## SOILS AND FOUNDATIONS

BNBC 2017, PART 6, CHAPTER 3

### Foundation Engineer vs. Geotechnical Engineer

**FOUNDATION ENGINEER**

A graduate Engineer with at least 5 (five) years of experience in civil engineering particularly in foundation design or construction.

**GEOTECHNICAL ENGINEER**

Engineer with Master’s degree in geotechnical engineering having at least 2 (two) years of experience in geotechnical design/construction or graduate in civil engineering/engineering geology having 10 (ten) years of experience in geotechnical design/construction.

Geotechnical Engineer > Foundation Engineer

### Working Stress Load combinations

· D + F (F = fluid load)

· D + H + F + L + T (H = earth pressure load, T = load due to temperature change)

· D + H + F + (Lr or R)

· D + H + F + 0.75(L + T ) + 0.75(Lr or R)

· D + H + F + (W or 0.7E)

· D + H + F + 0.75(W or 0.7E) + 0.75L + 0.75(Lr or R)

· 0.6D + W + H (needed to check uplifting)***

· 0.6D + 0.7E + H (needed to check uplifting)***

### Load combinations for foundation design

**Design Load :**

a) Shallow foundation design considering bearing capacity due to shear strength shall consider the most unfavorable effect of the following combinations of loading:

1. D + L

2. 0.75×[D + L + (W or E)] {0.75 = 1/1.33}

3. 0.9×D + Buoyancy Pressure

4. 0.6xD + W

b) Shallow foundation design considering settlement shall consider the most unfavorable effect of the following combinations of loading:

1. Sand: D + L

2. CLAY: D + 0.5 L

### Critical Depth for pile in sand

Dc=10D, for loose sand

Dc=15D, for medium dense sand

Dc=20D, for dense sand***

Applicable only in the method where no limiting value is mentioned

### Important notes

**Driven Precast Piles :**

· The minimum center-to-center pile spacing of 2.5B is recommended. The nominal dimensions and length of all the piles in a group should be similar. *****

· All piles shall be braced to provide lateral stability in all directions.

· Three or more piles connected by a rigid cap shall be considered as being braced (stable),

· A two pile group in a rigid cap shall be considered to be braced along the axis connecting the two piles.

· Piles supporting walls shall be driven alternately in lines at least 300 mm apart and located symmetrically under the centre of gravity of the wall load, unless effective measures are taken to cater for eccentricity and lateral forces, or the wall piles are adequately braced to provide lateral stability.

· Individual piles are considered stable if the pile tops are laterally braced in two directions by construction, such as a structural floor slab, grade beams, struts, or walls. *****

### Pile cap

1. Pile caps shall be made of reinforced concrete.

2. The soil immediately below the pile cap shall not be considered as carrying any vertical load.

3. The tops of all piles shall be embedded not less than 75 mm into pile caps and the cap shall extend at least 100 mm beyond the edge of all piles.

4. The tops of all piles shall be cut back to sound material before capping.

5. The pile cap shall be rigid enough, so that the imposed load can be distributed on the piles in a group equitably.

6. The cap shall generally be cast over a 75 mm thick levelling course of concrete.

7. The clear cover for the main reinforcement in the cap slab under such condition shall not be less than 50 mm.

### Structural capacity of driven precast pile section

Pile diameter/cross-section of a pile shaft at any level shall not be less than the designated nominal diameter/cross-section. The structural design of piles must consider each of the following loading conditions.

1. Handling loads are those imposed on the pile between the time it is fabricated and the time it is in the pile driver leads and ready to be driven. They are generated by cranes, fork lifts, and other construction equipment.

2. Driving loads are produced by the pile hammer during driving.

3. Service loads are the design loads from the completed structures.

### Maximum Allowable Stress on Driven Pile

RCC pile =0.33fc

Steel pile =0.25fy

Prestressed RCC pile =0.33fc-fpc

If Su < 10 kN/m2 Pile shall be designed As a long column

### Minimum rebar in driven pile

**Minimum Reinforcement in Driven Concrete Pile**: The maximum bending stress is produced while handling if the pile is pitched at the head. To prevent whipping during handling, length/diameter ratio of the pile should never exceed 50. The following reinforcement provisions may not be valid for laterally loaded piles or piles for uplift resistance.

1. Pile length < 30 times the least width : 1.00%

2. Pile length 30 to 40 times the least width : 1.5%

3. Pile length > 40 times the least width : 2%

### Hoops in driven pile

The lateral reinforcement resists the driving stresses induced in the piles and should be in the form hoops or links of diameter not less than 6 mm. The volume of lateral reinforcement shall not be less than the following:

· At each end of the pile for a distance of about three times the least width/diameter – not less than 0.6% of the gross volume of the pile.

· In the body of the pile – not less than 0.2% of the gross volume of the pile.

· The transition between closer spacing and the maximum should be gradual over a length of 3 times the least width/diameter.

### Concrete of driven pile

Minimum Grades of Concrete: The minimum 28 days cylinder strength of concrete for driven piles is 21 MPa. Depending on driving stresses, the following grades of concrete should be used.

· For hard driving – 28 MPa

· For easy driving – 21 MPa

**** Durability must be considered to select concrete mix ratio

### Bored Pile vs. Drilled Shaft?

· Same thing

· Sometimes large diameter piles called shaft or pier

### Bored pile rebar

**Minimum Reinforcement in Bored Concrete Pile**

The longitudinal reinforcement shall be of high yield steel bars (min f_y = 420 Mpa) and shall not be less than:

0.5% of A_c , for A_c ≤ 0.5 m2;

2500 mm2 , for 0.5 m2 < A_c ≤ 1 m2;

0.25% of A_c, for A_c > 1.0 m2;

Where, A_c is the gross cross-sectional area of the pile.

The minimum diameter for the longitudinal bars is 16 mm

### Bored pile rebar detailing

1. The assembled reinforcement cage should be sufficiently strong to sustain lifting and lowering into the pile bore without permanent distortion or displacement of bars

2. bars should not be so densely packed that concrete aggregate cannot pass freely between them.

3. Hoop reinforcement (for shear) is not recommended closer than 100 mm centres

4. Minimum concrete cover to the reinforcement periphery shall be 75 mm.

5. This guidance is only applicable for piles with vertical load.

### Group capacity of bored pile or drilled shaft in cohesive soil

**CAP IN FIRM CONTACT WITH GROUND**

· Reduction factor, ζ = 1.0

· The group capacity may then be computed as the lesser of

o the sum of the individual capacities of each shaft in group, or

o the capacity of an equivalent pier defined in the perimeter area of the group

· For the equivalent pier, shear strength shall not be reduced by any factor to calculate skin friction

**CAP IN FIRM CONTACT WITH GROUND OR GROUND IS SOFT**

1. Reduction factor, ζ = 0.67 for a center-to-center (CTC) spacing of 3B (B=pile dia)

2. Reduction factor, ζ = 1.0 for a CTC spacing of 8B (B=pile dia)

3. The group capacity may then be computed as the lesser of

(i) the sum of the individual capacities of each shaft in group, or

(ii) the capacity of an equivalent pier defined in the perimeter area of the group

For the equivalent pier, shear strength shall not be reduced by any factor to calculate skin friction

### Group capacity of bored pile or drilled shaft in cohesionless soil

CAP CONTACT WITH GROUND is not considered here

· Reduction factor, ζ = 0.67 for a center-to-center (CTC) spacing of 3B (B=pile dia)

· Reduction factor, ζ = 1.0 for a CTC spacing of 8B (B=pile dia)

· The group capacity may then be computed as the lesser of

(i) the sum of the modified individual capacities of each shaft in group, or

(ii) the capacity of an equivalent pier defined in the perimeter area of the group

For the equivalent pier, shear strength shall not be reduced by any factor to calculate skin friction

### Group capacity of driven pile

· Reduction factor, ζ = 1.0

· The group capacity may then be computed as the lesser of

(i) the sum of the individual capacities of each shaft in group, or

(ii) the capacity of an equivalent pier defined in the perimeter area of the group

For the equivalent pier, shear strength shall not be reduced by any factor to calculate skin friction

### Minimum pile spacing for bored pile

Center to Center Spacing:

Conflict with previous

If closer spacing is required, the sequence of construction shall be specified and the interaction effects between adjacent shafts shall be evaluated by the designer.

### Longitudinal Bar Spacing:

· The minimum clear distance between longitudinal reinforcement shall not be less than 3 times the bar diameter nor 3 times the maximum aggregate size.

· If bars are bundled in forming the reinforcing cage, the minimum clear distance between longitudinal reinforcement shall not be less than 3 times the diameter of the bundled bars.

· Where heavy reinforcement is required, consideration may be given to an inner and outer reinforcing cage.

### Bentonite slurry specification

In drilling of holes for all piles, bentonite and any other material shall be mixed thoroughly with clean water.

Bentonite slurry shall meet the Specifications as shown below

Density during drilling to support excavation, greater than 1.05 g/ml, Mud density Balance (ASTM D4380)

· Density prior to concreting, less than 1.25 g/ml, Mud density Balance (ASTM D4380)

· Viscosity, 30 – 90 Seconds, Marsh Cone Method (ASTM D6910)

· pH, 9.5 to 12, pH indicator paper strips or electrical pH meter (ASTM D4972)

· Liquid limit, > 450%, Casagrande apparatus (ASTM D4318)

### Drilling fluid level

Where a borehole is formed using drilling fluid for maintaining the stability of a boring, the level of the water shall maintain at a level not less than 2 m above the level of ground water.

### Bored pile construction sequence

· Augured cast-in-situ pile shall not be installed within 6 pile diameters centre to centre of a pile filled with concrete less than 24 hours old.

· Bored cast-in-situ concrete piles shall not be drilled/bored within a clear distance of 3 m from an adjacent pile with concrete less than 48 hours old.

· For concreting under water, the concrete shall contain at least 10 percent more cement than that required for the same mix placed in the dry.

### “Rabbit” for bored pile

Successful placement of concrete under water requires preventing flow of water across or through the placement site. Once flow is controlled, the tremie placement consists of the following three basic steps:

· The first concrete placed is physically separated from the water by using a “rabbit” or go-devil in the pipe, or by having the pipe mouth capped or sealed and the pipe dewatered.

· Once filled with concrete, the pipe is raised slightly to allow the “rabbit” to escape or to break the end seal. Concrete will then flow out and develop a mound around the mouth of the pipe. This is termed as “establishing a seal”.

· Once the seal is established, fresh concrete is injected into the mass of existing concrete.

### Methods of tremie concreting

**Tremie concreting method**

· The capped tremie pipe approach

· The “rabbit” plug approach

### Important notes

1. Tremies should be embedded in the fresh concrete a minimum of 1.0 to 1.5 m (3 to 5 ft) and maintained at that depth throughout concreting to prevent entry of water into the pipe.

2. Rapid raising or lowering of the tremie pipe should not be allowed.

3. All vertical movements of the tremie pipe must be done slowly and carefully to prevent “loss of seal”.

### Number of tests?

1. Integrity Test: In order to check the structural integrity of the piles Integrity tests shall be performed on the piles in accordance with the procedure outlined in ASTM D5882. For any project where pile has been installed, integrity tests shall be performed on 100% of the piles.

2. Axial Load Tests for Compression :

I. For a major project, at least 2% of pile's axial load test for compression shall be tested in each area of uniform subsoil conditions.

II. The load test on a pile shall not be carried out earlier than 4 weeks from the date of casting the pile.

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