Monday, February 11, 2013

Concrete Piles.

Concrete piles are the most widely used in the developed countries and may be cast in situ, precast, reinforced and prestressed.

(a) Precast
This type is commonly used where:

(i) The length required can be realistically predicted.
(ii) Lateral pressure from a stratum within the soil profile is sufficient to squeeze (neck) a cast-in-situ pile.
(iii) Where there are large voids in sections of the soil which would possibly have to be filled with an excessive amount of in situ concrete or could cause loss of support for wet concrete prior to setting.
(iv) For structures such as piers and jetties above water level on coastal, estuary and river sites.

Though precast piles can be manufactured on site it is  more common to have them designed, manufactured and installed by specialist subcontractors.

There are disadvantages in the use of precast concrete piles as follows:

(i) It is not easy to extend their length.
(ii) They are liable to fracture when driven into such obstacles as large boulders in boulder clay and the damage can remain out of sight.
(iii) Obstructions can cause the pile to deflect from the true vertical line.
(iv) There is an economic limit, restricted by buckling, of the unrestrained length of the pile.
(v) Noise and vibration caused by driving can cause nuisance and damage.
(vi) There can be large wastage and health and safety risks to the workforce caused by noise and vibration due to the need to cut off the projecting length after driving.
(vii) The accuracy of the estimated length is only proved on site when short piles can be difficult to extend and long piles can prove to be expensive and wasteful.
(viii) The relatively large rig required for driving often needs extensive hard-standings to provide a suitable surface for pile driving.

The advantages of precast concrete piles are:

(i) It is easier to supervise the initial quality of construction in precast than in situ.
(ii) The pile is not driven until the concrete is matured.
(iii) Stresses due to driving are usually higher than those due to foundation loading so that manufacturing faults are more easily discovered and, in effect, the  pile is preload tested (provided the defects can be detected).

The reinforcement, while adding to the load-bearing capa- city, is mainly designed to cope with handling, transporting and driving stresses.

(b) Cast in situ
There is an ever increasing variety of cast in situ piles offered by specialist piling subcontractors. The piles are
usually circular in cross-section and are regarded as small- diameter piles when their diameters are from 250–600 mm and larger-diameter piles when their diameters exceed  600 mm; large-diameter piles are now possible with dia- meters up to 3.0 m.

The advantages of cast in situ piles are:

(i) They can be constructed immediately, thus cutting out the time required for casting, maturing and delivering of precast piles.
(ii) There is no need to cut off or extend excessive lengths of the piles as they can be cast in situ to the required level.
(iii) They can be cast to longer lengths than is practical with precast piles.
(iv) Most obstructions can be hammered and broken through by the pile-driving techniques.
(v) The placing can cause less noise vibration and other disturbance compared to driving precast piles.
(vi) Soil taken from boring can be inspected and compared with the anticipated conditions.

The disadvantages of cast in situ piles are:

(i) It can be difficult to place and ensure positioning of any necessary reinforcement.
(ii) Concrete quality control is more difficult.
(iii) There is a danger of necking from lateral earth pressure.
(iv) Young concrete is susceptible to attack from some soil chemicals before it has set and hardened.

(c) Prestressed
Prestressed concrete in superstructure design is made of higher strength concrete, requires smaller cross-sectional area and can be made impact-resistant. The same results apply to prestressed piles relative to comparison with pre- cast reinforced piles. Their advantages compared to precast reinforced are:

(i) Handling stresses can be resisted by a smaller cross- section which can result in a more economical pile.
(ii) It is easier with the smaller section to achieve longer penetration into load-bearing gravels.
(iii) Tensile stresses that are generated up from the toe of the pile after the hammer blow can be compensated for by prestress.
(iv) The reduction of tensile cracking of the concrete can lead to greater durability.

The disadvantages of prestressed piles are:

(i) The smaller section provides less end bearing and total peripheral skin friction.
(ii) Deeper penetration into end-bearing strata (gravel, compact sand, etc.) may be necessary.
(iii) It is more difficult to extend the length of a precast driven pile.
(iv) As in prestressed concrete superstructure elements, stricter quality control in manufacture is necessary.

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