PILE FOUNDATION - 3 Mr Mohd Faiz b Mohammad Zaki

EMPIRICAL BEARING CAPACITYMost of the times, it is preferable and more practical to use empirical formula for pile design. The most ones are those derived from the results of standard penetration test (SPT) and cone penetration test (CPT).

CONTPile capacity based on SPT values

The formulas based on SPT tests have been proposed by Meyerhoff (1976). The end bearing capacity of the pile is given in the following equations:

CONTFor driven pile

For drilled pile

CONTWhere N' is the corrected SPT-N value near the pile base or within the range of 1B above the tip and 2B below the tip corrected with the overburden pressure, Df is embedded length of pile , and B is the smallest dimension of the pile.

Most piles have high Df /B ratios, thus the upper limit nearly always control.

CONTThe friction bearing of the pile depends on the type of piles. For large displacement pile such as solid driven piles, the friction is estimated as

CONTwhile for small displacement pile such as H driven piles or shell piles, the friction resistance is:

where N is the average SPT value along the embedded length of pile.

CONTNote that the equations are applicable for piles embedded in cohesionless soils because the standard penetration test does not give reliable estimation of pile capacity in cohesive soil.

Pile capacity based on CPT dataThe pile capacity can be estimated based on empirical formula developed based on the results of cone penetration test. Briaud and Miran (1991) proposed the end bearing capacity (qb ) formula as:

CONTWhere qca is the equivalent cone bearing at pile tip which is the average qc values from 1.5 times pile diameter above the pile tip to 1.5 pile diameter below the pile tip.

kc is the cone end bearing factor which depends on the soil and pile types.

CONT

The friction resistance is derived based on side friction (Nottingham and Schemertmann , 1975). The unit friction resistance for piles driven in cohesionless soil is given as:

CONT

CONTFor pile driven in cohesive soil:

where fsc is the local side friction, B is the smallest dimension of the pile, L is the length of pile embedment, and 's is the adhesion factor which depends of the soil and pile types and the types of cone. An average value of 0.5 can be used for this factor.

DYNAMIC FORMULAThe ultimate resistance of driven piles may be predicted based on the amount of energy delivered to the pile by the hammer and the resulting penetration of the pile.

The greater the resistance to drive the pile, the greater the capacity of the pile is to carry the load . In this case, the net kinetic energy is equal to the work done during penetration equal to the soil resistance.

CONT

Where W is the weight of hammer, h is the height of falling hammer, R denotes the soil resistance , and s is set, that is the average depth of penetration during the last blow count.

CONTHundreds of pile driving formulas have been proposed by researchers such as the Engineering News Record (ENR), US Navy, Gates, Danish , Eytelwein, etc .

Some of these formulas dating back to the 19th century such as that proposed by Wellington (1888) which is known as the ENR formula which come in the form :

CONT

where Wh is the weight of hammer, h is the falling height of the hammer, s and c are set and empirical constant respectively, and FS is the factor of safety.

The empirical coefficient c was given as 1.0 for a drop hammer (light hammer) and 0.10 for a single acting steam hammer (heavy hammer).

CONTThe most widely used dynamic formula in Malaysia is Hiley formula proposed in 1930 .

The formula takes into account the mechanism of impact between the hammer and the pile . Many formulas used Newton's theory of impact only, while Hiley tried to account for energy losses in the pile/hammer system (i.e. higher energy losses).

CONTwhere Wh, h, s and c are defined previously, e is the efficiency factor of the hammer, used to take into account energy losses during hammer drop, Wp is the weight of the pile, is the coefficient of restitution which takes into account the energy loss through cushion and pile cap.

CONTIt is to be noted also that the formula was developed for driving systems and piles in used in England in the 1930's (e, r and c values) for which the piles were driven into stiff clays and dense sands.

Thus, it is to be used with caution in different soils and elsewhere in the world.

Factors of safety of the order of 2 to 3 were suggested .

PILE LOAD TESTPile cannot be readily inspected once they have been constructed due to the variations in the bearing stratum.

It is not easy for the designer to be assured that the pile foundations are complying with the design .

Therefore, the pile test should be considered as a part of the design and construction process.

CONTTo determine the pile capacity directly in situ, an adequate number of pile load test depending on the extent of ground investigation program is required .

In this case, test piles may be installed and tested to prove the suitability of the pilling system and to confirm the design parameters inferred the site investigation .

CONTThe test pile should be driven at a location where soil conditions are known and where soil conditions are relatively poor.

Testing of test piles may involve integrity testing to check the construction technique and workmanship and for load testing to confirm the performance of the pile as a foundation element.

CONT

CONTMaintained Load Test (MLT) is the most common method and also known as the Standard Loading Procedure. This test needs a long duration, which can take 30-70 hours to complete.

In this method, the load-settlement relationship for the test pile is obtained by loading in suitable increments, allowing sufficient time between increments for settlement to be substantially complete.

CONTThe test shall be assessed by two-cycle static maintained load test to twice the safe working load.

The ultimate load is normally taken as the corresponding to a specified settlement, for example 10% of the pile diameter.

CONTIt is normal to include an unloading-reloading cycle, after 100 percent of the design load is reached, to establish the elastic rebound of the pile.

The final load is maintained for 24 hours, after which the pile is unloaded in increments.

CONT

PROBLEMS RELATED TO PILE FOUNDATIONUplift / Tension Resistance of Piles

Piles may be required to resist uplift forces. This is the case when structure is subjected to large overturning moments such as transmission towers , jetty structure or bridge abutments.

PROBLEMS RELATED TO PILE FOUNDATIONThe uplift force in piles is resisted by friction and the weight of pile itself. Additional uplift resistance may be obtained by under-ream or enlarge base of piles.

CONTA relatively few pulling test results were reported in literature. However, for practical purposes the resistance to uplift (Pu) can be calculated based on the fricti on resistance, plus the weight of piles (Wp) :

UNDER- REAMED PILESSome drilled piles are constructed with enlarged base in order to increase the end bearing capacity of the piles.

This type of pile is known as under-reamed pile. The size of the enlarged based is about three times the diameter of the stem.

CONTDue to the drilling process and the enlarged base of the pile , it is advised to disregard skin friction at the top of the pile (about 1.5 m) and at the enlarged part of the pile.

In addition, the skin friction along the stem over a length of 2B above the enlarged base should not be accounted for.

CONT

Negative Skin FrictionIf driven or bored piles are installed in compressible fill or any soil showing appreciable consolidation under its own weight, an additional load is working on the perimer of the pile.

This is referred as negative skin friction.

CONT

CONTThe negative friction or down drag of piles may also occur wherever piles are driven through, or adjacent to, recently placed fill.

Such fill may merely be to raise the existing ground level, or may be part of an embankment or a bridge approach .

The problem is normally associated with soft, lightly over consolidated deposits of clay .

CONTIn some sensitive clay, remolding of the soil during driving may lead to down drag of the piles, even where no fill is placed (Fellenius, 1972).

CONTThe negative skin friction (Qn) can be calculated based on the effective overburden stress distribution along the pile:

CONTIf there is no fill above the clay layer, then equation wiII reduce to:

EXAMPLE

PILE GROUPIn most cases piles are used in groups to transmit the structural load to the soil. A pile cap is constructed over a group of three or more piles.

Pile group works better than a single pile because a single pile usually does not have enough capacity and the installation of a pile sometimes results in some problems such as eccentricity which can produce moment to the pile.

PILE GROUPMultiple pile can minimize the eccentricity and provide redundancy, thus piles can continue to support the structure even if one pile is defective.

CONTIdeally the bearing capacity of the group of piles (Qug) should not be less than the capacity of a single pile times the number of pile.

However, if two piles are driven close to each other, then soil stresses caused by the piles tend to overlap, and the bearing capacity of both piles would be less than the sum of the capacity of the two piles .

CONTThus, piles need to be spaced relatively far apart so that the stresses do not overlap. This will results in a bigger size of a pile cap.

CONTMinimum allowable pile spacing of piles is often specified by design manual depending on the types of piles and the types of soil.

However, for highest efficiency. The center to center distance of 2 to 8 times the smallest dimension of pile is suggested.

CONTThe optimum distance of 3 times the diameter of circular piles or width of square pile is widely acceptable .