Part 6Structural Design

Section 2 Soils and Foundations

2.1 General

2.1.1 Clay — An aggregate of microscopic and sub-microscopic particles derived from the chemical decomposition and disintegration of rock constituents. It is plastic within a moderate to wide range of water content. The particles are less than 0.002 mm in size.

2.1.2 Clay, Firm — A clay which at its natural water content can be moulded by substantial pressure with the fingers and can be excavated with a spade.

2.1.3 Clay, Soft — A clay which at its natural water content can be easily moulded with the fingers and readily excavated.

2.1.4 Clay, Stiff — A clay which at its natural water content cannot be moulded with the fingers and requires a pick or pneumatic spade for its removal.

2.1.5 Foundation — That part of the structure which is in direct contact with and transmits loads to the ground.

2.1.6 Gravel — Angular, rounded or semi-rounded particles of rock or soil of particle size between 4.75 mm and 75 mm.

2.1.7 Peat — A fibrous mass of organic matter in various stages of decomposition generally dark brown to black in colour and of spongy consistency.

2.1.8 Sand — Cohesionless aggregates of angular, sub-angular, sub-rounded, rounded, flaky or flat fragments of more or less unaltered rocks, or mineral of size between 4.75 mm and 0.075 mm IS Sieve.

2.1.9 Sand, Coarse — Sand of particle size between 2.0 mm and 4.75 mm IS Sieve.

2.1.10 Sand, Fine — Sand of particle size between 0.075 mm and 0.425 mm IS Sieve.

2.1.11 Sand, Medium — Sand of particle size between 0.425 mm and 2.0 mm IS Sieve.

2.1.12 Silt — Fine-grained soil or fine-grained portion of soil which exhibits a little or no plasticity and has a little or no strength when air dried. The size of particles ranges from 0.075 mm to 0.002 mm.

2.1.13 Soft Rock — A rocky cemented material which offers a high resistance to picking up with pick axes and sharp tools but which does not normally require blasting or chiselling for excavation.

2.1.14 Soil, Black Cotton — Inorganic clays of medium to high compressibility. They form a major soil group in India. They are predominately montmorillonitic in structure and yellowish black or blackish grey in colour. They are characterized by high shrinkage and swelling properties.

2.1.15 Soil, Coarse Grained — Soils which include the coarse and largely siliceous and unaltered products of rock weathering. They possess no plasticity and tend to lack cohesion when in dry state. In these soils, more than half the total material by weight is larger than 75-micron IS Sieve size.

2.1.16 Soil, Fine Grained — Soils consisting of the fine and altered products of rock weathering, possessing cohesion and plasticity in their natural state, the former even when dry and both even when submerged. In these soils, more than half of the material by weight is smaller than 75-micron IS Sieve size.

2.1.17 Soil, Highly Organic and Other Miscellaneous Soil Materials — Soils consisting of large percentages of fibrous organic matter, such as peat, and particles of decomposed vegetation. In addition, certain soils containing shells, concretions, cinders, and other non-soil materials in sufficient quantities are also grouped in this division.

2.1.18 Total Settlement — The total downward movement of the foundation unit under load.

2.2 Ground Improvement

2.2.1 Ground Improvement — Enhancement of the in-place properties of the ground by controlled application of technique suited to subsoil conditions.

PART 6 STRUCTURAL DESIGN — SECTION 2 SOILS AND FOUNDATIONS

2.2.2 Injection — Introduction of a chemical/cementitious material into a soil mass by application of pressure.

2.2.3 Preloading - Application of loads to achieve improvement of soil properties prior to imposition of structural loads.

2.2.4 Soil Densification — A technique to densify cohesionless soils by imparting shocks or vibrations.

2.2.5 Soil Reinforcement — Rods, strips or fabrics incorporated within soil mass to impart resistance to tensile, shear and compressive forces.

2.3 Shallow Foundation

2.3.1 Back Fill — Materials used or reused to fill an excavation.

2.3.2 Bearing Capacity, Safe — The maximum intensity of loading that the soil will safely carry with a factor of safety against shear failure of soil irrespective of any settlement that may occur.

2.3.3 Bearing Capacity, Ultimate — The intensity of loading at the base of a foundation which would cause shear failure of the supporting soil.

2.3.4 Bearing Pressure, Allowable (Gross or Net) — The intensity of loading which the foundation will carry without undergoing settlement in excess of the permissible value for the structure under consideration but not exceeding safe bearing capacity.

The net allowable bearing pressure is the gross allowable bearing pressure minus the surcharge intensity.

NOTE — The concept of ‘gross’ and ‘net’ used in defining the allowable bearing pressure could also be extended to safe bearing capacity, safe bearing pressure and ultimate bearing capacity.

2.3.5 Factor of Safety (with Respect to Bearing Capacity) — A factor by which the ultimate bearing capacity (net) shall be reduced to arrive at the value of safe bearing capacity (net).

2.3.6 Footing — A structure constructed in brick work, masonry or concrete under the base of a wall or column for the purpose of distributing the load over a larger area.

2.3.7 Foundation, Raft — A substructure supporting an arrangement of columns or walls in a row or rows transmitting the loads to the soil by means of a continuous slab, with or without depressions or openings.

2.3.8 Made-Up Ground — Refuse, excavated soil or rock deposited for the purpose of filling a depression or raising a site above the natural surface level of the ground.

2.3.9 Offset — The projection of the lower step from the vertical face of the upper step.

2.3.10 Permanent Load — Loads which remain on the structure for a period, or a number of periods, long enough to cause time dependent deformation/settlement of the soil.

2.3.11 Shallow Foundation — A foundation whose width is generally equal to or greater than its depth. The shearing resistance of the soil in the sides of the foundation is generally neglected.

2.3.12 Spread (or Isolated or Pad) Foundation/Footing — A foundation which transmits the load to the ground through one or more footings. A spread footing (or isolated or pad) is provided to support an individual column. A spread footing is circular, square or rectangular slab of uniform thickness. Sometimes, it is stepped or haunched to spread the load over a large area.

2.3.13 Strip Foundation/Footing — A type of shallow foundation which provides continuous and longitudinal bearing for loads carried by vertical elements, such as continuous wall foundation beams or the like. A strip footing is also provided for a row of columns which are so closely spaced that their spread footings overlap or nearly touch each other. A strip footing is also known as continuous footing.

2.4 Pile Foundation

2.4.1 Allowable Load — The load which may be applied to a pile after taking into account its ultimate load capacity, group effect, the allowable settlement, negative skin friction and other relevant loading conditions including reversal of loads, if any.

2.4.2 Anchor Pile — A pile meant for resisting pull or uplift forces.

2.4.3 Batter Pile (Raker Pile) — The pile which is installed at an angle to the vertical using temporary casing or permanent liner.

2.4.4 Bored Cast in-situ Pile — Piles formed by boring a hole in the ground by percussive or rotary method with the use of temporary/permanent casing or drilling mud and subsequently filling the hole with reinforced concrete.

2.4.5 Bored Compaction Pile — A bored cast in-situ pile with or without bulb(s) in which the compaction of the surrounding ground and freshly filled concrete in pile bore is simultaneously achieved by a suitable method. If the pile is with bulb(s), it is known as under-reamed bored compaction pile.

2.4.6 Bored Pile — A pile formed with or without casing by excavating or boring a hole in the ground and subsequently filling it with plain or reinforced concrete.

2.4.7 Cut-off Level — It is the level where a pile is cut-off to support the pile caps or beams or any other structural components at that level.

2.4.8 Driven Cast in-situ Pile — The pile formed within the ground by driving a casing of uniform diameter, or a device to provide enlarged base and subsequently filling the hole with reinforced concrete. For displacing the subsoil the casing is driven with a plug or a shoe at the bottom. When the casing is left permanently in the ground, it is termed as cased pile and when the casing is taken out, it is termed as uncased pile. The steel casing tube is tamped during its extraction to ensure proper compaction of concrete.

2.4.9 Efficiency of a Pile Group — It is the ratio of the actual supporting value of a group of piles to the supporting value arrived at by multiplying the pile resistance of an isolated pile by their number in the group.

2.4.10 Initial Load Test — A test pile is tested to determine the load carrying capacity of the pile by loading either to its ultimate load or to twice the estimated safe load.

2.4.11 Negative Skin Friction — Negative skin friction is the force developed through the friction between the pile and the soil in such a direction as to increase the loading on the pile, generally due to drag of a consolidating soft layer around the pile resting on a stiffer bearing stratum such that the surrounding soil settles more than the pile.

2.4.12 Pile Spacing — The spacing of piles means the centre-to-centre distance between adjacent piles.

2.4.13 Precast Concrete Piles in Prebored Holes — A pile constructed in reinforced concrete in a casting yard and subsequently lowered into prebored holes and the annular space around the pile grouted.

2.4.14 Precast Driven Pile — The pile constructed in concrete in a casting yard and subsequently driven into the ground when it has attained sufficient strength.

2.4.15 Routine Test — Test carried out on a working pile with a view to check whether pile is capable of taking the working load assigned to it without exceeding permissible settlement.

2.4.16 Safe Load — It is the load derived by applying a factor of safety on the ultimate load capacity of the pile/pile group or as determined from load test.

2.4.17 Ultimate Load Capacity — The maximum load which a pile can carry before failure, that is, when the founding strata fails by shear as evidenced from the load settlement curve or the pile fails as a structural member.

2.4.18 Under-Reamed Pile — A bored cast in-situ or bored compaction concrete pile with enlarged bulb(s) made by either cutting or scooping out the soil or by any other suitable process.

An under-reamed pile having more than one bulb is termed as multi under-reamed piles. The piles having two bulbs may be called double under-reamed piles.

2.4.19 Working Load — The load assigned to a pile as per design.

2.4.20 Working Pile — A pile forming part of the foundation system of a given structure.