Foundations PDF

Foundations Module Leader: Graham Terry Learning Outcomes: Foundations At the end of this session you should be able to: Understand the different types of foundations Understand the relationship between ground conditions and types of foundations for low-rise buildings; Appreciate building...

Foundations Module Leader: Graham Terry Learning Outcomes: Foundations At the end of this session you should be able to: Understand the different types of foundations Understand the relationship between ground conditions and types of foundations for low-rise buildings; Appreciate building near trees Foundations: Building Regulations Introduction to Foundations A foundation is the base on which a building rests and its purpose is to safely transfer the load of the building to a suitable subsoil A foundation is made up of two parts. Foundation function The foundation is the lowest load-bearing part of a building. Partially or wholly built underground. The primary function of the foundations is to safely transmit the building loads to the supporting strata, spreading them over a sufficient area to ensure that the safe load bearing capacity of the soil is not exceeded. Be within the ability of the contractor to construct Loads The average loading for a two-storey domestic dwelling of traditional construction is 30 – 50 kN/m. Selection Criteria Selection Criteria The specific design of a foundation depends upon: Ground conditions; Structure and the way in which its loads are delivered to the foundation; Location: proximity to adjacent structures; consider the implication of the vibration and noise from the driven piles; Cost: choose the most economical design solution that effectively meets the performance; requirements. Selection Criteria: Ground Conditions The specific factors related to the nature of the ground that affect foundation selection are as follows: bearing capacity of the ground; depth of good strata; composition of the ground; and ground level and gradients Procedure in foundation design Assessment of the site conditions in the form of a site and soil survey Calculation of anticipated loads Choosing the foundation type taking into consideration Soil conditions The type of structure The structural loads Any construction problems. Foundations cast on clay Clay is the most difficult of all subsoils with which to deal Depths down to a depth of about 1m are subject to seasonal movements as the moisture content of the clay changes.  Foundation types Spread foundations Strip footings Pad footings Continuous pad and beam foundations Raft foundations Pile foundations Foundation types Strip Foundations Shear failure In a strip foundation Foundation design using the Building Regulations The width of a concrete strip foundation depends on the bearing capacity of the sub-soil and the load on the foundations The greater the bearing capacity of the subsoil the less the width of the foundation for the same load The minimum width of strip foundation is set out in table 10, taken from Approved Document A to the Building Regulations. Minimum depth of strip footings Except where strip foundations are founded on rock, the strip foundations should have a minimum depth of 0.45m to their underside to avoid the action of frost. This depth however, will commonly need to be increased in areas subject to long periods of frost or in order to transfer the loading onto satisfactory ground. In clay soils subject to volume change on drying (‘shrinkable clays’, with Plasticity Index greater than or equal to 10%), strip foundations should be taken to a depth where anticipated ground movements will not impair the stability of any part of the building taking due consideration of the influence of vegetation and trees on the ground. The depth to the underside of foundations on clay soils should not be less than 0.75m although this depth will commonly need to be increased in order to transfer the loading onto satisfactory ground. Minimum width of strip footings Foundation design using calculations The width of a strip foundation (W) can be determined using the following formula W = Load The size of pad foundations can be determined from the following formula Bearing capacity W= not less than that given in Table 10 AD A Load is 50kN/m Bearing capacity of gravel is 100kN/m2 Area of = Load base 50 𝑊= 100 Bearing capacity 𝑊 = 0.5𝑚  Calculating the building load What is the load imposed by a building weighing 120 tonnes, and… …with a rectangular floor plan of 6m x 9m? 𝑇𝑜𝑡𝑎𝑙 𝑙𝑜𝑎𝑑/𝑘𝑁 𝐴𝑣𝑒𝑟𝑎𝑔𝑒 𝑙𝑜𝑎𝑑 𝑝𝑒𝑟 𝑚 𝑟𝑢𝑛 𝑜𝑓 𝑤𝑎𝑙𝑙 = 𝑃𝑒𝑟𝑖𝑚𝑒𝑡𝑒𝑟/𝑚 Calculating the building load Foundation widths Shear failure In a strip foundation Foundation development Strip foundations Strip foundations (cont.) Strip foundations (cont.) Strip foundations (cont.) Strip foundations (cont.)  Stepped Foundations Sequence of construction Establish the construction site and mark out boundaries Secure the site to prevent unauthorised access, safeguard the public. The site entrance should contain lockable gates for added security. Ensure appropriate H, S and W systems and procedures are established. Remember site accommodation, services, storage areas etc One of the first operations is to strip the topsoil and store for later use (external works) as landscape material when the contract is nearly completed Hardcore (MOT type 1 or 2, depending upon specification) is delivered to site from aggregate supplies or quarries The base material is spread by machine and compacted to the required level to provide a working surface for plant and construction staff The site engineer can now set out the foundation lines ready for excavation. Profiles and string lines are used in this example but co-ordinates of building corners and total stations are also used. Narrow trenches require accurate setting out The trenches are then excavated using appropriate equipment Foundations excavated to required depth and note the foundation levels marked by timber pegs. The Building Inspector must inspect prior to the concrete being poured. Concrete delivered by readymix truck and transported to the foundation by 360 degree excavator. Alternatively, depending on the site, other methods include a crane and skip, a dumper, concrete pump or offload straight into the trench The higher the finish of the concrete, the more accurate the levels need to be. Completed foundation ready for the building lines to be established by the site engineer (sometimes by the bricklayer though). The concrete surface should be level to accept the substructure. Blocks loaded out by mechanical means ready for substructure construction. Note spot boards for bricklayer's mortar. The mortar can either be mixed on site by the bricklaying gang or delivered to site (ready mixed) in skips or silos. Substructure construction commences. Note the string line for line and level. Substructure walls- trench blocks Substructure walls- cavity blockwork Substructure construction to be level horizontally & vertically Cavity fill Substructure walls complete and ready for backfill. Note voids in substructure walls for services. Note profile boards at edge of site for earthworks levels Potential Problems Pad Foundations Isolated or pad foundations Foundation for columns. Where building loads are high, mass concrete foundations may prove Uneconomical and reinforcement will need to be used. Pad foundations Raft Foundations Raft foundations Foundation for poor soil conditions where: The bearing capacity is low or Where differential settlement may occur. Raft foundations Construction of a raft foundation Flat slab raft Edge beam raft Raft on sloping site Pile Foundations Pile and beam foundations Use where traditional strip foundations would not be suitable such as: Low bearing capacity of subsoil Made up ground High water table High moisture movement of subsoil Presence of highly compressible layers such as peat. Pile foundations Pile and beam foundations Pile and beam foundations (Cont.) Services through foundations Services through foundations Building near trees Building near trees can cause problems with foundations especially in clay soils The NHBC Standards give the technical requirements and recommendations for identifying hazards which need to be considered to ensure that dwellings foundations and associated works are carried out satisfactory. Building near trees (cont.) Where trees are in rows, this may be increased to 1.5 times the height of the tree. Building near trees (cont.) Summary of foundation types Strip foundations On rock, gravel, dense sand or stiff clay, narrow strip foundations are usual On soft clay or soft sandy clay, wider strip foundations are usually required Piled foundations On shrinkable clays with new or felled trees Where a firm layer of ground is at considerable depth i.e. where strip foundations would be uneconomic Where the water table is close to the surface of the ground Summary of foundation types Raft foundations On ground of low bearing capacity such as soft clay or silt In mining areas where subsidence is a risk On deep areas of fill where piling would be uneconomical Pad foundations Rarely used for domestic construction Suitable for individually loaded columns on rock, gravel, dense sand or clay. Any questions