2. Foundation

FunctionsClassification Soil treatmentUnderpinning construction

Functional requirement of a building

Foundation:A foundation is the base on which a building rests, and its purpose is to transfer the load of a building safely to a suitable sub soil.

Terminology:

Backfill: Materials excavated from site and if suitable used to fill in around the walls and foundations.

Bearing capacity: Safe load per unit area that the ground can carry.

Bearing pressure: The pressure produced on the ground by the loads.

Made up ground: Refuse, excavated rock or soil deposited for the purpose of filling in a depression or for raising the site above its natural level.

Settlement: Ground movement which may be caused by: -Deformation of the soil due to imposed load -Volume changes of the soil as a result of seasonal conditions -Mass movement of the ground in unstable areas

Load Bearing structure

A framed structure is composed of beams and columns. With the load being transferred into the beams via the floor and then down to foundations through the columns.

RCC Framed structure

Rural isolated locations for a single property Undeveloped land for a number of properties On land where previous properties have been demolished On infill locations behind existing properties

Enable detailed setting out to take place Modify the ground contours to remove excessive undulations Provide for services to properties and the access roads provide retaining structures To accommodate severe changes in ground levels Allow access to the both during construction and when occupied

Preliminary desk studyExamination and interpretation of air-photographsSite walk-over surveyDesign of ground exploration programmeExploration by trial pits and/or bore holesSoil and rock classification by sample description and index testingInsitu and laboratory testing of soils and rocks for mechanical and chemical propertiesPreparation of reports

Topography, Vegetation and DrainageDoes the site lie on sloping ground and if so what is the maximum slope angle?Are there springs, ponds, or watercourses on or near the site?Is there evidence of changes in ground level? Are trees or hedges growing in the area of construction?

Ground ConditionsWhat geological strata lie below the site and how thick are they?Is there available information on the strength and compressibility of the ground?Is the subsoil shrinkable clay?Is there any evidence of land slipping either on or adjacent to the site?Any mining or quarrying activities in the area?

What area will the building occupy?What foundation load is expected?How sensitive is the building to differential foundation movements?What soil information is required to design the foundation?Is specialist Geotechnical skill required

A number of simple tools are needed for site inspectionCompass Pocket penetrometerAbney levelAugerSpadeCamera

Abney level

Spade

Local EnquiriesEnquiries involve talking to people and visiting sources of reference materials. These include visit to libraries, planning authorities, universities etc.

Enquiries from public utilities provider, such as gas, electricity, water and telephone facilities.

Enquiries from authorities concerned for records and maps of the site showing the position of field boundaries, ponds, streams and pits are to be made. These records can usually be hand copied.

TRIAL PITA trial pit is an excavation of ground in order to study or sample the composition and structure of the subsurface. usually dug during a site investigation, a soil survey or a geological survey. Trial pits are dug before the construction. They are dug to determine the geology and the water table of that site.Trial pits are usually between 1 and 4 metres deep, and are dug either by hand or using a mechanical digger. Building and construction regulations clearly state that any trial pits that concede deeper than 1.2 metres should be secured against structural collapse, if they are to be entered by people.

Deep vane and seismic method

Soil test Exploration technique

The visual inspection of in-situ soil conditions.The detailed examination of soil variability, structure and weathering profile.The observation of water seepage and its measurement.The search for geological or archaeological features or existing foundations.Ease of excavation and stability of excavations.

Obtaining large bulk samples, block samples etc.

Methods of BoringAuger BoringShell and Auger BoringWash BoringPercussion BoringRotary Boring

Auger Boring

Shell and Augor Boring

Wash BoringRotary boring

Levelling a Construction Sites Cut and FillEnable setting out of the buildings Provide particular landscaping featuresProvide level areas for the construction of buildings Assist with the construction of roads

Leveling a Construction Sites

Setting Out of Simple BuildingEstablishing the position of the building on the site is known as setting out the building

The first task in setting out the building is to establish a base line to which all the setting out can be related. The base line very often coincides with the building line, which is a line, whose position on site is given by the local authority in front of which no development is permitted.

The first step in building setting out is to identify a base line according to the site layout plan. We can establish the base line considering the permanent structures and the relevant distances to structural parts from them as given in the drawingsStep 1: Obtaining of a Base Line

Setting out is done based on the principle of Whole to part. According to this principle the largest possible rectangle of the building is found and set up first. Then it is further divided into small parts completing the major setting out for the building.

Main instruments involved in this process are Theodolite, Steel and Linen Tapes, Arrows, Wooden pegs, Wire nails and Nylon threads.After establishing the base line, the main rectangle is set up using the pegs and theodolite. Arrows are used as temporary pegs and wooden pegs are driven for permanent pegs .90 angle is taken by the theodolite and Pythagoras rule is also commonly used for the processTheodolite Wooden peg Pythagoras theorem

Building ElementsThe important parts of a building are called the building elements. The building elements of houses include:

Foundations External walls Pitched roof Windows Partition walls Suspended floors Services

Excavation: Excavation of topsoil down to formation level (point of commencing floor), foundation trenches, (assuming strip foundations), and compaction of trench bottoms.

Concrete: In strip foundation, the concrete is laid as per the design.

Services: Route the position of incoming services (and occasionally outgoing ground floor WCs), including ducts through walls to inside floor level as needed, install below ground drainage system, form trenches for incoming water, electricity, gas, and telecommunications.

Brick work / Block work: Common bricks and in foundations trenches, facing bricks to outer skin up to DPC.

Filling: Stone hardcore to floor slab and inner part of the foundation trenches, sand blinding to hard-core surface.

Membranes: Damp proof membrane (DPM) polythene sheeting, turned over inner skin of external wall and any internal wall, which have the foundation.

Insulation: Possible rigid board insulation for floor slab.

Steps in Construction Activities..Concrete: To floor slab, after laying the hard core etc: (assuming solid floor slab).

Above external wall DPC

Brick Work/Block Work: Construct external wall block work, place the insulation and holding ties, lay brickwork, including building in lintels, cavity tie/weep holes arrangements and closing cavity. Construct block works to internal partitions all to the first floor level.

Wood works: Fix first floor joists, fix staircase.

Brick work/block work: Continue with external walls and window openings.

Roofing: Fix wall plate above inner external wall skin, locate and fix trussed rafter roof structure.

Partitions: Fix the first floor partition carcass.

Services: Provide first fix for electrical (cable in), provide first fix for plumbing and heating (pipe in).

Windows and external doors: Fix in position.

Finishes: Apply plaster board and insitu plaster finishes, apply floor screed to ground floor (if not powder floated).

Services: Second fix electrical (light, sockets and switches), second fix plumbing (sanitary appliances and fittings) and heating.

Fittings: Install cupboards, fit kitchen.

Painting: Externally and internally.

External works: Landscaping, grass laying, planting, boundary fencing etc.

Factors affecting choice of construction

Existing ground level. Floor loading. Bearing capacity of the subsoil.Type of foundation used.Required floor finish.

Super structure: which is above the ground

Substructure or foundation: below the ground level

The basic function of a foundation is to transmit the dead loads, superimposed loads (live loads), and wind loads from a building to the soil on which the loads are transmitted

Foundations are below ground level and support the weight of the building

Reduction of load intensity: Foundation distribute the loads of the superstructure to a larger area so that the intensity at the base, i.e. total load divided by the total area, does not exceed the safe bearing capacity of the sub soil.

Even distribution of load: Foundation distributes the non-uniform load of the super structure evenly to the subsoil.

Provision of level surface: Foundation provides the level surface over which the super structure can be built.

Lateral stability: It anchors the super structure to the ground thus imparting the lateral stability to the super structure.

Safety against undermining: It provides the structural safety against undermining or scouring due to burrowing animals and floodwater.

Protection against soil movements: Foundation minimizes or prevents the distress like cracks in the super structure, due to expansion and contraction.

DPC-Damp Proof CourseDamp proofing in construction is a type of moisture control applied to building walls and floors to prevent moisture from passing into the interior spaces. Damp problems are one of the most frequent problems encountered in homesDamp proofing is defined by the American Society for Testing and Materials (ASTM) as a material that resists the passage of water with no hydro-static pressure and waterproof as a treatment that resists the passage of water under pressure.

Generally damp proofing keeps moisture out of a building where vapor barriers keep interior moisture from getting into walls. Moisture resistance is not necessarily absolute: it is usually defined by a specific test method, limits, and engineering tolerances.

DPC-Damp Proof Course contdDamp proofing is accomplished several ways including:A damp-proof course (DPC) is a barrier in a masonry wall designed to resist moisture rising through the structure by capillary action such as through a phenomenon known as rising damp. The damp proof course may be horizontal or vertical. A DPC layer is usually laid below all masonry walls, regardless if the wall is a load bearing wall or a partition wall.

A damp-proof membrane (DPM) is a membrane material applied to prevent moisture transmission. A common example is polyethylene sheeting laid under a concrete slab to prevent the concrete from gaining moisture through capillary action. A DPM may be used for the DPC.

Integral damp proofing in concrete involves adding materials to the concrete mix to make the concrete itself impermeable.

Surface coating with thin water proof materials for resistance to non-pressurized moisture such as rain water or a coating of cement sprayed on such as shotcrete which can resist water under pressure.

Cavity wall construction, such as rain screen construction, is where the interior walls are separated from the exterior walls by a cavity.

Pressure grouting cracks and joints in masonry materials.

DPC-Damp Proof Membrane

DPC-Damp Proof Membrane

DPC-Damp Proof Membrane

DPC-Damp Proof Membrane

Spread footingsIsolated footings (Pad footings or Foundation)Combined footingsStrap footingsMat or raft foundations

Types of Shallow foundation

Strap footing

Shallow Foundation

Strip foundationsStrip foundations are used to support a line of loads, either due to a load-bearing wall, or if a line of columns need supporting where column positions are so close that individual pad foundations would be inappropriate

Spread / Strip / continuous footing

Pad foundationsPad foundations are used to support an individual point load such as that due to a structural column. Shape may be circular, square or rectangular.

They usually consist of a block or slab of uniform thickness, but they may be stepped or haunched if they are required to spread the load from a heavy column. Pad foundations are usually shallow, but deep pad foundations can also be used.

Pad or Isolated footing

Pad or Isolated footing

Raft foundationsRaft foundations are used to spread the load from a structure over a large area, normally the entire area of the structure. They are used when column loads or other structural loads are close together and individual pad foundations would interact.

Raft foundations

Deep foundations are those founding too deeply below the finished ground surface for their base bearing capacity to be affected by surface conditions, this is usually at depths > 3 m below finished ground level.

They include piles, piers and caissons or compensated foundations using deep basements and also deep pad or strip foundations.

Deep foundations can be used to transfer the loading to a deeper, more competent strata at depth if unsuitable soils are present near the surface.

Types

PilesPiersCaissonsCompensated Foundations

Pile foundation Foundation piles are groups of cylindrical or flat sections of wood, steel or concrete that are driven into soil to form part of a foundation. They are used when the soil near the surface is too weak to support the weight of the structure or building.

Types of PileEnd bearing pile

Friction piles

Types of Pile

Settlement reducing piles

Settlement Reducing pilesSettlement reducing piles are usually incorporated beneath the central part of a raft foundation in order to reduce differential settlement to an acceptable level.

Such piles act to reinforce the soil beneath the raft and help to prevent dishing of the raft in the centre

Concrete strip foundations are used to support and transmit the loads from walls as shown in Figure. The width of strip foundations can be calculated as follows:Minimum width of strip= Total load of building per metre (kN/m)/Safe bearing capacity of subsoil kN/m2Length of the wall:9mWidth of the wall:300mmHeight of the wall:3.0mImposed load on the wall: 20kN/mDensity of wall:20 kN/m3Calculate width of strip foundation?SBC=100kN/m2

Solution:Weight of wall per metre run = 3.0x0.3x20 = 18.0 kN/m Total load= Weight of wall per m run+ imposed load = 18.0+20 = 38.0 kN/m

Width of the strip= 38/100 = 0.38m.

Application Problems1. Calculate the minimum depth of foundation by Rankines formula to suit the following data.Safe bearing capacity of soil : 210 kN/m2Density of soil : 19 kN/m3Angle of repose of the soil : 33

Solution:Min depth of foundation by Rankines formula: hmin = p/w (1-sin / 1+sin )2 p- Bearing Capacity of soil, w- Unit weight of soil, - Angle of repose = 210/19 (1-sin33/1+sin33)2 hmin = 0.96 m.

Application Problems contd..

2. Calculate width of strip foundations to suit the following data.Length of the wall : 11 mWidth of the wall : 200 mmHeight of the wall : 3.25 mImposed load on the wall : 28 kN/mDensity of wall : 20 kN/m3SBC : 140 kN/m2

Application Problems contd..

Solution:Calculate weight of wall per metre run:Width of the wall : 0.2 mHeight of the wall : 3.25 mDensity of wall : 20 kN/m3Consider per metre length of the wallWeight of wall per metre run = 3.25x0.2x20 = 13.0 kN/m

Application Problems contd..

Imposed load on the wall = 28 kN/mSelf-weight of wall per metre run = 13.0 kN/m Total load per metre run = Weight of wall per m run+ imposed load

= 13.0+28.0 = 41.0 kN/m Calculate width of the strip:Width of the strip= Total load per metre run/ SBCWidth of the strip= 41/140 Width of the strip= 0.30m.

Application Problems contd..

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