R.C.C Foundations

R.C.C. FOUNDATION RITHIKA RAVISHANKAR ROLL NO.40 T.Y.B.ARCH ACADEMY OF ARCHITECTURE INTRODUCTION Most of the structures built by man are made of reinforced concrete. Here, the part of the structure above ground level is called as the superstructure, where the part of the structure below the ground level is called as the substructure. Footings are located below the ground level and are also referred as foundations. Foundation is that part of the structure which is in direct contact with soil. The foundations of buildings bear on and transmit loads to the ground. The foundation is that part of walls, piers and columns which is in direct contact with and transmit loads to the ground.  WHY IS FOUNDATION PROVIDED? To pass out building weight to the soil beneath evenly; To prevent differential settlement of building; To provide a plane surface for the convenience of construction; To make building substantial and durable by continuing the structure in the soil. 1 2. Types of Foundations Types of Pad Foundations Pad foundations are generally rectangular or square foundation to transfer load from structure to the ground. These are provided at shallow depth and are shallow foundations. Following are the types of pad foundations: Plain concrete is only an economic option where the loading is relatively light as T must equal P otherwise excessively thick pads are needed which is not economic. Fig: Plain concrete pad foundation Reinforced concrete enables relatively wide but shallow foundations, often designed to be square plan area to make the reinforcing cage easier to construct and place. Rectangular pads are used for eccentric/inclined loading (longer dimension parallel to direction of inclination/eccentricity). Fig: Reinforced concrete pad foundations Combined pad foundations are adopted close to a site boundary to enable the balancing effect of an internal column to be incorporated. Fig: Combined pad foundation Continuous pad exists when pads and the columns they support are fairly closely spaced. Extending the reinforcing between pads ensures longitudinal stiffness (resists differential settlement). Fig: Continuous pad foundations Pad and ground beam: here smaller isolated pads are connected by ground beams to provide structural rigidity. Fig: Pad foundation with continuous beam Based on the position with respect to ground level, Footings are classified into two types; 1. Shallow Foundations 2. Deep Foundations Shallow Foundations are provided when adequate SBC is available at relatively short depth below ground level. Here, the ratio of Df / B < 1, where Df is the depth of footing and B is the width of footing. Deep Foundations are provided when adequate SBC is available at large depth below ground level. Here the ratio of Df / B >= 1. The foundations of varying types depending upon load requirement, ground conditions and prevailing practices in the region. A strip foundation is a continuous strip of concrete under walls, an isolated footing is a concrete isolated base under piers and columns, a raft foundation is a continuous base under the whole of the building and a pile is a concrete column or pillar cast in or driven into the ground to support a concrete base or ground beam. Main activities of building foundation are:- To distribute building load to soil beneath To distribute the load uniformly To grapnel the structure to the ground to resist movement due to lateral force  To prevent sinking of the structure Strip foundation Strip Foundations consist of a continuous strip, made up of brick masonry/stone masonry/concrete formed centrally under load bearing walls. The continuous strip serves as a level base on which the wall is built and is of such a width as is necessary to spread the load on the foundations to an area of subsoil capable of supporting the load without undue compaction. Isolated foundation It is sometimes economical to construct a foundation of isolated piers or columns of brick or concrete supporting reinforced concrete ground beams in turn supporting walls, rather than excavating deep trenches and raising walls off strip foundations, some depth below ground. The isolated foundations are typical rectangular or trapezoidal block made up of reinforced concrete. In some places where burnt clay brick quality is good, these are made with burned bricks also. Raft foundation The Raft foundations consist of a raft of reinforced concrete under the whole of the building designated to transmit the load of the building to the subsoil below the raft. Raft foundations are used for buildings on compressible ground such as very soft clays, alluvial deposits and compressible fill material where strip foundations would not provide a stable foundation. A typical raft foundation comprises of beam column system along with huge slab below ground. Pile foundation Where the subsoil has poor or uncertain bearing capacity or where there is likely to be appreciable ground movement as with firm, shrinkable clay or where the foundation should be deeper than stay 2 m, it is often economical to use piles. A pile is a column of concrete either cast in or driven into the ground to transfer loads through the poor bearing soil to a more stable stratum. The piles support reinforced concrete beams off which load bearing wall are built. Short-bored piles For small buildings, for example on shrinkable clays where adjacent trees or the felling of trees makes for appreciable volume change in the subsoil for some depth, it is often wise and economical to use a system of short-bored piles for foundations. Short-bored, that is short-length piles are cast in holes augured by hand or machine Combined Column Footing These are common footings which support the loads from are provided when • SBC is generally less • Columns are closely spaced • Footings are heavily loaded In the above situations, the area required to provide isolated footings generally overlap. Hence, it is advantageous to provide single combined footing. In some cases, columns are located on or close to property line. In such cases footings cannot be extended on one side. Here, the footings of exterior and interior columns are connected by the combined footing. The combined footing for columns will be rectangular in shape if they carry equal loads. The design of rigid rectangular combined footing should be done in such a way that center of gravity of column loads coincide with the centroid of the footing area. If the columns carry unequal loads, the footing is of trapezoidal shape, as shown in Fig. (b). Types of Deep Foundations Deep foundations are provided when adequate SBC is available at large depths below GL. Pile Foundation Pier Foundation Well Foundation Bearing Capacity of Soil The safe bearing capacity of soil is the safe extra load which the soil can withstand without experiencing shear failure. The Safe Bearing Capacity (SBC) is considered unique at a particular site. But it also depends on the following factors: • Size of footing • Shape of footing • Inclination of footing • Inclination of ground • Type of load • Depth of footing etc. SBC alone is not sufficient for design. The allowable bearing capacity is taken as the smaller of the following two criteria • Limit states of shear failure criteria (SBC) • Limit states of settlement criteria Based on ultimate capacity, i.e., shear failure criteria, the SBC is calculated as SBC = Total load / Area of footing Usually the Allowable Bearing Pressure (ABP) varies in the range of 100 kN/m2 to 400 kN/m2. The area of the footing should be so arrived that the pressure distribution below the footing should be less than the allowable bearing pressure of the soil. Even for symmetrical Loading, the pressure distribution below the footing may not be uniform. It depends on the Rigidity of footing, Soil type and Conditions of soil. In case of Cohesive Soil and Cohesion less Soil the pressure distribution varies in a nonlinear way. However, while designing the footings a linear variation of pressure distribution from one edge of the footing to the other edge is assumed. Once the pressure distribution is known, the bending moment and shear force can be determined and the footing can be designed to safely resist these forces. Limit state of collapse is adopted in the design of isolated column footings. The various design steps considered are; • Design for flexure • Design for shear (one way shear and two way shear) • Design for bearing • Design for development length The materials used in RCC footings are concrete and steel. The minimum grade of concrete to be used for footings is M20, which can be increased when the footings are placed in aggressive environment, or to resist higher stresses. Cover The minimum thickness of cover to main reinforcement shall not be less than 50 mm for surfaces in contact with earth face and not less than 40 mm for external exposed face. However, where the concrete is in direct contact with the soil the cover should be 75 mm. In case of raft foundation the cover for reinforcement shall not be less than 75 mm. Minimum reinforcement and bar diameter: The minimum reinforcement according to slab and beam elements as appropriate should be followed, unless otherwise specified. The diameter of main reinforcing bars shall not be less 10 mm. The grade of steel used is either Fe 415 or Fe 500. Moments and Forces (a) In the case of footings on piles, computation for moments and shears may be based on the assumption that the reaction from any pile is concentrated at the centre of the pile. (b) For the purpose of computing stresses in footings which support a round or octagonal concrete column or pedestal, the face of the column or pedestal shall be taken as the side of a square inscribed within the perimeter of the round or octagonal column or pedestal. Bending Moment (a) The bending moment at any section shall be determined by passing through the section a vertical plane which extends completely across the footing, and computing the moment of the forces acting over the entire area of the footing on one side of the said plane. (b) The greatest bending moment to be used in the design of an isolated concrete footing which supports a column, pedestal or wall, shall be the moment computed in the manner prescribed in ***(a) at sections located as follows: a) At the face of the column, pedestal or wall, for footings supporting a concrete column, pedestal or wall; b) Halfway between the center-line and the edge of the wall, for footings under masonry walls; and c) Halfway between the face of the column or pedestal and the edge of the gusseted base, for footings under gusseted bases. Framework for circular column Depending on the type of soil, the sub-soil water table and the presence of surface water, four types of foundation will be used for each type of tower location. Classified as follows: Normal dry type : To be used for location in normal day cohesive or non-cohesive soils Wet type : To be used for locations- i) Where sub-soil water is met at 1.5 m or more below the ground line. Or ii) Which are in surface water for long periods with water penetration not exceeding one meter below the ground line. iii) In black cotton soils Partially sub-merged type: To be used at locations where sub soil water table is discovered between 0.75 meter below the ground line. Fully sub-merged type: To be used at locations where sub-soil water table is met at less than 0.75 meter below the ground line. In addition to the above, depending on the site conditions, other types of foundations may be introduced suitable for- Intermediate conditions under the above classification to effect more economy, or For locations in hilly and rocky areas. For locations where special foundations (well type or piles) are necessitated. The proposal for this shall be submitted by the contractor based on the Board. PROPERTIES OF EARTH TESTING OF SOIL It is desirable to undertake testing of soil for all the tower locations and report should be obtained about the sub-soil water table, as prevalent in the month of September and October type of soil encountered, bearing capacity of soil, possibility of submergence and other soil properties required for the correct casting of casing of foundations. Testing should be carried out about soil resistively in dry season and its record should be kept properly along with route alignment map. After soil investigation along the line alignment, the final quantities of foundation types should be worked out based on the soil investigation carried out and such foundations should be casted and installed only after proper checking and approval. EXCAVATION Except as specifically otherwise provided, all excavation for footings shall be made to the lines and grades of the foundation. The excavation walls shall be vertical and the pit dimensions shall be such as to allow a clearance of not more so as to maintain a clean sub-grade, until the footing is placed, using timbering, shoring or casing, if necessary. Any sand, mud, silt or other undesirable materials which may have accumulated in the excavation pit shall be removed before placing concrete. The soil to be excavated for tower foundation shall be classified as under- Normal soil : Soil removable by means of ordinary pick axes, shovels & spades such as types of soil found in gigantic plains, black cotton soil etc. Wet Soil: Soil where the sub soil water table is encountered within the range of foundation depth, the soil below the water table and that at locations where pumping or bailing out of water is required due to presence of surface water, will be treated as wet soil. Rocky Soil : Soft rocks-This will mean decomposed rock, hard gravel, kankar, lime stone, laterite or any other soil of similar nature which can be easily excavated with pick axe or spade. Hard rocks: Hard rock will be that which requires chiseling or drilling and blasting. Where rock is encountered, the holes for tower footings, shall preferably be drilled, but where blasting is to be resorted to as an economy measure, it shall be done with the utmost care to minimize the use of concrete for filling up the blasted area. All necessary precautions for handling and use of blasting materials shall be taken. Shoring of pits with shuttering will be done when the soil condition is so bad that there is likelihood of accident due to the falling of surrounding earth. However, the necessity of shoring of pits with shuttering shall be decided by the Supervising Engineer, depending upon the site conditions. Depending on the condition of water available in the pits, following methods of dewatering will be adopted Manual: Where dewatering is done by men with the help of buckets etc. Mechanical: Where dewatering is done by hand pump. Power driven: When engines or electrical power driven pumps with power input not less than half H.P. are used for dewatering. DETAILS OF FOUNDATIONS The thickness have been designed such as to satisfy that conditions specified herein. The thickness of concrete in the chimney portion of the tower footing would be such that it provides minimum cover of not less than 100 mm from any part of the stub angle to the nearest outer surface of the concrete in respect. Of all dry locations limiting the minimum section of chimney to 300 mm square. In respected of all wet locations chimney should have an all-round clearance of 150 mm from any part of the stub angle limiting to 450mm square minimum. The chimney top or muffing must be at least 225 mm above ground level and also the coping shall be extended up to the lower most joint level between the bottom lattices and the main corner leg of the tower. The spread of concrete pyramid or slabs will be limited to 45 deg. with respect to the vertical. At least 50 mm thick pad of size equal to the bases of pyramid with its side vertical will be provided below the pyramid to account for the unevenness of soils and impurities likely to be mixed in concrete due to direct contact of wet concrete with earth and for allowing stone aggregate reaching up to corner edges. This pad will also be provided in cases where pyramids are provided over concrete slabs. In case of fully submerged type foundation one base slab, not less the 200 mm thick has been provided. The minimum distance between the lowest edge of the stubs angle and the bottom surface of concrete footings shall not be less than 100 mm or more than 150 mm in case of dry locations and not less than 150 mm or more than 200 mm in case of wet locations. The portion of the stub in the pyramid has been provided with the cleats. Foundation failure due to Soil Movement When water present between soil particles is removed, the soil tend to move closer together. When water is absorbed by soil, the soil starts to swell. This movement of soil is based on the type of soil. Large movement is seen with clayey soils than sandy soils. These kind of movement of soil due to change in water content affects the foundation settlement. Foundation tends to settle to and excessive settlement of foundation may lead to differential settlement and damage to the structure. Soil movement can occur due to following: 1. Presence of vegetation or remains of old cut trees 2. Presence of mining areas 3. Shrinkable soils Remedies for foundation failure due to soil movement: 1. Use of pile foundations where the soil is shrinkable, so that forces are transferred to the hard strata or rock. 2. Taking the foundation levels down to avoid foundation on shrinkable soils. 3. The vegetation is removed from the construction site and its roots are removed. Any cavity due to roots of vegetation shall be compacted and filled with concrete. 4. Presence of any mining areas needs to be inspected and professional help shall be taken while construction new buildings in such areas. Foundation failure due Settlement of Soil Fill If the building is constructed on a newly developed land by soil filling, the foundation on such soils tend to settle more with time as long time is needed for such soil to settle and become compact to resist the loads from the building foundation. Remedies: It shall be ensured that such soils are adequately compacted before construction begins on them. The foundation depth shall be increased to the hard strata or rock below the filled soil or pile foundations shall be used to prevent subsidence of foundation. Shear and Bond i. The shear strength of footings is governed by the more severe of the following two conditions: a. The footings acting essentially as a wide beam, with a potential diagonal crack extending in a plane across the entire width, the critical section for this condition shall be assumed as a vertical section located from the face of the column, pedestal or wall at a distance equal to the effective depth of the footing in case of footings on soils and a distance equal to half the effective depth of footing for footing on piles. b. Two way action of the footing, with potential diagonal cracking along the surface of truncated cone or pyramid around the concentrated load: in this case, the footing shall be designed for shear in accordance with appropriate provisions discussed below. (Fig.2) Critical Section for Shear The critical section for checking the development length in a footing shall be assumed at the same planes as described for bending moment in B(3) and also at all other vertical planes where abrupt changes of section occur. If the reinforcement is curtailed, the anchorage requirements should be checked. iv. Thus according to the above provision, shear stress is to be checked for (i) one way action (i.e. beam shear) for which the governing section AB is at a distance d from the face of column or pedestal (fig.2 (a)) and (ii) two way shear (i.e. punching shear), for which the governing section is along the perimeter ABCD situated at a distance d/2 from the face of the column or pedestal  FACTOR OF SAFETY The required factor of safety depends upon: Type of structure permanent or temporary Sensitivity of structure Extent of soil exploration Nature of loading considered and assumption made in the design Extent of quality control during construction. It is recommended that the factor of safety should be between 2 and 4. NOMINAL REINFORCEMENT The nominal reinforcement for concrete sections of thickness greater than 1m shall be 360 sq. mm per meter length in each direction on each face. This provision does not supersede the requirement of minimum tensile reinforcement based on the depth of the section. Reinforcement Distribution in Footing In one-way RCC footing, the reinforcement is distributed uniformly across the full width of footing. In two-way square footings, the reinforcement extending in both directions is distributed uniformly across the full width of the footing. But in the case of two-way rectangular footings, reinforcement is distributed across the full width of footing in long direction, however for short direction, the reinforcement is distributed in the central band as per calculations below. The rest reinforcement in short direction is distributed equally on both sides of the central band. Where y is the long side and x is the short side of the footing CIRCULAR RAFT FOOTING Ring or circular rafts can be used for cylindrical structures such as chimneys, silos, storage tanks, TV-towers and other structures. In this case, ring or circular raft is the best suitable foundation to the natural geometry of such structures. The design of circular rafts is quite similar to that of other rafts. MACHINE FOUNDATIONS Machine foundations are special types of foundations required for machines, machine tools and heavy equipments which have wide range of speeds, loads and operating conditions. These foundations are designed considering the shocks and vibrations (dynamic forces) resulting from operation of machines. Following are the types of machine foundations generally used: 1. Block Type Machine Foundation: Following figure shows block type machine foundation. This type of foundation consists of a pedestal resting on a footing have has large mass and a small natural frequency. 2. Box or Caisson Type Machine Foundation: Box type foundation consists of a hollow concrete block as shown in figure below. The mass of this foundation is less than block type machine foundation as it is hollow. The natural frequency of the box type machine foundation is increased. 3. Wall Type Machine Foundation: This type of machine foundation consists of a pair of walls with a slab resting on top. This type of foundation is constructed of homogeneous materials. It is used for small machines and the machine is rested on the top slab. 4. Framed Type Machine Foundation: This type of machine foundation consists of vertical columns with horizontal frame at their tops. It is used for larger machines. The machines are rested on the top of frames. The vertical and horizontal members of this foundation can be constructed by different materials. 5. Non-Rigid or Flexible type of Machine Foundation Following figure shows the non-rigid or flexible foundation. From the practical point of view, the following requirements should be fulfilled. 1. The groundwater table should be as low as possible and groundwater level deeper by at least one-fourth of the width of foundation below the base plane. This limits the vibration propagation, groundwater being a good conductor of vibration waves. 2. Machine foundations should be separated from adjacent building components by means of expansion joints. 3. Any steam or hot air pipes, embedded in the foundation must be properly isolated. 4. The foundation must be protected from machine oil by means of acid-resisting coating or suitable chemical treatment. 5. Machine foundations should be taken to a level lower than the level of the foundations of adjoining buildings. SOIL TESTING Soil Classifications: Engineers dealing with soil mechanics devised a simple classification system that will tell the engineer the properties of a given soil. The unified soil classification system is based on identifying soils according to their textural and plasticity qualities and on their grouping with respect to behavior. Soils are usually found in nature as mixtures with varying proportion of particles of different sizes, each of these components contribute to the soil mixture. Soil is classified on the basis of: Percentage of gravel, sand, and fines. Shape of grain. Plasticity and compressibility characteristics. In the unified soil classification system (uscs) the soil is given a descriptive name and a letter symbol indicating its principal characteristics. Placement of solid into its respective group is accomplished by visual examination and laboratory tests. In the unified soil classification, the terms cobbles, gravel, sand, and fines (silt or clay) are used to designate the size ranges of soil particles. Soil particle size ranges from largest to smallest: Cobbles Gravel (Coarse + Fine) Sand (Coarse + Medium + Fine) Fines consisting of Clay or Silt Soil shear strength is made up of cohesion (water content, how sticky it is) and internal friction (based on size of grains). This is determined by triaxial compression testing Soil Groups: Soils are then grouped into three groups consisting of: Coarse Grained – divided into gravely soils (G) and sands and sandy soils (S) Fine Grained – divided based on their plasticity properties. (L,H) Highly Organic – are not subdivided. (Pt) Coarse Gained – are soils which composed of gravel and or sands and which contain a wide variety of particles. These are most suitable for foundations when well drained and well confined. They are soils with good bearing value. Particularly the G series (GW, GP, GM, GC). Identified on the basis of the percentage amount of gravel and sand. Fine Grained – are soils that are Silts and Clays (L,H). Contain smaller particles of silt and clay. These are suitable for foundations but require compactions. The most suitable of this series (L) is the CL. Based on their cohesive properties and permeability. Highly Organic – are soils that are usually very compressible and are not suitable for construction. They contain particles of leaves, grass, and branches. Peat, Humus, and swamp soil with highly organic texture are typical of this group (Pt). These are identified readily on the basis of color, texture, and odor. Moisture content is also very high in this type of soil. Soil names shown on the unified soil classification system are associated with certain grain size and textural properties. This is the case for the coarse grained soils. For silts and clay the names are based on the plasticity basis of the soil. Relevant information of samples taken by borings which can aid the geotechnical engineer in determination of foundations includes: 1. For coarse grain soil – the size of the particles, mineralogical composition, shape of grains, and character of the binder. For fine grained soils – strength, moisture, and plasticity. In the preliminary stages, a visual inspection can determine the behavior of the soil when used as component in the construction of a proposed building. Soil can be classified according to the classification categories of the unified soil classification system. (Later on laboratory testing can be performed). Strength and consolidation which make up the compaction characteristics of the soil determines its suitability forbuilding foundations. Soil Problems: The problem of uplift pressures in soil can be reduced by having well drained and free draining gravels (GW, GP). Uplift pressures can occur in fine grained soils consisting of silts and clays; such soils can cause heaving of foundations and formation of boils. Due to potential frost action Regardless of the frost susceptibility of the various soil groups, two conditions must be present simultaneously before frost action will be a consideration – a source of water during the freezing period and a sufficient period of the freezing temperature to penetrate the ground. In general silts and clays (ML, CL, OL) are more susceptible to freezing (as they contain moisture). Well drained granular soils are less susceptible to freezing and creating foundation problems. Due to drainage Characteristics The drainage characteristics of soils are a direct reflection of their permeability. The presence of moisture in base, sub-base and sub-grade materials may cause the development of pore water pressure and loss of strength. The gravelly and sandy soils with little or no fines (GW, GP, SW, SP) have excellent drainage characteristics. Fine grained soils and highly organic soils have poor drainage characteristics. Compaction: The sheepsfoot and rubber tired rollers are common pieces of equipment used to compact soils. Some advantage is claimed for the sheepsfoot roller in that it leaves a rough surface that affords better bond between layers. Granular soils consisting of well graded materials (GW, SW) furnish better compaction results than the poorly graded soils (GP, SP). Fine grained soils can also be compacted. For most construction projects of any magnitude, it is highly desirable to investigate the compaction characteristics of the soil be means of a field test section. Suitability of soils for foundations depends primarily on the strength, cohesion and consolidation characteristic of the soils. The type of structure, load and its use will largely govern the adaptability of a soil as a satisfactory foundation material. A soil might be entirely satisfactory for one type of construction but might require special treatment for other building. In general, gravel and gravely soils (GW, GP, GM, GC) have good bearing capacity and undergo little consolidation under load. Well graded sands (SW) usually also have good bearing capacity. Poorly graded sands and silty sands (SP, SM) have variable capacity based on their density. Some soils containing silts and clays (ML, CL, OL) are subject to liquefaction and may have poor bearing capacity and large settlements when subject to loads. Of the fine grained soil group CL is probably the better for foundations. Organic soils (OL and OH ) and highly organic soils (Pt) have poor bearing capacity and usually exhibit large settlement under load. Foundations: For most of the fine grained soils (containing silt and clays) it might be sufficient to use simple spread footings, it is largely depending on the magnitude of the load. The location of the foundations in relation to the soil (need to be aware of foundation walls and hydrostatic pressure as moisture is present in the soil). If the soil is poor and structure loads are relatively heavy, then alternate methods are required. Pile foundations might be required in some cases where fine cohesive silt and clay soil is present. (CH, OH). Sometimes it might be desirable and economically feasible to over excavate remove such soils that are not of bearing capacity; can remove compact and fill back or import other engineered soil. The geotechnical engineer based on borings will recommend suitable foundations systems or alternative solutions, also beating capacity, minimum depths, and special design or construction procedures might be established. Safe bearing capacity of soil equals to the ultimate bearing capacity divided by a safety factor (usually 2-4). ultimate bearing capacity is defined as the maximum unit pressure a soil can sustain without permitting large amounts of settlements. Bedrock has the highest safe bearing capacity. Well graded gravel and sand that are confined and drained have a safe bearing capacity of 3,000 – 12,000 PSF. Silts and clays have lower safe bearing capacity of 1,000 – 4,000 PSF. Soil tests required to determine safe bearing capacity of shallow foundations and raft foundations are discussed here. These tests are as per IS 6403 – 1981. Apart from ascertaining the highest level ever reached by the ground water table and tests for classification of soil as per IS1498 – 1970 based on grain size analysis as per IS2720 (Part –IV)– 1985, index properties of soil as per IS2720 (Part-V) – 1985, the following tests are required to determine safe bearing capacity based on shear strength consideration: 1. Standard penetration test as per IS2131 – 1991 for coarse grained / fine grained cohesion less soils with semi-pervious clayey soils (i.e.  soils with clay upto 30%). 2. Direct shear test (controlled strain) as per IS 2720 (Part – 13) – 1986. Consolidated un-drained tests for cohesive and for  soils and consolidated drained tests for cohesion less soils. The results may be compared with standard penetration test / static cone penetration test results. Since there is escape of pore water during box shear, partial drainage vitiates the consolidated un-drained test. Hence this test is not exact for semi-pervious soils such as clayey sands / silts (i.e. with clay more than 15% but less than 30%). For such soils, triaxial tests are required if shear strength is critical criterion. 3. Static cone penetration test as per IS 4968 (Part -3) – 1976 for foundations on non-stiff clayey soils such as fine grained soils (i.e. more than 50% passing through 75 micron sieve). In fine and medium coarse sands such tests are done for correlation with standard penetration test and to indicate soil profiles at intermediate points. 4. Unconfined compressive strength test as per IS 2720 (Part-10) – 1973 for highly cohesive clays except soft / sensitive clays. 5. Vane shear tests for impervious clayey soils except stiff or fissured clays. 6. Triaxial shear tests for predominantly cohesive soils. If shear strength is likely to be critical. SOIL TESTS REQUIRED FOR RAFT FOUNDATIONS: (As per Para 3 of IS 2950 (Part-1) – 1981. Apart from other tests for shallow foundations, the following soil tests are required especially for raft foundations: 1. Static cone penetration test as per IS 4968 (Part-3) – 1976 for cohesionless soils to determine modulus of elasticity as per IS 1888 – 1982. 2. Standard penetration test as per IS 2131 – 1981 for cohesionless soils and  soils to determine modulus of sub-grade reaction. 3. Unconfined compressive strength test as per 2720 (Part -10) – 1973 for saturated but no pre-consolidated cohesive soil to determine modulus of sub-grade reaction. 4. As specified in IS 2950 (Part -1) – 1981¸ plate load test as per IS 1888 – 1982 where tests at Sl. No. – 1 to 3 above are not appropriate such as for fissured clays / clay boulders. 5. In case of deep basements in pervious soils, permeability is determined from pumping test. This is required to analyze stability of deep excavation and to design appropriate dewatering system. The soil tests required for deep foundations are: 1. While the composition and depth of the bearing layer for shallow foundations may vary from one site to another, most pile foundations in a locally encounter similar deposits. Since pile capacity based on soil parameters is not as reliable from load tests, as a first step it is essential to obtain full information on the type, size, length and capacity of piles (including details of load – settlement graph) generally adopted in the locality. Correlation of soil characteristics (from soil investigation reports) and corresponding load tests (from actual projects constructed) is essential to decide the type of soil tests to be preformed and to make a reasonable recommendation for the type, size, length and capacity of piles since most formulae are empirical. 2. If information about piles in the locality are not available or reliable, it may be necessary to drive a test pile and correlate with soil data. 3. Standard penetration test (SPT) to determine the cohesion (and consequently the adhesion) to determine the angle of friction (and consequently the angle of friction between soil and the pile and also the point of resistance) for each soil stratum of cohesion less soil of  soil. 4. Static cone penetration test (CPT) to determine the cohesion (and subsequently the adhesion) for soft cohesive soils and to check with SPT result for fine to medium sands. Hence for strata encountering both cohesive and cohesion less soils, both SPT and CPT tests are required. 5. Vane shear test for impervious clayey soils. 6. Un-drained triaxial shear strength of undisturbed soil samples (obtained with thin walled tube samplers) to determine cohesion (c) and angle of internal friction () for clayey soils (since graphs for correlations were developed based on un-drained shear parameter). In case of driven piles proposed for stiff clays, it is necessary to check with the c and  from remoulded samples also. Drained shear strength parameters are also determined to represent in-situ condition of soil at end of construction phase. 7. Self boring pressure meter test to determine modulus of sub-grade reaction for horizontal deflection for granular soils, very stiff cohesive soils, soft rock and weathered or jointed rock. 8. Ground water condition and permeability of soil influence the choice of pile type to be recommended. Hence the level at which water in the bore hole remains are noted in the bore logs. Since permeability of clay is very low, it takes several days for water in the drill hole to rise upto ground water table. Ground water samples need to be tested to consider the possible chemical effects on concrete and the reinforcement. Result of the cone penetration test for the same soil show substantial scatter. Hence, they need to be checked with supplementary information from other exploration methods. Pressure meters are used to estimate the in-situ modulus of elasticity for soil in lateral direction. Unless the soil is isotropic, the same value cannot be adopted for the vertical direction. Subsurface investigations Subsoil conditions are examined using test borings, provided by soil engineer (geotechnical). Number of borings and location of borings depends on building type and site conditions. Typically for uniform soil conditions borings are spaced 100-150′ apart, for more detailed work, where soil footings are closely spaced and soil conditions are not even borings are spaced 50′ apart. Larger open warehouse type spaces, where fewer columns are present (long span) required less boring samples. Borings must extend to firm Strata (go through unsuitable foundation soil) and then extend at least 20 feet more into bearable soil. Location of borings samples are indicated on engineer plan. Borings are not taken directly under proposed columns. Borings indicate: depth, soil classification (according to the unified soil system), and moisture content and sometimes ground water level is shown as well. (Physical properties: particle size, moisture content, density). Soil report recommendation should be based on testing of materials obtained from on site borings and to include: Bearing capacity of soil. Foundation design recommendations. Paving design recommendations. Compaction of soil. Lateral strength (active, passive, and coefficient of friction). Permeability. Frost depth. Spread Footings: Used for most buildings where the loads are light and / or there are strong shallow soils. At columns there are single spot square pads where bearing walls have an elongation form. These are almost always reinforced. These footing deliver the load directly to the supporting soils. Area of spread footing is obtained by dividing the applied force by the soils safe bearing capacity (f=P/A). Generally suitable for low rise buildings (1-4 Stories). Requires firm soil conditions that are capable of supporting the building on the area of the spread footings. When needed footings at columns can be connected together with grade beams to provide more lateral stability in earthquakes. These are most widely used because they are most economical. Depth of footings should be below the top soil, and frost line, on compacted fill or firm native soil. Spread footings should be above the water table. Concrete spread footings are at least as thick as the width of the stem. As the weight of the building increases in relation to the bearing capacity or depth of good bearing soil, the footing needs to expand in size or different systems need to be used. Drilled Piers or Caissons: For expansive soils with low to medium loads, or high loads with rock not too far down, drilled caissons (piers) and grade beams can be used. The caissons might be straight or belled out at bottom to spread the load. The grade beam is designed to span across the piers and transfer the loads over to a column foundation. Caissons deliver the load to soil of stronger capacity which is located not too far down. Piles: For expansive soils or soils that are compressive with heavy loads where deep soils can not take the building load and where soil of better capacity if found deep below. There are two types of piles. 1. Friction piles – used where there is no reasonable bearing stratum and they rely on resistance from skin of pile against the soil. 2. End bearing – which transfer directly to soil of good bearing capacity. The bearing capacity of the piles depends on the structural strength of the pile itself or the strength of the soil, whichever is less. Piles can be wood, steel, reinforced concrete, or cast in place concrete piles. Cast in place piles are composed of hole drilled in earth and then filled with concrete, it is used for light loads on soft ground and where drilling will not cause collapse. Friction type, obtained from shaft perimeter and surrounding earth. Fig: Concreting of pile foundation The grade of concrete to be used for piling should be minimum M25 (or as as required at the site for load conditions) with the minimum cement content of 400 kg/m3. Mixing is carried out in mechanical mixer only. In case of piles subsequently exposed to free water or in case of piles where concreting is done under water or drilling mud using method other than tremie, 10% extra cement over the design grade of concrete at the specified slump is used subject to a minimum quantity of cement specified above. For the design purpose of bore cast-in-situ piles, the strength of concrete mix using above mentioned quantities of cement is taken as M20. Concreting for the piles is to be done with tremie of suitable diameter. Natural rounded shingle of appropriate size may be used as coarse aggregate. It helps to give high slump with less water cement ratio. For tremie concreting aggregates having nominal size more than 20mm should not be used. Mat Foundations: Reinforced concrete raft or mats can be used for small light load buildings on very weak or expansive soils such as clays. They are often post tensioned concrete. They allow the building to float on or in the soil like a raft. Can be used for buildings that are 10-20 stories tall where it provides resistance against overturning. Can be used where soil requires such a large bearing area and the footing might be spread to the extant that it becomes more economical to pour one large slab (thick), more economical – less forms. It is used in lieu of driving piles because can be less expensive and less obtrusive (i.e. less impact on surrounding areas). Usually used over expansive clays, silts to let foundation settle without great differences. EFFECT OF WATER TABLE ON SAFE BEARING CAPACITY OF SOIL The position of ground water has a significant effect on the bearing capacity of soil. Presence of water table at a depth less than the width of the foundation from the foundation bottom will reduce the bearing capacity of the soil. 1. When the water table is below the base of foundation at a distance ‘b’ the correction  is given by the following equation ; When b =0,  = 0.5 2. When water table further rises above base of foundation, correction factor  comes in to action, which is given by the following equation. When a =,  = 0.5 First let us begin with the correction factor  When water table is at a depth greater than or equals to the width of foundation, from the foundation bottom, the correction factor  is 1. i.e. there is no effect on the safe bearing capacity. Let us assume water table started rising then the effect of  comes in to action. The correction factor will be less than 1. When the water table reaches the bottom of foundation, i.e., when b = 0,  = 0.5. FAILURE OF FOOTINGS List of foundation failures of occurrence - things happening to the foundation Foundation inspectors and engineers need to agree on what terms are used to describe various foundation conditions. Articles throughout this website use and illustrate the foundation damage or failure terms listed below. Backfill height too high or premature backfill causing foundation buckling, leaning, or collapse Building relocation or set damage foundation crack or damage during building set, often impact damage Bulging foundation walls & bulging cracks - the center of the foundation wall arcs inwards towards the building; if the foundation materials are masonry block, brick or stone there will be horizontal cracks, most extreme at the inner-most point of bulging. See BULGE or LEAN MEASUREMENTS. Concentrated loads or point loads and their characteristic appearance as foundation damage Equipment damage (backfill, vehicles) causing foundation wall buckling, breaks, or leaning; equipment striking a building can also result in impact damage Excessive loading leading to foundation fractures (frost heaves can produce similar damage) Improper materials (soft brick, below grade) causing settlement, differential settlement, leaning, or tipping of foundation walls Interior cracks in buildings may be traced to foundation movement or damage Leaning or tipping foundation wall cracks & angles - the wall is said to be "rotating" or leaning inwards or outwards from an axis point that is usually the wall footing Movement or Foundation Damage indicators or signs can show up both in the foundation and as accentuated cracks higher in the building's walls or as opening/closing problems at windows or doors Settlement cracks in a foundation or masonry wall are due to differential settlement of the wall footings, poor original construction, water, nearby blasting operations Settlement cracks in a foundation may be traced to uniform or differential movement Severity or danger of foundation cracks or movement is discussed at FOUNDATION CRACK DICTIONARY Shallow/absent/undermined/cut footings, settlement & frost damage causing settlement, differential settlement, leaning, or tipping of foundation walls Shrinkage cracks: in concrete, concrete block, are usually not a structural concern, but are a possible point of water or radon entry Soil preparation errors - failure to compact soils, especially where foundations are constructed on fill, can lead to settling footings & slabs Types of shear failure of foundation soils: Depending on the stiffness of foundation soil and depth of foundation, the following are the modes of shear failure experienced by the foundation soil. 1. General shear failure (Fig.1 (a)) 2. Local shear failure (Fig.1 (b)) 3. Punching shear failure (Fig.1(c)) Fig.1: Shear failure in foundation soil Fig:  Curve in different foundation soils General Shear Failure This type of failure is seen in dense and stiff soil. The following are some characteristics of general shear failure. 1. Continuous, well defined and distinct failure surface develops between the edge of footing and ground surface. 2. Dense or stiff soil that undergoes low compressibility experiences this failure. 3. Continuous bulging of shear mass adjacent to footing is visible. 4. Failure is accompanied by tilting of footing. 5. Failure is sudden and catastrophic with pronounced peak in  curve. 6. The length of disturbance beyond the edge of footing is large. 7. State of plastic equilibrium is reached initially at the footing edge and spreads gradually downwards and outwards. 8. General shear failure is accompanied by low strain (<5%) in a soil with considerable  (>36o) and large N (N > 30) having high relative density (ID> 70%). Local Shear Failure This type of failure is seen in relatively loose and soft soil. The following are some characteristics of general shear failure. 1. A significant compression of soil below the footing and partial development of plastic equilibrium is observed. 2. Failure is not sudden and there is no tilting of footing. 3. Failure surface does not reach the ground surface and slight bulging of soil around the footing is observed. 4. Failure surface is not well defined. 5. Failure is characterized by considerable settlement. 6. Well defined peak is absent in  curve. 7. Local shear failure is accompanied by large strain (> 10 to 20%) in a soil with considerably low  (<28o) and low N (N < 5) having low relative density (ID> 20%). Punching Shear Failure of foundation soils: This type of failure is seen in loose and soft soil and at deeper elevations. The following are some characteristics of general shear failure. 1. This type of failure occurs in a soil of very high compressibility. 2. Failure pattern is not observed. 3. Bulging of soil around the footing is absent. 4. Failure is characterized by very large settlement. 5. Continuous settlement with no increase in P is observed in  curve. DRAINAGE METHODS SOIL PROTECTION General conclusions Controlled by many factors: Integrated decision-making and functioning of architects, structural engineers and foundation engineers; Building below the water table level is costly and sometimes damaging to the building; Building close to an existing structure to be avoided (any digging activity on either sites will affect one another and can lead to costly repairs); Column or wall load becoming more than that which can be supported by a shallow foundation (deep foundations are expensive) ; Uncertainties can be avoided by using larger factors of safety in design of foundations over soils Ranking of Soil for foundations: (from best to unsuitable): Sand & Gravel – Best Medium & Hard Clays – Good Silts & Soft Clays – Poor Organic Silt and Clays – Undesirable Peat – Unsuitable The greater the PI – Plasticity Index, Cohesiveness the greater the potential for shrinkage and swelling usually characteristic of clay like soils. Non-cohesive are granular soils consisting of gravel and sands. Cohesive soils are silts and clays, and also organic. Differential settlements in concrete foundations should be limited to ¼ to ½” maximum. Generally cost of foundations are 5% of total construction cost. Most economical where safe bearing capacity is at least 3000 PSF – Spread Footings.