resulting in n = 0.01 estimated by Eq. (7) for both weathering grades. In this study, it is hypothesized that stiffness and n-values increase as weathering decreases from higher to lower grades. Thus, stiffness of Grade IV and corresponding n- value are disregarded. Figure 6. n for different weathering grades Estimated n-values are plotted against weathering grades and fitted with a logistics function given by Eq. (8), as shown in Figure 6. n(i) = 1 / [1 + exp(- i)] (8) In Eq. (8), numeric values of 1 to 6 are assigned to weathering grades i, with i = 1 being Grade I (fresh rock) to i = 6 being Grade VI (soils). In addition, = and = -2.3 are used. Table 2 summarises n-values estimated from Eq. (7) and range of fitted n-values obtained from Figure 6. It can be seen that the 50-90% rock for Grade III obtained from Figure 6 is consistent with Little s (1969) weathering classification, which suggests 50-90% rock for Grade III. Fresh rocks of Jurong Formation include a wide variety of sedimentary rocks such as conglomerate, sandstone, siltstone and shale (TR 26:2010). Based on the borehole log of site-a and its proximity to site-b, it is assumed that the fresh rock at both site-a and site-b is siltstone.g rock for siltstone of Jurong Formation is estimated to be 15,830 MPa (Sharma et al. 1999). As described earlier, upper portion of Jurong Formation has been into residual soils of silty clay (Leong et al. 2003). The V s is measured to be about 150 m/s from CSW tests, which is in good agreement with typical V s of silty clays found in literatures (e.g., Vogelaar 2001). Table 2. Range of n estimated from Bukit Timah Granite residual soils Weathering grade G (MPa) n (Eq.7) Range of n (Figure 6) Soil (Grade VI) 1, (0 1% rock) Completely (Grade V) Highly (Grade IV) Moderately (Grade III) Slightly (Grade II) Fresh rock (Grade I) * n-value disregarded 2, (1 10% rock) 2, * (10 50% rock) 12, (50 90% rock) 23, (90 100% rock) 25, (100% rock) Table 3. Stiffness and weathering grade for Jurong Formation from siltstone Weathering grade Stiffness Range (MPa) Soil <200 (Grade VI) Completely 200 1,620 (Grade V) Highly 1,620 7,940 (Grade IV) Moderately 7,940 14,250 (Grade III) Slightly 14,250 15,830 (Grade II) Fresh rock 15,830 (Grade I) Using Eq. (2), the stiffness (G soil ) of silty clays can be calculated from the measured V s as 45 MPa. In this paper, G soil of the silty clays is assumed to be the stiffness of soils of Grade VI with 0% rock (n = 0) i.e., 45 MPa. Figure 6 suggests that soils of Grade VI consist of 0 to 1% rock in the soil matrix (n = 0 to 0.01). With G soil = 45 MPa (0% rock) and G rock = 15,830 MPa (100% rock), the upper limit of stiffness of Grade VI soils with 1% rock (n = 0.01) can be calculated as 200 MPa. Based on range of n-values shown in Figure 6, a range of stiffness for other weathering grades is subsequently computed and results are compiled in Table 3. Using the stiffness range estimated for each weathering grade, stiffness profiles obtained from CSW test for site-a and site-b are related to respective weathering grades as shown in Figure 7. Figure 7. shows that both sites have been heavily to a substantial depth. Site-A has been into Grade VI up to 7.5m deep and Grade V from 7.5 to 16m. Site-B was found to be into Grade VI to the entire depth investigated by CSW test. 899

7 Figure 7. Degree of weathering from CSW stiffness profiles 5 CONCLUSION It can be concluded that dispersion characteristics of a typical residual soil site is normally dispersive, i.e., the stiffness of each underlying layer increases with depth. It was also found that the stiffness profiles obtained from CSW tests correlate well with stiffness estimated from standard penetration test (SPT) and cone penetration test (CPT) data. In addition, weathering profiles of these sites were inferred from the stiffness profiles obtained from CSW tests. The veracity of Figure 6 needs to be further verified by a combination of seismic surveys and borehole logging for more residual soil sites. ACKNOWLEDGEMENTS The work described in this paper is part of a DSTA research project, PTRC-CEE/DSTA/ REFERENCES Aki, K. (1957). "Space and Time Spectra of Stationary Stochastic Waes, with Special Reference to Microtremors." Bulletin, Earthquake Research Institute, 35, Asten, M. W., and Henstridge, J. D. (1984). "Array Estimator and the Use of Microseisms for Reconnaissance of Sedimentary Basins." Geophysics, 49(11), Capon, J. (1969). "High-Resolution Frequency-Wavehumber Spectrum Analysis." IEEE, Gucunski, N., and Woods, R. D. (1992). "Numerical simulation of the SASW test." Soil Dynamics and Earthquake Engineering, 11, Imai, T., and Tonouchi, K. (1982). "Correlation of N-value with S-wave velocity and shear modulus." Proceedings of the 2 nd European symposium of penetration testing, Amsterdam, Lai, C. G., and Rix, G. J. (1998). "Simultaneous inversion of Rayleigh phase velocity and attenuation for near-surface site characterisation." National Science Foundation and U.S. Geological Survey, Georgia Institute of Technology. Leong, E. C., Rahardjo, H., and Tang, S. K. (2003). "Characterisation and Engineering Properties of Singapore Residual Soils." Characterisation and Engineering Properties of Natural Soils, Tan et al., ed., Swets & Zeitlinger B.V., Lisse, The Netherlands, Little, A.L. (1969). "The engineering classification of residual soils." Seventh International Conference on Soil Mechanics and Foundation Engineering, ISSMFE. Mexico, 1, Look, B. G. (2007). Handbook of Geotechnical Investigation and Design Tables, Taylor & Francis Group, London, UK. Maheswari, R., Boominathan, A., and Dodagoudar, G. R. (2010). "Use of Surface Waves in Statistical Correlations of Shear Wave Velocity and Penetration Resistance of Chennai Soils." Geotech Geol Eng, 28, Matthews, M. C., Hope, V. S., and Clayton, C. R. I. (1996). "The Use of Surface Waves in the Determination of Ground Stiffness Profiles." Proc. Instn Civ. Engrs Geotech, Engng, 119, Mayne, P. W., and Rix, G. J. (1993). "G max -q c Relationships for Clays." Geotechnical Testing Journal, 16(1), Nazarian, S. (1984). "In-situ Determination of Elastic Moduli of Soil Deposits and Pavement Systems by Spectral Analysis of Surface Wave Method," University of Texas at Austin. Okada, H. (2003). The Microtremor Survey Method, Society of Exploration Geophysicists, U.S.A. Sharma, J. S., Chu, J., and Zhao, J. (1999). "Geological and geotechnical features of Singapore: An overview." Tunnelling and Underground Space Technology, 14(4), Socco, L. V., and Boiero, D. (2008). "Improved Monte Carlo inversion of surface wave data." Geophysical Prospecting, 56, Technical Reference for Deep excavation (TR26: 2010), SPRING Singapore. Tokimatsu, K. (1995). "Geotechnical site characterization using surface waves." The first international conference on earthquake geotechnical engineering, I. K, ed., Balkema, Tokyo, Tokimatsu, K., Kuwayama, S., Tamura, S., and Miyadera, Y. (1991). "Vs Determination from Steady State Rayleigh Wave Method." Soils and Foundations, 31(2), Tokimatsu, K., Tamura, S., and Kojima, H. (1992). "Effects of multiple modes on Rayleigh wave dispersoin characteristics." Journal of Geotechnical Engineering, 118(10), Vogelaar, B.B.S.A. (2001). Cavity Detection, A Feasibility Study Towards the Application of Seismic Surface Wave Stack Methods for the Identification and Localization of Underground Voids. Master thesis, University of Utrecht, The Netherlands. Yoshida, Y., Ikemi, M., and Kokusho, T. (1998). " Empirical formulas of SPT blow counts for gravelly soils." Proceedings of the First International Symposium on Penetration, 2, Yuan, D., and Nazarian, S. (1993). "Automated Surface Wave Method: Inversion Technique." Journal of Geotechnical Engineering, 119(7),