Chapter 13Other Foundations, Substructures and Foundations for Special Structures

NBC 2016 (SP 7:2016) Volume 1

2016 · PDF page 586

Hover any section to copy its link or text, ask Copilot about it, or bookmark it.

Open the original PDF

13.1 Pier Foundations

13.1.1 Design Considerations

13.1.1.1 General

The design of concrete piers shall conform to the

PART 6 STRUCTURAL DESIGN — SECTION 2 SOILS AND FOUNDATIONS

requirements for columns specified in Part 6 ‘Structural Design, Section 5 Concrete’ of the Code. If the bottom of the pier is to be belled so as to increase its load carrying capacity, such bell shall be at least 300 mm thick at its edge. The sides shall slope at an angle of not less than $60^{\circ}$ with the horizontal. The least permissible dimensions shall be 600 mm, irrespective of the pier being circular, square or rectangular. Piers of smaller dimensions if permitted shall be designed as piles (see 8 and 9).

13.1.1.2 Plain concrete piers

The height of the pier shall not exceed 6 times the least lateral dimension. When the height exceeds 6 times the least lateral dimension, buckling effect shall be taken into account, but in no case shall the height exceed 12 times the least lateral dimension.

When the height exceeds 6 times the least lateral dimension, the deduction in allowable stress shall be given by the following formula:

f _ {\mathrm{c}} ^ {\prime} = f _ {\mathrm{c}} \left(1. 3 - \frac {H}{2 0 D}\right)

where

f _ {\mathrm{c}} ^ {\prime} = \text {reduced allowable stress},

f _ {\mathrm{c}} = \text {allowable stress,}

H = \text {height of pier, and}

$D =$ least lateral dimension.

NOTE - The above provision shall not apply for piers where the least lateral dimension is $1.8\mathrm{m}$ or greater.

13.1.1.3 Reinforced concrete piers

When the height of the pier exceeds 18 times its least dimension, the maximum load shall not exceed:

P ^ {\prime} = P \left(1. 5 - \frac {H}{3 6 D}\right)

where

$P^{\prime} =$ permissible load;

$P =$ permissible load when calculated as axially loaded short column;

$H =$ height of the pier measured from top of bell, if any, to the level of cut-off of pier; and

$D =$ least lateral dimension.

13.2 Design and construction of machine foundations, diaphragm walls, etc, shall be carried out in accordance with good practice [6-2(29)].

14 GROUND IMPROVEMENT

14.1 In poor and weak subsoils, the design of conventional foundation for structures and equipment

may present problems with respect to both sizing of foundation as well as control of foundation settlements. A viable alternative in certain situations, is to improve the subsoil to an extent such that the subsoil would develop an adequate bearing capacity and foundations constructed after subsoil improvement would have resultant settlements within acceptance limits. This method/technique is called ground improvement which is used to improve in-situ soil characteristics by improving its engineering performance as per the project requirement by altering its natural state, instead of having to alter the design in response to the existing ground limitations. The improvement is in terms of increase in bearing capacity, shear strength, reducing settlement and enhancing drainage facility, mitigating liquefaction potential, etc, of soil, as also in improving lateral capacity in case of deep foundations.

14.2 For provisions with regard to necessary data to be collected to establish the need for ground improvement at a site; considerations for establishing need for ground improvement methods; selection of ground improvement techniques; equipment and accessories for ground improvement; control of ground improvement works and recording of data, reference shall be made to good practice $[6-2(30)]$ . Annex K presents various methods of ground improvement along with principles, applicability to various soil conditions, material requirements, equipment required, results likely to be achieved and limitations. This table may be referred to as guidance for selecting the proper method for a situation {see also good practice $[6-2(30)]$ for details}.

14.2.1 For provisions relating to ground improvement by reinforcing the ground using stone columns so as to meet the twin objective of increasing the bearing capacity with simultaneous reduction of settlements, reference shall be made to good practice [6-2(31)].

14.2.2 Whenever soft cohesive soil strata underlying a structure are unable to meet the basic requirements of safe bearing capacity and tolerable settlement, ground improvement is adopted to make it suitable for supporting the proposed structure. Both the design requirements that is shear strength and settlement under loading, can be fulfilled by consolidating the soil by applying a preload, if necessary, before the construction of the foundation. This consolidation of soil is normally accelerated with the use of vertical drains. For provisions relating to ground improvement by preconsolidation using vertical drains, reference shall be made to good practice $[6-2(32)]$ .

14.2.3 Use of suitable geo-synthetics/geo-textiles may be made in an approved manner for ground improvement, where applicable. See also accepted standards [6-2(33)].

20D

H

)