Thursday 3 June 2010

Piling, Diaphragm and Retaining Wall Systems

Sheet Piling

Sheet piles are normally formed of reinforced concrete or steel

Reinforced concrete sheet piles are of value in the construction of permanent embankments to rivers, canals and other forms of water-related structures.

An interlocking mechanism joins piles together

Steel sheet piling is the most common form of sheet piling and is used in both temporary and permanent works.

It is used in such structures as cofferdams, retaining walls, river frontages, quays, wharves, dock and harbour works, land reclamation and sea defence works.

Advantages are: high structural strength combined with watertightness and can be easily driven into most types of ground.

Steel sheet piles are available in four basic forms in the UK:

· Normal sections

· Straight web sections

· Box sections

· Composite sections or high modulus piles.

Normal section sheet piles include Larssen and Frodingham sheet piles.

Straight web piling is used to construct cellular cofferdams.

Such piles are interlocked and driven to form cells which are then filled with gravel or broken rock.

Box piles are formed from two or more sheet pile sections welded together.

Used where local heavy loads are anticipated and can be positioned in a normal section pile wall

Composite sheet piling or high modulus section piling has been developed to support bending moments which are in excess of the capacity of normal sheet pile sections.

Methods of driving

When piles are being driven they have a tendency to lean in the direction of driving;

There are various ways in which piles may be guided during driving, the two principal methods are:

· Driving in panels

· Use of trestles and walings.

Pile driving equipment

· Percussion drivers - which includes diesel and air hammers

· Hydraulic drivers

· Vibratory drivers.

Examples of hydraulic drivers is the ‘Still Worker’ Designed by Tosa, in Japan, the ‘Still Worker’ is a hydraulic piling machine which installs and extracts sheet piles and H-piles.

Hydraulic drivers work on the principle of pushing sheet piles into the ground by means of a hydraulic ram acting against a firm reaction. The reaction is provided by the skin friction of piles already partially driven.

Vibratory drivers are very suitable for the initial driving of sheet piles, H-piles and tubular piles but must be avoided in heavy clays since the clay tends to dampen the vibrations.

The unit vibrates the piles, which reduces the resistance of the ground around them, allowing the piles to pass into the ground. The vibrations can be either low or high frequency depending on the nature of the soil - high frequency can reach 2300 vibrations per minute.

Continuous-flight augered piles with grout or concrete intrusion

Continuous-flight auger (CF A) piles are now widely available in sizes generally up 750 mm diameter, and exceptionally 900 mm diameter. The commonly used diameters are 300, 450, 600, and 750 mm. These piles offer considerable environmental advantages during construction. Vibration is minimal, and noise outputs are low. In permeable soils with a high water table, their use removes the need for concreting by the tremie method, and temporary support for the borehole walls using casing or bentonite slurry is not necessary. The method of pile construction is suitable in sands, gravels and clays.

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