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CLIENT: LAW COMPANIES GROUP, INC.
February 2002: Structural Engineer
Most structural engineers have undoubtedly questioned the merits of merging sophisticated structural analysis with foundation recommendations from geotechnical consultants that appear to be little better than general "rule of thumb" values with large and vague factors of safety.
The necessity of less precise foundation recommendations lies chiefly in the heterogeneity inherent in natural soil and in the often empirical nature of geotechnical design. Whereas the engineering properties of construction materials are relatively well defined and predictable, the engineering properties of soil and rock are usually expected to vary from location to location.
Numerous geotechnical design methodologies are based partly on theory and partly on empirical test results. Although local experience and understanding of the origins of geologic materials at project sites can help validate the applicability of specific design methodologies, without performance testing, the geotechnical engineer often has no recourse but to rely on conservative factors of safety that attempt to account for various uncertainties.
Site-specific performance tests are typically not cost-effective in most projects and using large factors of safety instead is usually a more economical way of mitigating the risk of uncertainties. However, for certain projects, performance testing of foundations may prove cost-effective and sometimes necessary. Performance testing enables more aggressive geotechnical design because actual factors of safety can be verified. More aggressive geotechnical design usually results in a reduction of the cost of foundations. In addition, performance testing may be necessary for project sites with unusual geologic materials or materials unlike those for which empirical design information is available.
Modern pile foundations are used in areas where less costly shallow spread?type foundations are not deemed feasible (pile foundations are usually used to control anticipated settlement or to extend to competent bearing materials). Because pile foundations are often used to support heavier and/or critical structures on marginal geologic materials, performance testing of pile foundations (pile load testing) is relatively common. Pile load tests are generally performed to either prove that piles are capable of sustaining the design load or to gain more detailed information that will enable a more efficient design.
TYPES OF PILE LOAD TESTS
Static and dynamic pile load tests can be performed on drilled or driven piles to evaluate either axial or lateral capacities. Static tests consist of loading piles and measuring deflection. Dynamic tests attempt to obtain static pile capacities generally using stress wave analyses of pile deflection caused by dynamic loads. The typical means and methods used in static tests and various dynamic pile load test methods, which are generally easier to perform and more economical, are discussed in the sections below.
As mentioned earlier, pile load tests are generally performed to either prove that piles are capable of sustaining the ultimate design load ("proof test") or to gain more detailed information that will enable a more efficient design ("load-deformation test"). For a proof test, a 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. If the deflection is within allowable levels, the test has "proved" that the pile is acceptable. Proof tests are generally performed during construction as the piles are installed.
Load-deformation tests, on the other hand, are usually performed during the design phase of projects prior to actual construction. For these tests, a pile is typically tested to failure and deformation (and often stress) is measured at several points along the pile shaft and at the pile tip as well as at the pile head. The detailed load-deformation data obtained allows more efficient design by reducing the factor of safety through better understanding of the site-specific properties.
STATIC PILE LOAD TESTS
Conventional static pile load tests in drilled or driven piles consist of constructing a reaction frame around the test pile and incrementally loading the pile, usually with a hydraulic jack. The reaction frame is anchored by at least two reaction piles. The test load is measured with load cells and pile head deformation is measured with strain gauges and surveying equipment.
For load deformation tests, strain gauges imbedded within the pile may be used to determine the load distribution along its length. Uplift and lateral load tests are performed by modifying the reaction frame and loading (jacking) the pile in the desired direction. Although costly and time-consuming, conventional load test generally provide the most reliable performance data because the loading method is similar to service loading.
For drilled piles, load tests using Osterberg cells may be a more cost-effective alternative to conventional static load tests. Osterberg cells are in essence large-diameter hydraulic jacks with built-in load cells that are 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 gauges and reference rods isolated from strain (sleeved) extending from the top of Osterberg cells to the ground surface. Strain gauges are also used to measure the opening of the cell.
Osterberg cells are typically not used for uplift testing because conventional uplift tests are generally less expensive in most cases. However, Osterberg cells are cost-effective for compression and lateral load tests because reaction piles or anchors are not required.
Single cells are typically used for compression proof tests. A load cell is 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. During the test, the cell is expanded near the bottom of the shaft, causing uplift above the cell and settlement below the cell.
Because the 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.
DYNAMIC PILE TESTS
The currently used dynamic pile testing methodology was 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. There are two types of dynamic pile testing: large-strain methods 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. Whereas low-strain methods to inspect pile integrity are limited to depths of about 20 times the pile diameter, large-strain methods can usually be used to evaluate the entire length of piles.
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. Large-strain dynamic pile testing is 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). 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.
The 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).
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.
Although dynamic testing can used to estimate static pile capacity for drilled piles, mobilizing a pile-driving hammer and rig is usually not cost-effective.
PSEUDO-STATIC LOAD TESTS
Test methodologies that combine the expediency of dynamic methods with loading similar to conventional load tests include the Statnamic® test and the Pile Load Tester. Although both methods are similar to dynamic tests in that test piles are impact loaded, these methods prevent wave propagation effects by spreading the transmitted energy over a longer period. Similarly to static load tests, these methods generate load-deformation (settlement) curves.
Compression and lateral pile capacities can be evaluated with the Statnamic test method. The test method consists of accelerating reaction masses in the direction opposite to the test load direction by igniting propellant fuel. A load cell measures the load and the deformation of is measured using surveying equipment. For safety reasons, reaction masses are enclosed within a metal casing filled with gravel or another materials used to dampen the return fall of the reaction masses.
Pile Load Tester
A pile load tester can only be used to evaluate compression pile capacities. For this test method, a large mass is dropped on top of the test pile. The mass with the coiled springs is dropped onto an anvil resting atop the test pile. On completion of the upward stroke (bounce), the mass is caught in its highest position by hydraulic clamps. The load is measured using a load cell and deformation is measured using surveying equipment.
The impact of the falling mass is softened and the energy transmission time is extended over a longer time period by the use of heavy coiled springs attached to the bottom of the mass. The springs enable the introduction of a slow-rising, long lasting blow to the pile without causing dynamic effects (wave propagation) present during dynamic load testing; wave propagation complicates interpretation. The springs spread the impact wave over about 200 to 400 milliseconds.
More detailed and precise geotechnical foundation recommendations can be developed using the pile load testing methods described above. The additional investment required to procure the more precise information often pays large dividends in material costs savings. At the very least, proof testing of pile foundations can provide additional peace of mind that the expected capacities in design are in fact available in the field.
Return to: 2002 Feature Stories