DISCUSSIONS AND CLOSURES

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1 DISCUSSIONS AND CLOSURES Downloaded from ascelibrary.org by on 02/24/18. Copyright ASCE. For personal use only; all rights reserved. Discussion of Axial Compression of Footings in Cohesionless Soils. I: Load-Settlement Behavior by Sami O. Akbas and Fred H. Kulhawy November 2009, Vol. 135, No. 11, pp DOI: / ASCE GT Marco Uzielli, A.M.ASCE 1 ; and Paul W. Mayne, M.ASCE 2 1 Georisk Engineering S.r.l., Via Giuseppe Giusti 16/5, Firenze, Italy. muz@georisk.eu 2 GeoSystems Group, Civil and Environmental Engineering, Georgia Institute of Technology, Atlanta, GA The discussers would like to commend the authors for their investigation of the characterization and parameterization of the load-settlement behavior of axially loaded footings on cohesionless soils because of its importance in geotechnical practice. In this discussion, the discussers would like to: 1 provide the results of a comparative assessment of the performance of loaddisplacement models in approximating the results of a set of fullscale load tests on large footings; 2 stress the importance of investigating load-displacement uncertainty by addressing quantitatively not only second-moment statistics of model parameters but also the correlation between the parameters themselves; and 3 examine the adequateness of load-displacement models for the generation of multivariate distributions for simulation purposes. A database of 30 footings on 12 different sands that were subjected to vertical loading in the field was used for these purposes. The origins of these sands included natural marine deposits, alluvial sediments, aeolian dune sands, and compacted sand fills. The footings included 18 square, 7 rectangular, and 5 circular shallow foundations, with the equivalent footing width B varying from 0.5 to 6 m. Only full-scale foundations were considered to avert difficulties associated with 1-g model tests and scaling problems e.g., Cerato and Lutenegger A summary of the sand sites, test locations, sand origin, index number in the database, footing sizes, embedment depth, groundwater depth, median grain size, and CPTU net cone tip resistance q net representative of capacity is given in Uzielli and Mayne The applied stress was normalized by q net ; settlement s was normalized by foundation width B. Three load-displacement model types were considered: Model type LIN linear model displacements s for shallow spread footings on sand by elastic theory s = q app B I 1 v 2 /E 4 where I=elastic displacement influence factor; =soil Poisson s ratio; and E =drained elastic soil modulus. The latter can be estimated using E =a E q net. Eqs. 4 and 1 are equivalent, with k 1 =a E / I 1 2. The PWR model follows the characteristic stress-normalized displacement curve suggested by Fellenius and Altaee 1994 and later supported by Phoon et al. 1995, Briaud and Gibbens 1999, Decourt 1999, Lutenegger and Adams Each of the three models was fitted to the data from the 30 tests in the source database using generalized leastsquares regression. An example regression is shown in Fig. 1. Figures 2 a and b show the bivariate plots of model parameters for HYP and PWR, respectively. The existence and magnitude of statistical correlation between model parameters was investigated quantitatively for the two-parameter models HYP and PWR. Kendall s tau was calculated for each couple of model parameters, yielding k2k3 = 0.38 for HYP and k4k5 =0.62 for PWR. Hence, with reference to the regression output samples, it was assessed that the magnitude of the direct statistical dependence between the PWR model parameters exceeds, in absolute value, that of the negative statistical dependence between the HYP parameters. The performance of the models was assessed comparatively through three quantitative criteria. First, the goodness of fit was parameterized by the coefficient of determination R 2 and the root mean squared error RMSE. The PWR model was shown to perform significantly better than LIN and also to outperform HYP. The total RMSE values were 0.737, 0.193, and 0.112; the mean Fig. 1. Example fitting of the three candidate load-displacement models to the load test q app /q net = k 1 Model type HYP hyperbolic model q app /q net = / k 2 + k 3 Model type POW power series model 1 2 q app /q net = k 4 k 5 3 where =s/b=pseudo-displacement. The LIN model is related to the conventional approach to evaluating magnitudes of foundation Fig. 2. Scatterplots of samples of model parameters for a HYP; b PWR JOURNAL OF GEOTECHNICAL AND GEOENVIRONMENTAL ENGINEERING ASCE / JANUARY 2011 / 109

2 R 2 was 0.707, 0.973, and for LIN, HYP, and PWR, respectively. Second, a second-moment analysis on the samples of model parameters was conducted. The authors correctly highlighted the importance of addressing prediction uncertainty, for instance through second-moment statistical analysis. The comparative assessment of the magnitude of prediction uncertainty in the LIN, HYP, and PWR models was pursued by comparing the sample coefficients of variation COVs of model parameters. The COVs of k 4 and k 5 were calculated at 0.29 and 0.17, respectively. These values are significantly smaller than those of COV k 2 =0.45 and COV k 3 =0.58 and, especially, COV k 1 =0.74. On the basis of the above, the discussers recommend the use of the power law model over the hyperbolic and especially the linear model for axially loaded footings on cohesionless soils. Briaud, J. L., and Gibbens, R. M Behavior of five large spread footings in sand. J. Geotech. Geoenviron. Eng., 125 9, Cerato, A. B., and Lutenegger, J Scale effects of shallow foundation bearing capacity on granular material. J. Geotech. Geoenviron. Eng., , Decourt, L Behavior of foundations under working load conditions. Proc. XI Panamerican Conf. on Soil Mechanics and Geotechnical Engineering, Foz do Iguaçu, Brazil, Vol. 4, Fellenius, B. H., and Altaee, A Stress and settlement of footings in sand. Geotechnical Special Publication GSP No. 40, Proc. ASCE Conf. on Vertical and Horizontal Deformations for Foundations and Embankments, College Station, TX, Vol. 2, Lutenegger, A. J., and Adams, M. T Characteristic loadsettlement behavior of shallow foundations. Proc., Int. Symposium on Shallow Foundations (FONDSUP), Vol. 2, Laboratoires des Ponts et Chaussées, Phoon, K. K., Kulhawy, F. H., and Grigoriu, M. D Reliabilitybased design of foundations for transmission line structures. Rep. TR , Electric Power Research Institute, Palo Alto, CA. Uzielli, M., and Mayne, P. W Serviceability limit state CPTbased design for vertically loaded shallow footings on sand, Taylor and Francis, London in press. Closure to Axial Compression of Footings in Cohesionless Soils. I: Load-Settlement Behavior by Sami O. Akbas and Fred H. Kulhawy November 2009, Vol. 135, No. 11, pp DOI: / ASCE GT Sami O. Akbas, M.ASCE 1 ; and Fred H. Kulhawy, Dist.M.ASCE 2 1 Assistant Professor, Dept. of Civil Engineering, Gazi Univ., Celal Bayar Bulvari, Maltepe, Ankara, Turkey soakbas@gazi.edu.tr 2 Professor Emeritus, School of Civil and Environmental Engineering, Hollister Hall, Cornell Univ., Ithaca, NY fhk1@cornell.edu The writers thank the discussers for their interest in the paper and look forward to seeing their data. During their research for the paper, the writers considered various models to approximate the load-settlement curves, including linear, power, and logarithmic functions. The detailed analyzes showed the hyperbolic model to be decisively the best. As indicated in the paper, r 2 values for the hyperbolic model fitting parameters a and b are greater than 0.95, except in the case only four case histories. This is also in agreement with the results for the HYP model given by the discussers. The writers also want to note two very important distinctions between their evaluations and the discussers : First, the characteristics of any compiled database significantly influence results. The writers employed a very large and comprehensive load test database that includes many more load tests conducted at a larger number of cohesionless soil sites than the discussers. As can be seen in Table 5 of the original paper, even for the same model, statistics for the fitting parameters change significantly when smaller selected groups of tests are considered. Note that for an extreme case that involves a database consisting solely of dense sands, a linear model can potentially be the most appropriate. Second, the discussers used a different parameter, the CPTU net cone tip resistance, to normalize the applied stress. The more appropriate normalizer to use is Q L2, the actual interpreted capacity from the load-displacement curve, which relates directly to the theoretical value of the bearing capacity, as shown in the companion paper Akbas and Kulhawy Direct use of field test parameters introduces additional uncertainties, and the CPTU cannot be employed in very coarse, dense, or cemented cohesionless soils. Even with the limited database and the CPTU normalizer, the difference in the performances of the HYP and PWR models as presented by the discussers is relatively minor, especially when the many sources of uncertainties are considered. Akbas, S. O., and Kulhawy, F. H Axial compression of footings in cohesionless soils. II: Bearing capacity. J. Geotech. Geoenviron. Eng., , Discussion of Behavior of Pile Groups Subject to Excavation-Induced Soil Movement in Very Soft Clay by D. E. L. Ong, C. F. Leung, and Y. K. Chow October 2009, Vol. 135, No. 10, pp DOI: / ASCE GT Francesco Castelli, Ph.D. 1 1 Researcher of Soil Mechanics, Dept. of Civil and Geoenvironmental Engineering, Univ. of Catania, Italy. fcastelli@dica.unict.it Piles surrounded by moving soil are relevant to many engineering applications, and several numerical and experimental studies have been conducted to analyze the behavior of piles subjected to soil movements. The origin of these movements can be related to excavation-induced soil displacement behind a retaining wall but also to liquefaction phenomena, basement excavation works for 110 / JOURNAL OF GEOTECHNICAL AND GEOENVIRONMENTAL ENGINEERING ASCE / JANUARY 2011

3 Fig. 1. Measured free-field lateral soil movements at different depths at 3 m behind the wall for Test 2 new buildings, embankment construction, and landslides. Soil movement significantly affects pile response, and the authors are to be congratulated for their contribution on the evaluation of the induced bending moment and deflection profiles because these characteristics are very important for the structural design of piles. It has been observed that the behavior of an individual pile in a group is different from that of a single pile because of the interaction between piles in the group through the surrounding soil. Brown et al. 1988, for example, introduced the term shadowing to indicate the phenomenon in which the lateral soil resistance on a pile in a trailing row is reduced because of the presence of the leading piles. In the paper, the authors report that in very soft clay the induced bending moment on an individual pile in a free-head pile group is always smaller than that on a corresponding single pile located at the same distance behind a retaining wall. This is attributed to the shadowing and reinforcing effects of other piles within the group. With a capped-head pile group, the induced bending moment on the front piles is moderated by the rear piles through the pile cap. To account for these pile group shadowing and reinforcing effects, the authors suggest a semiempirical soil movement moderation factor k s to correct the free-field soil movements. This suggestion seems to be in agreement with similar moderation factors employed by other researchers Maugeri et al. 1994; Lim 2001 in their analyses on pile subject to soil movements. The discusser, in the numerical simulation of the behavior of a pile group surrounded by moving soil Maugeri and Castelli 1999, proposed a reduction of the measured free-field soil movements because the interaction between a row of piles and the moving soil can itself reduce the soil movement. The amount of the reduction of soil movement must be related to pile diameter, pile spacing, and soil behavior. The soil movement will in turn displace the pile by a certain amount depending on the relative stiffness between the pile and the soil. Fig. 2. Comparison between measured and computed bending moment profiles for free-head single pile Test 2 and 4-pile group Test 12 at 3 m behind the wall The effect caused by a moving soil can be considered by taking into account the relative movement between the soil and the pile. If the soil mass moves and the pile movement y p is less than the soil movement y s, the soil exerts a driving force on the pile. If the pile movement y p is greater than the soil movement y s, the soil provides the resistance force p lim to the pile. The response of the soil-pile system can then obtained by the following governing differential equation: EI d4 y dz 4 p y p y s =0 1 where EI=pile stiffness; p=soil reaction per unit pile length; and z=depth. This equation can be solved by a numerical procedure based on a finite-element discretization, in which the pile load force per unit area FL 2 owing to relative pile-soil movement y p y s can be represented by a series of p-y curves on both sides of the pile shaft along its length. To take into account that the lateral pile response is nonlinear, the following p-y relationship could be adopted Castelli 2002; Castelli and Maugeri 2009 : p z = y p z y s z 1 E si z + y p z y s z p lim z where E si FL 2 =initial modulus of horizontal subgrade reaction; p and p lim FL 2 =mobilized and ultimate horizontal soil resistance, respectively. The hyperbolic p-y relationship in Eq. 2 is defined by the two parameters p lim and E si. In a single pile-soil interaction, the ultimate horizontal soil resistance p lim can be evaluated according to the well-known formulas existing in literature Matlock 1970; Poulos and Davis 1980, both for cohesive Broms 1964b and for cohesionless soils Broms 1964a. As far as the initial modulus of horizontal subgrade reaction E si is concerned, the relationships 2 JOURNAL OF GEOTECHNICAL AND GEOENVIRONMENTAL ENGINEERING ASCE / JANUARY 2011 / 111

4 proposed by Matlock 1970, Welch and Reese 1972, and Robertson et al for hyperbolic p-y curves can be used. According to Poulos and Davis 1980 and Castelli and Maugeri 2009, the modulus can also be evaluated using empirical relationships with the undrained shear strength c u, assuming that E si ranges from 150 to 400 c u. This numerical model has been used to back-analyze the centrifuge model test data on the free-head single pile Test 2 and the pile group Test 12 subject to the measured free-field lateral soil movements induced at 3 m behind the wall by excavation Fig. 1. Fig. 2 shows the comparison between the computed and measured pile bending moment profile of the front 3 m behind wall single pile and pile group. The preexcavation soil strength profile shown has been employed in the back-analysis. For the free-head 4-pile group Test 12, a soil movement moderation factor of 0.8 was required to achieve a reasonable agreement for the bending moment profile. This result seems to be in agreement with the range of variation proposed for the empirical soil moderation factor k s, considering that to correct the measured free-field soil movements the authors propose the value 0.7 for a 4-pile group. Broms, B. B. 1964a. Lateral resistance of piles in cohesionless soils. J. Soil Mech. and Found. Div., 90 3, Broms, B. B. 1964b. Lateral resistance of piles in cohesive soils. J. Soil Mech. and Found. Div., 90 2, Brown, D. A., Morrison, C., and Reese, L. C Lateral load behavior of pile group in sand. J. Geotech. Geoenviron. Eng., , Castelli, F Discussion of Response of laterally loaded largediameter bored pile groups by C. W. W. Ng, L. Zhang, and D. C. N. Nip. J. Geotech. Geoenviron. Eng., , Castelli, F., and Maugeri, M Simplified approach for the seismic response of a pile foundation. J. Geotech. Geoenviron. Eng., , Lim, J. K., Behavior of piles subject to excavation-induced soil movement. MEng thesis, National Univ. of Singapore, Singapore. Matlock, H Correlations for design of laterally loaded piles in soft clay. Proc. II Offshore Technical Conf., Houston, TX, Maugeri, M., and Castelli, F Discussion of Piles subjected to lateral soil movements by L. T. Chen and H. G. Poulos. J. Geotech. Geoenviron. Eng., 125 6, Maugeri, M., Castelli, F., and Motta, E Analysis of piles in sliding soil. Proc., 3rd Int. Conf. on Deep Foundation Practice Incorporating Pile Talk, CI Premier Pte Ltd., Singapore, Poulos, H. G., and Davis, E. H Pile foundation analysis and design, Wiley, New York. Robertson, P. K., Davies, M. P., and Campanella, R. G Design of laterally loaded driven piles using the flat dilatometer. Geotech. Test. J., 12 1, Welch, R. C., and Reese, L. C Laterally loaded behavior of drilled shafts. Research Rep. n , Center for Highway Research, Univ. of Texas. Closure to Behavior of Pile Groups Subject to Excavation-Induced Soil Movement in Very Soft Clay by D. E. L. Ong, C. F. Leung, and Y. K. Chow October 2009, Vol. 135, No. 10, pp DOI: / ASCE GT D. E. L. Ong 1 ; C. F. Leung 2 ; and Y. K. Chow, M.ASCE 3 1 Senior Lecturer, School of Engineering, Computing and Science, Swinburne Univ. of Technology, Sarawak Campus, Kuching, Sarawak, Malaysia. elong@swinburne.edu.my 2 Professor, Centre for Soft Ground Engineering, Civil Engineering Dept., National Univ. of Singapore, Singapore, cvelcf@ nus.edu.sg 3 Professor, Centre for Soft Ground Engineering, Civil Engineering Dept., National Univ. of Singapore, Singapore, cvechow@ nus.edu.sg The authors thank the discusser, F. Castelli, for his interest in their research study. Castelli presented a numerical method to predict the response of a pile with an imposed soil movement profile. The soil movement would in turn displace the pile by a certain magnitude depending on the relative stiffness between the pile and the soil. The numerical approach used by Castelli is essentially similar to that used by the authors see Chow and Yong 1996 with the main difference being the form of the p-y curves that describe the behavior of the soil. Chow and Yong 1996 used an elastic perfectly plastic p-y curve, whereas Castelli used a hyperbolic p-y curve. In Castelli s discussion, he back-analyzed the response of a single pile in Test 2 of the original paper and Ong et al and a front peripheral FP individual pile within a 4-pile group in Test 12 of the paper. Both of these piles are located 3.5 m behind the retaining wall, and owing to the excavation, the free-field soil movement profile that occurred is as given in Fig. 8 of the original paper. Preexcavation undrained shear strength is used in the back-analysis as this is usually the case in real-life situations. The results of Castelli s numerical analysis show reasonably good agreement with the back-analyses performed by the authors for the two cases mentioned above. In both the back-analyses performed by Castelli and the authors, a soil moderation or correction factor was not used for the single pile analysis in Test 2. However, a moderation factor of 0.8 and 0.7 was used by Castelli and the authors, respectively, for Test 12. The difference between the two proposed factors is considered too small to cause any large variation in the pile responses. The pile bending moment responses predicted by Castelli and the authors show reasonably close agreement, especially in terms of the profiles and magnitudes. This demonstrates that a soil moderation factor applied to the free-field soil movement is indeed required in the back-analysis using this type of numerical model to account for the pile reinforcing and shadowing effects in a pile group. Soil reinforcing effect can be physically observed in the development of soil arching in this study. Fig. 7 in the original paper shows the top view of the ground surface taken after the experiment of Test 12. The phenomena of soil arching and separation are evident, and these suggest that the clay in this upper clay layer may have yielded. This soil arching behavior is consistent with the numerical study by Chen and Martin It is believed that the development of this yield or plastic zone at the upper layer of 112 / JOURNAL OF GEOTECHNICAL AND GEOENVIRONMENTAL ENGINEERING ASCE / JANUARY 2011

5 the clay starts on both sides of the pile and extends to the front of the pile when the soil deformation increases as a result of excavation. This phenomenon can be further explained by the fact that the limiting soil pressure or the ultimate soil resistance at upper layers are typically much lower owing to near-surface effects, as suggested by Poulos 1995 and Pan et al Careful observation from Fig. 8 in the original paper also revealed the existence of a near-surface effect; the soil at the upper 2 m is observed to have moved more than the pile deflection beyond an excavation depth of 0.5 m. The reinforcing effect of a pile group can also be enhanced by capping the individual pile heads. By capping a pile group, the individual pile heads are forced to act in unison when subject to different magnitudes of soil movement. The front pile, which experiences a greater effect of soil movement, will be moderated by the rear pile through the pile cap. The interaction between the front and rear piles induces negative bending moment at the pile head but reduces the magnitude of bending moment developed along the pile and, subsequently, the pile group deflection. Shadowing effect within individual piles in a group is demonstrated in Fig. 13 in the original paper, which shows the magnitudes of single pile and pile group head deflections. The magnitudes of pile head deflection reduce as the size of the pile group increases and the shadowing effects become more significant. The relative position of an individual pile in a group is also important. For example, peripheral piles can effectively shield the center piles at the same distance behind the wall from the full exposure of soil movement, and thus, smaller bending moment is developed in the shielded piles. Castelli has further reaffirmed the approach used by the authors with his back-analysis of the experimental results. The implication for the design is that one has to be pragmatic when considering pile group shadowing and reinforcing effects by carefully factoring down the magnitudes of free-field soil movement obtained via inclinometer readings to prevent overconservatism when designing pile groups subject to soil movement. The original paper has adopted a practical approach by reducing the soil movement in an average sense as a function of the group size by applying the soil moderation factor to the entire free-field soil movement profile. Also, as the size of the pile group increases, the average distance from the center of the pile group to the wall increases. Hence, it is intuitive that the soil moderation factor reduces with increasing distance from the wall, depicting the reduction in soil movement at greater distance. By numerical back-analysis, the magnitude of the soil moderation factor is established to be 0.8 for a 2-pile group, 0.7 for a 4-pile group, and 0.5 for a 6-pile group in clay for this study. Nonetheless, this study is limited to pile groups exposed to soil movement within working limits when the soil displacements are reasonably small. Large-strain soil deformation or soil flow phenomena owing to failure of a retaining wall, as discussed by Leung et al. 2006, which places more consideration on limiting soil pressure on group piles, shall be discussed in a future paper. In conclusion, the reasonably close agreement of back-analysis results by Castelli and the authors can be attributed to the understanding of the following important points: 1. The choice of Young s modulus of soil can be estimated assuming that E ranges from 150 to 400 c u Poulos and Davis 1980; Chow and Yong 1996; Castelli and Maugeri 2009, where c u is the in situ undrained shear strength. This parameter is especially important given that this study is limited to piles exposed to soil movement within working limits when the soil displacements are reasonably small. 2. The limiting soil pressure p y or p lim can be evaluated using the method proposed by Poulos and Davis 1980, among others. However, this is not a critical issue in this study as the imposed soil movements are generally still within working limits. 3. The presence of pile reinforcing and shadowing effects in a pile group must be acknowledged. 4. Single pile analysis does not require any correction for the free-field soil movement. 5. A soil correction or moderation factor should be applied to free-field soil movement involving a pile group owing to the shadowing and reinforcing effects of other piles in the group. Castelli, F., and Maugeri, M Simplified approach for the seismic response of a pile foundation. J. Geotech. Geoenviron. Eng., , Chen, C. Y., and Martin, G. R Soil-structure interaction for landslide stabilizing piles. Comput. Geotech., 29, Chow, Y. K., and Yong, K. Y Analysis of piles subject to lateral soil movements. J. Int. Eng. Singapore, 36 2, Leung, C. F., Ong, D. E. L., and Chow, Y. K Pile behavior due to excavation-induced soil movement in clay. II: Collapsed wall. J. Geotech. Geoenviron. Eng., 132 1, Ong, D. E. L., Leung, C. F., and Chow, Y. K Pile behavior due to excavation-induced soil movement in clay. I: Stable wall. J. Geotech. Geoenviron. Eng., 132 1, Pan, J. L., Goh, A. T. C., Wong, K. S., and Teh, C. I Ultimate soil pressures for piles subjected to lateral soil movements. J. Geotech. Geoenviron. Eng., 128 6, Poulos, H. G Design of reinforcing piles to increase slope stability. Can. Geotech. J., 32 5, Poulos, H. G., and Davis, E. H Pile foundation analysis and design, Wiley, New York. JOURNAL OF GEOTECHNICAL AND GEOENVIRONMENTAL ENGINEERING ASCE / JANUARY 2011 / 113