Pile Load Testing

Pile Load Testing - Pile Load Testing Foundation Design CE...

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Unformatted text preview: Pile Load Testing Foundation Design CE 482 Introduction sophisticated structural analyses using “rough” geotechnical design parameters heterogeneity of soil (unlike construction materials) geotechnical design often empirical conservative factors of safety site-specific performance testing more aggressive design, verified safety factors Types of Load Tests proof-test pile is loaded to the ultimate design load (allowable design load times the factor of safety) and the deflection is measured at the pile head generally performed during installation load-deformation test pile typically tested to failure, deformation (and often stress) measured at several points along the pile shaft and at the pile tip as well as at the pile head detailed load-deformation data obtained allows more efficient design usually performed during the design phase Types of Load Tests static load pile & measure deflection dynamic static pile capacities generally using stress wave analyses of pile deflection caused by dynamic loads Static Pile Load Tests Conventional construction of reaction frame and incremental loading of the pile, usually with a hydraulic jack [Figure 1] test load measured with load cells and pile head deformation measured with strain gages and surveying equipment . Figure 1: typical setup for compressive testing Static Pile Load Tests Conventional uplift and lateral load tests performed by modifying reaction frame and loading (jacking) direction [Figures 2 and 3] costly and time-consuming but generally provide the most reliable performance data because the loading method is similar to service loading Figure 2: typical setup for uplift test Figure 3: typical setup for lateral test Static Load Tests Osterberg Cells often more cost-effective than conventional static load tests Osterberg cells are in essence largediameter hydraulic jacks with built-in load cells cast within the pile with twin reaction plates similar in diameter to the drilled pile at the top and bottom of the cell movement is measured using strain gages and reference rods isolated from strain (sleeved) extending from the top of Osterberg cells to the ground surface Static Load Tests Osterberg Cells typically not used for uplift testing because conventional uplift tests are generally less expensive Osterberg cells cost-effective for compression and lateral load tests because reaction piles or anchor are not required Static Pile Load Tests Osterberg Cells single cells typically used for compression proof tests load cell cast near the bottom of the pile and expanded to obtain load and deflection data some interpretation of the data is required because the test loading is differently from service loading Static Pile Load Tests Osterberg Cells during the test, the cell is expanded near the bottom of the shaft, causing uplift above the cell and settlement below the cell because cell loads are resisted by shaft resistance above the cell and pile end bearing below the cell, load-deformation data for the pile tip and pile shaft can be obtained independently multiple cells can be cast within a test pile to isolate end-bearing and shaft friction effects or to evaluate directional effects of shaft friction [Figure 4] Figure 4: installation of Osterberg cell at pile tip Dynamic Load Tests developed from research funded by the Ohio Department of Transportation and the Federal Highway Administration at the Case Institute of Technology in Cleveland, Ohio using measurements of strain and acceleration and the principles of wave mechanics, dynamic test methods are used to estimate static pile capacity, inspect pile integrity, and evaluate pile driving systems Dynamic Load Tests two types of dynamic pile testing: large-strain and low-strain methods low-strain methods are typically performed using hand-held hammers that measure pile top velocities and are used mainly to inspect integrity and length of concrete piles. Anomalies in the velocity record are used to evaluate pile integrity. low-strain methods to inspect pile integrity are limited to depths of about 30 times the pile diameter Dynamic Load Tests Large-Strain Methods Large-strain methods are used almost exclusively for driven piles to evaluate the driving system as well as for estimating static axial pile capacity Strain gauges and accelerometers are installed near the top of the piles and measurements are taken during pile driving Dynamic Load Tests Large-Strain Methods typically performed during the indicator pile program (The indicator pile program is a field test of the selected driving hammer and system to evaluate the driving criteria, driveability, and production rate) Dynamic Load Tests Large-Strain Methods because the cost of installing the strain gauges and accelerometers and monitoring the measurements is relatively inexpensive compared to the total cost of the indicator pile program, dynamic pile testing is a cost-effective way of optimizing the driving system and estimating static pile capacity for driven piles, optimization of the driving system may be as important as estimating pile capacities A typical setup is presented in Figure 5 Figure 5: typical PDA setup Dynamic Load Tests Large-Strain Methods measurements of strain are converted to force and the measurements of acceleration are converted to velocity for input into dynamic resistance equations to estimate static pile capacities. the most popularly used dynamic resistance equation is the Case Method (Goble et al., 1975) Dynamic Load Tests When a hammer or drop weight strikes the top of a foundation, a compressive stress wave travels down its shaft at a speed c, which is a function of the elastic modulus E and mass density The impact induces a force F and a particle velocity v at the top of the foundation The force is computed by multiplying the measured signals from a pair of strain transducers attached near the top of the pile by the pile area and modules Dynamic Load Tests The velocity measurement is obtained by integrating signals from a pair of accelerometers also attached near the top of the pile Strain transducers and accelerometers are connected to a Pile Driving Analyzer® (PDA), for signal processing and results Dynamic Load Tests As long as the wave travels in one direction, force and velocity are proportional: F = Zv, where: Z = EA/c is the pile impedance E is the pile material modulus of elasticity A is the cross sectional area of the pile c is the material wave speed at which the wave front travels Dynamic Load Tests Soil resistance forces along the shaft and at the toe cause wave reflections that travel and are felt at the top of the foundation The times at which these reflections arrive at the pile top are related to their location along the shaft The measured force and velocity near the pile top thus provide necessary and sufficient information to estimate soil resistance and its distribution Dynamic Load Tests Total soil resistance computed by the PDA includes both static and viscous components The static resistance can be obtained by subtracting the dynamic component from the total soil resistance The dynamic component is computed as the product of the pile velocity times a soil parameter called the Damping Factor The damping factor is an input to the PDA and is related to soil grain size Dynamic Load Tests Total soil resistance computed by the PDA includes both static and viscous components The static resistance can be obtained by subtracting the dynamic component from the total soil resistance The dynamic component is computed as the product of the pile velocity times a soil parameter called the Damping Factor The damping factor is an input to the PDA and is related to soil grain size Dynamic Load Tests The energy delivered to the pile is directly computed as the work done on the pile from the integral of force times incremental displacement ( ∫Fdu ) which is easily evaluated as force times velocity integrated over time ( ∫Fvdt ) Maximum compression stresses at the pile top come directly from the measurements The measurements also allow direct computation of the compression stress at the pile toe and the tension stresses along the shaft Dynamic Load Tests Pile integrity can be evaluated by inspecting the measurements for early tension returns (caused by pile damage) prior to the reflection from the pile toe; lack of such reflections assures a pile with no defects Dynamic Load Tests Large-Strain Methods a specific hammer blow can be analyzed using the Case Method and a soil model to estimate the shaft friction, end bearing, dynamic damping factors, and soil stiffness a computer program called CAPWAP® for Case Pile Wave Analysis Program from Goble, Rausche, Likins and Associates, Inc. can be used to perform this analysis the compression and uplift static pile capacities can then be estimated Dynamic Load Tests Large-Strain Methods although dynamic testing can used to estimate static pile capacity for drilled piles, the cost of mobilizing a pile-driving hammer and rig may make it inappropriate Weap In the 1950’s, E.A. Smith of the Raymond Pile Driving Company developed a numerical analysis method to predict the capacity versus blow count relationship and investigate pile driving stresses The model mathematically represents the hammer and all its accessories (ram, cap, cap block), as well as the pile, as a series of lumped masses and springs in a one-dimensional analysis The soil response for each pile segment is modeled as viscoelastic-plastic Weap All components of the system are thus realistically modeled The analysis begins with the hammer ram falling and attaining an initial velocity at impact This method is the best technique for predicting the relationship of pile capacity and blow counts (or set per blow), and the only method available to predict driving stresses Weap Improvements to Smith’s method include work by GRL to incorporate a thermodynamic diesel hammer model and residual stresses The GRL Wave Equation Analysis of Piles (GRLWEAP) program is based on Smith’s method The wave equation approach it is an excellent predictive tool for analysis of impact pile driving, but it has some limitations These are mainly due to uncertainties in quantifying some of the required inputs, such as actual hammer performance and soil parameters Pseudo-Static Tests impact energy prolonged to better simulate static loading Statnamic pile loaded by acceleration of reaction mass with ignition of propellant fuel Pile Load Tester spring system used to dampen impact and delay (prolong) energy transfer onto pile ...
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