PART I: AN EXPERIMENTAL STUDY INTO THE VISCOUS DAMPING RESPONSE OF PILE-CLAY INTERFACES

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1 PART I: AN EXPERIMENTAL STUDY INTO THE VISCOUS DAMPING RESPONSE OF PILE-CLAY INTERFACES V. B. L. Chin, Gue & Partner Sdn Bhd, Malayia; Formerly Monah Univerity, Autralia J. P. Seidel, Foundation QA Pty Ltd, Autralia; Formerly Monah Univerity, Autralia Abtract Dynamic method for forecating the drivability of pile and for verifying the capacity of intalled pile are premied on accurate modelling of the dynamic friction due to vicou damping at the pile-oil interface. Whilt variou dynamic friction model for the pile haft-oil interface have been propoed, thee model are either not baed on experimental data, or baed on experimental tudie with perceived limitation. In order to develop an improved and phyically baed vicou damping model, an experimental tudy ha been undertaken at Monah Univerity on the vicou damping repone of the pile-clay interface. Thi paper, which form Part I of the dicuion, report on the development of a novel tet device for imulating the dynamic repone of the pile-clay interface in the laboratory and the tet programme. Keyword: dynamic friction; pile-oil interface; vicou damping; dynamic pile teting 1. Introduction During a dynamic event, the pile head i loaded, forcing the pile haft downward. In the initial tage, the train in the oil adjacent to the pile wall i very mall, and the pile haft and the oil move in concert. A the pile move downward to a critical diplacement, the platic trength of the pile-oil interface i exceeded, reulting in a localized band of high hear train in the oil at the interface and the pile haft lip pat the oil. During the phae of relative movement between the pile and the oil, the pile-oil interface friction i found to be rate-dependent due to vicou damping. The rate dependent friction, t, meaured during dynamic hearing can be defined a the um of the vicou damping friction, d, and the tatic friction,, and can be expreed a follow: t = d + (1) The tatic friction i the friction meaured at a reference low hear rate and a uch i really a quai-tatic friction. In order to normalize the total friction and to quantify the degree of the vicou damping effect, the total friction i normalied by the tatic reitance a follow: t d + d = = +1.0 (2) For the ake of convenience, the ratio of the total friction to the quai-tatic tatic trength i referred to a the trength ratio from hereon. The numerical value of the trength ratio quantifie the proportion (or potentially the percentage) of the trength increae. It i ignificant to note that the dynamic interface trength aociated with the pile-driving event relate to the high diplacement or the reidual trength; a

2 uch, the quai-tatic interface trength for normalizing the dynamic trength hould correpondingly be the high diplacement trength. 2. Review of previou reearch Baed on hi experience, Smith (1960) propoed the following model: t = ( 1+ J Smith v) (3) where t = total friction [kpa] = intantaneou tatic friction [kpa] v = pile velocity [m/] J Smith = Smith damping parameter [/m] A modified verion of thi model i ued in CAPWAP and GRLWEAP where the intantaneou tatic friction i replaced with the ultimate tatic friction. It i noted that the Smith model i not baed on experimental tudie. All reearcher who have meaured the dynamic friction have unanimouly found that it i dependent on the pile velocity (v). Dayal and Allen (1975), and Litkouhi and Pokitt (1980) performed penetrometer tet whilt Heerema (1979) performed direct hear tet. They propoed model that are generally in the form of: N t = (1 + Jv ) (4) where = ultimate tatic friction [kpa] J = damping contant [(/m) N ] N = index [-] with a value of about 0.2 Heerema (1979) uggeted that the damping factor increae with decreaing hear trength of the clay. The remaining reearcher did not evaluate the dependence of the factor on oil parameter becaue of inufficient data in the individual tudie. The model which are baed on experimental tudie appear to have ome ignificant limitation. The tudie by Litkouhi and Pokitt (1980) and Dayal and Allen (1975) were baed on penetrometer tet performed at contant or near-contant velocity. Baed on data preented in Litkouhi and Pokitt (1980), the interface dynamic friction continued to increae with depth uch that it wa not poible to aociate a unique friction value to the particular velocity, and data for tet performed under the ame boundary condition were highly variable. The data from Dayal and Allen (1975) howed that a critical velocity exited below and above which the trength ratio increaed at dratically different rate. The likely caue of thee anomalie are hypotheied. The interface behaviour of the trailing pile haft wa apparently dependent on the way the leading tip deformed the oil. Alo, the penetration behaviour of the pile tip might have ubjected the interface to different normal tree depending on the rate of penetration, in which cae, the trength ratio would be a function of not only the penetration rate, but alo of conequent normal tre variation. Thu the interface component of the penetrometer tet would appear to be unuitable for tudying the interface-only behaviour. The data obtained by Heerema (1979) howed that the dynamic friction varied ignificantly even when the velocity wa almot contant, and howed exceive increae in friction (up to nine time the tatic friction at a pile velocity of 1.0m/) that

3 ha not been encountered in practice. A a particular ample wa ued in many tet, including the quai-tatic tet that took coniderable time to complete, the ample could have lot ignificant moiture a the tet were carried out. If o, thi effect would have been additional to the rate effect and would have been reflected in the friction-velocity relationhip propoed by Heerema. The tet by Dayal and Allen (1975), Heerema (1979) and Litkouhi and Pokitt (1980) were performed at contant or near-contant velocity. For each tet, the meaured dynamic friction wa interpreted to correpond to the contant or near-contant velocity uch that one tet yielded only one data point in the friction-velocity plot. Hence, the data from a collection of thee tet each performed at a different contant velocity form the friction-velocity plot. In theory, the tet would give the actual friction-velocity repone if and only if the intantaneou friction could be aociated with it correponding intantaneou velocity. However, there i indication that the interpretation of the dynamic friction in the work of Litkouhi and Pokitt, Dayal and Allen, and Heerema wa not unique and wa therefore quetionable. A detailed dicuion of their interpretation can be found in Chin (2003). Alo, ince the frictionvelocity relationhip wa obtained by combining data point from individual contant velocity tet, there i typically ignificant catter in the friction-velocity plot, which introduce ome uncertainty to the actual repone. In addition, uch tet do not imulate a typical pile driving event which involve tranient velocitie and varying acceleration, and therefore, did not allow any effect of acceleration on the dynamic friction to be invetigated. Benamar et al. (1991, 1992) and Benamar (1999) invetigated the dynamic repone of pile-clay interface by driving a miniature pile through the pecimen contained in a triaxial cell. Whilt they were able to continuouly record the intantaneou dynamic friction and the correponding intantaneou pile velocity during one ingle driving event, the friction-velocity repone obtained by Benamar et al. applie to the loading phae of a ingle driving event. At the beginning of their force-velocity curve, which i baed on the initial loading tage of the event, two effect occur concurrently. The firt effect i the dynamic or vicou effect. The econd effect i the incomplete mobilization of the quai-tatic reitance (that will only be fully mobilied after ufficient hear diplacement known a the quake ha been reached). Therefore, it i not poible to imultaneouly determine the intantaneou tatic reitance and vicou damping reitance for the data at the beginning of the force-velocity curve. Thi ha the implication that the dynamic friction cannot be normalized by the quaitatic friction, and that the force-velocity curve from Benamar et al. cannot be applied generally to other ituation. Therefore, a laboratory et-up that overcome the major limitation of thee previou tudie mut be developed, and baed on the data obtained uing thi et-up, the actual trength ratio-velocity relationhip for the pile-clay interface mut be obtained. 3. Development of laboratory et-up In an attempt to overcome ome of the limitation of the previou laboratory et-up, a novel tet device wa developed at Monah Univerity to perform quai-tatic and dynamic pile-oil interface tet. Key requirement were iolation of the pile tip repone from the pile haft-oil interface repone; one-to-one modeling of the

4 interface; facility for the teting of different pile material and pile of different roughne; tranient motion for determining the effect of velocity and acceleration on the dynamic friction; facility for controlling and meauring the tre acting normal to the interface in order to replicate variou tre condition; minimization of radiation damping by minimiing the thickne of the oil pecimen; repetitive teting of a particular pile-oil interface o that the clay would be remoulded a i the cae for both driven and drilled pile. Alo, the friction-velocity relationhip wa to be obtained by continuouly logging the dynamic friction and the velocity during a ingle driving event. It i the author belief that thi i the mot reliable approach to obtaining the dynamic repone of the interface in a driving event where the velocity of a particular egment of the pile increae to a maximum, then decreae to maller value before becoming negative during rebound. The friction-velocity repone obtained in thi way would directly give the functional relationhip between velocity and dynamic friction throughout the driving event. Thi approach require no interpretation of the appropriate interface friction, and enable direct aociation of interface friction with the correponding velocity. From the practical point of view, thi approach ha the advantage of only needing to perform a ingle tet to obtain multiple data point (that form the frictionvelocity relationhip) for a particular interface teted under a particular et of boundary condition. To fulfil the requirement, a cheme baed on the direct hear device wa adopted. A chematic of the device (for the dynamic tet configuration) i hown in Figure 1. Contant normal load i applied to the interface uing a loading platen in the vertical direction, and hear load i applied to the interface in the horizontal direction. The pile ection i locked onto a carriage. The carriage i coupled to an actuator o that it i puhed rather than impacted. The hear box accommodate a clay pecimen meauring 555mm x 160mm in plane and 40mm thick. A hown in Figure 1, the hear box i tople and bottomle o that the loading platen can come in contact with the top of the ample and the pile urface with the bottom of the ample. In the quai-tatic tet, the hear load i applied uing an Intron actuator. For the dynamic tet, the hear load i applied by a pecially deigned high-peed actuator, and the carriage and the actuator are topped mechanically. Further detail of the device can be found in Chin (2003). During the dynamic tet, the following are recorded uing high-peed data acquiition: the acceleration of the pile, the diplacement of the pile, the normal load on the interface, the vertical diplacement of the ample, and the force delivered by the high-peed ram to the carriage. The acceleration of the pile i integrated to give the velocity. The product of the acceleration and the ma of the body moved by the ram (i.e. the pile and the carriage) give the inertial force. The interface friction i obtained by deducting the inertial force from the force delivered by the ram to the carriage. 4. Tet Programme For a particular pile-clay interface, quai-tatic and dynamic tet were performed in order to etablih the effect of hear rate on the interface friction.

5 Figure 1 Configuration of the hear device for dynamic tet Three type of clay of low, medium and high platicity (according to the platicity chart) were teted. In order to invetigate the effect of the preconolidation tre, normal tre, OCR and hence the hear trength on the interface behaviour, thee parameter were varied. For a particular type of clay, ample with two preconolidation tree at 325kPa and 500kPa were fabricated, and for a particular preconolidated ample, normal tree ranging from 60 to 250kPa were applied to the ample during the tet. Thu, the effective OCR of the ample and the hear trength of the ample (which depended on the normal tre and the OCR) were varied. In order to determine the effect of the pile urface roughne on the interface behaviour, tet were conducted on a mooth concrete pile, rough concrete pile and a mooth teel pile. 5. Tet procedure In order to tet clay pecimen of known propertie, procedure were developed to fabricate clay pecimen with conitent propertie in the laboratory. The clay wa purchaed from a pottery material upplier in the form of dry powder. The powder wa mixed with water in a mixer to form a lurry. The lurry wa placed in a purpoebuilt mould for conolidation to a target preconolidation tre. The dynamic tet were performed at tranient velocitie up to 1.6m/. A the clay pecimen took a coniderable amount of time to fabricate, three-taged teting wa performed on the ame interface ample for the ake of efficiency. The multi-tage teting wa baed on the principle of multi-tage teting of oil in the tandard direct hear device a outlined in Head (1994) but with light variation.

6 The quai-tatic tet were performed at a hear rate of 0.01mm/. Water wa poured onto the pile ection and around the ample to prevent moiture lo at the interface. Under the applied normal tre, the pile-clay interface wa heared by moving the carriage uing the actuator until the reidual trength wa reached. 6. Concluion In order to improve the reliability of the dynamic method, reearch ha been undertaken to better model and characterize the dynamic repone of the pile-oil interface during pile-driving event. The approach adopted in the experimental program ha been to imulate the field repone a accurately a practically poible in the laboratory under controlled condition, and in a way which overcome the deficiencie perceived in previou reearch program. The repone of the pile-oil interface thu attained i dicued in an accompanying paper which form Part II of the dicuion. Acknowledgement The work decribed in thi paper wa funded by the Autralian Reearch Council (ARC). The firt author wa upported by Monah Univerity cholarhip. The upport of both ARC and Monah Univerity i gratefully acknowledged. Reference Benamar, A. (1999). Pile behaviour during driving. 8th Autralia New Zealand Conference on Geomechanic, Hobart, Tamania, Autralia, Autralian Geomechanic Society, Vol. 1, Benamar, A., Lepert, P., Levacher, D. and Boiard, P. (1991). Shaft reitance during driving in clay from laboratory tet. Proceeding of the 2nd International Conference on Recent Advance in Geotechnical Earthquake Engineering and Soil Dynamic, St. Loiu, Miouri, Vol., Benamar, A., Levacher, D. and Lepert, P. (1992). Phyical and numerical imulation of lateral haft friction along driven pile. Proceeding of the 4th International Conference on the Application of Stre-wave Theory to Pile, The Hague, The Netherland, Balkema, Vol., Chin, V. B. L. (2003). The dynamic repone of pile-oil interface during pile driving and dynamic teting event. PhD thei, Department of Civil Engineering, Monah Univerity, Victoria, Autralia. Dayal, U. and Allen, J. H. (1975). The effect of penetration rate on the trength of remolded clay and and ample. Canadian Geotechnical Journal, 12, Head, K. H. (1994). Manual of oil laboratory teting - Volume 2: Permeability, hear trength and compreibility tet. New York, John Wiley & Son, Inc. Heerema, E. P. (1979). Relationhip between wall friction, diplacement velocity and horizontal tre in clay and in and for pile driveability analyi. Ground Engineering, 12(1), Litkouhi, S. and Pokitt, T. J. (1980). Damping contant for pile driveability calculation. Geotechnique, 30(1), Smith, E. A. L. (1960). Pile-driving analyi by the wave equation. American Society of Civil Engineer Tranaction, 127(Part 1),

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