Under-Ream Pile Foundations: Objectives and Design | Soil Engineering

Under-ream piles are commonly used in India especially as a solution for structures in black cotton soil regions.

Objective and Applications of Under-Ream Pile Foundations:

Under-reamed pile foundations are bored cast in-situ concrete piles with one or more bulbs or under-reams. The bulb is formed by enlarging the stem of the pile near the bottom in a double conical shape, as shown in Fig. 20.30.

The purpose of providing the bulb is as follows:

1. To provide anchorage to the foundation for preventing its movement in a vertical direction due to alternate swelling and shrinkage of the soil in expansive soils caused by seasonal changes in the moisture content.

2. To provide additional bearing area through the enlarged bulb in firm strata, underlying the top weak or filled-up ground.

3. To obtain adequate load-carrying capacity for downward, upward, and lateral loads and moments.

4. To take the foundation below the scour level.

Under-ream piles are very popular in India as they provide an effective solution to the swelling and shrinkage due to seasonal changes in expansive soil regions. The anchorage provided by the under-ream pile prevents cracks in floors and walls of buildings in expansive soil areas. Single under-ream piles have been extensively used for single- or double-storied buildings. The provision of an additional bulb increases the load-carrying capacity by about 50%. Figure 20.30 shows single and double under-ream pile foundations.

Design Considerations of Under-Ream Piles:

Following are the design considerations of under-ream piles as per IS – 2911 (Part III)—1980:

1. The concrete used for under-ream piles shall have a slump of 10-15 cm for concreting in water-free unlined bore holes. For concreting by tremie, the concrete used should have a slump of 15-20 cm for better workability. M-15 or M-20 concrete with respective minimum cement content of 350 or 400 kg/m³ may be used.

2. The minimum length of under-ream piles, in deep deposits of expansive soils, should be 3.5 m below the ground level.

3. The diameter of the under-ream is kept as 2.5 times the stem diameter.

4. The maximum vertical spacing between under-reams is 1.5 times the diameter of the under-ream for piles up to 0.3-m diameter. For large-diameter piles, the spacing may be reduced to 1.25 times the stem diameter.

5. The topmost bulb should be at a minimum depth of two times the bulb diameter. For expansive soils, this depth should not be less than 1.75 m below the ground level.

6. The number of bulbs provided should be normally not more than 2.

7. The minimum center-to-center spacing of under-ream piles in a group is 1.5 times the diameter of the under- ream but usually kept at two times the under-ream diameter. For a pile group with piles at a spacing of 2 D_u, the group capacity may be taken equal to the sum of the load-carrying capacity of the individual piles in the group. For a pile group with piles at a spacing of 1.5 D_u, the safe load assigned per pile should be reduced by 10%.

Design of Under-Ream Piles:

The load-carrying capacity of under-ream piles is derived from the following three components:

1. Point-bearing resistance at the toe of the pile.

2. Skin friction resistance along the pile stem.

3. Skin friction on the soil cylinder between the extreme bulbs.

However, when piles are subjected to uplift, the point-bearing resistance at the toe will not be present.

Under-ream piles should be designed on the basis of following two considerations:

1. The piles should resist the imposed loads as a structural member with adequate factor of safety.

2. The soil supporting the pile should withstand the loads without shear failure or excessive settlements with ade­quate factor of safety.

Safe Load for Under-Ream Piles Using Static Formulae:

The estimation of safe load for under-ream piles is done in the following procedure:

1. Piles in Clays:

The safe load on the under-ream piles may be computed using the following three methods:

a. Computation of the ultimate load using Eqs. (20.61)-(20.66), based on soil properties and considering the point-bearing and skin friction resistance and applying adequate factor of safety.

b. Load test on the piles.

c. Safe load tables.

A factor of safety of 2.5 in compression and 3 in uplift should be used to obtain the safe load from the ultimate load.

Ultimate load capacity of piles for single under-ream piles –

Ultimate load capacity of piles for multiple under-ream piles –

where c_p is the cohesion of the soil at the base; A_p is the cross-sectional area of the pile at the toe (base); N_c is the bearing capacity factor, usually taken as 9; α is the reduction factor, usually taken as 0.5 for clays; A_bs is the surface area of cylinder circumscribing the under-ream bulbs applicable only for multiple under-ream piles; c_s is the average cohesion along the pile stem; A_s is the surface area of the pile stem; c_b is the average cohesion of soil around under-ream bulbs; and A_b is the cross-sectional area of the bulb excluding the stem.

A_b = π/4 x (D_u² – D²) …(20.63)

where D_u is the diameter of the under-ream and D is the diameter of the pile stem.

2. Piles in Sands:

Ultimate load capacity of piles in sands is given by –

where γ is the density of soil, d_f is the depth of foundation or the total length of the pile below ground level, B is the width of the pile or the diameter of the pile stem, d_r is the depth of the center of different under-ream bulbs below the ground level, K is the lateral earth pressure coefficient, usually taken as 1.75 for sandy soils, δ is the angle of wall friction or ɸ, d₁ is the depth to the center of the first under-ream, d_n is the depth to the center of the last under-ream, N_q is the bearing capacity factor taken from Fig. 20.13, and N_γ is the bearing capacity factor taken as per IS – 6403-1981.

From Eq. (20.64), ultimate load capacity of piles for single under-ream piles –

Similarly, ultimate load capacity of piles for double-under-ream piles –

3. Piles in c-ɸ Soil:

Ultimate load capacity for piles in soils having both cohesion and friction may be computed by combining Eqs. (20.61)-(20.66).

Safe Load from IS Code Tables:

The safe load on under-ream piles as recommended by IS – 2911 (Part III)-1980 in bearing, uplift, and lateral loads are given in Table 20.13. The values are applicable for medium-compact sandy soils (N = 10-30) and clayey soils of medium consistency (N = 4-8), including expansive soils and for piles with D_u = 2.5 D using M-15 concrete.

For dense sandy soils (N > 3) and stiff clayey soils (N > 8), the safe load in compression and uplift given in Table 20.13 should be increased by 25%. No increase is permitted for lateral thrust without detailed analysis. For piles in loose sandy soils (N = 4-10) and soft clayey soils (N = 2-4), the safe loads should be taken as 75% of those given in Table 20.13. For very loose sandy soils (N < 4) and for very soft clayey soils (N < 2), the safe loads should be taken as 50% of those given in Table 20.13.

Also, the safe loads should be reduced by 25% if the bore hole is full of drilling mud or groundwater during con­creting. No reduction will be necessary if the water is confined up to the shaft portion below the bottommost bulb.

The safe loads in compression and uplift given in Table 20.13 should be reduced by 15% for under-ream piles with a bulb of diameter twice that of the stem. No such reduction is necessary for lateral thrust.

Reinforcement for Under-Ream Piles:

Following recommendations are made by IS–2911(Part III)-1989 for reinforcement to be provided in under-ream piles:

1. The minimum longitudinal reinforcement in the stem portion is 0.4% of the cross-sectional area, with minimum 3 bars of 10-mm diameter mild steel or 3 bars of 8-mm diameter high yield strength deformed (HYSD) bars.

2. The maximum clear spacing of transverse reinforcement in the form of circular stirrups is the diameter of the stirrups or 30 cm, whichever is less.

3. The minimum clear cover for reinforcement is 4 cm under normal conditions and 7.5 cm in the case of concrete subjected to sulfate attack.

Construction of Under-Ream Piles:

The procedure for construction of under-ream piles is as follows:

1. Bore holes are made by earth augers. In ground with high water table with unstable bores, bentonite slurry may be used as a drilling mud. Casing may be required for permeable stratum overlying a thin clayey stratum; casing may be used over the permeable stratum. In very loose strata at the top, temporary casing of required length may be used during boring and concreting to avoid an irregular shape and widening of the bore hole.

2. After the bore is made to the required depth, the base may be enlarged made by using an under-reaming tool.

3. In empty bore holes, a small quantity of concrete is first poured to form a concrete layer of about 0.1-m thickness at the bottom. Reinforcement is then lowered and positioned correctly. Concrete is then poured to fill the bore hole. Care should be taken during compaction of concrete that soil is not scrapped from sides.

4. If a small quantity of water is present at the bottom of the bore hole, it should be bailed out and concreting should be done as in the case of an empty bore hole.

5. In case the pile bore is stabilized with drilling mud or by maintaining water head within the bore hole, concret­ing should be done by the tremie method.

6. A tremie pipe of minimum 15-cm diameter with a flap valve at the bottom should be used for placing concrete in bore holes full of drilling mud or groundwater. Concreting should be done immediately after the bore hole is made, within 12-24 h in the case of bores filled with drilling mud.

7. The top of concrete in a pile should be brought above the cutoff level to permit removal of all laitance and weak concrete before construction of the pile cap to ensure good concrete at the cutoff level for proper embedment of the pile into the pile cap.

Pile Caps:

The pile cap should be designed by assuming that the load from the column or pedestal is dispersed at 45° from the base of the column/pedestal up to the mid-depth of the pile cap.

The clear overhang of the pile cap beyond the edge of the outermost pile should be about 10-15 cm.

A leveling course of 7.5-cm thickness should be provided between the pile cap and the pile. The minimum clear cover for main reinforcement in the pile cap is 7.5 cm. The pile should project 4 cm into the cap concrete.

Grade Beams:

Grade beams are provided to support the walls of the building and transfer their load to under-ream piles. Thus, grade beams are provided, interconnecting all piles/pile caps and constructed monolithic with them.

The minimum depth of grade beams is 0.15 m. A minimum longitudinal reinforcement of 3 bars of 10-mm diam­eter mild steel or its equivalent HYSD bars both at the top and at the bottom should be provided. The maximum spacing of 6-mm diameter stirrups is 0.3 m, which should be reduced to 0.1 m at the door openings near the wall edge up to a distance of three times the depth of the beam.

Grade beams should be provided at a height of 80 mm above the ground level in expansive soils. Alternately, they should be provided over a leveling concrete course of 80-mm thickness, as shown in Fig. 20.31(a).

In the case of exterior beams over piles in expansive soils, a ledge projection of 75-mm thickness should be pro­vided on the outer side of the beam, extending 80 mm into the ground, as shown in Fig. 20.31(b).