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Important Guidelines To Design A Pile Foundation

Allowable Load

The load may be applied to a pile after taking into account its ultimate load capacity, group effect, allowable settlement, negative skin friction and other relevant loading conditions.

Bored Cast In-situ Pile —

A pile is formed by boring a hole in the ground by a percussive or rotary method with the use of temporary/permanent casing or drilling mud and subsequently filling the hole with reinforced concrete.

Initial Load Test

A test pile is tested to determine the load-carrying capacity of the pile by loading either to its ultimate load or to twice the estimated safe load.

Initial Test Pile —

One or more piles, which are not working for piles, may be installed if required to assess the load-carrying capacity of a pile. These piles are tested either to their ultimate load capacity or to twice the estimated safe load.

Routine Test Pile

A pile that is selected for load testing may form a working pile itself if subjected to routine load test up to one and 1.5 times the safe load.

Safe Load

It is the load derived by applying a factor of safety on the ultimate load capacity of the pile or as determined from the load test.

Ultimate Load Capacity

The maximum load which a pile can carry before failure, that is, when the founding strata fail by shear as evidenced from the load settlement curve or the pile fails as a structural member.

Working Load —

The load assigned to a pile as per design.

Depth of Borehole —

This should generally be not less than 10 m beyond the pile founding level.

How to determine the depth of liquefaction?

Design details of pile foundation

The design details of the pile foundation shall give the information necessary for setting out and layout of piles-

Cut-off levels,
Finished cap level,
Layout and orientation of pile cap in the foundation plan and
The safe capacity of each type of pile, etc.

How Boring Operation is done on the site?

Boring operations are generally done by percussion type rigs or rotary rigs using direct mud circulation or reverse mud circulation methods to bring the cuttings out. In soft layers and loose sands, the bailers and chisel method should be used with caution to avoid the effect of suction. For percussion boring using bailer chisel and for rotary rigs, stabilization of boreholes may be done either by the circulation or suspended mud.

What is Kentledge?

 – Deadweight used for applying a test load on a pile.

Vertical Pile Capacity –

The load-carrying capacity of a pile depends on the properties of the soil in which it is embedded. Axial load from a pile is normally transmitted to the soil through skin friction along the shaft and end-bearing at its tip

Horizontal Pile Capacity –

A horizontal load on a vertical pile is transmitted to the soil primarily by a horizontal sub-grade reaction generated in the upper part of the shaft. The lateral load capacity of a single pile depends on the soil reaction developed and the structural capacity of the shaft under bending. It would be essential to investigate the lateral load capacity of the pile using appropriate values of the horizontal sub-grade modulus of the soil.

How to determine the pile capacity?

The ultimate load capacity of a pile should be estimated by means of static formula based on soil test results. Pile capacity should preferably be confirmed by initial load tests [see IS 2911 (Part 4)].

How to determine the pile settlement?

The settlement of pile obtained at safe load/working load from load-test results on a single pile shall not be directly used for estimating the settlement of a structure.

The settlement may be determined on the basis of subsoil data and loading details of the structure as a whole using the principles of soil mechanics.

How to determine Vertical Load Capacity?

The ultimate load capacity of a single pile may be obtained by using static analysis, the accuracy being dependent on the reliability of the soil properties for various strata. When computing capacity by static formula, the shear strength parameters obtained from a limited number of borehole data and laboratory tests should be supplemented, wherever possible, by in-situ shear strength obtained from field tests.

What are negative skin friction and Drag downforce? 

When a soil stratum, through which a pile shaft has penetrated into an underlying hard stratum, compresses as a result of either it being unconsolidated or it being under a newly placed fill or as a result of remoulding during the installation of the pile, a drag-down force is generated along the pile shaft up to a point in depth where the surrounding soil does not move downward relative to the pile shaft. The existence of such a phenomenon shall be assessed and suitable correction shall be made to the allowable load where appropriate.

Structural Capacity of Pile –

The piles shall have the necessary structural strength to transmit the loads imposed on it, ultimately to the soil. In case of uplift, the structural capacity of the pile, that is, under tension should also be considered.

Buckling Analysis of Pile –

Where a pile is wholely embedded in the soil (having an un-drained shear strength not less than 0.01 N/mm2), its axial load carrying capacity is not necessarily limited by its strength as a long column. Where piles are installed through very weak soils (having an un-drained shear strength less than 0.01 N/mm2), special considerations shall be made to determine whether the shaft would behave as a long column or not. If necessary, suitable reductions shall be made for its structural strength following the normal structural principles covering the buckling phenomenon.

When the finished pile projects above ground level and is not secured against buckling by adequate bracing, the effective length will be governed by the fixity imposed on it by the structure it supports and by the nature of the soil into which it is installed. The depth below the ground surface to the lower point of contra-flexure varies with the type of soil. In good soil, the lower point of contra-flexure may be taken at a depth of 1 m below ground surface subject to a minimum of 3 times the diameter of the shaft. In weak soil (un-drained shear strength less than 0.01 N/mm2), such as, soft clay or soft silt, this point may be taken at about half the depth of penetration into such stratum but not more than 3 m or 10 times the diameter of the shaft whichever is more. The degree of fixity of the position and inclination of the pile top and the restraints provided by any bracing shall be estimated following accepted structural principles. The permissible stress shall be reduced in accordance with a similar provision for reinforced concrete columns as laid down in IS 456.

What is Fixed and Free Head Pile?

– A group of three or more pile connected by a rigid pile cap shall be considered to have fixed head condition. Caps for single piles must be interconnected by grade beams in two directions and for twin piles by grade beams in a line transverse to the common axis of the pair so that the pile head is fixed. In all other conditions, the pile shall be taken as free headed.

The minimum centre-to-centre spacing of piles –

The minimum centre-to-centre spacing of piles is considered from three aspects, namely,

  1. Practical aspects of installing the piles,
  2. Diameter of the pile, and
  3. Nature of the load transfer to the soil and possible reduction in the load capacity of piles group.

NOTE — In the case of piles of non-circular cross-section, the diameter of the circumscribing circle shall be adopted.

In the case of piles founded on hard stratum and deriving their capacity mainly from end-bearing, the minimum spacing shall be 2.5 times the diameter of the circumscribing circle corresponding to the cross-section of the pile shaft. In the case of piles resting on the rock, the spacing of two times the said diameter may be adopted.

Piles deriving their load-carrying capacity mainly from friction shall be spaced sufficiently apart to ensure that the zones of soils from which the piles derive their support do not overlap to such an extent that their bearing values are reduced. Generally, the spacing in such cases shall not be less than 3 times the diameter of the shaft.

Reinforcement in Pile –

The design of the reinforcing cage varies depending upon the installation conditions, the nature of the subsoil and the nature of load to be transmitted by the shaft-axial, or otherwise. The minimum area of longitudinal reinforcement of any type or grade within the pile shaft shall be 0.4 per cent of the cross-sectional area of the pile shaft. The minimum reinforcement shall be provided throughout the length of the shaft.

The curtailment of reinforcement along with the depth of the pile, in general, depends on the type of loading and subsoil strata. In the case of piles subject to compressive load only, the designed quantity of reinforcement may be curtailed at an appropriate level according to the design requirements. For piles subjected to uplift load, lateral load and moments, separately or with compressive loads, it would be necessary to provide reinforcement for the full depth of the pile. In soft clays or loose sands, or where there may be a danger to green concrete due to installation of adjacent piles, the reinforcement should be provided up to the full pile depth, regardless of whether or not it is required from uplift and lateral load considerations. However, in all cases, the minimum reinforcement specified in the above para shall be provided throughout the length of the shaft.

Piles shall always be reinforced with a minimum amount of reinforcement as dowels keeping the minimum bond length into the pile shaft below its cut-off level and with adequate projection into the pile cap, irrespective of design requirements.

Clear cover to all main reinforcement in the pile shaft shall be not less than 50 mm. The laterals of a reinforcing cage may be in the form of links or spirals. The diameter and spacing of the same are chosen to impart adequate rigidity to the reinforcing cage during its handling and installations. The minimum diameter of the links or spirals shall be 8 mm and the spacing of the links or spirals shall be not less than 150 mm. Stiffener rings preferably of 16 mm diameter at every 1.5 m centre-to-centre should be provided along the length of the cage for providing rigidity to the reinforcement cage. Minimum 6 numbers of vertical bars shall be used for a circular pile and the minimum diameter of the vertical bar shall be 12 mm. The clear horizontal spacing between the adjacent vertical bars shall be four times the maximum aggregate size in concrete. If required, the bars can be bundled to maintain such spacing.

An important factor in the Design of Pile Cap –

The pile caps may be designed by assuming that the load from a column is dispersed at 45º from the top of the cap to the mid-depth of the pile cap from the base of the column or pedestal. The reaction from piles may also be taken to be distributed at 45º from the edge of the pile, up to the mid-depth of the pile cap. On this basis, the maximum bending moment and shear forces should be worked out at critical sections. The method of analysis and allowable stresses should be in accordance with IS 456.

The pile cap shall be deep enough to allow for the necessary anchorage of the column and pile reinforcement.

The pile cap should be rigid enough so that the imposed load could be distributed on the piles in a group equitably

In the case of a large-cap, where differential settlement may occur between piles under the same cap, due consideration for the consequential moment should be given.

The clear overhang of the pile cap beyond the outermost pile in the group shall be a minimum of 150 mm.

The cap is generally cast over a 75 mm (as per IS 2911-1-2-2010) thick levelling course of concrete. The clear cover for the main reinforcement in the cap slab shall not be less than 60 mm.

The embedment of the pile into the cap should be 75 mm.

The design of the grade beam if used shall be as given in IS 2911 (Part 3).

What are the steels used in the Pile Foundation?

Reinforcement steel shall be any of the following:

  1. Mild steel and medium tensile steel bars conforming to IS 432 (Part 1),
  2. High strength deformed steel bars conforming to IS 1786, and
  3. Structural steel conforming to IS 2062.

When very soft marine clay or loose sand exists at bed level, it should be checked for potential liquefaction during an earthquake.

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