Pile Foundation

INTRODUCTION :

This type of foundation is useful: –

  1. In widely varying layers of strata,
  2. In different types of soils; where foundations are required to be taken to a certain depth,
  3. To avoid the undesirable effect of seasonal moisture changes, as in expansive soils
  4. To obtain adequate capacity for downward, upward and lateral loads;
  5. Or to take the foundations below scour level and for

For the satisfactory design and construction of pile foundation, the following information is necessary.

  1. Soil investigation data
  2. A quantitative indication of the expansive nature of the foundation soil, which may be obtained from the free swell
  • The general layout of the columns and load bearing walls sharing estimated loads,including moments and torques to the top of pile caps and
  1. A hydrological data, such as highest subsoil water level with seasonal variation, flood/ tide levels at different times of the year, depth of scour, amount and rate of seepage
  2. Chemical properties of soil and ground water, with particular attention to change of any deleterious effect on concrete or

The design particulars of pile foundations shall give the structural data of the orientations of the piles, the bearing capacity of each pile type, the levels at which each pile should finally be finished to suit the site conditions, details of pile caps and grid beams, together with winds coming on to them.

MATERIALS USED IN PILE FOUNDATION:-

  • Cement used in concrete piles should be OPC or as per requirements specially demanded, rapid hardening cement confirming to IS: 269-1966.
  • Steel requirement shall confirm to IS:226-1969, for bored compaction piles. The reinforcement cage shall be prepared by welding the hoop bars to withstand the driving Any reinforcement used in, cast in situ or bored piles should be made up into cages, sufficiently well wired to withstand handling without damage. The Bars should be so placed as not to impede the placing of concrete. The lateral ties or spiral binding should not be closer than 15 cm c/c. Reinforcement in the pile may be provided in the top section only or in entire length depending upon the manner of transmission of load by the pile to the soil and need not normally exceed 0.8% of the cross-sectional area of the pile. Care should be taken to maintain correct cover and alignment of reinforcement throughout the whole operation of placing of concrete.
  • Concrete:- The high slump (15 to 20 cm) concrete which is generally used shall not be less than M15 grade. For underwater concreting, the mix of concrete shall have 10% more cement than what applied for normal situations. The specifications of various grades of concrete shall be as laid down in IS: 456- 2002. Coarse aggregate may also be natural rounded shingle of approximate size. It helps to give high slump with less water-cement

Materials used should confirm to the requirement of IS 456-2002, code of practice for plain and reinforced concrete. For concrete to be placed through tremie pipe, gravel aggregates and fine aggregates from natural sources will be preferable to crushed aggregates, for better workability. For 20 cm diameter tremie pipes, a slump of 12.5 to 17.5 cm shall be allowed. If concrete is transported from a central mixing place, a higher slump will cause segregation.

PILE CAPS

  1. The pile cap along with the column pedestal shall be deep enough to allow for the necessary anchorage of column and pile
  2. The clear anchorage of the pile cap beyond the outermost pile in the group shall, normally be 10 to 15 cm, depending upon the pile
  • A levelling coarse of mass concrete of about 8 cm thickness may be provided under pile caps as
  1. The clear cover for the main reinforcement from the bottom of cap shall not be less than 6
  2. The reinforcement from the pile should be tied properly to the pile

CONCRETING

After completing the boring, the concreting shall be done as early as possible. Should a bore hole, which is not cased, be left unconcreted for more than two hours, it shall be cleaned thoroughly before placing the concrete. Concreting under water shall be done in one operation.

Concrete should preferably be placed by means of a tremie pipe in those bore holes which are not dry.

It should, however be ensured that, concrete entering the tremie pipe does not get mixed with the slurry. This is achieved by pouring ¼ kg of granulated vermiculite in the tremie pipe before pouring concrete. The vermiculite granules will form a plug separating concrete from the fluid below. The tremie pipe shall extend up to the bottom of the bore hole at the start and may be withdrawn in sections as the level of concrete rises in the bore holes, but its discharge end shall, at all times be embedded in the concrete to a minimum depth of 1 meter. Placing of concrete should be held concentric in the hole.

DISPOSAL OF MUD

The borehole should be maintained full with the drilling fluid, wherever used, throughout the concreting operation. Mud displacement from the borehole by the concrete shall be channeled away or pumped into suitable receptacles for disposal to waste or reuse.

WITHDRAWAL OF CASING:

Extraction of casing shall be done in such a way that no necking or shearing of the concrete in the shaft takes place. During the extraction of casing, slumping of concrete shall be observed and when required, additional quantity of concrete shall be poured so that the pile is formed up to at least 15 cm above the cut off level. During extraction of the casings, special slump records should be maintained.

ALLOWABLE WORKING LOADS:

  1. Vertical compressive load:-

2/3 of the minimum load causing net settlement of 2% of diameter, subject to the maximum net settlement being 25 cm, may be taken as the allowable working load on the pile. The settlement should, however, be restricted to 4% of the diameter. For this purpose, load increase shall be put in an equal time intervals.

Assessment on the basis of mechanical properties of soil:- In clayey soils, the working load may be determined as follow-

Working load = 3 x base area x shear strength at formation level

+ 015 to 0.20 x shaft area x average cohesion.

  1. Pull out load:-

Where design conditions necessitates the use of anchor piles, the allowable pull- out load should be determined by

  • Pullout test on a
  • Assessment on the basis of mechanical properties of

PERMISSIBLE TOLERANCES:

As a general guide, the permissible positional deviation for bored piles should not be greater than 7.5 cm for piles up to 75 cm diameter and not greater than 10 cm for piles of 180 cm diameter at the level of the bottom of the pile cap. For pile diameter in the range 75 cm to 180 cm , the tolerance may be interpolated

linearly. The deviation from the vertical plumb should not be more than 2 in 100 in ratio.

FINISHING OF PILE HEADS:

The top of pile should be brought up above the finished level to permit all laitance and weak concrete to be removed and to ensure that it can be properly keyed into the pile cap. The minimum distance of keying pile into pile cap shall be 5 cm. Any defective concrete in the head of the completed pile should be cut away and made good with new concrete. The clear cover between the reinforcement in the pile cap from the top of the pile shall be not less than 10 mm. The reinforcement in the pile should be exposed for a sufficient distance to permit it to be adequately bonded into the pile cap.

REPLACEMENT OF DEFECTIVE BORED PILES:

Defective piles shall be removed or left in place as judged convenient without affecting the performance of adjacent piles or capping above and additional piles shall be provided to replace them.

CASED PILES:

In case of boring with casing, the casing should be used atleast from the level of subsoil water. The casing shall be kept ahead of boring in case where there is danger of caving- in, due to sub-soil water entering into the bottom of the hole occuring due to difference in hydrostatic head.

Example for Bored cast in situ pile. Consider a pile of 500 mm diameter. Let the weight of chisel be 1000Kg. Area of pile = 0.1963 Sq.meter.

If fall of chisel limited to 2.0 meters,

Energy of each blow = 1 Tonne x 2.0 m = 2.0 Tonne

The energy of 2250 Tonne-meter / Sq.m. is converted into equivalent energy for 500mm diameter pile, Equivalent energy = 2250 x 0.1963 = 441.68 Tone meter.

To achieve this no. of blows of 1.0 Tonne chisel with 20. m fall will be;

= 441.68 / 2.0 = 220.84 blows (say 300 Nos.)

The no. of blows are increased to account for submerged weight of chisel and wire rope tension on drum while releasing the chisel so the chiseling criteria for 500 mm diameter pile will be:-

The penetration should be less than 10 cm for 300 blows of chisel with weight of 1.0 Tone and falling through a height of 2.0 m.. Generally 240 blows can be applied within 30 minutes while checking the chiseling criteria; the chisel shall be withdrawn after half an hour hole cleaned and penetration measured.

CALCULATION OF CHISELING ENERGY:

Diameter of pile (D)= 400mm. Weight of chisel (w) = 0.75 Tonne Height of fall (h)               = 1.5 m

No. of blows (n)             = 240. Penetration for ‘n’ blows = P

Energy per blow          = wh x w1/w2 x c W1= Unit weight of steel chisel in water.

W2=Actual unit weight of steel chisel in air. C = correction factor for a fall of chisel.

Therefore, for energy per blow

= 0.75 x 1.5 x 6.8/7.8 x 0.8 = 0.7846 Tonne -meter.

Area of 400 mm diameter of pile = pi x (0.4)2  /4 = 0.1257 m2. Therefore, equivalent energy for 400 mm diameter at pile for ‘n’ blows.

= 240 x0.7846/ 0.1257

= 1498.06 T.m/m2

SOCKETING STRATUM:

It is defined as the layer where the penetration is equal to or less than 10 cms for chiseling input energy of 750T.m./m2 of pile area.

This criteria must be satisfied for three consecutive trials. The socketing layer is assumed to have started from the end of such third consecutive trial.

FOUNDING STRATUM:

It is defined as the layer where the penetration is equal to or less than 10 cms for chiseling input energy of 2250 Tm/m2 of pile area. This criteria must be satisfied for three consecutive trials. The founding strata is assumed to have been reached after third successful consecutive trial. In the event where founding stratum as defined above is not encountered, the pile would be sealed after ensuring minimum socketing length i.e. 5D where D is diameter of pile in the acceptable socketing stratum, to realize various load carrying capacities. This means for 60 cm diameter pile the socketing length will be 3.0 m. If criteria of 2250 Tm/m2 per 10cm is not satisfied.

DETAILS OF CHISELING CRITERIA

For drop chisel using Bailer / Bentonite method:-

The weight of chisel in Kgs shall not be less than 1.6 to 2 x Diameter of pile (mm). (For 600mm diameter pile, 1 Tonne chisel is accepted).

The weight of chisel shall be checked by actually weighing it on a weigh Bridge. The chisel shall be of non-energy absorption type.

The founding strata is considered suitable if penetration is less than 10cm after an energy input of 2550 Tm/m2 of the pile base area.

During the checking of founding strata, the fall of the chisel shall be limited to 2.0m. This criteria shall be checked and satisfied at least twice consecutively. The method of working out the chiseling criteria is as follows.

Chiseling energy per 10 cm penetration

=     Equivalent energy x10

——————————-

Penetration

= 1498.06 x10 / p T.m./m2/10 cm.

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