ASSESSMENT OF SHEAR WAVE VELOCITY FROM INDIRECT INSITU TESTS

Transcription

1 Proceedings of Indian Geotechnical Conference IGC-2014 December 18-20, 2014, Kakinada, India ASSESSMENT OF SHEAR WAVE VELOCITY FROM INDIRECT INSITU TESTS Kant, L., M. Tech Student, Department of Earthquake Engg. IIT Roorkee, Mukerjee, S., Visiting Faculty, Department of Earthquake Engg. IIT Roorkee, Saran, S., Professor (Retd.), Department of Earthquake Engg. IIT Roorkee, ABSTRACT : This paper presents the results of an insitu experimental programme including SPT,SCPT, MASW and the Cross Borehole Test at two sites. The soil at one site was of the SM type whereas at the other site it was layered between CL and SP. Empirical relationships have been developed for assessing shear wave velocity, at different depths, from SPT and SCPT using results of cross borehole tests conducted at the same location and depths as a standard. The correlations developed herein were found to be in good agreement with those reported in literature. The results of the MASW tests were found to be at about 10 to 15 percent variance with the cross borehole test results. Keywords ; Shear wave Velocity, SPT, CPT, DCPT, MASW, Cross Borehole Test INTRODUCTION Analyses of geotechnical engineering problems related to foundations and soil structure interaction require an accurate evaluation of the shear wave velocity of the foundation soil. The cross borehole test ideally provides the required soil data for such problems. This test provides greater measurement accuracy and control as the observed waves travel through a particular stratum with less interference from nearby refracting horizons. In India the cross borehole test is not popular because of its cost and limited availability of the equipment. The Standard Penetration Test (SPT) is a popular and widely used insitu penetration test, though it is plagued by many problems that affect its accuracy. It is used primarily because of its low cost and usefulness in obtaining various soil parameters like angle of internal friction, relative density etc. It can also be used to evaluate settlement, bearing capacity, shear strength, dynamic shear modulus, liquefaction potential of cohesionless soils and undrained shear strength of cohesive soil from existing correlations. The Cone Penetration Test (CPT) is another commonly used insitu penetration test. The CPT is more popular when compared to the SPT as a method for geotechnical soil investigation because of its increased accuracy, speed of deployment, continuous soil profiling and reduced cost when compared to other soil testing methods. Empirical correlations have been developed by investigators, between SPT-N value or CPT tip resistance and sleeve resistance values and the insitu shear wave velocities. In developing these correlations various influencing factors like depth, corrected SPT-N values, overburden pressure, etc, have been considered in various combinations. Correlations giving the highest value of the correlation coefficient are adopted. These correlations have been validated, by investigators, from the results of cross borehole tests. Multichannel analysis of surface wave (MASW) test has been gaining acceptance as it is more economical and an easy to conduct alternative test for evaluating insitu shear wave velocity. Results from the MASW are generally more reliable than SPT or SCPT results and only slightly less accurate, about 15 percent for uniform strata, than the cross borehole test results (Anderson-2010). An additional advantage of this method is that it can be conducted in areas which are not accessible for installing drilling rigs.

2 L. Kant, S. Mukerjee and S. Saran EXEPERIMENTAL PROGRAMME At two sites located in the IIT Roorkee campus, it was proposed to conduct Standard Penetration Tests (SPT), Cone Penetration Tests (CPT), Multichannel Analysis of Surface Wave (MASW) and cross borehole tests, upto a depth of 9 meters. The procedures recommended by the Indian Standard Code IS: and IS: 4968-Part-III were followed for conducting standard penetration tests (SPT) and cone penetration tests (CPT) respectively. Soil samples collected from the SPT sampler, at different depths, were used for conducting soil classification tests. Table 1 indicates the site soil classification, insitu densities and SPT-N values for site A. Table 2 indicates the site soil classification, insitu densities and SPT-N values for site B. Tables 3 and 4 indicate the cone tip resistance and sleeve friction resistance obtained from CPT at different depths, for the sites sites A and B respectively. Table 1: SPT results for site A. Table 2: SPT results for site B. Depth (m) Soil Type Insitu density (gm/cc) SPT-N Corrected 0.75 SP SP CL CL CL SP SP Where, CL= Lean Clay, SP=Poorly graded sand,n =Corrected SPT-N due to overburden pressure. Table 3: CPT results for site A. N Depth (m) Soil Type Insitu density SPT-N Corrected N Depth(m) q c (kpa) (q c +f s )kpa f s (kpa) (gm/cc) SM SM SM SM SM SM SM SM SM Where, q c =Cone tip resistance, f s =Sleeve friction resistance. Where, SM= Silty Sand, N =Corrected SPT-N due to overburden pressure. 414

3 Table 4: CPT results for site B. Depth(m) q c (kpa) (q c +f s ) kpa f s (kpa) Where, q c =Cone tip resistance, f s =Sleeve friction resistance. ASSESSMENT OF SHEAR WAVE VELOCITY FROM INDIRECT INSITU TESTS depth wise shear wave velocity profile for the sites A and B were obtained and are reported in Table 5 and 6 respectively. Figure 1 : Dispersion Curve for Site A. The procedure for conducting the Multichannel Analysis of Surface Wave test (MASW) consists of the following steps. a) Acquisition of dispersive Rayleigh wave test data, b) Construction of a dispersion curve, c) Obtain the shear-wave velocity profile from the constructed dispersion curve, using the Inversion technique. The accuracy of the shear wave velocity profile for a site depends upon the accuracy of the constructed dispersion curve. To conduct the MASW test, 9 geophones, were placed along a straight line laid out on the field, at a spacing of 3metres centre to centre. Seismic waves were generated by the impact of a 16 kg sledge hammer on a metal plate, placed on the ground. Records from 10 impacts were stacked and recorded in a data acquisition system (Soilspy).In this test the effective parameters are (i) distance between impact source and the first receiver(geophone), (ii) distance between impact source and the last receiver and (iii) geophones spacing. These parameters were selected as per the recommendations given by Park et al.(1999).the acquired Rayleigh wave data was then analyzed using the Grilla software package, which is designed to generate a depth wise shear wave velocity profile. Figures 1 and 2 show the dispersion curves for the sites A and B respectively. Using the inversion technique the Figure 2 : Dispersion Curve for Site B. Table 5: Shear wave profile for site A. Depth at the Thickness [m] Vs [m/s] bottom of the layer (m)

4 L. Kant, S. Mukerjee and S. Saran Table 6: Shear Wave Velocity Profile for Site B. Depth at the Thickness [m] Vs [m/s] bottom of the layer (m) Cross Borehole Test (CH) is limited to the determination of the velocity of the horizontally travelling shear waves at a site. A depthwise shear wave velocity profile for a site can be obtained from this test. This test can be considered to be a standard for shear wave velocity determination and is often used to evaluate velocities obtained from other tests. A set of three boreholes laid out along a line and spaced at 3.0 metres, centre to centre, are needed for this test. Figure 3 shows a schematic diagram of the test set up. Boreholes should be drilled vertically and cased with a PVC pipe. The annular space between the casing and the outer soil should be grouted with a material having the same density as the soil. A Bision type borehole hammer is now lowered into the first borehole, to the desired depth, the hammer shoes extended hydraulically to lock the hammer in place. Borehole geophones are then lowered into the other two boreholes and locked at the same depth with the help of a pneumatically inflatable rubber bladder. Shear waves are generated by operating the hammer and the corresponding shear wave arrivals are recorded at the borehole geophones. Figure 4 shows a typical record. The time difference between the generation of the shear wave and its arrival at the geophone, for a particular borehole spacing, gives the shear wave velocity at the depth under consideration. The borehole hammer shoes are now retracted, the hammer lowered to the next depth and locked again by extending the shoes. Similarly the borehole geophones are lowered to the same depth by deflating and then inflating the rubber bladders. Now the test is repeated at this new depth. In this manner the test is carried out for the entire depth of the borehole. Results of cross borehole tests conducted at the two sites A and B are given in Tables 7 and 8 respectively. Figure 3: Schematic diagram of the Cross borehole test Ground motion Reverse Polarity Figure 4: Waveforms from the Cross Borehole Test at the site A. 1E 2E 3E 4E 5E 6E 7E 8E 9E 10E 11E 12E 416

5 Table 7: Cross borehole test results for site A. Depth(m) V s = D/Δt (m/s) Table 8: Cross borehole test results for site B. Depth(m) Vs = D/Δt (m/s) ANALYSIS A site specific correlation between shear wave velocity (V s ) and any independent parameter, using either the Standard Penetration Test (SPT) data or the Cone Penetration Tests (CPT) data can be developed in two ways as mentioned below; a) Modifying existing correlation equations. b) Developing a new correlation based on the site-specific data. If limited data for shear wave velocity (V S ) and the independent parameter are available for a site, available equations may be modified by adjusting their coefficients and exponential values to match the site-specific data. However, if sufficient site specific shear wave velocity (V S ) and SPT or CPT data is available, it is possible to develop new sitespecific correlation equations. ASSESSMENT OF SHEAR WAVE VELOCITY FROM INDIRECT INSITU TESTS In this study, new correlations have been developed using the Power law. This Power law is of the form, Vs=A * X 1 B * X 2 C... (2) Where, A, B, C,... are Constants, X i is an independent Variable (i=1, 2..) and Vs is the Shear Wave Velocity. The experimental data generated at the sites A and B were used to develop unique empirical relations to estimate the shear wave velocity at the site; considering soil depth, over burden pressure and SPT-N values for the Standard Penetration Test (SPT) and considering soil depth, Tip Resistance(q c ) and Sleeve Friction Resistance (f c ) for the Cone Penetration Test (CPT).Empirical equations developed based on SPT and CPT are listed in Tables 9 and 10 respectively, for site A. Similarly empirical equations developed based on SPT and CPT are listed in Tables 11 and 12 respectively, for site B. Table 9: SPT based Models developed for Site A Sl. Functions Equation for Shear wave Velocity (Vs) r- value 1 ƒ(n) N ƒ(n ) N ƒ(n,h) N H ƒ(n,h) N H ƒ(n, σ v ) N σ v ƒ(n. σ v ) N σ v N ƒ(n, σ v,h) σ v H N ƒ(n', σ v,h) σ v H where, N= SPT-N value, N = Corrected N value for over burden pressure, σ v = Effective over burden pressure, H=Depth (metre ), r= Correlation Coefficient. 417

6 L. Kant, S. Mukerjee and S. Saran Table 10: CPT based Models developed for Site A Sl. Function Equation for Shear wave Velocity (Vs) r- value 1 ƒ(q c ) q c ƒ(f c ) f c ƒ(fr) FR ƒ(tr) 69.82tr ƒ(tr,h) H tr ƒ(q c,h) H q c ƒ(f c,h) H f c ƒ(fr,h) H 0.12 FR ƒ(q c,f c ) f c q c ƒ(q c,f c,h) q c f c H ƒ(fr,h,tr) tr FR H ƒ(q c,f c,tr) q 0.03 c f 0.12 c tr ƒ(q c,h,tr) tr q 0.03 c H ƒ(f c,h,tr) H 0.13 f c tr ƒ(q c,f c,h,tr) q c f c H tr Where, q c =Cone resistance (kpa), f c =Sleeve friction (kpa), tr=total resistance= q c + f c in kpa, H=Depth (metre), FR= Friction ratio= f c / q c, r= Correlation Coefficient. Table 11: SPT based Models developed for Site B Sl Function Equation for Shear wave r-value. Velocity (Vs) 1 ƒ(n) N ƒ(n ) N ƒ(n,h) H N ƒ(n,h) H 0.01 N ƒ(n, σ v ) N 0.23 σ v ƒ(n. σ v ) N 0.24 σ v ƒ(n, σ v,h) 83.17N 0.28 σ v H N 0.28 σ ƒ(n', σ v v,h) H Where, N= SPT-N value, N = Corrected N value for over burden pressure, σ v = Effective over burden pressure, H=Depth (metre ), r= Correlation Coefficient. Table 12: CPT based Models developed for Site B Sl. Function Equation for Shear wave Velocity (Vs) r- value 1 ƒ(q c ) q c ƒ(f c ) f c ƒ(fr) FR ƒ(tr) tr ƒ(tr,h) 12.30H tr ƒ(q c,h) H q c ƒ(f c,h) f 0.02 c H ƒ(fr,h) H FR ƒ(q c,f c ) 83.17q c f c ƒ(q c,f c,h) 39.81H f c q c ƒ(fr,h,tr) 6.4tr 0.45 H FR ƒ(q c,f c,tr) 37.15q c f c tr ƒ(q c,h,tr) 6.16q 0.07 c H -0.2 tr ƒ(f c,h,tr) 7.58tr 0.47 H fc ƒ(q c, f c, H,tr) 6.91 q c f c H tr Where, q c =Cone resistance (kpa), f c =Sleeve friction (kpa), tr=total resistance= q c + f c in kpa, H=Depth (metre), FR= Friction ratio= f c / q c, r= Correlation Coefficient. Shear wave velocities obtained from the cross borehole test were plotted on X-axis, shown as measured shear wave velocity (Vs), whereas the predicted shear wave velocities, obtained from the best developed SPT based model, were plotted on the Y-axis for site A as in Figure 5. Similar plots using CPT data and MASW data are shown in Figures 6 and 7 respectively for site A. Similar plots were developed for site B and are shown in Figures 8, 9 and 10. Figure 5: Predicted Vs from SPT for site A. 418

7 ASSESSMENT OF SHEAR WAVE VELOCITY FROM INDIRECT INSITU TESTS Figure 6: Predicted Vs from CPT for site A. Figure 9: Predicted Vs from CPT for site B. Figure 10: Predicted Vs from MASW for site B. Figure 7: Predicted Vs from MASW for site A. CONCLUSION Correlation equations have been developed to assess insitu shear wave velocity from the results of Standard Penetration Tests (SPT) and Cone Penetration Tests (CPT) carried out at the two sites. The suggested correlation equations are reported in Tables 13 and 14 for sites A and B respectively. A comparison of shear wave velocity obtained from MASW and the cross borehole test indicates that the results were in good agreement. The values from MASW were about 10 to 15 percent lower than the cross borehole results. Figure 8: Predicted Vs from SPT for site B. 419

8 L. Kant, S. Mukerjee and S. Saran The widely used Standard Penetration Test (SPT) or Static Cone Penetration Test (SCPT) can readily be used to assess the depthwise distribution of shear wave velocity at a site using the relationships developed herein. Similarly the MASW test can be used with confidence as an alternative to the more expensive cross borehole test for an evaluation of the insitu shear wave velocity, as demonstrated herein. Table 13: Suggested Models developed for Site A ( Soil Type-SM) Equation r-value 3. Kant, L. (2014), Assessment of Shear Wave Velocity from Indirect Insitu Tests, M Tech Thesis, Department of Earthquake Engineering, IIT Roorkee. 4. Kramer, S (1996). Geotechnical Earthquake Engineering,Simon & Schuster. 5. Mitra, V.K (1995), Determination of Dynamic Shear Modulus by Cross Borehole Test, M.E. Thesis, Department of Earthquake Engineering, IIT Roorkee. V s =169.18N H V s =234.74q c f c H tr r = Correlation coefficient value Table 14: Suggested Models developed for Site B ( Soil Type-layered CL and SP) Equation r-value V s =83.17N 0.28 σ v H V s =6.91 q c 0.01 f c H tr r = Correlation coefficient value ACKNOWLEDGEMENT The authors are grateful to IIT Roorkee for providing the facilities and finances for conducting this investigation. REFERENCES 1. IS: , Method for Standard Penetration Tests for Soils, Indian Standards Institution, Manak Bhawan, new Delhi, India. 2. IS: 4968-Part-III, Method for Cone Penetration Tests for Soils, Indian Standards Institution, Manak Bhawan, new Delhi, India. 420