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Thursday, May 19, 2011

Pile testing mechanisms

In the present status quo there is a vast number of pile testing mechanisms are available. Pile testing mechanisms are divided into two major groups which are namely Pile Loading Test and Integrity Testing. The pile loading tests are carried out to determine the settlement under working load, to determine the bearing capacity and also as proof of acceptability. Integrity testing is a method which is used to test the integrity of the piles. Basically methods are available which either test the pile material itself or test the pile and soil together. Again, some methods require prior methods, whereas other does not. The main piling mechanisms can be summarized follows.

In the package one of the Southern Transport Development Project Sonic Logic test is carried out to test the piling process. The test is based on the measurement of time taken to pass a sonic pressure waves horizontally between a transmitter and a receiver. The transmitter and the receiver are made of piezoelectric ceramics which move within the two parallel, vertical, normally about 42 diameter tubes. Before the test is carried out the tubes are filled with water and the transducers are kept in the same horizontal plane. The difference in propagation times of the sound waves between the two tubes are represented as a linear line with the help of an oscilloscope. An area of weak concrete is detected by a marked fainting of signals and the unusual lengthening of the travel time.

Piling process

There are many piling processes available in the world. It may differ from the type of pile used. In the Southern Transport Development Package one, bored are used for the construction. There are seven piers whereas two constructed in the Kalu Ganga. Therefore two different procedures are carried out in the piling process.

Some of the piles are constructed in soft grounds. As a result of it many of them are end bearing piles which goes straight forward towards the bed rock. There are a couple of friction piles also. The type of pile is governed by the type of structure, ground conditions, ground water table and many more factors. But the process of piling slightly differs from the type of pile.

Piling process

Foundation piles are load-bearing members made of steel, concrete, timber or a combination of those materials. They're typically used where the foundation soils are too unstable or compressible to provide adequate support for the structure. Once it's in place, foundation piling transfers loads from the substructure to the firmer underlying layers of soil or rock.

Excavation

First, the soil within the guide casing to a depth of approximately in is removed by machine or hand excavation to permit the top of the bore to be filled with the clay/silt suspension. Then the pile boring is commenced by dropping of a hammer under only gravity while maintaining the verticality. The hammer is dropped through a height maximum up to 2m depending on the material being removed.

As the excavation is advanced gradual thickening of the clay/silt slurry occurs due to the increased amount of material in suspension. Simultaneously the bore is charged with fresh slurry. This process results in the pile bore being cleansed of suspended excavation material and excavation once again proceeds.

Cofferdam Construction

Cofferdams or temporary dikes are often required when excavation work is done in stream channels, in unstable soils that may cave in without proper support, and near existing structures that might be disturbed by the excavation operation. Section 206.09 outlines the construction requirements governing the use and design of cofferdams and of temporary dikes. Cofferdams and temporary dikes are paid for either as a part of the structure excavation pay item or as a separate pay item.

Plans for the construction of cofferdams and temporary dikes must be submitted by the contractor and approved by the Central Office. If the contractor begins construction before the plans are approved, the contractor runs the risk of not being paid for that work in the event the plans are rejected or revised.

A typical cofferdam consists of sheet piling that has been driven deep enough to prevent scour and sufficiently braced to keep water and soil out of the excavation. Cofferdams must also be large enough to permit pile driving and the construction of forms when those two items are required.

Cofferdams need to be as watertight as possible so the water that's pumped out stays out. The contractor must be careful not to pump out any concrete ingredients once concrete placement begins. No pumping is permitted for 24 hours after concrete placement unless approved by the Engineer.

They should never be constructed simply to provide a convenient working platform on which to construct the footing.

Once the footing and stem have been constructed, cofferdams can be removed. The contractor usually has the option of either pulling out the sheet piling entirely or cutting it off two feet below finished ground level. In any case, the contractor has to be careful not to disturb or damage the newly-poured footing and stem during the removal operation.

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