Estimation of Vertical Subgrade Reaction Modulus from CPT and Comparison with SPT for a Liquefiable Site in Christchurch
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INTERNATIONAL SOCIETY FOR SOIL MECHANICS AND GEOTECHNICAL ENGINEERING This paper was downloaded from the Online Library of the International Society for Soil Mechanics and Geotechnical Engineering (ISSMGE). The library is available here: https://www.issmge.org/publications/online-library This is an open-access database that archives thousands of papers published under the Auspices of the ISSMGE and maintained by the Innovation and Development Committee of ISSMGE. The paper was published in the proceedings of the 12th Australia New Zealand Conference on Geomechanics and was edited by Graham Ramsey. The conference was held in Wellington, New Zealand, 22-25 February 2015. Estimation of vertical subgrade reaction modulus from CPT and comparison with SPT for a liquefiable site in Christchurch N. Barounis1, CEng MICE MIPENZ and Brett Menefy2, CertETn, AIPENZ 1Opus International Consultants, 20 Moorhouse Avenue, Christchurch, New Zealand, email: [email protected] 2Opus International Consultants, 20 Moorhouse Avenue, Christchurch, New Zealand, email: [email protected] ABSTRACT A methodology introduced in 2013 (Barounis et al.) for estimating the coefficient of subgrade reaction for sands from Cone Penetration Test (CPT) data was applied for a liquefiable residential site in Christchurch. The data gathered from a site investigation comprising of four CPT’s and one borehole with Standard Penetration Test’s (SPT’s). The site consists of relatively uniform soil formations. The results from the CPT’s are compared with the SPT borehole results. The SPT results are analysed by using the methods proposed by Scott (1981) and Moayed and Janbaz (2011). The three methods are applied on different sizes of foundations: square footing, rectangular footing and strip footing. The results from the CPT methodology are presented and compared with the other two methods. The proposed method can be of use in foundation design after applying suitable conversion factors which are also presented. Keywords: Subgrade reaction coefficient, CPT, foundation, sand, SPT 1 INTRODUCTION The earthquake activity in the Canterbury area, New Zealand, between September 2010 and June 2011 resulted in extensive CPT investigations within the affected City of Christchurch. As Christchurch is currently under rebuild, there is a high demand both for strengthening of existing foundations and design of new foundations for schools, churches, residential, commercial and industrial properties in limited amount of time and budget (Opus International Consultants Ltd., 2010 – 2014). For these reasons, a rapid method for the estimation of the vertical subgrade reaction modulus k [N/m3] (others may use the terminology subgrade reaction coefficient) for sands from CPT data was introduced in 2013 by Barounis et al. The k obtained from CPT data will carry the symbol KCPT while the resulted subgrade reaction for a given foundation will carry the following symbols throughout this paper: KFCPT for CPT data KFSCOTT for SPT data KFM&J for SPT data This paper presents the results of the proposed method applied at a liquefiable site in Papanui, Christchurch. The CPT results are compared with SPT results executed within a borehole at the same site. 2 ADVANTAGES AND DISADVANTAGES OF THE PROPOSED METHOD The proposed method offers the following advantages over plate loading testing (Barounis et al., 2013): 1. Inexpensive 2. Quicker 3. No requirement for kentledge or reaction loads 4. No dewatering of excavations 5. Independence from plate load size effects 6. Higher exerted pressure (up to 40MPa, nominal 100MPa) 7. All soil types can be tested 8. Testing at greater depths 9. Influence of stress field at depth is reflected in the measurement 10. Testing to soil failure 11. k versus depth is delineated 12. Statistical analysis on obtained values 13. A pair of (qc, δ) values for penetration between 1 and 2 centimetres gives 50 to 100 measurements per meter 14. KCPT can be correlated with consistency of clays, Dr of sands, SPT N and soil behaviour 15. Measured KCPT can be transformed to a reliable KFCPT 16. May prove useful for liquefaction potential assessment. It also has the following disadvantages compared to plate loading testing and common laboratory tests (Barounis, 2013): 1. CPT measures properties at soil failure; 2. Plate load, unconfined and triaxial compression test and consolidation test measure properties within elastic range and/or at soil failure; 3. KCPT is considerably higher than Kplate due to the smaller probe diameter used; 4. Limited capacity to penetrate through hard soils or refuse on gravels may result in termination of the test with insufficient KCPT values. 3 BACKGROUND THEORY During cone penetration, δ, the soil is initially compressed and then sheared to failure at a stress equal to the measured value of qc. The rate of cone penetration of two centimetres per second classifies this type of testing as strain controlled undrained in-situ testing, similarly to the laboratory triaxial UU test and unconfined compression test. For the case of cone penetration with δ=10 mm, each soil layer of thickness T can be perceived as a number of vertical springs 10 mm long connected in series like a chain (see Figure 2). Each soil spring is assumed having an ultimate vertical reaction coefficient value of KCPT, which is fully mobilized during cone penetration. The cone penetration test offers the ability to measure this ultimate reaction coefficient KCPT for the complete depth of the CPT profile as both qc and δ are measured. The mathematical definition for KCPT can be described by using a qc versus δ graph shown in Figure 1. The slope of the straight line connecting the origin of the axes and the value of qc is KCPT, defined as 3 qc/δ and carries units of N/m . This resembles elasto-plastic soil behaviour when loaded. At qc the soil fails, as this is proved during testing. At this failure point the soil is considered as behaving as a spring which has reached its ultimate stress limit. The duration of testing for 1cm of soil is between 0.5 to 1 seconds. A well maintained and calibrated CPT probe can let us assume with a lot of confidence that for each centimetre tested a straight line can be always drawn as shown in Figure 1. 50 q 45 c 40 35 30 25 (MPa) c q 20 15 10 5 0 0246810 δ (mm) Figure 1. Definition of KCPT by using a qc/δ graph for δ = 10 mm Figure 2. Delineation of K from CPT with depth For foundation design the KCPT value needs to be scaled before it is used as the foundation size is several orders of magnitude greater than the diameter of the CPT probe used. The applicable scaling factor is discussed in the next paragraph. 4 ESTIMATION PROCEDURE FOR KCPT AND KFCPT The KCPT and KFCPT can be estimated in five steps: Step 1: Calculation of KCPT KCPT = qc/δ (1) 3 where, KCPT = coefficient of subgrade reaction from CPT [N/m ] qc = tip cone resistance measured at any depth [MPa] δ = cone penetration applied [mm] For δ = 10 mm and qc is in MPa then, 3 KCPT = 100qc [MN/m ] (2) Step 2: Transformation of KCPT to a reference 300 mm load plate value KCPT(0.3) KCPT(0.3) = KCPT x (DCPT/300) (3) where, KCPT(0.3) = coefficient of subgrade reaction for a 300 mm load plate based on CPT measurements DCPT = the diameter of the cone used [mm] 300 = the reference plate diameter [mm] KCPT = the subgrade reaction coefficient calculated according to step 1 depending on the 3 penetration increment δ used [N/m ]. For DCPT = 35.7 mm, KCPT (0.3) is approximately 12% of KCPT (scaling factor of 0.119). Step 3: Conversion of KCPT(0.3) to KFCPT KFCPT = KCPT(0.3) x ) (4) where, m = L/B (Bowles, 1997), applicable to strip, pad or. raft foundations on medium dense sand. . For the same types of foundations on sands of any relative density: KFCPT = KCPT(0.3) x ( ) (5) where, B1 = 0.3 m (reference plate width) and B = actual foundation width [m]. Step 4: Divide the estimated KFCPT by a conversion factor, CF to obtain the value for foundation design: KDESIGN = KFCPT/CF (6) The recommended CF values for sands is discussed in paragraph six. 5 APPLICATION OF THE METHOD AT A SITE IN CHRISTCHURCH 5.1 Geotechnical site investigations The method was applied at a site on Paparoa Street, Christchurch at an existing residential building that suffered structural damage due to the recent Canterbury earthquakes. The property is situated 3.7km northwest of Christchurch CBD. The geotechnical site investigations undertaken in 2014 included: A sonic drilled borehole to a depth of 15m; SPT was undertaken at 1.5m depth intervals Four CPT’s with pore pressure measurements to effective tip refusal (CPT01 is shown in Figure 2, qc and KCPT versus depth are shown). The conducted site investigations and testing is shown in Table 1. Table 1: Conducted site investigations and testing Investigation depth below Site investigation Testing ground level (m) SPT with hammer efficiency Borehole 01 (BH01) 15.0 Er = 90.7%1 CPT 1 (1.5m away from BH01) 9.48 CPT 2 (20m away from BH01) 9.32 Pore pressure CPT 3 (35m away from BH01) 9.58 measurements CPT 4 (45m away from BH01) 9.30 1 Hammer efficiency determined from calibration certificate of the SPT hammer used. Figure 3: CPT01 and KCPT profiles versus depth 5.2 Ground conditions The published geology map of the area (Brown and Weeber, 1992) indicates the site is predominantly underlain with alluvial sand and silt overbank deposits (Yaldhurst Member of the Springston Formation). The typical ground profile of the site has been inferred based on the site investigations, and is shown in Table 2.