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EVALUATION OF EXISTING DRILLED SHAFTS WITH IMPULSE RESPONSE METHOD Prepared by Richard J. Finno
Northwestern University Civil Engineering DepartmentA Summary of a Presentation made at the 3rd ITI Bridge NDE Users Group Conference
Held at CALTRANS headquarters, Sacramento, CA
June 24, 1994
There are several non-destructive evaluation techniques commonly used for quality control of driven concrete piles and drilled shafts where the pile heads are accessible. The most common of these are parallel seismic, sonic logging, sonic logging and impulse response techniques. These methods are based on the propagation of stress waves through concrete. All the four tests are limited to some degree in their application. When evaluating existing foundations, the presence of a pile cap or other structure often times makes the pile heads inaccessible and the introduces further complications with the interpretation of the results. The parallel seismic and sonic logging techniques are direct transmission methods. The parallel seismic method relies on measuring the first arrival of stress waves generated at the surface of a structure with a transducer lowered down a core hole placed adjacent to the structure under evaluation. The main limitation of this method is the cost of installing the core hole. The sonic logging method involves measuring the propagation time of ultrasonic signals between vertical tubes cast into a shaft during construction. The main limitation, as with the parallel seismic method, is the cost of installing the access tubes, particularly when they are not placed as the shaft is constructed. The sonic echo and impulse response techniques are surface reflection methods in that they rely on measuring reflections of stress waves generated at the surface of the structure that reflect off either boundaries of the structure or anomalies within the structure. The main limitation of these two methods is the signal attenuation which inevitably occurs, especially in structures founded in very stiff soils. Because the impulse response methods appear to have the most technical and economic potential of the four methods for evaluating existing drilled shafts where access to the shaft head is precluded by the presence of a pile cap or other structure, it is evaluated in more detail. The impulse response is a stress wave reflection method which relies on the measurement of both stress wave reflections and low-strain hammer impact force. Prior to a test, the surface of the pile head must be prepared at the hammer impact point at the center of the shaft and at the geophone location near the perimeter of the shaft about 3 to 6 inches from the edge. The hammer and the geophone are connected to a data acquisition card inside a PC. The hammer impact triggers the PC to start recording both the force of the hammer and the velocity measured by the geophone. The hammer impact induces transient vibrations with frequencies as high as 2000 Hz. Both the force and velocity responses are digitally converted with a fast fourier transform to the frequency domain for analysis. Results are presented as a plot of velocity divided by force, or mobility, versus frequency. The frequency change between the resonant peaks allows one to calculate the length of the shaft assuming one knows the longitudinal wave speed in the shaft. The average value of the mobility allows one to compute the shaft impedance, which is a function of the density of the concrete, the shaft cross-sectional area and the longitudinal velocity of the stress wave through the concrete. Additionally, the low strain dynamic stiffness can be calculated from the low frequency portion of the mobility versus frequency curve. A case study describes impulse response tests obtained on a number of drilled shafts both after the shaft was constructed and after grade beams and walls were built. A total of 16 shafts were evaluated in the inaccessible-head condition. Free-head results were obtained for 12 of these shafts. The shafts ranged from 55.8 to 65.3 ft in length and were nominally 2.5 to 8 ft in diameter. All shafts had 60 degree bells at the base which were set in a hard clay/dense silt stratum. All shafts were temporarily cased resulting in the upper 20 to 27 ft of the shafts to be enlarged by at least 0.5 ft. All the foundation concrete had a specified design strength of 4,000 psi, but actual strengths ranged from 4,700 to 7,100 psi. The toe of the shaft could be identified for all the free-headed tests which had length to diameter ratios from 10 to 16. The ability to determine shaft integrity for the inaccessible shafts depended on both the configuration of the grade beams and the length to diameter ratio of the shafts. The toe of the shaft could be identified for thickness of pile cap to diameter of shaft ratios as high as 2.7. In general, the more rigidly a shaft head was held, the greater the signal attenuation. Larger grade beams and multiple grade beams attached to the same shaft head increased the attenuation of the shaft response. To provide an opportunity to analyze the results of inaccessible head tests in a more controlled environment, a test section is to be constructed at the National Geotechnical Engineering Test Site at Northwestern University. This test section will consist of 5 drilled shafts with diameters ranging from 2 to 3 ft and lengths varying from 40 to 90 ft. Individual shafts in two groups of two shafts each will be connected with pile caps and one single shaft also will be constructed with a cap. Provisions for sonic logging and parallel seismic tests will be included in the test section. Two of the shafts will be constructed with defects - a reduced cross-section and a thin, soil-filled joint. The former defect represents a typical construction deficiency while the latter represents a performance-related defect induced by excessive lateral loads. The main variables to be considered in the test section are the length to diameter ratio of the shafts, the pile cap depth to shaft diameter ratio, and the bottom condition of the shafts (soft clay, hard clay or dolomite).
Subject index terms
- Piles and foundations
Author contact information
Richard J. Finno
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