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Learn how foundations safely sustain and transmit loads, considerations for choosing a foundation, and different types of foundations for various soil types. Key topics include bearing capacity, settlement, differential settlement, reinforcement, and types of foundations.
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FUNCTION OF A FOUNDATION • Safely sustain and transmit to the ground, the combined • Dead loads, • Superimposed loads (live loads) • Wind loads. In such a way as limit settlement and failure of the underlying ground • To provide a level surface to start the building on. • To anchor very light structures to the ground
CONSIDERATIONS • Requirements to consider before choosing a foundation • Type of structure to be supported • Load generated by the structure (live anddead) • The function of the structure • Soil type • Water table level • Frost heave in sandy soils • Water content in clay soils • Hot weather (+/- 30mm movement) • Effect of trees
SUBSOIL TYPES • Subsoil can be divided into 3 main groups • Rock • Non cohesive type soils(granular type) • Gravel and sand • Cohesive type soils (putty like) • Clay • These different subsoil's react to loading differently
ROCK • Most rock would be considered very good as a sub-strata to build on • However it would be very important to establish the type of rock • Depth of rock • Area of rock in relation to the site
NON COHESIVE SOILS • Soils such as sands and gravels • They have no plasticity and lack cohesion especially when dry • Under pressure from loads these soils compress and consolidate rapidly by the expulsion of water and the rearrangement of the particles • Problems can arise due to frost heave when soils are waterlogged • Depth of foundation
COHESIVE SOILS • Such soils as clay • Smooth and greasy to touch • have high plasticity • Cohesive soils can contain high levels of water • Increasing the pressure on a soil (e.g. building a house on it) drives the water out of the soil • In cohesive soils settlement can occur over a period of years
THE EFFECTS OF TREES • A trees roots rend to be as deep as its branches are high. • For this reason a tree can be a major cause of subsidence.
TERMINOLOGY • BEARING PRESSURE • Pressure on the soil caused by the building load • BEARING CAPACITY • The load which the ground can carry • Both are measured in kN/m2
BEARING CAPACITY/PRESSURE • Bearing pressure about 1/3 the bearing capacity of the soil • Safety factor bearing pressure is calculated on a given 1 metre run of the structure weight on foundation per 1m run Safe bearing capacity
COMPACTION • The act of increasing the density and strength of a material by the application of impact forces (e.g heavy roller)
SETTLEMENT • Downward movement of the soil, or any structure on it, as a result of soil consolidation, usually caused by the load applied by the structure
DIFFERENTIAL SETTLEMENT • Where settlement of a building occurs at different rates. • Can lead to cracking in walls especially near windows and doors.
DIFFERENTIAL SETTLEMENT • Differential settlement can be caused by • Foundation failure. • Building on soils of varying bearing capacity. • Foundations at uneven depths. SUBSIDENCE • Downward movement of the ground caused by imposition of internal forces (e.g. mining, water)
REINFORCEMENT • Concrete is strong in compression but weak in tension and where tension occurs it is usual to introduce steel bars to provide the strength which concrete lacks. • If the tensile strength of the concrete is exceeded then cracking will occur resulting in a weak and unsuitable foundation.
REINFORCEMENT • The steel must be free from oil, rust, paint, mud and other substances which might impair the bond between the steel and the concrete. • Reinforcing bars are usually mild steel or high tensile steel and can be deformed or twisted which gives greater frictional resistance than round bars. • It is important the steel is fixed securely to avoid displacement during the pouring of concrete. The bars are tied together with soft wire at intersections and spacers of small precast concrete blocks or plastic rings ensure the correct cover of concrete(50 mm)
TYPES OF FOUNDATIONS SHALLOW FOUNDATIONS • Strip • Traditional strip • Wide strip • Deep strip • Stepped • Raft DEEP FOUNDATIONS • Piled
TYPES OF FOUNDATIONS • Strip Foundation consists usually of a continuous strip of concrete, under a load bearing wall. • It serves as a level base on which the wall is built. • It is of such a width is as necessary to spread the load over an area of subsoil capable of supporting the structure.
STRIP FOUNDATIONS • STRIP • Traditional • Wide • Deep • Stepped • Used where bearing capacity is good • rock, gravel, dense sand, stiff clay
STRIP FOUNDATION DIMENSIONS • All strip foundations must be 3 times the width of the wall it is supporting. • Foundations must be 300mm in depth. • Foundation must be 600mm below ground level. • To prevent frost heave.
STEPPED STRIP FOUNDATION • Used on sloped site to avoid overly deep foundation
DEEP STRIP FOUNDATION • This type of foundation is used to reduce the expense of constructing an Traditional strip foundation at depths of over one meter. • Laying blocks in a narrow trench can be very difficult. (especially squaring) • By filling the trench to within 150mm of the top although more concrete is used, less blockwork, labour and mortar are used which evens out the cost. • There can also be time savings because of quicker completion. • Used where the bearing capacity of the soil is good.
WIDE STRIP FOUNDATION • Used where the bearing capacity of the soil is low. (marshy boggy type land) • The wide strip is used to spread the load over a greater area. • Need for transverse steel reinforcing bars to withstand the tensions that will arise. • The depth of the foundation below ground is the same as that for the traditional strip.
RAFT FOUNDATIONS • Used where soil has very low bearing capacity or in mining areas where subsidence is a risk. • Where piling would be uneconomical. • The raft will cover the entire area of the building. • It consists of a slab of concrete up to 400 mm in thickness and is widened further under the load bearing walls.
RAFT FOUNDATIONS • The primary advantage over strips is that it acts as a single unit thus eliminating differential settlement, however they are expensive. • The ground at the edge of the raft must be protected from deterioration from the weather and this can be achieved in a number of ways; • Laying concrete paving all around the building. • Deepening the edge beam A • Laying a field drain in a trench filled with a suitable material. B Even though settlement has taken place there are no cracks in the walls. Foundation failure has not occurred.
RAFT FOUNDATIONS • The level of the base of the raft is usually within 300 mm of the surface. (hence the name raft or floating foundation) • Reinforcement is in the form of two layers of mesh (A393) one at the top one at the bottom. • A polythene membrane should separate the concrete from the sand blinding to prevent loss of water from the fresh concrete.
RAFT FOUNDATIONS • High tensile steel reinforcement beams under load bearing walls. • Boot shaped beam around the edge of the house (to provide step)
RAFT FOUNDATIONS • Note how the positions of the rooms are built up to allow extra depth under the walls. • A completed raft
PILED FOUNDATION • These can be defined as a series of columns constructed or inserted into the ground to transmit the loads of a structure to a lower level of subsoil. • The lack of suitable foundation conditions may be caused by, • A firm layer of soil is found below a soft layer • Shrinkable clays with felled trees • Areas with a high water table • Presence of layers of highly compressible subsoil such as peat and recently placed filling materials which have not fully consolidated.
CLASSIFICATION OF PILES • Piles may be classified by their basic design function or by their method of construction;
PILED FOUNDATIONS • Piles in some cases are the only way in which a suitable foundation can be achieved in a particular site. • They are however very expensive and piling is a very specialized operation.
