CPTic_CSM8. A spreadsheet tool for identifying soil types and layering from CPTU data using the I c method. USER S MANUAL. J. A.

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1 CPTic_CSM8 A spreadsheet tool for identifying soil types and layering from CPTU data using the I c method. USER S MANUAL J. A. Knappett (2012) This user s manual and its associated spreadsheet ( CPTic_CSM8.xls ) accompanies Craig s Soil Mechnics, 8 th Edition (J.A. Knappett & R.F. Craig). The spreadsheet CPTic_CSM8 is an implementation of the methodology outlined in: Robertson, P.K., and Wride, C.E. (1998). Evaluating cyclic liquefaction potential using the cone penetration test. Canadian Geotechnical Journal, 35(3):

2 1. INTRODUCTION This manual will explain how to use the spreadsheet analysis tool CPTic_CSM8.xls to determine soil type and layering from CPT soundings using the latest iteration of the I c method (Robertson and Wride, 1998). The key features of the spreadsheet are: Ability to input high resolution digital data from modern CPTU soundings as a simple text file with up to 2500 data lines (e.g. 50 m penetration at 20 mm/s at a sampling frequency of 1 Hz); Input of estimated unit weight data for effective stress calculation up to 10 distinct layers with hydrostatic and artesian pore pressures (and therefore compatible with Stress_CSM8.xls). Automatic production of an A4 output sheet showing test data, initial stress/pore water conditions in the ground and soil classification with depth. The worksheet will produce a plot of soil type against depth which the user may then use to interpret the soil layering. The plot is colour-coded to assist with this. This manual is structured as follows: Section 2 The basic structure of both the workbook (CPTic_CSM8.xls) and the worksheet used to perform the analyses will be described and the principle of operation will be highlighted. Section 3 This section will describe the validation of the spreadsheet tool using a series of field test records published in the literature which have been digitised, processed using CPTic_CSM8.xls and compared to borehole logs. 2. PROGRAMME DESCRIPTION The spreadsheet analysis tool consists of four worksheets. The first, Data input, is the worksheet which is used to interact with the spreadsheet. There are various cells for inputting unit weight, water table location and pore water pressure conditions which are used to calculate approximate profiles of effective vertical stress (σ v ) and in-situ pore pressure (u 0 ) for use in CPTU normalisation (conversion of q t, f s and u 2 to Q t, F r and B q, see Section 6.6 and Figures 6.10 and 6.12 of the main text). The second worksheet, Data output, produces an A4 (landscape) output sheet summarising the assumed stress profile and initial excess pore pressure distributions within the ground, the original CPTU data (q t, f s and u 2 ) plotted against depth and the soil behaviour type classification with depth using the I c method. The final two worksheets, Calculations 1 and Calculations 2 perform the required calculations to determine soil type as a function of depth. The structure of the workbook and the layout of the Data input sheet are shown in Figure 1. Data input: In the Project ID section, basic information relating to the project or example being analysed is inputted, along with details of the user who prepared the calculations for auditing purposes. This information is not required for analysis, but appears in the header boxes when the output sheet is printed, and should be included as a matter of course. 2

3 The Soil & groundwater data section is where all of the information required for calculation is input by the user. Up to 10 soil layers, each with a distinct unit weight can be included; however, as the spreadsheet is to be used to determine the layering, it is usually sufficient to initially assume a single layer. Based on the interpreted profile and/or the results of other testing (e.g. boreholes), the layer profiles may be subsequently refined to provide the best estimate of Q t, F r and B q for material property determination (Section 7.5 of the main text). Unit weights should be input with Layer 1 being the uppermost layer. The depth of the WT below ground level (BGL) is also input (N.B. for WT below the ground surface, the value should be positive). If a surcharge (e.g. due to fill material) is acting on the surface to increase the total stress, the thickness and unit weight may be entered. If only the surcharge pressure is known, an equivalent unit weight and thickness should be input, the product of which will give the appropriate pressure. If a layer is under artesian pressure, the level of the piezometric surface above ground level (AGL) can be input this is the height AGL of the water level as measured in a standpipe sunk into the artesian layer, and is therefore suited to the input of measurements taken directly from the field. If the WT is below ground level, the spreadsheet will automatically calculate the additional head in the artesian layer and add this to the hydrostatic pressure to get the correct overall pore water pressure in the layer. Finally, the user can enter a depth range over which the resulting stress/pressure profiles will be plotted, by varying the values of z min and z max (N.B. as for WT depth, these values should be positive to indicate depths BGL). The CPTU data section consists of 2500 rows which are editable into which the digital CPTU data should be imported. This may be done by copy-and-paste from another worksheet or from an ASCII format text file (.txt,.dat, etc.) using the data import tool (Data > Import External Data in Microsoft Excel). The raw data should be in four columns: Column 1: Depth (m) Column 2: Corrected tip resistance, q t (MPa) Column 3: Sleeve friction, f s (MPa) Column 4: Pore water pressure measured at the shoulder, u 2 (MPa) The text file should have no more than 2500 data lines, and should be truncated or resampled at a lower sampling rate prior to import if necessary. It should be noted that the current version of CPTic_CSM8.xls does not support the import of uncorrected tip resistance q c and in-programme conversion to q t. Data output: In this sheet, the data from the Project ID section of Data input is copied across, and the graphs arising from the calculations are plotted over the depth range specified by z min and z max in Data input. The print area has been set such that the worksheet will automatically be scaled to fit a single sheet of A4 paper (landscape) when the printing. Calculations 1: This worksheet calculates the total and effective vertical stress profiles within the ground based on the assumed data provided in the Data input sheet (Soil & groundwater data section) using a simple finite difference method. This worksheet is identical to the calculations worksheet in Stress_CSM8.xls. 3

4 Calculations 2: This worksheet interpolates σ v0, u 0 and σ v0 from Calculations 1 at each CPTU datapoint, normalises the data to determine Q t, F r and B q and finds I c for each point. A lookup table is then used to assign a soil classification number (as defined in Figure 6.10 of the main text), which is plotted as a function of depth in Data output. All of the worksheets are protected so that only data input cells can be edited by the user. This is to prevent accidental over-typing of formulae which may affect the functionality of the spreadsheet. However, the protection is not password protected, and so may be removed (Tools > Protection > Unprotect Sheet in Microsoft Excel) if users wish to investigate the calculation procedures used. Figure 1: Workbook structure 3. VALIDATION AGAINST CASE HISTORIES To illustrate the usefulness, versatility and accuracy of the spreadsheet for use in ground investigation, five CPTU records published in the literature have been digitised, processed using CPTic_CSM8.xls and compared to visual observations of soil type and profile from borehole logs at the same sites. The Companion Website contains sample workbooks for each of these case histories, including the digitised CPTU data. The database used herein include sites with stiff marine clays, soft organic clays, and layered and inter-bedded sands and clays. 4

5 Case history 1 Layered sand over clay, Vancouver, Canada The original CPTU data was reported by Campanella and Robertson (1981); it is also reproduced in Lunne et al. (1997). The corrected cone resistance (q t ) sleeve friction (f s ) and pore water pressures measured at the shoulder (u 2 ) are shown in Figure 2. The maximum penetration reported was 30 m BGL. In the absence of other data, the total and effective stresses were determined using an initial estimate for the unit weight of 17 kn/m 3 over the full depth of soil investigated (30 m). The water table was observed to be at approximately 2 m BGL. The resulting plot of soil classification number (between 1-7) against depth is shown in Figure 3. The data suggest a layer of relatively clean sand to a depth of 15 m, underlain by clay. There is some intermixing of the soils suggested between m, i.e. increasing fines content of the sand as it transitions to clay, which is to be expected. A siltier sand is also suggested in the upper 2 m of the deposit. A borehole log recorded at the same site is also shown in Figure 3, which confirms the overall layering, including the siltier material towards the surface and the thin layer of silty sand between m. Figure 2: CPTU data for case history 1 5

6 Figure 3: Soil classification from CPTic_CSM8 (left) and borehole observations (right), case history 1 Case history 2 Glacial till with inter-bedded sand layers, Cowden, UK The original CPTU data and borehole log in this case history is reported by Lunne et al. (1997). The corrected cone resistance (q t ) sleeve friction (f s ) and pore water pressures measured at the shoulder (u 2 ) are shown in Figure 4. The maximum penetration reported was 24 m BGL. In the absence of other data, the total and effective stresses were determined using an initial estimate for the unit weight of 22 kn/m 3 over the full depth of soil investigated (24 m). The water table was observed to be at approximately 1 m BGL. The resulting plot of soil classification number against depth is shown in Figure 5. The data suggest a silty clay down to 10 m, transitioning to a finer clay from m. There is then a distinct layer of (silty) sand from m, clay from m, then another 2 m thick layer of sand from m. Below this level the soil reverts to clay. A borehole log recorded at the same site is also shown in Figure 5. This confirms the inter-bedded granular layers within a soil that is predominantly finegrained. The levels and thicknesses of these layers differ slightly from the CPTU interpretation as the borehole and CPT investigations were not recorded at precisely the same location (i.e. the same hole). The CPTU is not able to directly describe the fine soil as a glacial till based on I c, but it does correctly identify that the soil has a clayey matrix and will behave in a clay-like way (e.g. low permeability, undrained shear strength etc.). 6

7 Figure 4: CPTU data for case history 2 Figure 5: Soil classification from CPTic_CSM8 (left) and borehole observations (right), case history 2 7

8 Case history 3 Stiff, heavily overconsolidated clay, Madingley, UK The original CPTU data and borehole log in this case history is reported by Lunne et al. (1997). The corrected cone resistance (q t ) sleeve friction (f s ) and pore water pressures measured at the shoulder (u 2 ) are shown in Figure 6. The maximum penetration reported was 21 m BGL. In the absence of other data, the total and effective stresses were determined using an initial estimate for the unit weight of 19 kn/m 3 over the full depth of soil investigated (21 m). The water table was observed to be at approximately 1 m BGL. The resulting plot of soil classification number against depth is shown in Figure 7. The data suggest a that the site is silty clay, with perhaps lower silt content below 10 m, and also identify a thin deposit of silty sand material between 3 4 m. A borehole log recorded at the same site is also shown in Figure 7. This confirms the CPTU interpretation, though the layer of sand (2.5 m thick in the borehole) was recorded deeper. This is expected to be due to level variations of this unit across the site with the borehole and CPT investigations not being recorded at precisely the same location. They would be expected to be from the same geological unit. The CPTU cannot say anything about the strength or overconsolidation of the clay based on I c alone, but the data could be further interpreted as outline in Chapter 7 of the main text to provide greater insight into these parameters. Figure 6: CPTU data for case history 3 8

9 Figure 7: Soil classification from CPTic_CSM8 (left) and borehole observations (right), case history 3 Case history 4 Soft, lightly overconsolidated clay, Lilla Mellösa, Sweden The original CPTU data was reported by Larsson and Mulabdic (1991); it is also reproduced in Lunne et al. (1997). The corrected cone resistance (q t ) sleeve friction (f s ) and pore water pressures measured at the shoulder (u 2 ) are shown in Figure 8. The maximum penetration reported was just under 14 m BGL. In the absence of other data, the total and effective stresses were determined using an initial estimate for the unit weight of 15 kn/m 3 over the full depth of soil investigated (15 m). The water table was observed to be at approximately 1 m BGL. The resulting plot of soil classification number against depth is shown in Figure 9. The data suggest a uniform clay below a depth of about 2 m. It can be seen from Figure 8 that q t and f s are both very high between 0 2 m. This corresponds to a desiccated crust of material which is identified from the borehole log which is also shown in Figure 9. The spreadsheet has identified this material as silty clay, but this cannot be considered reliable in crustal materials which may be partially saturated and have other macro-fabric features which distort the response of the soil to penetration of the cone. Care must therefore be taken when interpreting the results of the spreadsheet in near surface soils, and data in the top few metres are not normally relied upon in geotechnical practice. 9

10 Figure 4: CPTU data for case history 4 Figure 9: Soil classification from CPTic_CSM8 (left) and borehole observations (right), case history 4 10

11 REFERENCES Campanella, R.G. and Robertson, P.K. (1981) Applied cone research. Cone Penetration Testing and Experience. Proc. ASCE National Convention, St Louis, Larsson, R. and Mulabdic, M. (1991) Piezocone tests in clay. Swedish Geotechnical Institute Report no. 42, Linköping. Lunne, T., Robertson, P.K. and Powell, J.J.M. (1997) Cone Penetration Testing in Geotechnical Practice. E & FN Spon, London. Robertson, P.K., and Wride, C.E. (1998). Evaluating cyclic liquefaction potential using the cone penetration test. Canadian Geotechnical Journal, 35(3):