9 Construction Monitoring and Testing Methods of Driven Piles Manjriker Gunaratne

Page 363 9 Construction Monitoring and Testing Methods of Driven Piles Manjriker Gunaratne CONTENTS 9.1 Introduction 364 9.2 Construction Techniques Used in Pile Installation 365 9.2.1 Driving 365 9.2.2 In Situ Casting 367 9.2.3 Jetting and Preaugering 367 9.3 Verification of Pile Capacity 367 9.3.1 Use of Pile-Driving Equations 367 9.3.2 Use of the Wave Equation 368 9.4 Pile-Driving Analyzer 372 9.4.1 Basic Concepts of Wave Mechanics 374 9.4.2 Interpretation of Pile-Driving Analyzer Records 375 9.4.3 Analytical Determination of the Pile Capacity 378 9.4.4 Assessment of Pile Damage 380 9.5 Comparison of Pile-Driving Formulae and Wave-Equation Analysis Using the 384 PDA Method 9.6 Static Pile Load Tests 386 9.6.1 Advantages of Load Tests 392 9.6.2 Limitations of Load Tests 392 9.6.3 Kentledge Load Test 392 9.6.4 Anchored Load Tests 393 9.7 Load Testing Using the Osterberg Cell 394 9.7.1 Bidirectional Static Load Test 394 9.8 Rapid Load Test (Statnamic Pile Load Test) 400 9.8.1 Advantages of Statnamic Test 400 9.8.2 Limitations of Statnamic Test 400 9.8.3 Procedure for Analysis of Statnamic Test Results 401 9.8.3.1 Unloading Point Method 402 9.8.3.2 Modified Unloading Point Method 405 9.8.3.3 Segmental Unloading Point Method 405 9.8.3.4 Calculation of Segmental Motion Parameters 406 9.8.3.5 Segmental Statnamic and Derived Static Forces 407 9.9 Lateral Load Testing of Piles 408 9.10 Finite Element Modeling of Pile Load Tests 411 9.11 Quality Assurance Test Methods 413 9.11.1 Pile Integrity Tester 413 9.11.1.1 Limitations of PIT 414 Page 364 9.11.2 Shaft Integrity Test 414 9.11.3 Shaft Inspection Device 416 9.11.4 Crosshole Sonic Logging 417 9.11.5 Postgrout Test 417 9.11.6 Impulse Response Method 418 9.12 Methods of Repairing Pile Foundations 420 9.12.1 Pile Jacket Repairs 420 9.13 Use of Piles in Foundation Stabilization 422 9.13.1 Underpinning of Foundations 422 9.13.2 Shoring of Foundations 423 References 424 9.1 Introduction Depending on the stiffness of subsurface soil and groundwater conditions, pile foundations can be constructed using a variety of construction techniques. The most common techniques are (1) driving (Figure 9.1), (2) in situ casting and preaugering (Figure 9.2), and (3) jetting (Figure 9.3). Due to the extensive nature of the subsurface mass that it influ- FIGURE 9.1 Driven piles. (From www.vulcanhamrner.corn. Withpermission.) Page 365 FIGURE 9.2 Cast-in situ piling. (From www.gdonalcom. With permission.) ences, the degree of uncertainty regarding the actual working capacity of a pile foundation is generally much higher than that of a shallow footing. Hence, geotechnical engineers constantly seek more and more effective techniques of monitoring pile construction to estimate as accurately as possible the ultimate field capacity of piles. In addition, pile construction engineers and contractors are also interested in innovative monitoring methods that would reveal information leading to (1) on-site determination of pile capacity as driving proceeds, (2) distribution of pile load between the shaft and the tip, (3) detection of possible pile or driving equipment damage, and (4) selection of effective driving techniques and equipment. 9.2 Construction Techniques Used in Pile Installation 9.2.1 Driving The most common technique for installation of piles is driving them into strong bearing layers with an appropriate hammer (such as Vulcan, Raymond) system. In order for this technique to be effective, the hammer and the pile must be able to withstand the driving stresses. Although driving can be monitored using the specified penetration criteria (Section 9.3.1) to assure safe conditions, nowadays the technique of pile driving is commonly accompanied by the pile- driving analysis method of monitoring (Section 9.4). Specific details of hammers and hammer rating is found in Bowles (1995). Page 366 FIGURE 9.3 (a) Jetted piles.(From www.state.dot.nc.us.Withpermission.) (b) Preaugared concrete pile. (From www.iceusa.com. With permission.) Page 367 9.2.2 In Situ Casting When the subsurface soil layers are relatively strong, it is common to install significantly large-diameter piles and using boring techniques. For caissons, this is the only viable installation method (Chapter 7). Depending on the collapsibility of the soils and availability of casings, in situ casting can be performed with or without casings. In cases where casing is desired, drilling mud (such as bentonite) is an economic alternative. More construction details of cast-in situ piles are found in Bowles 9.2.3 Jetting and Preaugering Although driven piles are installed in the ground mostly by impact driving, jetting or preaugering can be used as aids when hard soil strata are encountered above the estimated tip elevation required to obtain adequate bearing. However, the final set is usually achieved by impact driving the last few meters, an exercise that somewhat restores the possible loss of axial load bearing capacity due to jetting or preaugering. Nonetheless, it has been reported (Tsinker, 1988) that impact-driven piles have better load bearing characteristics than jetted- driven piles under comparable soil conditions. This is possible due to the soil in the immediate neighborhood first liquefying as a result of the excessive jet water velocity and subsequently remolding with the dissipation of excess pore pressure. The original in situ soil structure and the skin-friction characteristics are significantly altered. During the jetting process, some water also infiltrates onto the neighborhood maintaining a high pore pressure there. Thus, the creation of liquefaction and filtration zones, known as the zone of combined influence of jetting, is expected to result in a reduction of the lateral load capacity. Consequently, although pile jetting may be effective as a penetration aid to impact driving in saving time and energy, the accompanying reduction in the lateral load capacity will be a significant limitation of the technique. Similar inferences can be made regarding preaugering as well. 9.3 Verification of Pile Capacity There are several methods available to determine the static capacity of piles. The commonly used methods are (1) use of pile-driving formulae, (2) analysis using the wave equation, and (3) full-scale load tests. A brief description of the first two methods will be provided in the next two subsections. 9.3.1 Use of Pile-Driving Equations In the case of driven piles, one of the very early methods available to determine the load capacity was the use of pile-driving equations. Hiley, Dutch, Danish, Janbu, Gates, and modified Gates are some of pile-driving formulae available for use. For more information on these, the reader is referred to Bowles (1995) and Das (2002). Of these equations, one of the formulae most popular ones is the engineering news record (ENR) equation, that expresses the pile capacity as follows: (9.1) Page 368 where n is the coefficient of restitution between the hammer and the pile (<0.5 and >0.25), Wh is the weight of the hammer, WP is the weight of the pile, s is the pile set per blow (in inches), C is a constant (0.1 in.), Eh=Wh(h), h is the hammer fall, and eh is the hammer efficiency (usually estimated by monitoring the free fall). It is seen how one can use Equation (9.1) to compute the instant capacity developed at any given stage of driving by knowing the pile set (s), which is usually computed by the reciprocal of the number of blows per inch of driving. It must be noted that when driving has reached a stage where more than ten blows are needed for penetration of 1 in. (s=0.1 or at “refusal”), further driving is not recommended to avoid damage to the pile and the equipment. Example 9.1 (This example is solved in British units. Hence, please refer to Table 7.9 for appropriate conversion to SI units.) Develop a pile capacity versus set criterion for driving a 30 ft concrete pile of 10 in. diameter using a hammer with a stroke of 1 ft and a ram weighing 30 kips (kilopounds). The weight of the concrete pile=¼ π(10/12)2(30)(150)(0.001) kips=2.45 kips Assume the following parameters: n=0.3 Hammer efficiency=50% Substituting in Equation (9.1), 9.3.2 Use of the Wave Equation With the advent of modern computers, the use of the wave-equation method for pile analysis, introduced by Smith (1960), became popular. Smith’s idealization of a driven pile is elaborated in Figure 9.4. The governing equation for wave propagation can be written as follows: (9.2) where ρis the mass density of the pile, E is the elastic modulus AP is the area of cross section of the pile, u is the particle displacement, t is the time, z is the coordinate axis along the pile and R(z) is the resistance offered by any pile slice, dz. The above equation can be transformed into the finite-difference form to express the displacement (D), the force (F), and the velocity (υ), respectively, of a pile element i at time t as follows: D(i, t)=D(i, t−Δt)+V(i, t−Δt) (9.3) F(i,t)=[D(i, t)−D(i+1, t)]K (9.4) V(i, t)=V(i, t−Δt)+[Δtg/w(i)][F(i−1, t)−F (i, t)−R(i, t)] (9.5) Page 369 FIGURE 9.4 Application of the wave equation.

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