General requirements For design of pile foundation as per BNBC 2006 the following must be considered
- A plan or layout showing clearly the designation of all piles by identification must be provided
- All details of individual pile must be recorded
- A copy of such plan and details must be provided at the site during construction
- Installation of pile foundations shall be under the direct supervision of a competent engineer
- All piles shall be braced to provide lateral stability in all directions considering that the three or more piles connected by a rigid cap as being braced and the two-pile group by a rigid cap is braced only two pile direction
- Pile cross section shall be of sufficient size and strength to withstand driving stresses
- Pile diameter, length and reinforcement must be maintained during construction
Design considerations
The allowable axial load and lateral loads on piles shall be determined by an established method of analysis or load test. The allowable load on a pile shall be determined by considering least value of the following factors
- The capacity of the pile as a structural member
- The capacity of pile as considering soil strata underlying the pile cap
- The capacity of pile by load test
Pile design method: Two type of pile design formula
- Static bearing capacity formula
- Dynamic formula
- Pile load test
Negative skin friction
Piles installed in compressible fill or soft soil subject to compression shall be designed against additional downdrag, generally known as negative skin friction of the compressible soil must be considered where required.
Lateral capacity of pile: Lateral capacity of pile must be determined where the pile subjected to lateral load considering single pile and following considerations
- Soil failure
- Structural capacity of the pile
- Deflection of pile head
- Allowable load shall not be more than one-half of the test load which produces gross lateral movement of 25 mm during load test
Spacing of pile
- The minimum spacing in such cases shall be 2.5 times the diameter of the pile shaft where pile capacity is governed by end bearing
- The minimum spacing in such cases shall be 3.0 times the diameter of the pile shaft where pile capacity is governed by skin frictional resistance
- The minimum spacing in such cases shall be 2.0 times the diameter of the pile shaft where pile
Batter piles
Batter piles shall be used to transfer inclined load and horizontal forces. Factor of safety. A minimum factor of safety 2.5 shall be considered when static formula is used to determine ultimate bearing capacity.
Reinforcement
Precast driven pile
- Minimum longitudinal reinforcement is one and a half percent of the concrete section with minimum 4 Nos arranged symmetrically
- Lateral ties 6mm bar with 75mm spacing or equivalent
- Concrete clear cover shall be of not less than 50mm
- Concrete clear cover shall be of not less than 70mm where piles are exposed to sea water
Bored cast-in-situ pile
- Reinforcement shall be placed to provide at least 75 mm of clear cover
- Minimum vertical reinforcement in bored cast-in-situ piles shall be 4-D13mm bars
- Minimum embedment is half of the length
Under-reamed bored cast-in-situ pile
- Minimum vertical reinforcement in under-reamed bored cast-in-situ pile shall be 0.4 %
- Full embedment of the length
Under-reamed compaction pile
- Minimum of 4-D12mm bars shall be provided in general
- Minimum of 6-D12mm bars shall be provided for piles of length exceeding 5m and diameter exceeding 375mm
Pile cap design
- Must be reinforced concrete
- Pile top shall be embedded not less than 75mm
- Pile cap shall extend at least 100mm beyond the edge of pile
- Pile cap shall be rigid enough, so that the imposed load can be distributed on the piles in a group equitably
- The pile cap shall be cast over a 75mm thick levelling concrete
- The clear cover of such concrete shall be at least 60mm