IGC. 50 th INDIAN GEOTECHNICAL CONFERENCE ESTIMATION OF FINES CONTENT FROM CPT PARAMETERS FOR CALCAREOUS SOILS OF WESTERN INDIAN OFFSHORE

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1 5 th 5 th INDIAN GEOTECHNICAL CONFERENCE 17 th 19 th DECEMBER 215, Pune, Maharashtra, India ESTIMATION OF FINES CONTENT FROM CPT PARAMETERS FOR CALCAREOUS SOILS OF WESTERN INDIAN OFFSHORE Alankar Srivastava 1, R. K. Ghanekar 2 ABSTRACT Cone Penetration Test (CPT) or CPT with the option of measurement of pore water pressure i.e. Piezo Cone Penetration Test (CPTU) is the most widely used and accepted in-situ soil investigation tool in offshore. The applications of this test in soil investigation process are, a) to find out subsurface stratigraphy including soil classification, b) to estimate soil design parameters, and c) to provide the results for direct use in geotechnical design. This test has three major advantages over the traditional combinations of boring, sampling and other tests. It gives a) continuous or near continuous data, b) repeatable & reliable penetration data, & c) cost economy. Different soils respond to cone penetration differently which is reflected in the measured cone parameters. Hence, the classification performed based on measured cone parameters is called Soil Behaviour Type (SBT) classification. This classification system generally matches with the traditional soil classification systems. In addition, in recent years, several correlations have been proposed for estimating the soil fines content from CPTU based parameters (e.g. Robertson & Wride 1998; Idriss & Boulanger 28; Cetin & Ozan 29, Yi, F. 214). Actually these CPT / CPTU based correlations have been developed with the objective of assessing the liquefaction potential of sandy deposits which is significantly influenced by their fines content. However, such correlations can be gainfully used for classification purpose as well, especially where only CPT / CPTU data are available. In Western Indian Offshore, the normal practice for soil investigation is to have two bores one shallow bore hole up to a depth of around 3-4 m for continuous CPTU and another deep bore hole approximately 5 m apart for alternate CPTU and sampling up to a depth of around m. Hence for initial depths ranging between to 4m, soil samples as well as PCPT data both are available in a near continuous manner & thus fines content may be found out with the help of laboratory testing of soil samples. But for depth ranges below 3-4 m, where only CPTU data are available and no samples are available (due to alternate CPTU and sampling), such correlations can help in estimating fines content of the soils. Besides classification, estimate of fines content is also important for evaluating pile skin friction 1 Alankar Srivastava, IEOT, Oil and Natural Gas Corporation Ltd., Navi Mumbai, India, srivastava_alankar@ongc.co.in 2 R. K. Ghanekar, IEOT, Oil and Natural Gas Corporation Ltd., Navi Mumbai, India, rkghanekar@gmail.com

2 Alankar Srivastava and R. K. Ghanekar in calcareous sands. However, being area specific, the available correlations in literature may or may not be fully applicable to the Western Indian Offshore. The paper presents the results of evaluation of reported correlation between parameters derived from measured CPTU parameters and fines content, using data from top 4 m of soil profile from various fields of western Indian offshore. Results of attempts to develop correlations for specific application in western Indian offshore calcareous soils are also presented. Keywords: CPT, CPTU, In-situ Soil Investigation, Sampling, Soil Classification, Fines Content, Correlations.

3 5 th 5 th INDIAN GEOTECHNICAL CONFERENCE 17 th 19 th DECEMBER 215, Pune, Maharashtra, India ESTIMATION OF FINES CONTENT FROM CPT PARAMETERS FOR CALCAREOUS SOILS OF WESTERN INDIAN OFFSHORE Alankar Srivastava, Executive Engineer (Civil), IEOT, ONGC, srivastava_alankar@ongc.co.in R. K. Ghanekar, General Manager (Civil), IEOT, ONGC, rkghanekar@gmail.com ABSTRACT: Cone Penetration Test (CPT) / Piezo Cone Penetration Test (CPTU) is the most versatile and frequently used in-situ test in offshore soil investigations. In the past few years, researchers across the globe have proposed various correlations for estimating the fines content of soil from CPTU based parameters. These CPTU based correlations have been developed basically for the assessment of liquefaction potential of sandy deposits. However, such correlations can also be gainfully used in soil classification, especially where only CPTU data are available. This paper discusses the results of evaluation of published correlations, for the soil data from top 4 m of soil profile from various fields of Western Indian Offshore. Results of attempts to develop new correlation(s) for the specific application in calcareous soils of Western Indian offshore are also presented. INTRODUCTION Cone Penetration Test (CPT) or CPT with measurement of pore water pressure i.e. Piezo Cone Penetration Test (CPTU), is the most versatile and worldwide accepted in-situ test in offshore geotechnical investigations. Major advantages of the test are continuous data sampling, repeatability, reliability and economic efficiency. In CPT, a cone followed by a cylindrical sleeve is pushed into the ground at a constant rate. The resistance offered by the soil to the penetration of cone and friction on the sleeve are recorded. In CPTU, pore pressure generated is also recorded generally just behind the cone. Different soils respond to cone penetration differently which is reflected in the measured cone parameters (Tip Resistance, Sleeve Friction & Pore Water Pressure). Based on these measured/derived CPT/CPTU parameters, various soil classification systems have been proposed by the researchers. This method of soil classification is called Soil Behaviour Type (SBT) classification. This classification system generally matches with the traditional soil classification systems and is increasingly being used all over the world as supplementary information to the laboratory tests for classifying the soils. In addition, soil design parameters may also be estimated using various correlations (based on measured/derived CPT/CPTU parameters), proposed by various researchers. In last few years, some correlations have also been proposed for estimating the fines content of soil (silt and clay fraction) from CPT/CPTU based measured/derived parameters (e.g. Robertson & Wride, 1998 [1]; Idriss & Boulanger, 28 [2]; Cetin & Ozan, 29 [3], Yi, 214 [4]). Actually these CPT/CPTU based correlations have been developed with the objective of assessing the liquefaction potential of sandy deposits which is significantly influenced by their fines content. However, such correlations may be gainfully used for soil classification purpose as well, especially where only CPT/CPTU data are available.besides classification, estimation of fines content is also important for evaluating pile skin friction in calcareous sands such as found in western Indian offshore.

4 Alankar Srivastava and R. K. Ghanekar In the present paper, the available published correlations are evaluated for the soils from various fields of Western Indian Offshore. Efforts made to develop new correlation(s), specifically for the area of Western Indian Offshore, are also presented. SOIL INVESTIGATION PRACTICE IN WESTERN INDIAN OFFSHORE In soil investigations in Western Indian Offshore for oil and gas platforms, the normal procedure followed is to have two bore holes first one is a shallow bore hole in which CPTU (with standard 1 cm 2 A.P. van den Berg cone) is performed in a continuous manner up to a depth of around 3-4 m from the seafloor and the second one is a deep bore hole (5 m apart from the first bore hole) in which for the first 3-4m depth, only sampling is done in an almost continuous manner and below this level, both CPTU and sampling is done in an alternate way up to a depth of around m. It is assumed that the soil conditions are same at the two boreholes. For the initial depths ranging between. to about 4m, the soil samples are available in a near continuous manner and so it may be easily tested in the laboratory and soil classification may be done. But for the depths below 3-4 m, where only CPTU data are available and no samples are available (due to alternate CPTU and sampling), proposed correlations (between CPTU Parameters and Fines Content) may help in estimating the fines content of the soils. These estimated fines content will act as supplementary information in soil classification. However, being area specific, the available correlations in literature may or may not be fully applicable to the Western Indian Offshore. A study was undertaken to first evaluate the available correlations, for the soil data of top 4m of the soil profiles (where both CPTU data and soil samples were available) from different locations of Western Indian Offshore. The second objective of the study was to attempt to develop some new correlations specifically for western offshore calcareous soils, if required. For evaluation, three published correlations are selected. The following sections present the details of the selected correlations, the offshore soil data, evaluation of the selected correlations and the attempts to develop some new correlation(s) using regression analysis. SELECTED CORRELATIONS Robertson and Wride (1998) [1] Robertson and Wride (1998) have proposed a correlation between Soil Behaviour Type (SBT) Index (I ) and Apparent Fines Content (FC). Since CPTU parameters are influenced by more than just Fines Content, Robertson and Wride term it as Apparent Fines Content rather than Fines Content. The relationship is as follows- FC = % for I < 1.26 (1a) ( ) 3.25 FC % = 1.75I 3.7 if I is between 1.26 and 3.5 (1b) FC = 1% for I > 3.5 (1c) FC = 5% if I is between 1.64 and 2.36, and FR <.5% (1d) The expression for I proposed by Robertson and Wride (1998) [1] is as follows:- ( ) ( ) I = 3.47 log Qtn + log FR (1e) Where Q tn is the Normalized Cone Resistance and F R is the Normalized Friction Ratio, ( ) ( ) n Q = q σ / p. p / σ' tn t v a a v f s FR = X1% ( qt σv ) (1f) (1g) Parameter q t is the corrected cone resistance, f s is the measured sleeve friction, σ v is the Total Overburden Pressure and σ ' v is the Effective Overburden Pressure. p a = Atmospheric pressure, n is the Stress Exponent which varies with the soil behaviour type [1], n =.381 I +.5 σ ' / p.15 ( ) ( ) v a Idriss and Boulanger (28) [2] On the basis of the data from Suzuki et al. (1998)

5 5 th 5 th INDIAN GEOTECHNICAL CONFERENCE 17 th 19 th DECEMBER 215, Pune, Maharashtra, India [5], Idriss and Boulanger (28) have proposed a correlation between Soil Behaviour Type (SBT) Index (I ) and Fines Content (FC): 2.6 FC = 2.8I % (2a) ( ) Yi (214) [4] Based on the data collected from 11 nos. of project sites as well as from various published papers and reports, Yi (214) proposed a correlation between Soil Behaviour Type (SBT) Index (I ) and Fines Content (FC): FC (%) = % for I < 1.31 (3a) I 2.5 FC (%) = 42.I sin π 1.19 for1.31 I < 2.5 (3b) FC (%) 83.3I = for 2.5 I < 3.1 (3c) FC (%) = 1% for I 3.2 FC (%) = 5.FR for1.31< I 2.36 and F R <.6%, (3d) (3e) THE DATA The soil data used in this study have been taken from the Soil Investigation Projects of 32 nos. of locations from 8 different Fields of Western Indian Offshore, explored and drilled by ONGC. The fields are located in the WGS 84 UTM zones 42 and 43 off the coast of Mumbai Region. As stated earlier, the selected data are from the top about 4. m of soil profile at each location. The CPTU raw (measured) data have been taken from the soil investigation projects, corrected for seabed reference and pore water pressure effects and then, further processed to get various derived CPTU parameters like Pore Pressure Ratio B q, Normalized Cone Resistance Q t, and Normalized Friction Ratio F R etc. EVALUATION OF SELECTED CORRELATIONS Figure 1 shows the evaluation of the 3 selected correlations for the soil data from Western Indian Offshore. In the figure, Fines Content (FC) values (determined from laboratory tests) are plotted against the calculated values of Soil Behaviour Type (SBT) Index (I ) along with the published correlations between FC and I. From the figure, it becomes quite clear that the data are scattered and none of the correlations predict the Fines Content values well. The correlation by Robertson and Wride, 1998 [1] and Idriss and Boulanger, 28 [2] plot generally above the western Indian offshore data. Whereas the Yi, 214 [4] correlation plots towards the lower side of the data. From Fig. 1 it is also observed that data is sparse in FC range of between 1 to 6 %. REGRESSION ANALYSIS USING DERIVED CPTU PARAMETERS Since the predictions by the available published correlations are not very good for the Fields of Western Indian Offshore, development of some new correlations was attempted using regression analysis. Hence, single and multi-variable regression analyses were performed using the derived CPTU parameters. The results of the regression analyses are presented in Table 1. In addition to the derived parameters defined earlier, some additional parameters were used in the regression analyses which are defined below- ( ) ( ) 2 2 Ic = 3.47 log Qt + log FR

6 Alankar Srivastava and R. K. Ghanekar 5 Soil Behaviour Type Index (I ) Zone 2: Organic Soils - Peats Zone 3: Silty Clay to Clay Zone 4: Clayey Silt to Silty Clay Data from Western Indian Offshore Robertson & Wride (1998) Idriss & Boulanger (28) F. Yi (214) Zone 5: Silty Sand to Sandy Silt Zone 6: Clean Sand to Silty Sand Zone 7:Gravelly Sand to Sand Fines Content (FC) (%) Fig. 1 Evaluation of selected correlations for the soil data from western Indian Offshore Table 1 Results of Regression Analyses (relationship between FC and CPTU Parameters) Equation A B C Results of Regression Analysis (Relationship Between FC and CPTU Parameters) n R 2 S.E FC = I FC = 1.288I c FC = 116.4B q D FC = B q I c E u2 u FC = I u + c F u2 u FC = I σ' + c v G u2 u FC = I σ' + v Note: n is number of data points, R 2 is Coefficient of Determination and S.E. is the Standard Error

7 5 th 5 th INDIAN GEOTECHNICAL CONFERENCE 17 th 19 th DECEMBER 215, Pune, Maharashtra, India ( q σ ) t v Qt = σ ' v u 2 u Pore pressure ratio, Bq = q σ Where u 2 is pore water pressure measured behind the cone (i.e. between cone and friction sleeve) and subsequently corrected for seabed reference and u is equilibrium pore water pressure. In addition to B q, two more pore pressure parameters (u 2 -u )/u and (u 2 -u )/σ' v were also used. As can be seen from Table 1, correlation between fines content and SBT Index, I, as recommended by Robertson and Wride, 1998 [1], represented by Eq. A, shows a reasonably good strength with R 2 of.68. Correlation of fines content with SBT index (without the use of exponent n) I c surprisingly resulted in a stronger relation (Eq. B). A possible reason for this could be the dominance of data in the higher I c range (where n tends to become closer to 1) which could influence the results of statistical analysis. It is reasonable to expect that fines content can be one of the factors influencing the pore pressure generated due to penetration of the cone. A direct correlation attempted between fines content and pore pressure parameter, B q, also showed a reasonable correlation with R 2 value of.61 (Eq. C). Hence, a multi-variable regression was attempted to predict fines content from B q and I c which resulted in a stronger correlation with R 2 value of.76 (Eq. D) than Eqs. B or C. Attempts were also hence made to use two other normalized pore pressure parameters i.e. u 2 -u /u and u 2 -u /σ v to try and improve the strength of the correlations. Results of these attempts are represented by Eqs. E and F with R 2 values of.77 and.78 respectively. It is interesting to note that the best correlation (Eq. F) involves, besides all the three CPTU measured parameters, effective overburden pressure σ v. DISCUSSION To make a comparison, Eqs. A and B were plotted on the data and correlations presented in Fig. 1 t v (and re-plotted in Fig. 2). It is observed that Eqs. A and B appear to give more reasonable fits for the data used in this study from western Indian offshore compared to the published correlations. A limitation of the data, as observed earlier, is thinner population of data between FC ranges of 1 to 6%. To check if denser population of data in higher FC range is influencing the results of statistical analysis, measured FC values were plotted against predicted values using Eqs. A, B, D, E and F which are presented in Figs. 3, 4, 5, 6 and 7 respectively. As can be observed from the figures, Eqs. A and B predict somewhat better for FC range of 1 to 6% while latter three equations predict better for higher FC range. It must be noted that reasonable estimates of FC from CPTU parameters is more important for the intermediate range of FC (e.g. 1 to 6%) for western Indian offshore calcareous soils where prescribed design pile skin friction for mixed soils depends on the fines content and, where liquefaction potential is not a major concern. Soil Behaviour Type Index (I / Ic) Data from Western Indian Offshore Robertson & Wride (1998) Idriss & Boulanger (28) F. Yi (214) Equation-A ( FC = I 3.63 ) Equation-B ( FC = Ic ) Zone 2: Organic Soils - Peats Zone 3: Silty Clay to Clay Zone 4: Clayey Silt to Silty Clay Zone 5: Silty Sand to Sandy Silt Zone 6: Clean Sand to Silty Sand Zone 7:Gravelly Sand to Sand Fines Content (FC) (%) Fig. 2 Comparison of selected correlations with Equation-A and Equation-B for the soil data from Western Indian Offshore

8 Alankar Srivastava and R. K. Ghanekar 1 Equation-A ( FC = I 3.63 ) 1 Equation-D ( FC = Bq+47.99Ic ) % % -2% -1% % +1% -2% -1% Fig. 3 Predicted Fines Content from Equation -A Fig. 5 Predicted Fines Content from Equation D 1 Equation-B ( FC = Ic ) 1 Equation-E ( FC = ( u 2 -u /u ) Ic ) % % -2% -1% % +1% -2% -1% Fig. 4 Predicted Fines Content from Equation B Fig. 6 Predicted Fines Content from Equation E

9 5 th 5 th INDIAN GEOTECHNICAL CONFERENCE 17 th 19 th DECEMBER 215, Pune, Maharashtra, India Equation-F( FC = ( u 2 -u /σ'v) Ic ) A limitation of the data used is that data for FC range from 1 to 6% are not very well represented. It is believed that predominance of data in higher FC range has influenced the strength of the correlations derived % +1% -2% -1% Considering the greater importance of predicting fines content reasonably for middle range of FC for western Indian offshore soils, it is presently proposed to use Eqs. A and B in soil investigations. It is imperative that in future, inclusion of more data for mixed soils (which presently are not well represented in the dataset) will be taken up and the equations shall be re-evaluated Fig. 7 Predicted Fines Content from Equation F CONCLUSIONS Data from about 4 m depth below seafloor, from 32 nos. of locations, from 8 different fields of western Indian offshore have been used in this study to evaluate the applicability of the three published correlations (between FC and I ) for the calcareous soils of Western Indian Offshore. It is observed that (for the data used) none of the published correlations are directly applicable to predict the value of Fines Content (FC) from I. Hence, specifically for the soils of western Indian offshore area, some new correlations have been attempted by performing single/multi variable regression analyses on derived CPTU parameter(s). A total of seven correlation equations have been derived with strength of R 2 ranging from.61 to.78. Eqs. A, D, E and F show a decent relationship with R 2 of.68 or more. The best correlation statistically for the data is represented by Eq. F (R 2 =.78). ACKNOWLEDGEMENT Authors are grateful to the management of ONGC for granting permission to publish the paper. REFERENCES 1. Robertson, P.K. and Wride, C.E. (1998), Evaluating cyclic liquefaction potential using the cone penetration test, Canadian Geotechnical Journal, 35: Idriss, I.M. and Boulanger, R.W. (28), Soil Liquefaction During Earthquake, Earthquake Engineering Research Institute, EERI Publication MNO Cetin, K.O. and Ozan, C. (29), CPT-Based Probabilistic Soil Characterization and Classification, Journal of Geotechnical and Geoenvironmental Engineering, Vol 135, No Yi, F. (214), Estimating soil fines contents from CPT data, 3 rd International Symposium on Cone Penetration Testing, May 12-14, 214- Las Vegas, Nevada. 5. Suzuki, Y., Sanematsu, T. and Tokimatsu, K. (1998), Correlation between SPT and seismic CPT. Proc. Conference on Geotechnical Site Characterization, Balkema, Rotterdam, pp

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