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Basement Structure

Defination: • A basement is one or more floors of a building that are

either completely or partially below the ground floor.Uses: • Basements are typically used as a utility space for a • Basements are typically used as a utility space for a

building where such items as the furnace, water heater, breaker panel or fuse box, car park, and air-conditioning system are located; so also are amenities such as the electrical distribution system, and cable televisiondistribution point.

Design Considerations

• Excavation • Basement construction • Water proofing of basement• Insulation of basement• Insulation of basement

Excavations

Open Excavations:• Temporary support or timbering to the sides of

the excavation. • One method is to use battered excavation sides • One method is to use battered excavation sides

cut back to a safe angle of repose thus eliminating the need for temporary support works to the sides of the excavation.

• The main disadvantage of this method is the large amount of free site space required.

Open Excavations:

Perimeter Trench Excavations: • In this method a trench wide enough for the

basement walls to be constructed is excavated and supported with timbering as required.

• It may be necessary for runners or steel sheet piling to be driven ahead of the excavation work.

• This method can be used where weak subsoils are encountered so that the basement walls act are encountered so that the basement walls act as permanent timbering whilst the mound or dumpling is excavated and the base slab cast.

• Perimeter trench excavations can also be employed in firm subsoils when the mechanical plant required for excavating the dumpling is not available at the right time.

Perimeter Trench Excavations:

Complete Excavations: • This method can be used in firm sub-soils

where the centre of the proposed basement can be excavated first to enable the basement slab to be cast thus giving protection to the subsoil at formation level.

• The sides of excavation to the perimeter of • The sides of excavation to the perimeter of the basement can be supported from the formation level using raking struts or by using raking struts pitched from the edge of the basement slab.

Complete Excavations:

Excavations:

Excavations:

Types of Basement ConstructionRetaining Wall and Raft Basements :

Box and Cellular Raft Basements :

Piled Basements :

Deep Basements Construction:

• Basements can be constructed within a cofferdam or other temporary supported excavation up to the point when these methods become uneconomic, unacceptable or both due to the amount of necessary temporary support work.

• Deep basements can be constructed by installing diaphragm walls within a trench and providing permanent diaphragm walls within a trench and providing permanent support with ground anchors or by using the permanent lateral support given by the internal floor during the excavation period.

• Temporary lateral support during the excavation period can be provided by lattice beams spanning between the diaphragm walls.

Deep Basements Construction:

Basements Construction with Permanent Lateral Support:

Basements Construction with Temporary Lateral Support:

Water Proofing BasementsDense Monolithic Concrete walls and floors :• The main objective is to form a watertight basement using dense high

quality reinforced or pre-stressed concrete by a combination of good materials, good workmanship, attention to design detail and on site construction methods.

• If strict control of all aspects is employed a sound watertight structure can be produced but it should be noted that such structures are not always water vapour proof.

• If the latter is desirable some waterproof coating, lining or tanking should be used. The watertight ness of dense concrete mixes should be used. The watertight ness of dense concrete mixes depends primarily upon two factors:-

1. Water/cement ratio.2. Degree of compaction.

• Concrete with a free water/cement ratio of 0.5 is watertight and although the permeability is three time more at a ratio of 0.6 it is for practical purposes still watertight but above this ratio the concrete becomes progressively less watertight.

• For lower water/cement ratios the workability of the mix would have to be increased, usually by adding more cement, to enable the concrete to be fully compacted.

Dense Monolithic Concrete walls and floors :Admixtures:• If the ingredients of good design, materials and

workmanship are present watertight concrete can be produced without the use of admixtures.

• If admixtures are used they should be carefully chosen and used to obtain a specific objective:-

• 1. Water-reducing admixtures … used to improve workability

• 2. Retarding admixtures … slow down rate of hardening• 2. Retarding admixtures … slow down rate of hardening• 3. Accelerating admixtures … increase rate of hardening

… useful for low temperatures … calcium chloride not suitable for reinforced concrete.

• 4. Water-repelling admixtures … effective only with low water head, will not improve poor quality or porous mixes.

• 5. Air-entraining admixtures … increases workability … lowers water content.

Dense Monolithic Concrete walls and floors :

Water proofing at Joints:

Basement slabs:• These are usually designed to span in two

directions and as a consequence have relatively heavy top and bottom reinforcement.

• To enable them to fulfil their basic functions they • To enable them to fulfil their basic functions they usually have a depth in excess of 250mm.

• The joints, preferably of the construction type, should be kept to a minimum and if water bars are specified they must be placed to ensure that complete compaction of the concrete is achieved.

Dense Monolithic Concrete walls and floors :

Water proofing at Joints:

Basement slabs:

Dense Monolithic Concrete walls and floors :Water proofing at Joints:

Basement walls:• Joints can be horizontal and/or vertical according

to design requirements. • A suitable water bar should be incorporated in the

joint to prevent the ingress of water. joint to prevent the ingress of water. • The top surface of a kicker used in conjunction

with single lift pouring if adequately prepared by exposing the aggregate should not require a water bar but if one is specified it should be either placed on the rear face or consist of a centrally placed mild steel strip inserted into the kicker whilst the concrete is still in a plastic state.

Dense Monolithic Concrete walls and floors :Water proofing at Joints:

Basement walls:

Water Proofing BasementsMastic Asphalt Tanking :

• The objective of tanking is to provide a continuous waterproof membrane which is applied to the base slab and walls with complete continuity between the two applications.

• The tanking can be applied externally or • The tanking can be applied externally or internally according to the circumstances prevailing on site.

• Alternatives to mastic asphalt are polythene sheeting: bituminous compounds: epoxy resin compounds and bitumen laminates.

External Mastic Asphalt Tanking :• This is the preferred method since it not only

prevents the ingress of water it also protects the main structure of the basement from aggressive sulphates which may be present in the surrounding soil or ground water.

Internal Mastic Asphalt Tanking :• This method should only be adopted if external • This method should only be adopted if external

tanking is not possible since it will not give protection to the main structure and unless adequately loaded may be forced away from the walls and/or floor by hydrostatic pressure.

• To be effective the horizontal and vertical coats of mastic asphalt must be continuous.

External Mastic Asphalt Tanking :

Internal Mastic Asphalt Tanking :

Water Proofing BasementsDrained Cavity System :• This method of waterproofing basements can be used for

both new and refurbishment work.• The basic concept is very simple in that it accepts that a

small amount of water seepage is possible through a monolithic concrete wall and the best method of dealing with such moisture is to collect it and drain it away.

• This is achieved by building an inner non-load bearing wall • This is achieved by building an inner non-load bearing wall to form a cavity which is joined to a floor composed of special triangular tiles laid to falls which enables the moisture to drain away to a sump from which it is either discharged direct or pumped into the surface water drainage system.

• The inner wall should be relatively vapour tight or alternatively the cavity should be ventilated.

Drained Cavity System :

Basement Insulation• Basement benefit considerably from the insulating

properties of the surrounding soils. However that alone is insufficient to satisfy the typical requirements for wall and floor respectively.

• Refurbishment of existing basements may include insulation within dry lined walls and under the floor screed or particle board overlay. This should incorporate an integral vapour control layer to minimise risk of integral vapour control layer to minimise risk of condensation.

• External insulation of closed cell rigid polystyrene slabs are generally applied to new construction. These slabs combine low thermal conductivity with low water absorption and high compressive strength. The external face of insulation is grooved to encourage moisture runoff. It is also filter faced to prevent clogging of the grooves. Backfill is granular.

Basement Insulation