INTERNATIONAL SOCIETY FOR MECHANICS AND

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This is an open-access database that archives thousands of papers published under the Auspices of the ISSMGE and maintained by the Innovation and Development Committee of ISSMGE. Proceedings of the 16th International Conference on Soil Mechanics and Geotechnical Engineering © 2005–2006 Millpress Science Publishers/IOS Press. Published with Open Access under the Creative Commons BY-NC Licence by IOS Press. doi:10.3233/978-1-61499-656-9-2115

CAPWAP testing – theory and application Expérimentation du programme CAPWAP - théorie et applications

T.A.L. Green ARQ (Pty) Ltd, South Africa M.L. Kightley Testing and Analysis Ltd, United Kingdom

ABSTRACT CAPWAP (Case Pile Wave Analysis Program) is an analytical method used in the dynamic testing of driven precast piles. Named af- ter the Case Western Reserve University in the USA, this method combines measured field data with wave equations to predict the static of piles. Through an iterative process the analysis yields a complete model of the resistance in the pile, distin- guishing between the dynamic and static resistance. This provides a wealth of useful information particularly with regard to the de- flection of the pile under load, the integrity of the pile shaft during driving and the performance of the drop hammer, driving system and pile material. A number of precast concrete piles were tested during the course of two piling projects recently completed in South Africa. Although the use of such testing may be commonplace in some parts of the world, it is still relatively new to this country. The results of these recent tests were therefore extremely beneficial in assessing the accuracy of the original design as as the value of the CAPWAP method in analysing the capacity of piled foundations. This paper describes the theory and practical application of the CAPWAP analysis and presents some of the results and knowledge gained during recent testing in South Africa. RÉSUMÉ Le logiciel CAPWAP (Case Pile Wave Analysis Program) pour l'analyse des pieux par la méthode de la propagation d'ondes est une méthode d'essai dynamique des pieux préfabriqués battus. Son nom rappelle qu'elle fut mise au point à l'Université de la Case Wes- tern Reserve aux Etats Unis d'Amérique. La méthode applique la théorie de l'équation d'ondes aux mesures relevées sur le chantier pour prédire la charge statique des pieux. Par un procédé itératif, l'analyse fournit un modèle complet de la résistance du pieu, en dis- tinguant entre résistance statique et résistance dynamique. Le logiciel fournit une somme de renseignements utiles, en particulier con- cernant la déformation du pieu sous charge, l'intégrité du fût du pieu pendant le battage et les comportements du mouton de battage, du système de battage et des matériaux constituant le pieu. Un certain nombre de pieux préfabriqués en béton ont été testés durant deux projets de pieux récemment terminés en Afrique du Sud. Bien que l'usage de ce logiciel soit très courant dans certaines parties du monde, il est relativement nouveau ici. Le résultat de ces récents essais a donc été bénéfique pour vérifier la précision du dimensi- onnement d'origine des pieux et aussi la valeur de la méthode CAPWAP. Cette communication décrit la théorie et l'application prati- que de l'analyse par CAPWAP, elle présente quelques uns des résultats obtenus et les connaissances acquises pendant cette expéri- mentation.

1 INTRODUCTION crete pile is being driven and what is measured when the pile is dynamically tested. Although well established in Europe and the United States, the use of the CAPWAP (Case Pile Wave Analysis Program) 2.1 Stress Waves and Piles method of analysis in the dynamic testing of driven precast piles is still relatively new to the South African piling industry. Consider what happens when a pile top is struck by a pile- Through an iterative process this analytical method yields a driving hammer. A compressive stress wave is generated in the complete model of the resistance in the pile, distinguishing be- pile and the pile top moves downward. This compressive stress tween the dynamic and static resistance. wave and associated downward movement travels along the pile A number of precast concrete piles were tested during the re- with a constant velocity or “wave speed” (c), approximately cent construction of a new berthing quay for Durban Harbour 4000m/s for concrete. The travelling stress wave also causes a and the expansion of the Mondi Kraft Paper Plant in Richards movement in the particles of the pile known as the particle ve- Bay. The results of these tests have been used to assess the ac- locity (v), usually around 2m/s for concrete. The travelling curacy of the original design as well as the value of the CAP- stress wave causes a strain (ε) in the pile shaft. There is an im- WAP method in analysing the capacity of piled foundations and portant relationship between c, v & ε. During a short time pe- predicting the load-deflection characteristics of the piles. This riod (t) the stress wave travels a distance L=ct and the pile par- paper describes the theory behind dynamic pile testing and more ticles move a distance ∆L=vt (i.e. a length of pile ct has been specifically the CAPWAP method of analysis. The interpreta- strained and shortened by a length vt) therefore: tion and analysis of typical CAPWAP tests from testing at Dur- ε = ∆L/L = vt/ct = v/c (1) ban Harbour and Mondi Kraft is then briefly explored. Equation (1) is significant in that ε and v are two quantities which can be measured during dynamic . Equation 2 THEORY (1) can be developed further by multiplying both sides of the equation by the pile elastic modulus (E), the pile cross-sectional Dynamic load testing of piles is a practical application of one- area (A) and substituting �EA = F (Hoeke’s Law) leads to the dimensional wave mechanics as applied to an elastic rod and expression: has been explained by many authors (Davis et al., 1987). This F = v [EA/c] = vZ = V (2) section is included to give a feel for what happens when a con-

2115 Where F = force (from ε measured), v = velocity (from ac- a best match is obtained between the computed pile top force celeration measured) and the term EA/c includes pile properties and the measured pile top force (Fig. 2). Once this point has collectively known as the pile impedance (Z). For ease of data been reached the CAPWAP modelling procedure is complete analysis and direct visual inspection (v) is multiplied by (Z) to and the resulting model is a mathematical representation of the give a proportional pile top velocity in force units (V). Thus in pile and soil surrounding it. This model can then be examined a uniform elastic pile the wave speed (c) is constant and the pile to give information about the expected performance of the pile particle velocity (v) is proportional to the pile force (F) at a under static loading conditions i.e. end bearing, shaft resistance point. Dynamic pile testing results are seen as F & V versus and the distribution thereof, pile top and toe static load versus time plots. Because (c) is constant the time taken for the stress deflection et al. wave to travel from the pile top to the pile toe is given by 2L/c where (L) is the length of pile below the measuring gauges. Therefore any reflections seen in the F & V time plots before the 2L/c time must be from along the pile shaft and by consid- eration of their time of arrival a point of origin on the pile shaft can be determined relative to the pile top. In a uniform pre-cast concrete pile any measured divergence of force (F) and velocity (V) before the stress wave is reflected from the pile toe (2L/c) will generally either be due to soil resis- tance, which will be seen as a gradual long term increase in (F) and decrease in (V) or be due to a reflection from a small crack in the pile shaft possibly indicating the location of a mechanical joint or damage to the pile shaft which will show as a brief in- Figure 2. CAPWAP analysis best match for Pile TP350 at Mondi Kraft, crease in V and reduction F. Reflections just after 2L/c and Richards Bay, tested during installation. later are from the pile toe and contain information about the pile end bearing resistance (Fig. 1). 3 EQUIPMENT AND TEST PROCEDURE

Dynamic pile testing is now well known by the piling commu- nity worldwide. The equipment that was used during testing at the sites in Durban and Richards Bay is manufactured by Pile Dynamics Inc., USA and is known as the Pile Driving Analyzer (PDA). Testing was carried out at the Durban site in 2002 using a version of the PDA known as the PAK. The PAK allows the expert on site to fully control the test with the facility to review and analyse all test data. The pile testing at Richards Bay in 2003 was carried out on site by a technician, but controlled re- motely and in real-time by an expert in the UK using a version of the PDA known as the PAL-R (Likins et al., 2000). Using this equipment a mobile phone link is established between the Figure 1. Pile TP350 at Mondi Kraft, Richards Bay tested during instal- PAL-R and a remote computer and the test data sent live from lation. The pile joint and shaft are easily seen in the measured the site to the expert in the UK for data quality review and im- pile top Force and Velocity versus Time records. (For. Msd = Force mediate analysis and reporting. measured and Vel. Msd = Velocity measured). On site two accelerometers and two strain transducers are bolted in sets to opposite sides of the pile shaft at a minimum of 2.2 Soil Resistance two pile widths or diameters below the top of the pile. The The above theory is very useful in the evaluation of dynamic gauges are then attached to the PDA via a main cable and the pile test data. However with regard to static soil resistance the test hammer delivers a set of blows to the pile. The data is nature of soil (solid particles, air and water) means that the soil viewed and stored on site and in the case of the PDA-W instan- resistance measured on the pile shaft and toe are total resis- taneously transmitted to the expert. Instant transmission and tances consisting of static resistance and transient or velocity viewing of data from all four gauges allows any data quality is- dependant dynamic resistance. The CAWAP program (Rausche sues to be addressed immediately. et al., 2000) is required to separate the static and dynamic soil The PDA using the program PDA-W processes the resultant resistances from the total soil resistance whilst also providing pile top strains and accelerations in real-time to give pile top information about the static load deflection characteristics of the force and velocity versus time data for each hammer blow from pile. which all other information is derived and computed (Fig.2). The CAPWAP procedure starts with building a mathematical Pile top force and velocity records provide all the information model for the pile and soil around the pile. The pile model con- required to assess the quality of the pile and the ground support- sists of a series of elements usually around 1m in length having ing the pile. Pile hammer performance is measured directly and the characteristics of the pile material. Each pile model ele- obtained from the computation of the integral of the product of ment, below ground, has a soil resistance consisting of an elas- the measured pile top force and velocity against time. Pile tic-plastic spring (static resistance) and a linear dashpot (dy- compression and tension stresses are evaluated by dividing the namic resistance). The pile top force and velocity (velocity pile top force by pile cross-sectional area, likewise pile tension measured) and pile response (force measured) is known, while stresses and pile integrity can be determined by evaluation of the soil that produces the response is unknown. The analysis the pile top measurements. begins by imputing a starting soil model for the pile shaft ele- ments and the pile toe. One of the measured quantities (veloc- ity) is applied to the pile top and the program computes the re- 4 TESTING AT DURBAN HARBOUR sultant complementary quantity (force computed) which is compared graphically with the force measured. This process is The construction of 52 concrete caissons to provide a new repeated progressively adjusting the model soil resistance until berthing quay for Durban Harbour in South Africa called for rapid construction and launching of caissons, each with a dead

2116 weight of up to 3600 tonnes (Stoll, 2003). These were supported Table 1: Test Results of Dynamic Pile Test (WL = Working Load & on concrete ground beams and launched into the water by slid- MSR = Mobilised Soil Resistance) ing them on Teflon coated steel plates. Driven precast concrete Design Mobilised Soil Resistance Predicted Deflection Test piles were required under the beams due to the compressible na- Load No: Shaft Toe Total @ WL @ MSR ture of the underlying . (kN) (kN) (kN) (kN) (mm) (mm) 4.1 Project Background TB3-43 1600 1358 1522 2880 12.0 21.9

The site is located within the city of Durban, along the eastern This time the load-deflection graphs generated by the CAP- coast of South Africa. An existing layer of fill up to 3,5 WAP analysis was compared graphically (Fig. 4) to that gener- metres thick covers the site and is underlain by variable estua- ated by the pile deflection theory using soils information extracted rine and clays, typical of this area. Cretaceous siltstone from an adjacent . occurs at a depth of about 34 metres. The piling contract was awarded to a local specialist piling Figure 4. Predicted load-deflection graphs after Everett 1991 and load- contractor, Ground Engineering Ltd (GEL) towards the end of 2002. Although static load tests were required, it was soon real- 3500 ised that with the time required to install tension anchors to pro- 3000 vide a reaction load, the testing would only yield results towards 2500 the end of the driving programme. It was therefore agreed to employ Dynamic Pile testing methods to assess the load carry- 2000 1500 ing capacities of the piles. The actual testing and analysis was Load(kN) conducted by Testal (Testing and Analysis Ltd, a UK based 1000 company specialising in dynamic pile testing), in conjunction 500 with ARQ (Pty) Ltd (a local consulting company responsible for 0 the design of the piles). A conventional static load test was also 0 5 10 15 20 conducted, enabling comparison between the two testing meth- (TB3-43) Dynamic - Deflection (mm) ods. Side (E) End (E) Total (E) Side (C) End (C) Total (C) The piles were required to withstand vertical compressive loads of 1300kN to 2100kN and tension loads of up to 400kN. deflection curves from the CAPWAP analysis. To achieve this, 350 x 350mm square piles were installed to depths of 14 - 38m below natural ground level using a 7.1 At first glance the correlation between the theory and the re- Tonne Twinwood V100D Series 7 hydraulic drop hammer typi- sults of the CAPWAP analysis do not appear as good as that cally using a drop height of 600mm. Piles were cast in sections achieved with a static load test. Closer inspection however, leads up to 13m in length and joined with a steel plate and wedges. to some interesting conclusions. Firstly, the transfer of load at the base of the pile predicted by the theory ties in closely with that measured by the CAPWAP 4.2 Test Results, Analysis and Interpretation analysis, while the deviation between the curves is actually due to A conventional static load test was conducted on Pile TB3-4, the lower rate of side shear transfer of load onto the pile. which deflected 7mm under its design load of 1300kN. The load- During pile driving, excess positive pore water pressures are deflection graph generated during this test was then compared to often generated, particularly in fine grained soils such as clays, that generated during design (API, 1987) using load transfer func- or even fine sands. These pore water pressures reduce the tions as proposed by (Everett, 1991) and soils information ex- soil’s , thereby reducing the side shear of side re- tracted from an adjacent borehole. As can be observed from Fig- sistance to pile penetration, and thus the pile capacity at the time ure 3 below, there is an excellent correlation between the load- of driving. As this dissipates, the pile capacity deflection curve predicted by the theory and that measured in the increases. This effect is known as soil setup. static load test, thus confirming the accuracy of the soil parame- However, piles driven into dense saturated silts or fine sands ters and other assumptions used in the design. can generate negative pore water pressures towards the toe of the pile. As this negative pore pressure dissipates with time, the pile capacity decreases. This effect is known as soil relaxation. 2500 In the case of pile TB3-43, tested above, the pore water pres-

2000 sure has not yet had sufficient time to dissipate resulting in the initial discrepancy between the side shear transfer predicted by

1500 the theory and that measured by the CAPWAP analysis. As the pore water pressure dissipates over time, the correlation between 1000 Load(kN) the theory and test results will improve.

500

0 5 TESTING AT MONDI KRAFT 0 2 4 6 8 10 12 14 16 18 20 (TB3-4) Static - Deflection (mm) In 2003, Mondi Kraft South Africa announced a major ex- Side Resistance End Bearing Total Capacity Cycle 1 Cycle 2 pansion of their Mill in Richards Bay. This expansion was to Figure 3. Predicted design load-deflection curves and actual load- almost double the production of this plant in order to cater for deflection curves from a static load test for Pile TB3-4. their rapidly growing export market. The solution was to consist of driven cast in-situ piles and square precast concrete piles. Although numerous dynamic pile tests were conducted, for the purposes of this paper only one of these tests is discussed further due to its proximity to a known borehole. This test was conducted on a working pile several days after its installation (Table 1).

2117 5.1 Project Background transfer that can be observed once the deflection is greater than 15mm. Ground Engineering Ltd (GEL) was again awarded the contract to install the precast piles. Although not specifically required by the client, GEL made the decision to employ dynamic pile test- 3000 ing to provide their client with complete assurance that the piles were indeed capable of carrying the required loads. Also infor- 2500 mation gathered during the testing could be used in future con- 2000 tracts where similar work was required and would allow further evaluation of the dynamic pile testing system under local condi- 1500 tions. Load(kN) 1000 The driven precast piles installed by GEL were typically re- quired to withstand compressive loads of 2000kN and tension 500 loads of up to 300kN. To achieve this, 26m long 350x350mm 0 square piles were installed to depths of 22 - 25m, again using 0 2 4 6 8 10 12 14 16 18 20 the 7.1 Tonne Twinwood V100D Series 7 hydraulic drop ham- P47A - Deflection (mm) mers, with a drop height of 600mm. Side (E) End (E) Total (E) Side (C) End (C) Total (C) The site is located adjacent to the existing Mondi Kraft Plant, approximately 10km west of Richards Bay, situated along the eastern coast of South Africa. This area is underlain by un- Figure 5. Predicted load-deflection after Everett 1991 and load- consolidated sediments to a depth of 22 – 25m below natural deflection from the CAPWAP analysis for Pile P47A. ground level, and thereafter Cretaceous siltstone and sandstone. 6 SUMMARY AND CONCLUSIONS 5.2 Test Results, Analysis and Interpretation Although almost a dozen tests were completed, only two of the With the severe time constraints prevalent in most piling con- piles tested are discussed further. KP100 was installed in the Kiln tracts, the availability of a quick and cost effective means of area, while P47A was installed in the Digester Plant area. A testing the capacity of the piles is an extremely valuable tool. summary of the tests results are included in Table 2 below: That said, it is important to ensure that the results generated by such a system are in fact correct, conforming with both the de- Table 2: Dynamic Pile Test Results from Mondi Kraft sign and other test methods. Based on the results obtained from the tests conducted in Design Mobilised Soil Resistance Predicted Deflection Test South Africa, the CAPWAP system fulfils this requirement for Load No: Shaft Toe Total @ SLS @ MSR rapid, cost-effective and accurate testing of precast concrete (kN) (kN) (kN) (kN) (mm) (mm) piles. The results cannot however be taken at face value, and in- P47A 2000 1720 1080 2800 11.5 17.5 terpretation by personnel suitably experienced in pile design is KP100 2000 1250 1200 2450 14.5 20.5 required due to the effect of pore water pressures on the meas- KP100* 2000 3890 510 4400 7.0 44.0 ured pile capacity. Working within these limitations can then provide extremely useful information, both from a quality con- trol and design point of view. As discussed in section 4.2, excess positive pore water pres- It is hoped that over the course of the next year, further op- sures are often generated during the installation of driven precast portunities will be available to further test this system. Thereby piles, reducing the soil’s effective stress and thereby reducing the increasing our knowledge of both the program and our pile de- pile capacity at the time of driving. As this pore water pressure sign methodology. dissipates, the pile capacity increases. The information gathered by the PDA calculates the pile ca- pacity at the time of the testing. Therefore a selection of piles REFERENCES should be re-tested one or two days after the initial tests. These re- tests (or Restrike Tests) are used to determine the extent of the Davis R. A., Mure J. N. and Kightley M. L. 1987. The Dynamic Analy- soil setup or soil relaxation effect, and hence the long term pile sis of Piled Foundations using the CAPWAPC Method. Ground En- capacity. One of the tests conducted on KP100 (denoted KP100* gineering. November 1987/Volume 20/Number 8. in Table 2 above) is an example of such a restrike test. Note the Everett J. P. 1991. Load transfer functions and pile performance model- improvement in capacity of pile KP100 between installation and ling. Proceedings of the Tenth Regional Conference for Africa on the restrike test. Soil Mechanics and Foundation and Foundation Engineering and Pile P47A is located in close proximity to a known borehole, the Third International Conference on Tropical and Residual Soils. and as such is well suited for further analysis. A detailed design Maseru 23-27 September. pp 229-234. was conducted using the borehole information to generate load- Likins G., Rausche F. and Goble G. G, 2000. High Strain Dynamic Pile Testing Equipment and Practice. Proceedings of the Sixth Interna- deflection curves which can be compared to those produced by tional Conference on the Application of Stress-Wave Theory to Piles the CAPWAP analysis (Fig. 5). September 2000 327p ISBN 90 5809 150 3 The same trends observed in Durban can once again be ob- Rausche F., Robinson B. and Liang L., Automatic Signal Matching with served in this comparison. The end transfer of load measured by CAPWAP. Proceedings of the Sixth International Conference on the the CAPWAP conforms almost perfectly to that predicted by Application of Stress-Wave Theory to Piles September 2000 53p the theory. Furthermore, as this pile was tested during installa- ISBN 90 5809 150 3 tion we would expect the side shear transfer to be lower than API 1987. Recommended Practise for Planning, Designing and Con- that predicted by the theory, pending the dissipation of the ex- structing Fixed Offshore Platforms, API Recommended Practise 2A cess pore water pressure, which is indeed the case. (RP2A), Seventeenth Edition, April 1987. Stoll A, 2003. Case Study: Piles and Pile Testing for a Caisson Launch- It should also be noted that the CAPWAP analysis indicates ing Dock, Durban. Proceedings of the First African Young Geo- a maximum side shear capacity of 1720kN at 15mm for the pore technical Engineers Conference April 2003. water pressure at installation. This reduced maximum side shear is lower than the final case, with the result that more load is transferred to the end of the pile causing the deviation in end

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