Quality Control of Deep FoundationsAssociation of Construction and Development
February 15, 2013 — 1,964 views
Civil structures, especially tall buildings, have deep foundations for apparent reasons of rigidity, safety, and reliability of the concerned structures. Quality control of deep foundations is a critical component in the construction of such structures.
Deep foundations need to be tested for their quality as they augment the building entirely. For this purpose, non-destructive testing methods (NDT) have been developed. As the name suggests, the non-destructive testing methods utilized do not affect the structure under testing as it does not damage or destroy the test specimen. Testing is carried out in order to identify flaws such as cracks/voids, to check the endurance of the structure, and to pre-test the structure prior to complicated tests.
Non-destructive testing can be of many types: Dynamic Load Testing (DLT), Pile Integrity Testing (PIT), and Sonic Logging by cross hole method (CSL).
Dynamic Load Testing (DLT)
The test involves striking of the head of the pile with a striking hammer or a similar object of weight. The response of the pile soil is recorded and analyzed in terms of head of pile force and velocity with the help of advanced bolt-on transducers which are reusable. This test is applicable on various types of piles. The equipment which energizes the transducers and records the outputs is so robust and weathered that it can be lodged at the back of a small vehicle which makes it portable.
There are many benefits of DLT:
Some types of data such as that on end bearing components of soil resistance and skin friction distribution, which are tedious to obtain by static load tests, can be effectively acquired by DLT.
- Pile integrity can be assessed using DLT.
- DLT can be used to investigate bizarre driving behavior.
- Operating efficiency of the driving hammer can be monitored.
- DLT is a quick and effective option.
- As many as ten piles can be tested on a single day.
- DLT induces minimal disruptions to the piling operations.
- Piles installed on water can also be tested effectively using DLT.
Pile Integrity Testing (PIT)
This test is also termed as SIT, TNO, and Acoustic Integrity Test. It helps to decide on the integrity of a pile foundation by evaluation. Major defects like cracks, necking, soil inclusions, and even unknown lengths of piles supporting the buildings can be ascertained.
The test comprises of a small hand held hammer to be struck on the head of the pile. The acceleration caused by this impact and the reflecting shock waves is recorded using an accelerometer. A time graph is generated using these data on a hand held computer which can be stored for further analysis. The signal obtained can be amplified exponentially with time to prioritize the same signal that is reflected along the pile toe and pile shaft, thus removing any signal attenuation in the pile. Changes in the signal magnitude due to discontinuities may be determined from the velocity vs. time graph. Number of tests is recommended for each pile to eliminate errors in the results.
There are many benefits of PIT:
- Early detection of defects and quick remedies. This method is a quick and economical one.
- Testing can be done on any accessible pile.
- Up to 200 piles can be tested in a single day.
Crosshole Sonic Logging (CSL)
During this test, ultrasonic probes are lowered to access tubes and as the probes are drawn out to the surface, a data processor records the ultrasonic from a source in one tube to a receiver in another tube.
The probes need to be inserted inside steel tubes (preferably) or PVC pipes (38 mm to 50mm I.D). At a point wherein energy transferred is almost constant or wherein there is a very small change in the transit time with changing depth, the acoustic parameters of the concrete between the tested tubes can be interpreted as uniform. Defects are identified whenever there is a delay in signal arrival time and lower energy at a specified depth. These defects are indicated on the screen.
The Benefits of CSL are:
Large foundation elements can be more effectively tested using CSL method. CSL tests can be efficiently performed on piles that are scheduled for static loading by top-down load application or bi-directional testing.