DISCUSSIONS AND CLOSURES

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Discussion of “Effect of Gas on PorePressure in Wet Landfills” by Scott Merry,Wolfgang Fritz, Muniram Budhu, andKrzysztof JesionekMay 2006, Vol. 132, No. 5, pp. 553–561.DOI: 10.1061/�ASCE�1090-0241�2006�132:5�553�

Xiong Zhang1 and Mingjiang Tao2

1Alaska University Transportation Center, Dept. of Civil and Environ-mental Engineering, Univ. of Alaska Fairbanks, Duckering BuildingRoom 251, Fairbanks, Alaska 99775-5900. E-mail: ffxz3@uaf.edu

2Assistant Professor, Dept. of Civil and Environmental Engineering,Worcester Polytechnic Institute, Worcester, MA 01609. E-mail:taomj@wpi.edu

This paper presented a numerical solution to an interesting phe-nomenon that excess pore water pressure is caused by the gasgenerated in wet municipal landfills. The proposed equivalentfluid weight has valuable applications in evaluating the slope sta-bility of wet landfills. However, it is discussers’ opinion that it isnot necessary to use finite-difference method for calculating theexcess pore pressure. In this discussion, a close-form solution tothe same problem is proposed and some discussions will bepresented.

With the assumptions used in the paper, the landfill can bemodeled as a saturated soil column with a water chamber and agas chamber at the bottom of the column �see Fig. 1�. Waterbelow the soil column is squeezed out due to gas generation. Thevolume of the gas in the gas chamber is initially zero and willincrease as more gas is generated at the bottom of the column.

In the paper it is assumed that the model is one dimensional,the landfill above the bottom control stack is always saturated,and both the skeleton and the water are assumed to be incom-pressible. Consequently, the governing differential equation forthe water flow in the saturated landfill with rigid skeleton can beexpressed as follows

�2h

�z2 = 0 �1�

where h=hydraulic head; and z=coordinate in the vertical direc-tion. The left-hand side of Eq. �1� represents the difference inwater flowing in and out of a soil element while the right-handside of Eq. �1� represents the rate of change in water storage,which is equal to zero since both the soil skeleton and water areassumed to be incompressible. Integrating Eq. �1�, one can have

�h

�z= C1 �2�

where C1 should be negative since the resulted water flow isupward. It can also be noticed from Eq. �2� that the absolute valueof C1 is equal to the hydraulic gradient driving the water flow out

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of the waste landfills. Integrating Eq. �2�, one can have

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h = C1z + C2 �3�

C1 and C2 in Eqs. �2� and �3� are integration constants. Accordingto Bernoulli’s equation and considering assumption 5 in the paper,h=z+uf /� fluidg, which gives

uf = � fluidg��C1 − 1�z + C2� �4�

where uf =pore-fluid pressure. The water table is at the top of thesoil column �z=0� and the corresponding absolute water pressureis equal to the atmospheric pressure �uf = Patm�, which gives

C2 =Patm

� fluidg�5�

At the bottom of the soil column �z=−L�, the corresponding porewater pressure is

uf = � fluidgL�1 − C1� + Patm �6�

The volume of water flowing out of the soil column is equal to

Vf = vAt = − Kwaste,satiAt = − Kwaste,sat

�h

�zAt = − Kwaste,satC1At

�7�

Fig. 1. Schematic diagram of column of saturated control volumeswith gas being created in only the bottom of the soil column

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Now let us consider the gas generation process. The water

flow is due to the gas generation, and therefore the volume of thegenerated gas is equal to the volume of water flow out of the soil�V=−Kwaste,satC1At�. The gas pressure is equal to the pore-waterpressure at the bottom of soil column �P=� fluidgL�1−C1�+ Patm�since the pressure in the gas bubble is considered to be still inequilibrium with the surrounding leachate. The number of molesof gas within the bottom of the soil column can be expressed as

n = Qg�waste�x�y�z�gas

MWt �8�

Fig. 2. Comparison between authors’ and discussers’ solutions on thedependence of equivalent fluid unit weight: �a� density of the waste;�b� gas flux rate; and �c� saturated hydraulic conductivity of the waste

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u

The ideal gas law is considered to be still applicable to the gen-erated gas �PV=nRT�, which gives

− �� fluidgL�1 − C1� + Patm�Kwaste,satC1At = Qg�waste�x�y�z�gas

MWRTt

�9�

From Eq. �9�, two roots of Eq. �9� can be obtained, and the nega-tive one is the solution of interest

Fig. 3. Dependence of equivalent fluid unit weight on: �a� depth ofthe waste generating gas; and �b� thickness of landfills

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C1 =

�� fluidgL + Patm� −��� fluidgL + Patm�2 + 4� fluidgLQg�waste�z�gas

MWKwaste,satRT

2� fluidgL�10�

N

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As can be seen from Eq. �4�, three distinct components contributeto the absolute pore-water pressure versus depth: the atmosphericpressure constant with depth, the linearly increasing static fluidpressure due to the gravity, and the linearly increasing excesspore-water pressure due to the formation of gas. This is consistentwith the results in the paper. The solution does not have the sin-gularity mentioned in the paper since the water flow reachessteady state instantaneously. It can also be noticed that the equiva-lent unit weight proposed in the paper actually represents theslope of the absolute pore-water pressure versus depth; that is

� fluid,equivalent = � fluidg�1 − C1� �11�

From Eqs. �10� and �11�, it can be seen that factors influencing theequivalent unit weight include: � fluid, �waste, �gas, L, Qg, Kwaste,sat,and �z. Fig. 2 indicates that the results calculated by the close-form solution Eq. �11� are the same as those obtained in the paper.�z is taken as 1 m for the calculation in Fig. 2.

In the original paper, the influence of �z and L on the excesspore-water pressure is not discussed. In fact, the values of �z andL have great influence on the equivalent unit weight and thus onthe excess pore water pressure. Besides, the influence of �z and Lon the excess pore-water pressure will have an important impli-cation on the design of wet landfills. In the paper, it is apparentthat no distinction is made between the element size of finite-difference method and �z. The physical meaning of �z is thedepth of the waste mass below the saturated soil column thatgenerates gas while L is overlaying thickness of the landfillsabove the bottom waste layer generating the gas. The bigger thevalue of �z, the higher the generated excess pore water pressure,which is illustrated in Fig. 3�a�. Similarly, the larger the value ofL �the thicker the overlying layer�, the smaller the excess pore-water pressure will be resulted, as illustrated in Fig. 3�b�.

Conclusion

A simple close-form solution to the phenomenon in the originalpaper is presented in this discussion, with which the factors thatsignificantly affect the equivalent fluid unit weight and the excesspore-water pressure can be readily identified and studied. Theinfluence of other two important factors ��z and L� on the equiva-lent fluid unit weight is also added.

Acknowledgments

The discussers thank gratefully for the first author for providingthe information regarding the paper and his valuable discussion.The discussers also thank Mr. Rohit Raj Pant for his valuable

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Closure to “Effect of Gas on PorePressures in Wet Landfills” by Scott Merry,Wolfgang U. Fritz, Muniram Budhu, andKrzysztof JesionekMay 2006, Vol. 132, No. 5, pp. 553–561.DOI: 10.1061/�ASCE�1090-0241�2006�132:5�553�

Scott Merry1; Wolfgang U. Fritz2; Muniram Budhu3; andKrzysztof Jesionek4

1Senior Geotechnical Engineer, Kleinfelder, Inc., 2015 N. Forbes Road,Suite 103, Tucson, AZ 85745. E-mail: smerry@kleinfelder.com

2Project Engineer, NCS Consultants, LLC, 640 W. Paseo Rio Grande,Tucson, AZ 85737. E-mail: wufritz@msn.com

3Professor, Dept. of Civil Engineering and Engineering Mechanics, Univ.of Arizona, 1209 E. Second St., Tucson, AZ 85721. E-mail: budhu@u.arizona.edu

4Associate, GeoSyntec Consultants, Inc., 475 14th Street, Suite 450,Oakland, CA 94612-1940. E-mail: kjesionek@geosyntec.com

The writers would like to thank the discussers for their interest inthe paper and their further evaluation of the equations.

The writers are intrigued by the agreement between the closed-form and finite-difference solutions for the equivalent fluid unitweight, which varied by less than 0.001 kN·m3 for input valuesof engineering interest. The finite-difference method, a forwarddifference Euler method in the original paper, is often thought ofas an approximate method. However, the agreement by these twosolutions in determining the equivalent unit weight demonstratesthat using simple programming in a common spreadsheet canprovide an approximation that does not lack in computationalaccuracy, at least in terms of engineering significance. Hence, webelieve that the finite-difference method will be a very powerfultool in providing solutions to the more complex models that wecontinue to investigate.

The discussers also state that the values of �z and L have greatinfluence on the equivalent unit weight. While we acknowledgethat there is dependence, we do not agree that their influenceis great. Reasonable variations in �z result in a range of theequivalent unit weight of about 9.8–11.5 kN·m3 while variationsin the height of the saturated column of waste, L result in a rangeof equivalent unit weight of 10.2–10.9 kN·m3. As pointed out inthe original paper, variation in the density of waste provides arange in the resulting equivalent unit weight of about10.1 to 10.6 kN·m3 �see Fig. 4�a��, which is similar in magnitudeto that created by variations in L or �z. Rather, the two param-eters that result in significant differences of the equivalent unitweight are the gas generation rate and the hydraulic conductivityof the waste �see Figs. 4�b� and 4�c��. Variations of engineeringsignificance in these two parameters result in wide variations inthe equivalent unit weight. Hence, these two parameters are ofprimary engineering interest and should be the focus of furtherstudies.

Erratum

An erratum to the paper is the gas generation rate presented inTable 1 as 0.037 m3/kg/sec. Numerically, this gas generation ratecorresponds to a yearly rate; therefore, the proper gas generation

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rate value should be 1.173�10 mgas /kgwaste / sec.

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Discussion of “Particle Shape Effectson Packing Density, Stiffness, andStrength: Natural and Crushed Sands”by Gye-Chun Cho, Jake Dodds, andJ. Carlos SantamarinaMay 2006, Vol. 132, No. 5, pp. 591–602.DOI: 10.1061/�ASCE�1090-0241�2006�132:5�591�

Man T. Bui1 and Jeffrey A. Priest21Ph.D. Student, School of Civil Engineering and the Environment, Univ.

of Southampton, Highfield, Southampton, SO17 1BJ, U.K. E-mail:Man.Bui@soton.ac.uk

2Senior Research Fellow, School of Civil Engineering and theEnvironment, Univ. of Southampton, Highfield, Southampton, SO171BJ, U.K. E-mail: J.A.Priest@soton.ac.uk

In their paper entitled “Particle Shape Effects on Packing Density,Stiffness, and Strength: Natural and Crushed Sands,” the authorshighlight the results of tests conducted �using bender elementswithin an oedometeric cell� to measure shear wave velocity, Vs, asa function of applied mean effective stress, ��, for a variety ofsand specimens with differing particle shapes. By plotting Vs

against �� and applying Eq. �3� �p. 596 of the original paper� tothe data points, the authors obtain values for �, defined as Vs at1.0 kPa, and the � exponent reflecting the sensitivity of Vs to ��.The values of � and � are then plotted as functions of roundness,sphericity, and regularity. The resulting plots are used by the au-thors to suggest that there is a correlation between �, �, andparticle shape, leading to the conclusion that small strain stiffnessdecreases with irregularity.

However, the following discussion suggests that the author’sconclusion may not be justified. As is well known, the small strainstiffness of soils is significantly influenced by void ratio. As earlyas 1963, Hardin and Richart suggested that � parameter of bothOttawa sand �rotund sand� and crushed quartz sand �extremelyangular sand� was a linear function of void ratio. Extensive re-search has been undertaken over the last 40 years into the factorsaffecting the small strain stiffness of soils and has shown thatalong with �� and shear strain amplitude, void ratio is also animportant parameter. However, there seems to be no mention of avoid ratio correction factor that has been used by other research-ers �Hardin and Richart 1963; Hardin and Drnevich 1972;Kokusho et al. 1982; Jamiolkowski et al. 1995; Lo Presti et al.1997; Shibuya et al. 1997� to eliminate this effect from thoseobserved. It has also been shown that the � exponent is also afunction of void ratio �Clayton et al. 2005�. This suggests thatalthough the correlation observed by the authors is real, the causeof this may not be attributable to particle shape but instead tovariations in void ratio.

Reworking the authors’ data, the shear wave velocity, Vs, ofthe sand samples �in Table 1, p. 592� has been back calculated fortwo arbitrary values of �� �e.g., 100 kPa and 400 kPa� by usingthe values of � and � given by the authors, and Eq. �3� of thepaper. The values of Vs have then been plotted against their cor-responding values of roundness, sphericity and regularity. Theplots �Fig. 1� suggest that the correlations between Vs and particlecharacteristics are limited. The small coefficients of determination�R2=0.3295 at 100 kPa and R2=0.1054 at 400 kPa� in Fig. 1�c�indicate a poor correlation between particle regularity and Vs. It isalso difficult to conclude that there is a correlation betweenroundness and Vs, which increases with roundness smaller than

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�Fig. 1�a��. Similarly, values of Vs do not tend to increase withsphericity. In fact, Fig. 1�b� even shows that Vs at 400 kPa de-creases with sphericity. It is noted that the irregular data of PonteVedra and Jekyll Island sands have been excluded and are notplotted in Fig. 1. It therefore appears to the writers that the shearwave velocity data presented by the authors may be a functionprimarily of void ratio variation, and may in fact �on the basis ofthe evidence presented� be poorly correlated with the particleshape data. We would be grateful for the authors’ comments.

References

Clayton, C. R. I., Priest, J. A., and Best, A. I. �2005�. “The effects ofdisseminated methane hydrate on the dynamic stiffness and dampingof a sand.” Geotechnique, 55�6�, 423–434.

Hardin, B. O., and Drnevich, V. �1972�. “Shear modulus and damping insoils: measurement and parameter effects.” J. Soil Mech. and Found.Div., 98�6�, 603–624.

Hardin, B. O., and Richart, E. F., Jr. �1963�. “Elastic wave velocities ingranular soils.” J. Soil Mech. and Found. Div., 89�1�, 33–65.

Jamiolkowski, M., Lancellotta, R., and Lo Presti, D. C. F. �1995�. “Re-marks on the stiffness at small strains of six Italian clays.” Pre-failureDeformation of Geomaterials, Vol. 1, S. Shibuya, T. Mitachi, and S.Miura, eds., Balkema, Rotterdam, The Netherlands, 817–836.

Kokusho, T., Yoshida, Y., and Esashi, Y. �1982�. “Dynamic properties ofsoft clay for wide strain range.” Soils Found., 22�4�, 1–18.

Fig. 1. Shear wave velocity versus particle shape; values of Vs com-puted from the values of � and � provided in Table 1 with Eq. �3�from original paper

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Lo Presti, D. C. F., Jamiolkowski, M., Pallara, O., Cavallaro, A., and

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Pedroni, S. �1997�. “Shear modulus and damping of soils.” Geotech-nique, 47�3�, 603–617.

Shibuya, S., Hwang, S. C., and Mitachi, T. �1997�. “Elastic shear modu-lus of soft clays from shear wave velocity measurement.” Geotech-nique, 47�3�, 593–601.

Closure to “Particle Shape Effectson Packing Density, Stiffness, andStrength: Natural and Crushed Sands”by Gye-Chun Cho, Jake Dodds, andJ. Carlos SantamarinaMay 2006, Vol. 132, No. 5, pp.591–602.DOI: 10.1061/�ASCE�1090-0241�2006�132:5�591�

Gye-Chun Cho1; Jake Dodds2; andJ. Carlos Santamarina3

1Associate Professor, Dept. of Civil and Environmental Engineering,Korea Advanced Institute of Science and Technology �KAIST�,Daejeon 305-701, Republic of Korea �corresponding author�. E-mail:gyechun@kaist.edu

2Civil Engineer, National Resources Conservation Service, Price,UT 84501.

3Professor, School of Civil and Environmental Engineering, GeorgiaInstitute of Technology, Atlanta, GA 30332. E-mail:carlos.santamarina@ce.gatech.edu

The discussers suggest an alternative interpretation of the shearwave velocity versus stress data, taking into consideration pack-ing density and void ratio correction functions f�e�. The link be-tween the writers’ and discussers’ approaches is identified in thefollowing analysis, which augments the discussion presented inthe original manuscript �p. 597�:• Low perturbation shear wave propagation through a soil mass

is a small-strain, constant fabric phenomenon. Therefore, it is a“measure of state” �i.e., contact stiffness and fabric�.

• When stresses are increased, the contact stiffness increases aswell �i.e., flatter contacts, hertzian-type behavior�. In addition,fabric changes take place if the strain level exceeds the elasticthreshold strain. Therefore, the change in shear wave velocityreflects the increase in contact stiffness and fabric changes.

• At constant fabric, the �-exponent captures the stress sensitiv-ity to contact stiffness. For uncemented coarse grains, the ex-ponent may range from �=1/6 for smooth spherical particles�Hertz contact� to �=1/4 for cone-to-plane contacts, as inangular particles.

• If the coordination number increases when the effective con-fining stresses increase, the �-exponent exceeds the values in-dicated above by as much as �0.05 or even �0.1. Therefore,higher �-exponents are expected for the more angular particlesgiven their wider range in emax-to-emin and higher compress-ibility that suggest a more pronounced increase in coordinationupon loading. �Note: To minimize the effect of changes in voidratio, all specimens were tested under dense conditions �rela-tive density �90%�; refer to original paper�.

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One may attempt to correct Vs-�� data for void ratio changesto remove the contribution of fabric changes to the �-exponent.However, one should determine the correction function f�e� foreach soil. Selecting a single function for all soils adds noise to theanalysis and weakens correlations. Therefore, we opted for ana-lyzing the data without f�e� corrections, and to retain void ratioeffects in both � and � parameters.

Finally, the complete statement of the conclusion �pp. 597and 600� indicates that particle irregularity leads to “decreasein small-strain stiffness ��-coefficient�, yet increased sensitivityto the state of stress ��-exponent�.” Recognizing that the�-coefficient implies low confining stress �1 kPa, Eq. �3��,both observations are supported by the figure presented by thediscussers.

Discussion of “Raked Piles—Virtues andDrawbacks” by Harry G. PoulosJune 2006, Vol. 132, No. 6, pp 795–803.

DOI: 10.1061/�ASCE�1090-0241�2006�132:6�795�

William J. Neely, P.E., M.ASCE1

1Vice President, The Reinforced Earth Company, 1660 Hotel CircleNorth, Suite 304, San Diego CA 92108. E-mail: wneely@reinforcedearth.com

The author is commended for drawing attention to the influenceof vertical and/or lateral ground movements on the behavior ofraked piles through the use of an elegant parametric study. Thisdiscussion presents the results of field measurements of axialstrains in a series of vertical and raked precast concrete pilessubjected to vertical ground movements associated with consoli-dation of a thick clay layer. Consolidation of the clay layer wasdue to a temporary surcharge fill.

The field measurements presented in this discussion wereoriginally presented as part of lecture notes on the use of preload-ing techniques in the development of sites underlain by soft soils.The notes were prepared by Dr. P. Davies �Davies 1978� for alecture series in South Africa in the late 1970s. As far as thediscusser is aware, the information relating to the performance ofvertical and raked piles subjected to vertical ground movementssummarized here has not been formally published.

The test piles were installed at a site where soil conditionscomprise about 10 m of sand underlain by a layer of clay approxi-mately 20 m thick �see Fig. 1�. A 3.5 m high surcharge fill wasused to accelerate consolidation of the clay. The test piles com-prised 1,000-kN capacity precast concrete piles installed in pairs.Two piles were installed vertically, two were installed at a rake of1 in 4 �rake angle of 14.0°� and two piles were installed at a rakeof 1 in 8 �rake angle of 7.1°�. One pile in each pair was coatedwith a bitumen slip-coat over the upper 15 m of its length in orderto determine the effectiveness of the slip-coat in reducing down-drag loads due to negative skin friction.

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Each test pile also included a central inclinometer tube andfive sets of acoustic strain gages �denoted as Groups 1 through 5in Fig. 1� arranged to permit determination of axial forces andbending moments in the piles. Other instrumentation includedborehole extensometers to measure the distribution of verticalmovements within the clay layer as it compressed under theweight of the surcharge fill.

The fill was placed in early 1974 and was removed about18 months later. Data from the three arrays of boreholes exten-someters are shown in Fig. 2, where it can be seen that most ofthe vertical ground movement is due to compression within theupper 6 m of clay. The development of settlement with time forone of the vertical arrays of borehole extensometers is presentedin Fig. 3.

The response of the two vertical piles to the ground move-ments in the clay layer is illustrated in Fig. 4. As expected, sig-nificant axial loads developed in both piles as a result of thecompression of the clay layer, although for the slip-coated pile theaxial load was only about one-third of that for the uncoated pile.

Fig. 3. Typical time-settlement behavior

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Fig. 1. Soil conditions, test piles, and instrumentation

Fig. 2. Profiles of vertical ground movements

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Also, the removal of the surcharge resulted in the development ofaxial tension in the uncoated pile, as the direction of movementbetween pile and soil was reversed.

Fig. 5 shows the development of axial compression loads inthe two piles installed at a rake of 1 in 8. It can be seen that theaxial load in the coated pile �pile #1� is practically the same as forthe uncoated pile, demonstrating that coatings designed to mini-mize negative skin friction may not be very effective on rakedpiles installed through soils undergoing vertical movements. Theaxial loads developed in the two piles raked at 1 in 8 are greaterthan that measured in the uncoated vertical pile, confirming oneof the trends indicated by the author’s analysis that vertical loadsincrease with increasing rake angles for piles subjected to verticalground movements.

The axial compression in the piles increases even further as therake is increased to 1 in 4; see Fig. 6. In this case, the slip-coat

Fig. 4. Development of axial strain in vertical piles

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has no measurable effect on the axial load developed in responseto the vertical ground movement. Comparing the measured axialstrains in Figs. 5 and 6 indicates that increasing the rake anglefrom 7.1° �1 in 8� to 14.0° �1 in 4� resulted in an increase in axialcompression of about 50%.

For uncoated piles, the data in Figs. 4 and 6 suggest that, forthe case of vertical ground movements, the axial compression in apile installed at a 1 in 4 rake may be as much as four times thatdeveloped in a comparable vertical pile. Indeed, a common causeof foundation failure is due to the use of raked piles installedthrough soils that are still consolidating. As the soil flows arounda raked pile, the resulting lateral pressures on the pile shaft maybe sufficiently great to break the pile. Both the author’s analysisand Davies’ �1978� field measurements summarized here serve toemphasize the need for considerable caution when raked piles areinstalled where vertical ground movements are anticipated.

Fig. 5. Development of axial strains in piles raked at 1 in 8

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Closure to “Raked Piles—Virtues and

Drawbacks” by Harry G. PoulosJune 2006, Vol. 132, No. 6, pp. 795–803.

Harry G. Poulos1

1Senior Principal, Coffey Geotechnics, 8/12 Mars Rd., Lane Cove West,NSW Australia 2066. E-mail: harry�poulos@coffey.com.au

The writer is very grateful to Neely for making available theresults of the field tests that appear to have originated in SouthAfrica several years ago. These data yet again corroborate Terza-ghi’s views on the abiding value of field measurements. The datapresented by Neely demonstrate a number of points of practicalsignificance for pile design, including the following:1. Vertical ground movements can induce significant axial loads

in a pile;2. For vertical piles, the presence of a slip coating reduces these

axial loads considerably;3. For raked piles, the induced axial loads can be substantially

greater than for a vertical pile;4. For raked piles, the slip coating has a far less beneficial ef-

fect in reducing axial loads than it does for a vertical pile.It is gratifying that the field data presented by Neely are consis-

Fig. 6. Development of axial strains in piles raked at 1 in 4

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tion should be exercised if raked piles are to be used in situationswhere ground movements are likely to develop. It is the writer’sexperience that many foundation problems are caused by groundmovements that are not recognized. This can be a particular prob-lem where the foundation design is carried out by professionalswhose primary experience is not in geotechnical engineering andwho fail to recognize that loadings in piles caused by groundmovements can frequently be more severe that those due to con-ventional structural loadings. The apparent advantage of rakedpiles in increasing lateral stiffness can often be far outweighed bythe significant increases in axial force and bending moment in-duced in the raked piles by ground movements.

Discussion of “Termination Criteria forJacked Pile Construction and Load Transferin Weathered Soils” by L. M. Zhang,C. W. W. Ng, F. Chan, and H. W. PangJuly 2006, Vol. 132, No. 7, pp. 819–829.DOI: 10.1061/�ASCE�1090-0241�2006�132:7�819�

Feng Yu1 and Yun-Fang Yang2

1Lecturer, College of Civil Engineering and Architecture, Zhejiang Sci-Tech Univ., Hangzhou 310018, P.R. China. E-mail: pokfulam@zstu.edu.cn

2Associate Professor, College of Civil Engineering and Architecture,Zhejiang Sci-Tech Univ., Hangzhou 310018, P.R. China.

The authors have taken an effort to investigate the terminationcriteria of piles jacked into residual soils. Some results presentedby the paper seem questionable and discussion with the authors tomake further clarification would be helpful. The authors at-tempted to correlate the ultimate bearing capacity �i.e., Pult� withthe final jacking force �i.e., PJ�. Based on the data attained fromthe field test and other existing data, the authors proposed a re-gression equation as follows

Pult

PJ= 1.32 −

12.48

��1�

where �ratio of pile slenderness. Eq. �1� should have provideda convenient approach to decide the termination of jacking instal-lation. However, the relationship was derived inappropriatelywith respect of the following aspects.1. The authors only conducted two pile tests in residual soils in

Hong Kong. The remainder 142 data were extracted from thefield-test results in Guangdong Province in China �Lin andWang 2004�. The soil profiles in Hong Kong and Guangdongare not similar. Table 1 shows the soil profiles of three siteslocated in the Pearl River Delta in Gaungdong. Jacked pilesused in that area usually penetrate through a relatively thicksoft soil layer such as sludge to reach the bearing stratum,which is silty clay or silty sand for most cases. The groundconditions in Hong Kong are relatively stiff. Both the allu-vium and granitic saprolite are generally regarded as granularsoils.

2. The difference in ground condition and required pile’s capac-ity in Hong Kong and Guangdong draws toward a great dis-crepancy in the type and length of jacked piles as well as thecapacity of jacking devices. In China, including Guangdong,

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piles installed by jacking are mostly prestressed concrete

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pipe piles. Jacking machines with capacities of3,000–5,000 kN are generally adequate. Jacking very longconcrete piles is commonly unfavorable. The database col-lected by Lin and Wang �2004� mainly consists of concretepiles with lengths of 10–25 m. Most of the projects involv-ing pile-jacking in Hong Kong use long steel H-piles andlarge-capacity jacking machines, such as the instrumentedpile tests reported by Yang et al. �2006a,b�. The capacities ofthe jacking machines were 8,000–9,000 kN. The lengths ofthe piles ranged from 25 to 42 m, closely depending on thedepth and thickness of the bearing stratum.

Due to the great difference in the type of piles and thecharacteristic of embedded soils, jacked piles in Hong Kong

Table 1. Soil Profiles of Three Sites in Guangdong, China

Reference Superincumbent soil laye

Liang and Huang �2003�:Zhongshan, Guangdong

Fill

Silty clay

Sludgy soil

Coarse sand

Gravel

Zhuang and Xu �2002�:Shenzhan, Guangdong

Fill

Clayey sandy

Sandy clay

Completely decomposed gra

Li �1999�:Zhuhai, Guangdong

Sludgy soil

Silty sand

Fine sand

Sandy clay

Table 2. Resistances of Jacked Piles in Hong Kong and Guangdong

Description of soil

Lin and Wang �2004�:Guangdong, China

Silty sand

Fine sand

Medium sand

Coarse sand

CDG soils Clay

Sandy cla

Yu �2003�:Hong Kong

Alluvium

CDG soils

Notes: qs and qp denote the ultimate shaft resistance and toe resistancstatistically proposed with 95% confidence.

Table 3. Comparison of the Methods for Pile Tests in Hong Kong and C

JGJ 94–94 method, China

Loading sequence Normally one loading cycle; loading by an in�1/15–1/10� predicted Pult.

Termination of loading When �1� the settlement within one step of lofive times of the one within the previous stepsettlement within one step of loading exceedsthe one within the previous step, and the settunstable in 24 h.

Note: Q, Pd, A, E, L, and D represent the pile-head load, pile’s design c

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and Guangdong differ in capacities considerably. Table 2compares the ultimate shaft and toe resistances of jackedpiles in the two regions. The resistances measured inHong Kong are found much larger than the counterparts inGuangdong.

3. Determination of the ultimate bearing capacities also de-pends on the procedures and acceptance criteria of static loadtests. Table 3 compares the methods for pile-load tests in aslow maintained-load manner in China and Hong Kong. Dis-crepancies are found between the JGJ 94–94 �1995� methodand BD ordinance �GEO 1996�, which may cause differentbearing capacities determined for given final jacking forceand pile’s slenderness ratio.

Thickness of soil layer�m� SPT-N value

0.5–10.1 —

0–6.4 —

23.2–37.2 3.9

0–14.3 11.3

4.3–18.8 —

0.7–3.5 3–7

0.4–7.6 2–34

0.5–11.4 6.1–29

0.6–9.2 30–47.2

14.4–33.5 —

0–7.1 —

0–6.0 —

3.8–6.2 —

qs �kPa�

qp �MPa�

L16 m L�16 m

22–86 1.4–3.0 3.0–4.6

2.5–4.8 4.4–6.5

54–94 3.6–6.3 6.3–8.0

74–116 3.7–8.4 8.4–10.3

30–80 1.8–4.6 3.1–6.4

50–90 1.9–4.7 3.4–6.7

3.9–101.3 — —

102.2–377.5 14.8–57.1

ectively. The ultimate resistances cited by Lin and Wang �2004� were

BD ordinance, Hong Kong

nt of Three loading cycles; loading by an increment of 0.5Pd to1.0Pd at the 1st cycle and to 2.0Pd at the 2nd cycle and tofailure at the 3rd cycle; unloading at the end of each cycle;maintaining load at 2.0Pd for at least 72 h during the 2ndcycle.

exceeds� theimes ofis

When �1� the settlement within one step of loading exceedsQL /AE+D /120+4 �in mm�; or �2� the residual settlementupon unloading exceeds D /120+4 �in mm�.

, cross-sectional area, elastic modulus, embedded length, and equivalent

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In addition, the two regions use different approaches toderive pile’s design capacities �i.e., Pd� from Pult. Chinesetechnical codes employ the term of pile’s standard capacity,which is statistically derived from Pult through a series ofload tests. Pdstandard capacity divided by a coefficient ofintegrated subentries equal to 1.6–1.7. Engineers in HongKong commonly use a global factor of safety of at least 2.0to transfer Pult to Pd. Eq. �3� in the paper

Pd

PJ= 0.57 −

6.24

��2�

was established on the basis of a majority of test data inChina. However, this equation follows the approach adoptedin Hong Kong to determine Pd.

4. The termination criteria for jacked piles will affect their bear-ing capacities. The Technical Specification for Jacked Pilesin Guangdong recommends a termination criterion as fol-lows: �1� jacked piles should be subjected to repeated jackingto the final jacking force; the magnitude of PJ is in the rangeof 2.0–3.2Pd and the number of jacking cycles should be nomore than three; and �2� the duration of maintaining PJ ateach cycle is about 5–10 s. The termination criterion cur-rently adopted in Hong Kong has been described in thepaper. The two criteria deviate in the magnitude and durationof PJ as well as the number of jacking cycles.

The build-up of bearing capacity relies on the dissipation ofexcess pore pressure and the rehabilitation of soil disturbance,especially for jacked piles whose shaft frictions are considerable�Yang et al. 2006a�. The principle of soil mechanics indicates thatsoils ever undergoing higher pressure will deform less than thosewithout such experience during the subsequent loading process.An adequate duration of PJ can greatly reduce the residual settle-ment of jacked piles �Li et al. 2003�. Therefore, besides the mag-nitude of PJ, the duration of PJ and the number of jacking cycleswill influence the settlement of piles during load tests and conse-quently affect the bearing capacities.

Based on the fact described, one may conclude that jackedpiles and their application background in Hong Kong and Guang-dong are dissimilar. Combining case histories in the two regionsto obtain a unified regression equation is inappropriate. In thediscussers’ opinion, the relationship between Pult and PJ is com-plicated and closely associated with many factors rather thanmerely the ratio of pile slenderness. For example, Han �1996�reported two concrete square piles with the same dimensions�L=23.8 m, D=451 mm� jacked into the same ground of siltyclay. The measured values of Pult were both 800 kN while PJ wasrespectively, 750 and 900 kN. It implied that the magnitude ofPult / PJ was respectively 1.07 and 0.89 but the ratio of pile slen-derness remained the same to be 52.8.

The capacity of a jacked pile is essentially related to its finaljacking force. An experiential relationship in the form of Eq. �1�or Eq. �2� is attractive to engineers, since they can design thedimensions of pile and the required jacking force to achieve adesired capacity, or estimate the bearing capacity of a jacked pileaccording to its dimensions and penetration record. In fact, therehave been various recommendations in correlating the final jack-ing force to pile’s capacity in different regions in China, withinwhich the soil conditions and pile type are similar. For instance,in Shunde, Guangdong, such relationship has been suggested as

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�Zhang 2004�

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Pult

PJ= 1.25 −

12

��3�

and in Xi’an the relationship was described as below �Zhang2004�

Pd = 0.7603PJ − 315.6 �4�

A long list of such kind of relationships can be found. Whatshould be kept in mind is that all of them are for local use only.Presentation of a universal relationship with few parametersis impracticable since the application background variessignificantly.

China is one of the countries widely using the jack-piling tech-nique. However, the vast experience on jack piling in China couldonly be a reference to Hong Kong and direct combination of thetest results in the two regions would lead to misunderstanding. Arelationship suitable for Hong Kong demands accumulating morelocal construction experience and field-test database. One shouldbe prudent in suggesting such a format of relationship. A rationalapproach is to recommend a certain relationship between Pult andPJ applicable to a specified category, which may be classified bythe ground conditions, the type and dimensions of piles, the pro-cedures of load tests, as well as the termination criteria of jacking.To derive reasonable relationships, a great deal of field-test workshould be carried out. Careful classification of the results is alsorequired to state an explicit background for application.

References

GEO. �1996�. “Pile design and construction.” GEO publication No. 1/96,Hong Kong.

Han, X. J. �1996�. “Experimental research on static pressure and bearingcapacity of statically pressed piles.” J. Building Structures, 17�6�,71–77 �in Chinese�.

JGJ 94–94. �1995�. “Technical code for building pile foundations.” Chi-nese Academy of Building Science, Beijing, China �in Chinese�.

Li, K. S., Ho, N. C. L., Than, L. G., and Lee, P. K. K. �2003�. Casestudies of jacked piling in Hong Kong, Centre for Research and Pro-fessional Development, Hong Kong.

Li, X. R. �1999�. “Determination of the shear strength indices and bearingcapacity of soft soil in Hezui-Niupo Bay of Zhuhai.” GuangdongGeology, 14�1� �in Chinese�.

Liang, Y. W., and Huang, H. �2003�. “Geotechnical engineering assess-ment for foundation of Shakou Bridge under reconstruction in Zhong-shan City.”Guangdong Geology, 18�3�, 49–56 �in Chinese�.

Lin, B. H., and Wang, L. �2004�. “Research on bearing capacity mecha-nism of statically pressed precast concrete piles.” J. Building Struc-tures, 25�3�, 120–124 �in Chinese�.

Yang, J., Tham, L. G., Lee, P. K. K., Chan, S. T., and Yu, F. �2006a�.“Behaviour of jacked and driven piles in sandy soil.” Geotechnique,56�4� 245–259.

Yang, J., Tham, L. G., Lee, P. K. K., and Yu, F. �2006b�. “Observedperformance of long steel H-piles jacked into sandy soils.” J. Geo-tech. Geoenviron. Eng., 132�1�, 24–35.

Yu, F. �2003�. “Field tests on jacked steel H-piles in Hong Kong.” Tech-nical Rep., Dept. of Civil Engrg., Univ. of Hong Kong, Hong Kong.

Zhang, M. Y. �2004�. Research and application on jacked piles, ChinaBuilding Material Industry Press, Beijing �in Chinese�.

Zhuang, Q. R., and Xu, L. W. �2002�. “Selection and comparison ofpile-tip bearing stratum for Shiguan Industrial Park of Shenzhen.”

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Guangdong Geology, 17�2�, 33–36 �in Chinese�.

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Closure to “Termination Criteria for JackedPile Construction and Load Transfer inWeathered Soils” by L. M. Zhang,C. W. W. Ng, F. Chan, and H. W. PangJuly 2006, Vol. 132, No. 7, pp. 819–829.DOI: 10.1061/�ASCE�1090-0241�2006�132:7�819�

L. M. Zhang1; C. W. W. Ng2; F. Chan3; and H. W. Pang4

1Associate Professor, Dept. of Civil Engineering, The Hong Kong Univ.of Science and Technology, Clear Water Bay, Hong Kong. E-mail:cezhangl@ust.hk

2Professor, Dept. of Civil Engineering, The Hong Kong Univ. of Scienceand Technology, Clear Water Bay, Hong Kong.

3MPhil Research Student, Dept. of Civil Engineering, The Hong KongUniv. of Science and Technology, Clear Water Bay, Hong Kong.

4Chief Structural Engineer, Hong Kong Housing Authority, 33 FatKwong Street, Homantin, Kowloon, Hong Kong.

The writers are grateful to Dr. Y. Feng and Dr. Y. F. Yang for theirdiscussion on determination of final jacking forces. Both the writ-ers and the discussers agree that a rational approach is to recom-mend a certain relationship between required pile capacity Pult

and final jacking force PJ applicable to a specific site condition.Yet, it is important not to overwhelm the understanding of funda-mental behavior of jacked piles by only attempting to establishrules of thumb for a particular site condition. In this closure,issues regarding static loading test data for establishing the em-pirical relationship between Pult and PJ, ground conditions, deter-mination of pile capacity, and development of termination criteriaare addressed.

Available Test Data and Ground Conditions

The static loading tests in Fig. 13 of the paper include the twofield tests conducted in completely decomposed granite �CDG� inHong Kong reported in the paper, seven additional field tests inCDG in Hong Kong, five centrifuge pile load tests in dry CDG, aswell as 142 field tests in Guangdong reported by Lin and Wang�2004�. The sources of these data have been detailed in the paper.The discussers overlooked the seven additional field tests in CDGin Hong Kong and the five centrifuge pile-load tests in dry CDG.

The primary intention of Fig. 13 and Eq. �1� is to establish anempirical relationship between Pult and PJ for piles founded ingranular soils �i.e., silt, silty sand, and sand�. Certainly, the dataare very scattered due to many sources of uncertainty, such as: �1�differences in ground conditions and the presence of clay-silt-sand mixtures; �2� the type and dimensions of piles; �3� proce-dures of load tests and methods for interpreting the failure load;and �4� ignorance of setup effects.

The statement made by the discussers, “Unlike the CDG soilsin Hong Kong, those in Guangdong were classified into clay,sandy clay and gravel clay, all attributing to cohesive soils,” isperhaps an example of misunderstanding between practices inMainland China and elsewhere. For example, the CDG in Shen-zhen �a city neighboring Hong Kong� and the CDG in Hong Kongare outcomes of the same weathering process of granite. In thelatest Shenzhen Standard “Code for Investigation and Design ofFoundation” �Shenzhen Bureau of Construction 2005�, soils de-rived from decomposition of granite are classified as gravellyclay, sandy clay, and clay, respectively, if the content of particles

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equal to 20%, and zero. Following the Unified Soil ClassificationSystem �ASTM D2487�, however, the same soil that is classifiedas sandy clay or clay in Shenzhen would be classified as siltysand or silty-to-fine sand. A straightforward classification basedon grain-size distribution and plasticity would be preferred forbetter communication.

The discussers appear to emphasize that the ground conditionsin Hong Kong are entirely different from those in Guangdong andhence the behavior of jacked piles in Hong Kong are differentfrom those in Guangdong. This is not the authors’ experience. InHong Kong indeed heavy steel H-sections are used as jacked pilesand high pile capacity is achieved by jacking the piles to denseCDG �usually silty sand or fine-to-medium-sand� using largejacking rigs. Yet the fundamental behavior of the jacked pilesfounded in Hong Kong soils is still similar to that of jacked pilesconstructed elsewhere.

Through field tests in various ground conditions and labora-tory model tests �e.g., de Nicola and Randolph 1997; White andLehane 2004; Yang et al. 2006�, a jacked pile, compared with anidentical driven pile, has the following common features:1. The shaft resistance of the jacked pile is significantly larger

than that of the driven pile;2. The axial response of the jack pile is stiffer than that of the

driven pile;3. The jack pile is more brittle at failure; and4. The jack pile tends to plug more if its toe condition is

open-ended.These common features are not limited to a particular soil type.

The writers do consider that the possible different behavior ofreinforced concrete piles, prestressed pipe piles, and steel-H pilesin South China and Hong Kong should be investigated further interms of soil displacement during pile penetration. Soil plugsform between the flanges of a steel H-pile during jacking. Yet,how the soil plugs affect the toe and shaft resistances has not beenfully understood. In addition, how the pile capacity changes withtime after pile jacking should be quantified.

Determination of Pile Capacity

The discussers raised a good issue on the effects of load testprocedure, method for interpreting failure load, and method fordetermining the design load or factored load. The Chinese Tech-nical Code for Building Pile Foundation �JGJ 94-94� �Ministry ofConstruction 1995� recommends three methods for interpretingthe failure load �i.e., the turning point method, the settlementincrement-log-time method, and the specified-displacementmethod.� A description of the procedure of and interpretationmethods for pile load tests in JGJ 94-94 can be found in Zhanget al. �2003�. In Hong Kong, a maintained load test method andthe acceptance criteria specified in the Code of Practice for Foun-dations �Buildings Department 2004� are generally adopted forprivate and public housing developments. The Code of Practicefor Foundations uses the Davisson criterion and a residual settle-ment criterion to define pile failure. The pile-head residual settle-ment after the removal of the maximum test load should notexceed the greater of �D/120+4� mm and 25% of the maximumpile-head settlement during the test, where D is the least lateraldimension of the pile. Yet only the Davisson criterion was used tofind the failure loads for the tests in Hong Kong in Fig. 13, be-cause the residual settlement criterion may not be adopted else-where. Please note that several aspects of the information in Table3 in the discussion, for example on loading sequence, termination

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therefore should be updated against the latest Code of Practice forFoundations, which is available online.

The differences in the methods for interpreting failure loadswill certainly introduce errors in the pile capacity for those small-diameter jacked piles involved. The bias caused by failure criteriahas been examined by Zhang et al. �2005�. The settlementincrement-log-time method in JGJ 94-94 tends to interpret largerpile capacity than the Davisson criterion. Note that a mixed use ofthe three interpretation methods in JGJ 94-94 �e.g., Zhang et al.2003� among the large number of static loading tests in Guang-dong cited in Fig. 13 would also lead to added uncertainty ininterpreted failure loads even if JGJ 94-94 was strictly followed.One way to cope with various sources of uncertainty is to definea 95% confidence-level relationship in Eq. �2�, which can be re-fined when more data for a specific site condition is available.

The paper focuses on the ultimate capacity of jacked piles. Arelationship between design load and final jack load �i.e., Eq. �3��is introduced because the global safety factor approach is stillused in Hong Kong for pile foundation design. If limit-state de-sign is used, then an appropriate relationship between factoredload and final jacking force is required, as indicated by the dis-cussers. A description of the limit-state design methodology inJGJ 94-94 can be found in Lin and Wang �2004� and Zhang et al.�2003�.

Development of Termination Criteria

Jacked piles have been used widely in Mainland China and otherplaces, but were introduced to Hong Kong only recently. Thetermination criteria reported in the paper are largely similar tothose adopted in Guangdong. Indeed, the results of the limitedfiled tests in Hong Kong and the centrifuge tests in Fig. 13 followthe trend of the large number of tests in Guangdong quite well.

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The proposed equations provide a starting point for Hong Kongconditions. Refinement can be made in the future to accommodatespecific local conditions �e.g., ground conditions, construction de-tails, and acceptance criteria� by accumulating more local con-struction experience and field test data.

References

Buildings Department. �2004�. Code of practice for foundations, Build-ings Department, Hong Kong, �http://www.bd.gov.hk/english/documents/index_crlist.html�.

de Nicola, A., and Randolph, M. F. �1997�. “The plugging behaviour ofdriven and jacked piles in sand.” Geotechnique, 47�4�, 841–856.

Lin, B. H., and Wang, L. �2004� “Research on berning capacity mecha-nism of statically pressed precast concrete piles.” J. of Building Struc-tures, 25�2�, 120–124 �in Chinese�.

Ministry of Construction. �1995�. Technical code for building pile foun-dations JGJ 94–94, Ministry of Construction, Beijing �in Chinese�.

Shenzhen Bureau of Construction. �2005�. Code for investigation anddesign of foundation, Shenzhen Bureau of Construction, Shenzhen,Guangdong, China.

White, D. J., and Lehane, B. M. �2004�. “Friction fatigue on displacementpiles in sand.” Geotechnique, 54�10�, 645–658.

Yang, J., Tham, L. G., Lee, P. K. K., Chan, S. T., and Yu, F. �2006�.“Behaviour of jacked and driven piles in sandy soil.” Geotechnique,56�4�, 245–359.

Zhang, L. M., Li, D. Q., and Tang, W. H. �2005�. “Reliability of boredpile foundations considering bias in failure criteria.” Can. Geotech. J.,42�4�, 1086–1093.

Zhang, L. M., Liu, J. L., and Zhang, Z. M., �2003� “The Chinese limitstate design code for building pile foundations JGJ 94–94: A com-parative study.” Proc., LSD2003: International Workshop on LimitState Design in Geotechnical Engineering Practice �CD-ROM�,Phoon, K. K., Honjo, Y., and Gilbert, R. B., eds., World Scientific,Singapore.

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