Unit 6: Building Technology in Construction Topic Understand foundation design and construction...

Unit 6: Building Technology in Construction

Topic

Understand foundation design and construction

Today’s Objective

1.Understand the impact that soil conditions have on foundation design


Two main soil characteristics affect the choice of foundation:-

1.Maximum loads that can be carried without failure

2.Excessive settlement

Foundation Design: Soil Condition

Key term: SettlementThe way the soil reacts to having a load placed upon it – usually the soil ‘sinks’ a little due to the extra weight placed upon it, although this settlement should not affect the building a great deal.

Key term: CompressibilityThe ability of soil to compress and withstand a load imposed on it.


Foundation Design: Soil Movement

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How many different types of soils can you list?

•Rock

•Gravel

•Sands

•Silt

•Chalk

•Clay

•Peat

Which do you think will be the best material on which to put a foundation?


Site Investigation: Ground Investigation

1. RockHard solid formations, often impervious, such as granite or sandstone. Form

excellent strata on which to build. 2. Granular- Sand/gravel: non-cohesive granular material. Can have good ground bearing capacity if well compacted. Good water dissipation

and run off, and often very stable.3. Cohesive-Clay Good ground bearing, with impervious characteristics preventing water from

passing between strata. Often found at depths close to surface of the earth making it economical, and reasonable, on which to build low-rise structures.

4. Organic-PeatSoft, permeable and unsuitable surface for foundations. Can indicate the presence

of water table and groundwater conditions that can make the ground unstable.5. Filled or made groundGood quality, clean, hard-core rubble. Graded, selected and compacted in layers to

engineer’s levels means that ground stabilisation is possible with this method.


Site Investigation: Ground Investigation



Non cohesive soil•Eg Sand/gravel•Stronger and higher bearing capacity and low compressibility compared to cohesive soil •Settlement occurs instantaneously•May be suitable for any foundation type.

Poor Soil•Eg. Peat, silt, soft alluvial, filled ground•Poor load bearing and settlement characteristics•There are techniques that can be used to stabilise the ground and improve the load bearing capacity•Normally the unsuiable soil is excavated and the founations are sited on a better load bearing ground at a lower level



Rock – Strongest material on which to put a foundation•Solid rock only needs to be levelled (and it can take some doing) for most house foundations. •A thin layer of blinding can then be laid to provide a level base for bricklayers. •Cavities and fissures may need filling. •There is a potential problem of differential movement if rock and other materials are supporting the foundation. For example, a predominantly clay subsoil, with rocky outcrops, could pose problems.

Cohesive soil•Eg. Clay•Reasonable bearing capacity •Prone to long term settlement•Sensitive to changes in moisture content, prone to swelling/shrinkage


Foundation Design: Soil ConditionContaminated SoilList some examples of contaminated land•Landfill sites•Gasworks•Sewage farms and works•Scrap yards•Industrial sites

•Primary concern is health•Some aggressive contaminants can damage buried services and structures

Common approaches to contaminated land•Removal –costly and disposal needs to be made to licenced tips. Permanent.•Capping –Sealing of material by a clean material 1m deep. More Economic.



Field identificationField assessment of structure & strength

Possible foundation problems

Gravels

Particle size 2mm to 60mm Some dry strength indicates presence of clay

Loose, easily removed by shovel 50mm square stakes can be well driven into the ground

Loss of fine particles in water bearing ground

Gravel – Non-cohesive soil





Sands

Particle size 0.06mm to 2mm - when dry, clean sands break down. Sand particles are just visible to the naked eye and feel gritty between the fingers.

Compact, requires pick for excavation. 50mm square stakes will only penetrate a little way

Frost heave , especially in fine sands. Excavation below water table can wash out sand and cause runs and local collapse.



Chalk

White and thus readily identifiablePlastic - shattered, damp and slightly compressible or crumbly. Solid - needing a pick for removal

Frost heave is a risk in chalk, floor slabs are particularly at risk if constructed in cold weather. Swallow holes can also form.





Silts

Particle size 0.002mm to 0.06mm - not normally distinguishable to the naked eye. Silts feel slightly gritty; moist lumps can be moulded with the fingers but not rolled into threads. Shaking most lumps in the hand brings water to the surface. Silts also dry rapidly.

A soft silt can easily be moulded with the fingers; a firm silt requires strong finger pressure.

Frost heave , especially in fine sands. Excavation below water table can wash out sand and cause runs and local collapse.





Clays

Smooth and plastic to the touch. Sticky when moist. Hold together when dry. Wet lumps immersed in water soften without disintegrating Soft clays either uniform or show horizontal laminations.

Harder clays often fissured, the fissures opening slightly when the overburden is removed or a vertical surface is revealed, eg, by a trial pit.

5 types: very soft - exudes between fingers when squeezed

soft - easily moulded with the fingers

firm - moulded with strong finger pressure

stiff - cannot be moulded by the fingers

hard - brittle or tough

Shrinkage and swelling caused by vegetation and long term settlement by consolidation. Some clays contain sulfates which can attack concrete. Drainage can be poor.

Movement down slopes, most soft clays lose strength when disturbed.




Peat

Fibrous, black or brown; often smelly, and very compressible and water retentive.

Soft - very compressible, firm - compact Very low bearing capacity, peats shrink and swell considerably, high compressibility leads to high settlement.






Fill

Miscellaneous material including rubble, mineral waste, previously excavated material, decaying wood and other building debris.

To be avoided unless carefully compacted. May contain chemicals which either burn or are toxic.

Fill


Foundation Design: Soil Classification

Complete Worksheet



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Foundations need to be designed with sufficient density to resist what forces in the ground?•Acidic attack/corrosion through nitrates – all foundations should use acid and sulphate resistant concrete•Rise in water level can cause float•Lack of water can cause shrinkage•Ground heave –water level freezing – compressible materials are placed in excavations and foundations ton enable the ground to take up this differential movement without affecting the structure.•Differential Settlement – caused by buildings exerting differing levels of point loads on the earth at their foundations. Flexible joints and opportunities for building to accept movement will be incorporated into the design.



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Key Terms

Settlement:- The vertical downward displacement at the base of a foundation or structure due to soil consolidation, normally caused by the load applied by the structure. Shrinkage:- Soil shrinkage is the reduction of water content which causes a change in volume of soil. It is most evident in clay soils during long periods of dry weather, compounded by moisture abstraction from vegetation.

Heave:- Soil expansion due to the increase of water content which causes a change in volume of soil. It occurs during wet weather and is compounded by previous removal of moisture-dependent trees that would otherwise effect some drainage and balance to subsoil conditions.



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Trees ~ damage to foundations.



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Foundations on poor ground

What are the options?1.Excavate the poor soil away until ground of good bearing capacity is found2.Improve the soil so it can accept the load of the building

How can the soil be improved?

•Vibro-compaction of poor soil•Vibro-replacement of poor soil•Dynamic compaction of poor granular soil


Foundation Materials

Cement WaterAggregate

A foundation must be constructed of a durable material of adequate strength.

Experience has shown that the most suitable material is concrete.

What are the key requirements of a foundation?

What is concrete?

Concrete is a mixture of cement, aggregates and water in controlled proportions.



Cement is manufactured from clay and chalk and is the matrix or binder of the concrete mix. Cement powder can be supplied in 50kg air-tight sealed bags which require a damp free store, or alternatively it can be delivered in bulk by a tanker and pumped into a 12 to 50 tonne storage silo.



Aggregates can be either natural rock which has disintegrated or crushed stone or gravel. •Coarse aggregate is defined as a material which is retained on a 5mm sieve. •Fine aggregate is defined as a material which passes a 5mm sieve.

What is aggregate?



Water must be of drinking water quality.

Mixes are expressed as a ratio e.g. 1:3:6 / 20mm which is 1 part cement, 3 parts of fine aggregate, 6 parts of coarse aggregate with 20mm maximum size of coarse aggregate for the mix.

Water is added to start the chemical reaction and to give the mix workability. •The amount of water used is called the Water/Cement Ratio and is usually about 0.4 to 0.5. •Too much water will produce a weak concrete of low strength whereas too little water will produce a concrete mix of low and inadequate workability.



Compete worksheet


Next Lesson

You can now complete P3 of your assignment.

Criteria P3 &Edexcel Guidance

You need to describe the principles of foundation design. This should convey an understanding of how subsoil conditions influence foundation design and knowledge of different foundation types.

Tutor Guidance 1. Explain the principles of foundation design, how foundations spread the load of the building over the ground the materials and techniques used in the construction of the substructure for low-rise domestic and commercial buildings.

2. Describe how different subsoil conditions influence the following foundation designa) Strip Foundationb) Piled Foundationc) Raft Foundation.

Date post:	17-Jan-2016
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Unit 6: Building Technology in Construction Topic Understand foundation design and construction...

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