Seediscussions,stats,andauthorprofilesforthispublicationat:http://www.researchgate.net/publication/268590430 DevelopmentoftheUWA-05DesignMethod forOpenandClosedEndedDrivenPilesin SiliceousSand CONFERENCEPAPER·OCTOBER2007 DOI:10.1061/40902(221)12 CITATIONS 3 DOWNLOADS 269 VIEWS 85 3AUTHORS,INCLUDING: BarryMichaelLehane UniversityofWesternAustralia 92PUBLICATIONS702CITATIONS SEEPROFILE JamesSchneider ConsultingEngineer 28PUBLICATIONS205CITATIONS SEEPROFILE Availablefrom:BarryMichaelLehane Retrievedon:07August2015 1 INTRODUCTION AND BACKGROUND The authors, at the request of the American Petrole- um Institute (API) piling sub-committee, recently conducted a review of methods for the assessment of the axial capacity of driven offshore piles in sili- ceous sand. The review, which is described in detail in Lehane et al. (2005a) and involved the develop- ment of an extended database of static load tests, evaluated the existing API recommendations (API- 00) and three Cone Penetration Test (CPT) based methods namely: Fugro-04 (Fugro 2004), ICP-05 (Jardine et al. 2005) and NGI-04 (Clausen et al. 2005). A new design method, referred to as UWA- 05, emerged following the evaluation exercise and is the described in this paper and in Lehane et al. (2005b). The assessment of the predictive performance of API-00, Fugro-04, ICP-05 and NGI-04 against the new UWA pile test database indicated the following trends (which are described in detail in Lehane et al. 2005a): 1. All three CPT based design methods considered (Fugro-04, ICP-05 & NGI-04) had significantly better predictive performance than the existing API recommendations, which were seen to lead large under-predictions in dense sands and be- come progressively non-conservative as the pile length (L) or aspect ratio (L/D) increased. 2. Despite the CPT based methods having a broad- ly similar predictive performance against the new database of load tests, their formulations re- lating the pile end bearing with the cone tip re- sistance (q c ) are notably different. Formulations for shaft friction also differ significantly in de- tail, although all assume a near-proportional re- lationship between local shaft friction (τ f ) and q c and allow for the degradation of τ f with distance above the pile tip (h) due to friction fatigue. 3. The ICP-05 method indicated the lowest coeffi- cient of variation (COV) for calculated to meas- ured capacities (Q c /Q m ) of 0.32, when an equal weighting is given to each pile test in the data- base. However, the relative performance of each method for various categories within the data- base is less clear. For example, NGI-04 predic- tions appear best for open-ended piles in com- pression while Fugro-04 and ICP-05 provide comparable predictive accuracies for open-ended piles in tension. 4. When account was taken of the relative reliabil- ity of the pile test data (using a carefully de- signed weighting procedure), the methods listed below for each category of pile lead to the low- est probability of failure: API-00: closed-ended piles in compression Fugro-04: closed-ended piles in tension ICP-05 & NGI-04: open-ended piles in com- pression ICP-05 & Fugro-04: open-ended piles in ten- sion 5. API-00 gives the lowest probability of failure for closed-ended piles in compression partly be- cause the method generally under-predicts the capacity of the database piles to a significant de- gree. However, while the same average level of under-prediction also applies to API-00 predic- tions for closed-ended piles in tension, the esti- mated probability of failure is larger than the three alternative CPT design methods. The UWA-05 method for prediction of axial capacity of driven piles in sand B. M. Lehane, J.A. Schneider and X. Xu The University of Western Australia (UWA), Perth ABSTRACT: This paper describes a new method for evaluating the axial capacity of driven piles in siliceous sand using CPT q c data. The method is shown to provide better predictions than three other published CPT based methods for a new extended database of static load tests. The design expressions incorporate the most important features currently accepted as having a controlling influence on driven pile capacity at a fixed time after installation (e.g. the effects of soil displacement, friction fatigue, sand-pile interface friction, dilation at the shaft and loading direction) and are seen to reduce to a simplified form for typical (large diameter) off- shore piles. 6. The ICP-05 method displays a tendency to under- predict pile base capacities (when assuming ca- pacity solely from annular end bearing) and to be- come potentially non-conservative for tension ca- pacity as the pile aspect ratio (L/D) increases. The Fugro-04 method indicates a tendency to under- predict compression capacities for long piles and to over-predict base capacities in loose sand. The examination of the three CPT based methods coupled with a review of their various deficiencies and a careful examination of the new extended data- base of static load tests prompted the authors to pro- pose the UWA-05 method presented here. This method is believed to represent a significant im- provement on Fugro-04, ICP-05 and NGI-04 meth- ods. Particular comparisons are made with ICP-05, which Lehane et al. (2005a) adjudged to have a marginally better predictive performance than the other two CPT based methods. 2 THE UWA-05 DESIGN METHOD FOR PILES IN SAND 2.1 End Bearing Factors that were considered in the development of the UWA-05 proposals for base capacity evaluation of closed and open-ended piles are listed in the fol- lowing. These proposals are based on the analyses reported in Xu & Lehane (2005) and Xu et al. (2005). The base capacity is defined as the pile end bearing resistance at a pile base movement of 10% of the pile diameter, q b0.1 . 2.1.1 Closed-ended piles The strong direct relationship between the end bearing resistance of a closed-ended driven pile and the cone tip resistance, q c , has been recog- nised for many years and arises because of the similarity between their modes of penetration. Given the difference in size between a pile and a cone penetrometer, a correlation between q b0.1 and q c requires use of an appropriate averaging technique to deduce an average value of c q . Xu & Lehane (2005) show that, for many stratigra- phies encountered in practice, c q may be taken as the average q c value taken in the zone 1.5 pile diameters (D) above and below the pile tip. Xu & Lehane (2005), however, also show that when q c varies significantly in the vicinity of the pile tip (i.e. within a number of diameters), the Dutch averaging technique (Van Mierlo & Koppejan 1952, Schmertmann 1978) provides the most consistent relationship for end bearing and should be employed to calculate c q . A simplified (and conservative) means of deter- mination of the Dutch c q value is provided in Lehane et al (2005b), which may be more practi- cal when using CPT data collected offshore, which are often not continuous. The values of q b0.1 for driven piles are less than c q because the displacement of 0.1D is insuffi- cient to mobilise the ultimate value (of c q ). The findings of Randolph (2003), White & Bol- ton (2005), and others, are consistent with the UWA-05 proposal to adopt a constant ratio of q b0.1 / c q for driven closed-ended piles. The UWA-05 design equation for the end bearing of a closed-ended pile, with diameter D, is given as: 2 1.0bb D 4 qQ where q b0.1 / c q = 0.6 (1) 2.1.2 Open-ended piles Salgado et al. (2002), Lehane & Gavin (2001, 2004), and others, have shown that a relatively consistent relationship between q b0.1 for a pipe pile and the CPT q c value becomes apparent when the effects of sand displacement close to the tip during pile driving are accounted for. This installation effect is best described by the incre- mental filling ratio (IFR) measured over the final stages of installation- and is referred to here as the final filling ratio (FFR). As the FFR ap- proaches zero, q b0.1 approaches that of a closed- ended pile with the same outer diameter. The displacement induced in the sand in the vi- cinity of the base is most conveniently expressed in the terms of the effective area ratio A rb * , de- fined in Equation 2c. This ratio depends on the pile’s D/t (diameter to wall thickness) ratio and the FFR value, varying from unity for a pile in- stalled in a fully plugged mode to about 0.08 for a pile installed in coring mode with D/t of 50. Lehane & Randolph (2002), and others, have shown that, if the length of the soil plug is greater than 5 internal pile diameters (5D i ), the plug will not fail under static loading, regardless of the pile diameter. Experimental data and numerical analysis indi- cate that the resistance that can develop on the tip annulus at a base movement of 0.1D varies be- tween about 0.6 and 1.0 times the CPT q c value (e.g. Bruno 1999, Salgado et al. 2002, Lehane & Gavin 2001, Paik et al 2003, Jardine et al. 2005). Lehane & Randolph (2002) suggest that the base resistance provided by the soil plug for a fully coring pile (with FFR =1) is approximately equivalent to that of a bored pile. Recommended values of q b0.1 /q c for bored piles range from 0.15 to 0.23 (Bustamante & Gianeselli 1982, Ghionna et al. 1993). These rati- os are not dependent on the pile diameter. The value of c q should be evaluated in the same way as that employed for closed-ended piles, but using an effective diameter (D * ) related to the ef- fective area ratio, A rb * i.e. D * = D × A rb *0.5 . There are relatively few documented case histo- ries that report the incremental or final filling ra- tios. In the absence of FFR measurements, a rough estimate of the likely FFR may be obtained using equation 2d (see Xu et al. 2005). The UWA-05 proposal for end bearing of driven pipe piles is provided in Equation (2). This proposal is developed in Xu et al. (2005) and shown to com- pare favourably with the existing database of base capacity measurements for open-ended piles. 2 1.0bb D 4 qQ (2a) * rbc1.0b A45.015.0q/q (2b) 2 2 i * rb D D FFR1A (2c) 2.0 i m5.1 )m(D ,1minFFR (2d) where D i is the inner pile diameter. 2.2 Shaft Friction Factors that were considered in the development of the UWA-05 method for shaft friction are discussed in Schneider & Lehane (2005) and Lehane et al. (2005a). These are now summarised as follows: Local shaft friction (τ f ) shows a strong correlation with the cone tip resistance (q c ). This correlation, which has been observed directly in instrumented field tests has been employed successfully in well known design methods, such as that proposed by Bustamante & Gianiselli (1982). The shaft friction that can develop on a displace- ment pile is related to the degree of soil dis- placement imparted during pile installation. The higher capacity developed by the new generation of screw piles compared to that of a bored and continuous flight auger piles is just one example of this effect. The degree of displacement imparted to any giv- en soil horizon is related to the displacement ex- perienced by that horizon when it was located in the vicinity of the tip. This level of displacement can conveniently be expressed for both closed and open-ended piles in terms of an ‘effective ar- ea ratio’, A rs * , which is unity for a closed ended pile and, for a pipe pile, includes displacement due to the pile material itself and the additional displacement imparted when the pile is partially plugging or fully plugged during driving. White et al. (2005) use a cavity expansion analogy to deduce that the equalized lateral effective stress is likely to vary with the effective area ratio raised to a power of between 0.30 and 0.40. The incremental filling ratio (IFR) is a measure of soil displacement near the tip of a pipe pile and depends on a number of different parameters, in- cluding soil layering, pile inner diameter, pile wall thickness, plug densification or dilation, and installation method. For the (limited) database of IFRs reported, the mean IFR over the final 20D of penetration (where most friction is generated) can be reasonably approximated using Equation (3e) for relatively uniform dense to very dense sands in the database. After displacement of the sand near the tip in a given soil horizon and as the tip moves deeper, the radial stress acting on the pile shaft (and hence the available τ f value) in that horizon re- duces. This phenomenon, known as friction fa- tigue, is now an accepted feature of displacement pile behaviour (e.g. see Randolph 2003). The rate of radial stress and τ f reduction with height above the tip (h) depends largely on the magnitude and type of cycles imposed by the in- stallation method. White & Lehane (2004) show that the rate of decay is stronger for piles experi- encing hard driving and much lower for jacked piles, which are typically installed with a relative- ly low number of (one-way) installation cycles. White & Lehane (2004), and others, also show that the rate of degradation with h is greater at higher levels of radial stiffness (4G/D) and there- fore τ f at a fixed h value (i.e. after a specific number of installation cycles) in a sand with the same operational shear modulus (G) reduces as D increases. The foregoing, plus the tendency for hammer se- lection to be such that the number of hammer blows is broadly proportional to the pile slender- ness ratio (L/D), suggest that τ f may be tentative- ly considered a function of h/D. This approxima- tion is supported by field measurements such as those provided in Lehane et al (2005a), and is al- so compatible with the occurrence of a ‘critical depth’ at an embedment related to a fixed multi- ple of the pile diameter (such as 20D proposed by Vesic 1970 and a number of workers). The same approximation is employed by the ICP-05 and Fugro-04 design methods. Based on the former point, the ICP-05 method proposes that τ f varies in proportion to (h/D) -c , where c = 0.38. However, given that this value of c was estimated on the basis of field tests with jacked piles (Lehane 1992 and Chow 1997) where the type and number of cycles imposed is less severe than is typical of driven piles, a higher value of c is considered more appropriate for off- shore pile. Strong indirect evidence in support of this observation is also apparent in Lehane et al (2005a), which shows that the Fugro-04, ICP-05 and NGI-04 progressively under-predict the shaft capacity of jacked piles as the pile length increas- es The radial effective stress acting on a driven pile increases during pile axial loading and its magni- tude (when τ f is mobilised and dilation has ceased) increases as the pile diameter reduces, the sand shear stiffness around the pile shaft increas- es and the radial movement during shear (dila- tion) of the sand at the shaft interface increases. These increases are not significant for offshore piles (with large D) but need to be considered when extrapolating from load test data for small diameter piles in a database. The recommenda- tions of the ICP-05 method are considered rea- sonable for assessment of the increase in radial stress (∆σ' rd ), but with a modified expression for the shear stiffness derived from the CPT data. τ f varies in proportion to tan δ cv (where δ cv is the constant volume interface friction angle between the sand and pile); this δ cv value, which should be measured routinely, increases as the roughness normalized by the mean effective particle size (D 50 ) increases. Verification of the dependence of τ f on tan δ cv has been provided by Lehane et al. (1993), Chow (1997), and others. In the absence of specific laboratory measurements of δ cv . UWA-05 recommends the trend shown on Figure 1, which is the same as that employed by ICP-05 but with an upper limit on tanδ cv value of 0.55 (due to the potential for changes in surface roughness during pile installation). The shaft friction that can develop on a pile in tension is smaller than that which can be mobi- lised by a pile loaded in compression for the rea- sons described by Lehane et al. (1993), de Nicola & Randolph (1993) and Jardine et al. (2005). Because of the shortage of high quality measure- ments of τ f very close to the tip of a driven pile and the variable and inconsistent trends shown by the available measurements, one simplifying op- tion is to assume τ f is constant over the lower two diameter length of the pile shaft for both closed and open-ended piles in tension and compression. Shaft capacity increases with time as shown by Axelsson (1998), Jardine et al. (2005a), and oth- ers. Lehane et al (2005a) show, however, that rate of increase over the period 3 days to 50 days is not statistically significant for the UWA database of load tests. A design time of 10 to 20 days is considered appropriate for shaft friction calculat- ed using UWA-05. The UWA-05 design equations for shaft capacity of driven piles arose from the foregoing considera- tions and are expressed as follows: dzDQ fs (3a) cvrdrc c cvrff tan'' f f tan' (3b) 5.0 3.0 * rscrc 2, D h maxAq03.0' (3c) 2 2 i * rs D D IFR1A (3d) 2.0 i mean m5.1 )m(D ,1minIFR (3e) DrG4' rd (3f) where cv = constant volume interface friction angle ' rf = radial effective stress at failure ' rc = radial effective stress after installation and equalization ' rd = change in radial stress due to loading stress path (dilation) f / f c = 1 for compression and 0.75 for tension G/q c = 185·q c1N -0.75 with q c1N =(q c /p a )/(' v0 /p a ) 0.5 p a = a reference stress equal to 100 kPa ' v0 = in situ vertical effective stress r = dilation (assumed for analyses=0.02mm, as for ICP-05) 20 22 24 26 28 30 32 0.01 0.1 1 10 Median Grain Size, D 50 (mm) Interface Friction Angel, cv tan < 0.55 Employed for database evaluation UWA-05 recommendation Figure 1. cv variation with D 50 (modified from ICP-05 guide- lines) 3 PREDICTIVE PERFORMANCE OF UWA-05 The UWA database of static loads tests, as discussed in Lehane et al (2005a & b), was employed to assess the predictive performance of the proposed UWA-05 method. The predictions described employed equa- tions (1), (2) and (3) with the following additional considerations: Measured interface friction angles, when availa- ble, were adopted. Figure 1 was used in the ab- sence of measured δ cv values. When the incremental filling ratio (IFR) was rec- orded, A rb * was assessed using the mean IFR val- ue measured over the final 3D of pile penetration while the value of A rs * was assessed from the mean IFR value recorded over the final 20D of penetration. In the absence of IFR data, A rb * and A rs * were evaluated using Equations 2d & 3e. The database included 74 load tests at sites where CPT q c data were measured. Pile test data at sites containing micaceous, calcareous and residual sands were excluded from consideration – as were sites for which only Standard Penetration Test data were available. The database included substantially more pile tests than used for verification of the Fugro-04, ICP-05 and NGI-04 design methods and was sub- divided into the following four categories: (a) Closed-ended piles tested in compression (b) Closed-ended piles tested in tension (c) Open-ended piles tested in compression (d) Open-ended piles tested in tension A detailed presentation and discussion of this statis- tical analysis, which was conducted for API-00, Fugro-04, ICP-05 and NGI-04, as well as for UWA- 05 is presented in Lehane et al. (2005a & b) and may be briefly summarized as follows: (i) For the database taken as a whole (i.e. including all pile categories), the UWA-05 method pre- dicts a mean ratio of calculated to measured ca- pacity (Q c /Q m ) of 0.97 and the lowest overall coefficient of variation (COV) for this ratio of 0.29; this compares well with the respective COVs of 0.32, 0.38, 0.43 and 0.6 for ICP-05, Fugro-04, NGI-04 and API-00. (ii) The UWA-05 method has the lowest COV for Q c /Q m of all five methods for each of the four pile test categories (except for closed-ended piles in compression where UWA-05 and ICP- 05 have the same COV for Q c /Q m ). (iii) The COV of 0.19 for Q c /Q m of the UWA-05 method for open-ended piles in compression is significantly lower than the corresponding COV of 0.25 of ICP-05. (iv) UWA-05 shows no apparent bias of Q c /Q m with pile length (L), pile diameter (D), pile aspect ra- tio (L/D) and average sand relative density. One of the factors giving rise to the superior per- formance of the UWA-05 method for pipe piles is the inclusion of the effective area ratio terms in the expressions for base and shaft capacities of open ended piles. This is not surprising given the acknowledged importance of soil displacement on capacity and the fact that many of the database piles showed evidence of partial plugging. However, giv- en that the incremental filling ratio (IFR) is not commonly measured in practice, the sensitivity of the predictive performance to the IFR parameter employed was re-examined and a summary of this exercise is provided in Table 1. It is clear from Table 1 that the estimation of IFR using the empirical equations 2d & 3e, rather than direct use of the measured IFRs to deduce A r * val- ues, has only a minimal impact on the COV values for Q c /Q m . It may also be inferred that the assump- tion in UWA-05 of a fully coring pile (i.e. IFR=1) for the database piles (most of which had diameters less than 800mm) will lead, on average, to a 20% under-prediction of capacity. Such an under- prediction is in keeping with observed levels of par- tial plugging of (smaller diameter) database piles and suggests that other design methods, such as ICP- 05, which may provide a good fit to the existing da- tabase of load tests, but do not include an appropri- ate soil displacement term (such as A r * ), will over- predict the capacity of full scale offshore piles. Table 1: Sensitivity of pipe pile capacity to A r * (A rb * and A rs * ) 4 PREDICTIONS FOR OFFSHORE PILES The UWA-05 method simplifies to the following form for full scale offshore piles, as IFR=1 and the dilation term (∆σ’ rd ) can be ignored. dzDD 4 qQQQ f 2 1.0bsbcomp (4a) dzD75.0Q ftens (4b) rc1.0b A45.015.0qq (4c) cv 5.0 3.0 rcf tan2, D h maxAq03.0 (4d) 2 2 i r D D 1A (4e) Lehane et al. (2005b) examined the implications of equation (4) and assessed its performance against existing API recommendations and ICP-05 (the best Method for calculation of A r * Mean Q c /Q m COV for Q c /Q m Open-ended piles in compression Using Equations 2d & 3e for all tests 0.99 0.23 Assuming IFR= 1 0.81 0.24 Using measured IFR when available 0.98 0.19 Open-ended piles in tension Using Equations 2d & 3e for all tests 0.97 0.26 Assuming IFR=1 0.77 0.22 Using measured IFR when available 0.91 0.23 performing of the three CPT based methods consid- ered). This examination indicated that equation (4) provides a more conservative extrapolation than ICP-05 for shaft capacity from the existing database (of relatively small diameter piles – with a mean D of about 0.7m) to typical offshore piles used in prac- tice. Equation (4) also predicts higher base capaci- ties than ICP-05 because of its assumption that a pile plug with a length greater than 5 diameters will not fail under static loading. It is also noteworthy that Equation (4) tends to provide lower capacities than API-00 in loose sands, but higher capacities for dense sands in compres- sion. API-00 and UWA-05 predictions for tension capacity in dense sands are broadly similar for pile lengths in excess of 20m. However, the UWA-05 method, unlike API-00, does not show any predic- tion bias with L, D, L/D and D r . 5 CONCLUSIONS This paper has shown that the UWA-05 method: (i) is a significant improvement on existing API recommendations; (ii) provides better predictions for a new extended database of load tests than the ICP-05, Fugro- 04 and NGI-04 CPT based design approaches; (iii) employs soundly based formulations that draw on the considerable recent developments in our understanding of displacement piles in sand; (iv) provides formulations that enable a rational ex- trapolation beyond the existing database base of load tests. 6 REFERENCES Axelsson G. 1998. 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