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Transportation Curriculum Coordination Council |
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Chapter 10
Post Installation Integrity Testing
Contents
- This Chapter contains an overview of the post installation integrity testing methods.
- A short quiz is provided at the end of the Chapter.
Learning Objectives
When you have completed this Chapter, you will be able to:
- Identify and describe, in general, the various post installation integrity and load tests
Now the shaft is in, we need to ascertain its structural integrity. Will it carry the load it was designed for? Are there defects within the shaft caused by errors in construction?
There are two basic methods to test shafts, those being:
- Load Tests - these are test to determine if the shaft, as constructed, will carry the loads designed for.
- Integrity Tests - these are tests to evaluate the soundness or "integrity" of the constructed shaft.
Typically the Inspector is not involved in these post installation tests, except to document they have been completed. However, in some instances, the specifications may require some involvement.
Load Tests
Load tests come in several different types, which are used to determine different load carrying or resistance capacity. The three typically methods of load tests are:
Axial load tests - tests to determine if the shaft can carry the load imposed without settling.
Lateral load tests - these are test that test the shafts resistance to lateral forces.
Uplift tests - these tests are the opposite of axial, in that rather than push downward on the shaft, it is pulled upward to determine its resistance to being "pulled out".
Pictured above is a Statnamic Load test being performed on a shaft for a new bridge.
Pictured here is a lateral load test on a shaft to verify ship-impact capacity.
In these tests, reaction loads are jacked against, applying loads incrementally, and the movement measured and documented.
For axial, the load is applied downward.
For lateral tests, the load is applied from the side.
For uplift, the load is applied upward, as in pulled.Some of the more common tests used are Static Load Test, Statnamic Load test and Osterberg Load test.
Static Load Test
The photo above is of a typical, simple arrangement for loading a drilled shaft laterally. Two companion shafts are used to support the load from the reaction beam. The test shaft is pushed away from the reaction shafts, not pulled toward them (which might produce excessive stress overlaps in the soil). In a conventional test, shown at the right, reaction (anchor) shafts are installed on either side of the test shaft (two or four can be used). The anchor shafts should normally be constructed first. Hydraulic jacks are placed on top of the test shaft, usually on a steel plate that is carefully leveled. A reaction frame spans the anchor shafts, as shown. Potential disadvantages of this method are that it is relatively expensive compared to the other methods and the capacity is limited because of the use of the reaction frame. The conventional method can also be used to conduct uplift, or "pullout" test.
In a conventional test, shown above, reaction (anchor) shafts are installed on either side of the test shaft (two or four can be used). The anchor shafts should normally be constructed first. Hydraulic jacks are placed on top of the test shaft, usually on a steel plate that is carefully leveled. A reaction frame spans the anchor shafts, as shown. Potential disadvantages of this method are that it is relatively expensive compared to the other methods and the capacity is limited because of the use of the reaction frame. The conventional method can also be used to conduct uplift, or "pullout" test.
Statnamic Load Test
An alternate way of testing drilled shafts is the Statnamic® test method. The principle of operation is shown to the right. Heavy masses on top of the shaft are accelerated upward by a propellant. This produces a force against the masses equal to the mass of the accelerated masses time the magnitude of the acceleration and an equal and opposite force on the top of the shaft. On the lower right is a photo of a Statnamic test being performed.
Pictured above are the reaction weights (rings) and the propellant (charge) for a Statnamic Test.
Pictured to the right is a Statnamic Test just after the charge was setoff. The rings, which are now above the casing were originally set even with the top of the casing
Osterberg Load Cell
In the Osterberg Cell method the cell must be cast into the shaft at the time of construction, which means that the shafts to be tested must be identified in advance, unlike the Static or Statnamic.
Shown above is the principle of the operation of the Osterberg Cell. The Osterberg Cell rests on top of the reaction socket. Other configurations can be used to test end bearing only or to test both end bearing and side resistance.
This photo shows one 3000-ton cell being used to test a socket in soft rock. The socket diameter is 60 inches, so the 2-inch steel plates on either side of the Osterberg Cell are 59 inches in diameter. In this case the objective of the test was to find the ultimate side shearing resistance in the soft rock.
Integrity Tests
Just as Load tests come in several different types, so do Integrity tests. Most are non-destructive and are used to identify anomalies or defects in installed shafts.
Performing Sonic Echo test. Larger hammers, such as in this case, are used for deeper tests
Very large defect in shaft found by Sonic Echo test.
The most commonly used Integrity Tests are:
Sonic Echo / Impulse-response- test is performed on installed shaft- quick, easy and inexpensive.
Above is a schematic of a pulse-echo (sonic-echo) test. The principle is obvious from the sketch. Advantages of the test are that it can be done on virtually any shaft without prior planning (no access tubes need be placed in the shaft) and is quick and inexpensive. Disadvantages are that it is prone to showing false positives and to missing fairly large voids or inclusions in the concrete. It is essentially 100 per cent accurate only if the void or inclusion covers about half of the cross-sectional area of the shaft and is reasonably thick (say 18 inches (0.5 m) or thicker) and the test is performed correctly. This test is not usually effective in locating deep defects (depth > 60 feet (20 m) and cannot detect contact problems between the concrete and the soil or rock. False positives in this method come from changes in cross-section that are not associated with an anomaly, from changes in concrete modulus (such as at the interface between concrete placed from two different trucks), from changes in the stiffness of the soil or rock surrounding the shaft, which also dissipate sonic energy, and from testing technique errors such as setting the sensor on weak or powdery concrete.
Sonic Echo test being performed on a shaft over water. Note the small hammer being used to strike the shaft.
Cross-hole Acoustic (CSL) - these are test are performed in the access tubes installed on the rebar cage and is much more accurate than Sonic Echo testing.
A primary use of access tubes is in the performance of cross-hole acoustic tests (usually ultrasonic in air but sonic in concrete), sometimes called cross-hole sonic log tests or CSL tests. "Shots" are made from a source that generates acoustic energy to an energy receiver in another tube at the same elevation, as depicted to the right. Both the time of travel from the source tube to the receiver tube and the amount of energy transferred between tubes are indicators of the presence of either sound concrete or defective concrete. Good coverage of the interior of the cage can usually be achieved, however, little information on concrete outside the cage can be obtained.
Several variations on this method are practiced by highly skilled specialists, involving placing source and receiver at different elevations to develop a three-dimensional profile of the interior of the shaft, in a process referred to as tomography.
This method can be performed fairly quickly and is often more definitive than the pulse-echo method. However, as mentioned above, shafts to be tested must be identified in advance of construction to permit installation of the access tubes.
Sensors used in performing the CSL test.
Gamma-Gamma - these tests are also performed in access tubes with a nuclear density instrument. This test is also more definitive than the Sonic Echo test.
Another successful down-tube integrity test is the gamma-gamma, or backscatter gamma test, illustrated to the right. The device is a nuclear density meter that must be calibrated frequently. It measures density in the concrete to about 100 mm (4 inches) from the edge of the tube. Newer devices can reportedly measure density to about twelve inches from the tube, but that characteristic is of little use if the tube is less than twelve inches from the edge of the shaft. A disadvantage of the device is that it does not "shoot" across the shaft as does a CSL device, so it does not test the entire cross-section, and it is sensitive to being placed too close to a longitudinal rebar. Otherwise, it is a very definitive test.
This, like the CSL tests, requires advance identified of the shafts to be tested to allow for access tube installation.
Nuclear density source being lowered into access tube for gamma-gamma testing.
Coring - this is the most destructive of the common tests as a drill rig cores the shaft and the retrieved concrete cores are examined. This can be performed on any shaft and does nor require pre-installed instrumentation.
Coring of drilled shafts can be used as an independent integrity test method, or it can be used to attempt to confirm the presence of defects that appear as anomalies on pulse-echo records.
Coring is performed by setting a drill rig over the finished shaft, and then performing continuous core runs, typically 5 ft. (1.5 m) in length, to the bottom of the shaft. The individually retrieved cores are then set out, end to end, which gives a picture of the shaft concrete, etc.
The bottom left picture is an unacceptable shaft, based upon coring results and the bottom right picture is an acceptable shaft. Notice how the cores from the acceptable shaft are more intact and solid.
Coring is not full-proof, however, as cores can bypass serious defects. So, coring is a way of potentially confirming that the shaft is defective but not that it is not defective.
Very careful coring is sometimes an effective way to investigate whether there is a soft base in the drilled shaft.
Unacceptable
Acceptable
I you have completed Chapter 10 and am ready to take the Quiz
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