Driven Precast Concrete Piles · NBC 2016 (SP 7:2016) Volume 1

9.1 Provisions of 8 except 8.3.11 shall generally apply.

9.2 Design of Pile Section

9.2.1 Design of pile section shall be such as to ensure the strength and soundness of the pile against lifting from the casting bed, transporting, handling and driving stresses without damage.

9.2.2 Any shape having radial symmetry will be satisfactory for precast piles. The most commonly used cross-sections are square and octagonal.

9.2.3 Where exceptionally long lengths of piles are required, hollow sections may be used. If the final condition requires larger cross-sectional area, the

PART 6 STRUCTURAL DESIGN — SECTION 2 SOILS AND FOUNDATIONS

hollow sections may be filled with concrete after driving in position.

9.2.4 Wherever, final pile length is so large that a single length precast pile unit is either uneconomical or impracticable for installation, the segmental precast RCC piles with a number of segments using efficient mechanical jointing could be adopted.

Excessive whipping during handling precast pile may generally be avoided by limiting the length of pile to a maximum of 50 times the least width. As an alternative, segmental precast piling technique could be used.

The design of joints shall take care of corrosion by providing additional sacrificial thickness for the joint, wherever warranted.

9.2.5 Stresses induced by bending in the cross-section of precast pile during lifting and handling may be estimated as for any reinforced concrete section in accordance with relevant provisions of Part 6 'Structural

Design, Section 5 Concrete, Subsection 5A Plain and Reinforced Concrete' of the Code. The calculations for bending moment for different support conditions during handling are given in Table 5.

Table 5 Bending Moment for Different Support Conditions(Clause 9.2.5)
Sl No.(1)	Number of Points of Pick Up(2)	Location of Support from End in Terms of Length of Pile for Minimum Moments(3)	Bending Moment to be Allowed for DesignkN.m(4)
i)	One	0.293 L	4.3 WL
ii)	Two	0.207 L	2.2 WL
iii)	Three	0.145 L, the middle point will be at the centre	1.05 WL
NOTE — W = weight of pile, in kN.L = length of pile, in m.

9.2.6 The driving stresses on a pile may be estimated

by the following formula:

\left| \begin{array}{l} \frac {\text {Driving resistance}}{\text {Cross - sectional area of the pile}} \times \left[ \frac {2}{\sqrt {n}} - 1 \right] \\ \text {where} \end{array} \right|

$n =$ efficiency of the blow (see 9.2.6.1 for probable value of $n$ ).

NOTE — For the purpose of this formula, cross-sectional area of the pile shall be calculated as the overall sectional area of the pile including the equivalent area for reinforcement.

9.2.6.1 The formula for efficiency of the blow, representing the ratio of energy after impact to striking energy of ram, $n$ , is:

a) where $W$ is greater than $P.e$ and the pile is driven into penetrable ground,

n = \frac {W + \left(P \cdot e ^ {2}\right)}{W + P}

b) where $W$ is less than $P.e$ and the pile is driven into penetrable ground,

n = \left[ \frac {W + (P \cdot e ^ {2})}{W + P} \right] - \left[ \frac {W - (P \cdot e)}{W + P} \right] ^ {2}

The following are the values of $n$ in relation to $e$ and to the ratio of $P / W$ :

Sl No. (1)	Ratio of P/W (2)	e = 0.5 (3)	e = 0.4 (4)	e = 0.32 (5)	e = 0.25 (6)	e = 0 (7)
i)	$\frac{1}{2}$	0.75	0.72	0.70	0.69	0.67
ii)	1	0.63	0.58	0.55	0.53	0.50
iii)	$1\frac{1}{2}$

NOTES

1 $W =$ mass of the ram, in tonne and $P =$ weight of the pile, anvil, helmet, and follower (if any), in tonne.

2 Where the pile finds refusal in rock, $0.5P$ should be substituted for $P$ in the above expressions for $n$ .

3 $e$ is the coefficient of restitution of the materials under impact as tabulated below:

a) For steel ram of double-acting hammer striking on steel anvil and driving reinforced concrete pile, $e = 0.5$ .

b) For cast-iron ram of single-acting or drop hammer striking on head of reinforced concrete pile, $e = 0.4$ .

c) Single-acting or drop hammer striking a well-conditioned driving cap and helmet with hard wood dolly in driving reinforced concrete piles or directly on head of timber pile, $e = 0.25$ .

d) For a deteriorated condition of the head of pile or of dolly, $e = 0$ .

9.3 Reinforcement

9.3.1 The longitudinal reinforcement of any type or grade shall be provided in precast reinforced concrete piles for the entire length. All the main longitudinal bars shall be of the same length and should fit tightly into the pile shoe, if there is one. Shorter rods to resist local bending moments may be added but the same should be carefully detailed to avoid any sudden discontinuity of the steel which may lead to cracks during heavy driving. The area of main longitudinal reinforcement shall not be less than the following percentages of the cross-sectional area of the piles:

a) For piles with a length less than 30 times the least width: 1.25 percent,

b) For piles with a length 30 to 40 times the least width: 1.5 percent, and

c) For piles with a length greater than 40 times the least width: 1.5 percent.

9.3.2 Piles shall always be reinforced with a minimum amount of reinforcement as dowels keeping the minimum bond length into the pile shaft below its cut-off level and with adequate projection into the pile cap, irrespective of design requirements.

9.3.3 Clear cover to all main reinforcement in pile shaft shall be not less than 50 mm. The laterals of a reinforcing cage may be in the form of links or spirals. The diameter and spacing of the same is chosen to impart adequate rigidity of the reinforcing cage during its handing and installations. The minimum diameter of the links or spirals shall be 8 mm and the spacing of the links or spirals shall be not less than 150 mm. Stiffner rings preferably of 16 mm diameter at every 1.5 m centre-to-centre to be provided along the length of the cage for providing rigidity to reinforcement cage. Minimum 6 numbers of vertical bars shall be used for a circular pile and minimum diameter of vertical bar shall be 12 mm. The clear horizontal spacing between the adjacent vertical bars shall be four times the maximum aggregate size in concrete. If required, the bars may be bundled to maintain such spacing.

9.4 Additional provision for prestressed concrete piles shall be as per good practice [6-2(25)].

9.5 For detailed information regarding casting and curing, storing and handling, control of pile driving and recording of data, reference shall be made to good practice [6-2(25)].

9.6 Non-Destructive Testing

For quality assurance of concrete piles, non-destructive integrity test may be carried out prior to laying of beam/caps, in accordance with good practice [6-2(24)].

PART 6 STRUCTURAL DESIGN — SECTION 2 SOILS AND FOUNDATIONS

10 PRECAST CONCRETE PILES IN PREBORED HOLES

10.1 Provisions of 9 except 9.3 shall generally apply.