The Pile Load Test is the most reliable method of determining the load carrying of a pile. This test can be performed either on a working pile that forms the foundation of the structure or on a test pile.
Loads on Piles:
Following are the loads which are to be taken into account while designing a pile.
- Direct vertical load coming from the superstructure.
- Impact stresses developed during the process of pile driving.
- Stresses developed during handling operations.
- Bending stress developed due to the curvature of a pile.
- Bending stresses developed due to the eccentricity of loads coming on the pile.
- Lateral forces due to the wind, waves, currents of water, etc.
- Impact forces due to the ice sheets or bergs.
- Impact forces due to ships, in case of marine structures.
- Force due to the uplift pressure.
- Earthquake forces.
Pile Load Test Method:
The sets up for the load test on a pile consist of two anchor piles provided with an anchor girder or a reaction girder at their top as shown in Fig. The test pile is generally installed between two anchor piles in such a manner in which the foundation piles are to be installed. The test pit should be at least 3B or 2.5 m clear from the anchor piles.
The toad is applied through a hydraulic jack resting on the reaction girder. The measurements of the settlement of the pile are recorded with the help of three dial gauges, with respect to a fixed reference mark.
The test is conducted after a period of 3 dales after installation of the test pile in sandy soils, and after a period of one month after the installation of the test pile in silts and soft days.
This is because by driving the test pile the soil properties are altered and with the passage of time much of the original properties are restored.
The load is generally applied in an equal amount of increment and that is about 20 % of the allowable load. Settlements should be recorded with three dial gauges.
Each load increment is maintained till the rate of movement of the pile is not more than 0.1 mm per hour in sandy soils and 0.02 mm per hour in layer soils or a maximum of two hours (IS: 2911 — 1979).
For each load inclement settlements are observed at 0.5, 1, 2, 4, 8, 12, 16, 20, 60 minutes. The loading should he continued up to twice the safe load or the load at which the total settlement reaches a specified value.
The load is removed in the same decrements at 1 hour interval and the final rebound is recorded after 24 hours after the entire load has been removed.
The measured values of the settlement are plotted against the corresponding values of Load to obtain the load settlement curve. Fig. shows a typical load settlement curve (firm line) for loading as well as unloading obtained from a pile load test.
For given load, the net settlement (Sn) is given by,
Sn = St – Se
Where, Sn = Net Settlement
St = Total Settlement or Gross Settlement
Se = Elastic Settlement (rebound)
Fig. shows two loads-settlement curves obtained from a pile load tests on two different soils. The ultimate load Qu may be determined as the abscissa of the point where the load settlement curve changes to a steep straight line. Alternatively, the ultimate load Qu is the abscissa of the point of intersection of initial and final tangents of the load settlement curve. The allowable load is usually taken as one-half of the ultimate load.
According to IS: 2911 a 1974 (Parta4), the allowable load may be taken as one of the following whichever is less.
- 50 % of the load at which the total settlement is 10 % of the diameter of the pile.
- Two—thirds of the final load at which total settlement is 12 mm.
- Two–thirds of the load which causes a net settlement of 6 mm.
The limiting settlement criteria are also sometimes specified. Under the load twice the allowable load, the net settlement should not exceed 20 mm or the gross settlement should not exceed 25 mm.
Causes of Failure of Piles:
Following are the most common causes of failure of piles:
- Absence of statistical data regarding the nature of soil strata through which the pile is to be driven.
- The actual load coming on the pile is more than the design load.
- Bad workmanship in case of cast-in-situ cements concrete piles.
- Attack by insert etc. on wooden piles.
- Breakage due to over especially in case of the timber piles.
- Buckling of piles due to removal of side support, inadequate lateral support, etc.
- Lateral forces (wind, waves, currents, etc.)
- Damage due to abrasion resulting from the absence of suitable protective covering.
- Improper choice of types of piles.
- Improper choice of the method of driving the pile.
- Improper classification.
- Insufficient reinforcement or misplacement in the case of the R. C. C. piles.
- The pressure of soft strata just below the tips of piles.
- Misinterpretation of the results obtained during the pile load test.
- Wrongful use of pile formula for determining its load-bearing capacity.
Some Predominant Causes of Failure of the R. C. C. Piles:
- Improper designed concrete mix.
- Use of the wrong type of cement for the corrosive environment etc.
- Insufficient concrete cover to reinforcement.
- Early removal of concrete forms to expose green concrete to attack of salts
- Use of aggregate that reacts with the types of cement.
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