Pile Group Bearing Capacity Settlement Pile Cap Design December, 2016

Pile Group Capacity To support a large foundation load and eccentric loading, piles are often installed in groups. Because of the close spacing between individual piles in a pile group, the soil zones that are affected by the piles, known as the stress zones, overlap. Efficiency of PG:

Factors affecting the spacing b/n piles Stress Overlap Cost of Pile Cap Effeciency ( η )

Ultimate Pile Group Capacity is:

It is noted that the soil encompassed in the pile group moves (shears) with the pile group, and the skin resistance along the pile group block is between the soil in the pile group and the soil surrounding the pile group. Therefore, the cohesion and the internal friction angle of the soil should be used to determine fs.

Major Change?

Settlement of Pile Groups The settlement of a pile group includes the elastic compression of the piles, elastic (immediate) settlement of the soil at the toe of the piles, and consolidation settlement of the soils around and beneath the pile group. Pile groups that are supported in cohesionless soils are subjected only to immediate (elastic) settlement; consolidation settlement is negligible. Pile groups that are supported in cohesive soils are subjected to both immediate (elastic) settlement and consolidation settlement.

The settlement of a pile group is much greater than the settlement of a single pile because the soil zone affected by the loading adjacent to and beneath the pile group is much larger

Elastic compression of piles

Empirical equations for pile group settlement using field penetration data. The settlement of a pile group in granular soils using SPT data is:

Consolidation settlement of a pile group in saturated cohesive soil Terzaghi and Peck (1967) proposed that the consolidation settlement of a pile group in satu- rated clay could be evaluated using an equivalent footing situated at a depth of L∕3 above the pile toe. It is assumed that the soil in the top two-third of the pile length is not subjected to consolidation settlement. It is also assumed that the vertical stress increase from L∕3 above the

2V:1H Method

The consolidation settlement is caused by vertical stress increase. When the vertical stress increase, Δ s’ Z, at a certain depth is less than one-tenth of the in-situ effective stress s’ 0 is assumed the consolidation settlement is negligible. Therefore, consolidation settlement should be considered for a depth that is either Δ s’ Z = 0.1 s’ 0, or there is an underlying firm stratum whose compressibility is negligible

Step-by-step,

Review!!

For overconsolidated soils

Simplified Pile Design Process (GEO)

Pile Cap Has a function of spreading the load from a compression or tension member onto a group of piles, Accommodates deviates from the intended positions of piles, Bridge over defective pile by redistributing the loads

Pile Cap…(cont’d) Calculations has to be conducted by strut-and-tie model or flexural methods (EC-2: Section 9.8.1) Flexural Method- the pile cap is treated as inverted beam/slab Strut-and-tie model – when span-to- depth ratio is less than 2 and beam theory is not appropriated

Beam Theory Assumptions In the design of pile caps, the following assumptions are usually made: i.The pile cap is perfectly rigid. ii.The pile heads are hinged to the pile cap. As a result no bending moment is transmitted from the pile cap to the piles. iii.The deformations and stress distributions are considered planar.

Actions pile cap The forces that have to be considered in the analysis and design of a pile cap are a)Reaction from the piles (considered as series of concentrated loads) b)Column loads and moments c)Weight of the cap d)Weight of overlaying soil, if the cap is below the ground surface.

Pile Cap…(cont’d) Calculate Forces on Piles

Or simplifying

Depth of Pile Cap To start with? From Practice… Or D =(2 diameter of pile +100) for d less than or equal to 500mm D = (1/3) (8 diameter of pile +600)mm for d greater than or equal to 550mm

Design for one way shear The shear capacity of a pile cap should be checked at the critical section taken to be 20% of the pile diameter inside the face of the pile. Sometimes, the shear resistance must be calculated at a distance d from the face of the column, which is the same for beams and slabs.

The shear force is the sum of the loads acting outside of the section considered.

Reduction factor (a v /d) can be applied when the the load is close to the support. A large portion of the load is transfer by strut action than by bending and shear action. Refer Section of EC2. Or it is possible to proceed by strut-and-tie model.

Design for Punching Shear The shear capacity has to be checked at the critical section and face of the column, The same method as that of spread footings, Refer Section

Refer back the lessons on footing and flat slabs

For the computation of shear on any section through the pile cap, the following is recommended by EBCS-2: a. Entire reaction from any pile whose center is located at half the pile diameter or more outside the section shall be considered as producing shear on that section. b. Reaction from any pile whose center is located at half the pile diameter or more inside the section shall be considered as producing no shear on that section c. For intermediate positions of pile center, the portion of the pile reaction to be considered as producing shear on the section shall be based on straight line interpolation between full value at half the pile diameter outside the section and zero value at half the pile diameter inside the section.

General Principles

Detailing Issues? EBCS

EC

Some EC-2 Provisions