A construction of any building requires a site investigation preferably at the earliest stage possible but definitely during the design stage. The investigation may be from a simple examination of the surface soil to a few shallow trial pits, to a detailed study of the soil and ground water, including chemical analysis to a considerable depth below the surface by means of boreholes and tests, in situ and/or laboratory of the materials encountered.
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The main objective of a site investigation is to examine the ground conditions so that the most appropriate type of foundations can be selected. A site investigation starts with a desk study and a walk-over survey to establish the general geology of the site, and continues with an examination of the geotechnical properties of the ground. The reason for site investigations is to assist in the location of building and to determine ground conditions. Site investigation is concerned specially with the ground conditions. In our build we must ensure we avoid historic buildings, areas of outstanding natural beauty, rail and river crossings and land severance towards the Talbot area where there is going to be residential, commercial and industrial sections .
The soil and rock descriptions should be as defined in BS5930 and should contain the information described below:
Rock Descriptions – The acronym makers came up with CGTSWROS in a moment of inspiration
Colour – Same terminology as for soils with principal and secondary
Grain Size – Range of sizes present and the dominant sizes.
Texture & Fabric – Porphyritic, crystalline, granular, glassy, amorphous, homogeneous and many more as described in BS5930
Structure – Dependant on the type of rock, references are made to BS5930. Discontinuities in the rock can be caused by the drilling action, weathered surfaces indicate natural and clean surface indicate recent fractures.
Weathering – Engineers grade from 1-6 with 1 being fresh and 6 being residual soil with all the rock converted to soil.
Rock Type – Reference should be made to BS5930
Other Stratigraphic information, geological period, presence of fossils or coral seams.
Strength – Defined as from field observations.
There are many different types of rocks with different bearing capacities:
rock type
Av. Bearing capacity in terms of newtons per mm2
Solid formed igneous rocks
10.25
Hard sandstone and limestone
4.25
Undisturbed slate
3.20
Hard shales and soft sandstones
2.14
Clay shales
1.05
Hard solid chalk
0.65
According to Barry’s:
Hard formed igneous rocks have a very high bearing capacity and little likelihood of foundation failure.
Hard sandstones and limestones are, when massively bedded, stronger than good concrete and it is rare that their full bearing capacity is utilised.
Hard shales, formed from clayey or silty deposits by intense natural compaction, have a fairly high allowable bearing pressure.
Soft sandstones depending on the cement material have a very variable allowable bearing pressure.
Soft shales are intermediate between hard cohesive soils and rockes.They are liable to swell on exposure to water and soften.
Chalk includes a variety of materials composed mainly of calcium carbonate and the allowable bearing pressure may vary widely.
Thinly bedded limestones and sandstones, which are stratified rocks, often separated by clays or soft shales, have a variable allowable bearing pressure depending on the nature of the separating material. (Emmitt, 2008,p.53-54)
As well as the above table rocks are split into three different categories:
Sedimentary: sandstones (including conglomerates), some hard shales and limestones.
Metamorphic: some hard shales, slates, schists and gneisses.
Igneous: granite, dolerite and basalt.
Soil Description – Often remembered using the acronym MCCSSOW.
Moisture Content – Dry, slightly moist, moist, very moist or wet.
Colour – This is an indicator of chemical and mineralogical content.
Consistency – Loose or dense and other descriptions dependant on soil type.
Structure – Bedding laminates fissure, joints, fractures, shear zones etc.
Soil Type – Given by particle sizes as described in BS5930
Origin – Try and identify geological area and stratigraphic unit.
Groundwater Conditions – Depth to groundwater and any other observations.
Soils are classified in two categories: non-cohesive and cohesive.
Non-cohesive: means the soil has no shear strength if no confinement. Barry states (2008) that they are soils that are composed mainly of or combinations of sand and gravel consist of largely siliceous, unaltered products of rock weathering. Non-cohesive soils laterally confined to prevent spread of the soil under pressure .It has no plasticity and tend to lack cohesion especially when dry therefore from a construction point of view non-cohesive soils are good as foundation and as construction material although serious thought should be given to ground water level and any flow of water, ground water level near the foundation level will affect the density of packing of the soil and flow of water could wash out finer particles of the soil and change the grading in both cases reducing the bearing capacity (p.56)
Non-cohesive soils are usually larger in size than cohesive soils, below the table shows non cohesive soils, sizes and their bearing capacity
Non-cohesive soil types
Grain sizes in millimetres
Av.bearing capacity in terms of newtons per mm2
Compact gravels
2 to 60
0.53
Loose gravels
2 to 60
0.25
Compact coarse sands
0.6 to 2
0.45
Loose coarse sands
0.6 to 2
0.20
Compact medium sands
0.2 to 0.6
0.40
Loose medium sands
0.2 to 0.6
0.17
Compact fine sands
0.06 to 0.2
0.25
Loose fine sands
0.06 to 0.2
0.11
Cohesive soils descriptions: Cohesive soils are dense and tightly bound together by molecular attraction. They are plastic when wet and can be moulded, but become very hard when dry. Proper water content, evenly distributed, is critical for proper compaction. Cohesive soils usually require a force such as impact or pressure. Silt has a noticeably lower cohesion than clay. However, silt is still heavily reliant on water content. Cohesive soils are susceptible to slow volume changes foundation.
Below the table shows the different types of cohesive soils. Sizes and their bearing capacity
Cohesive soil types
Grain size in millimetres
Av.bearing capacity in terms of newton’s per mm2
Firm uniform silty sands
0.006 to 0.2
0.10
Silt and alluvial earth
0.002 to 0.6
0. 07
Very stiff and hard clays
Up to 0.002
0.50
Stiff clay and sandy clays
0.002 to 0.6
0.30
Firm clay and sandy clays
0.002 to 0.6
0.15
Medium soft clays and silts
0.002 to 0.6
0.11
Soft clays and silts
0.002 to 0.6
0.08
Very soft clays and silts
0.002 to 0.6
0.05
As well as the above table for cohesive soils on page A diagram 1 at the back of this assignment the soil triangle shows the different classifications for the percentage of sand, clay and silt mixed together
TASK 4
The primary functional requirements of foundations are strength and stability. To comply with building regulations the combined dead, imposed and wind loads of the building should be safely transmitted to the ground without causing movement of the ground that may impair the stability of any part of another building. Loading is concerned with the bearing strength of the ground relative to the loads imposed on it by the building. The foundation should be designed so that the combined loads from the building are spread over an area of the ground capable of sustaining the loads without undue movement. The building should also be constructed so ground movement caused by swelling, shrinking or freezing of the subsoil or landslip or subsidence (other than subsidence arising from shrinkage) will not impair the stability of any part of the building. (Emmitt, 2008, p61)
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Non-cohesive soils such as gravels, coarse, medium and fine sands have no plasticity and tend to lack cohesion, especially when dry. Under pressure from the loads on the foundations the soils in this group compress and consolidate rapidly by some rearrangement of the particles and the expulsion of water. The larger the particles the greater the bearing capacity however ground water level or flow of water coming into contact with the soil near the foundations could wash out the smaller particles greatly reducing the bearing capacity causing the foundation either to break and collapse causing severe subsidence.
Cohesive soils such as clays and silt are plasticity, under the pressure from the loads on the foundations the soils in this group compress very gradually by the expulsion of water or air making the building settle gradually during erection Seasonable variations in groundwater and vigorous growth of trees and shrubs will cause appreciable shrinkage in clays, hard winters will cause clay to go hard possibly causing cracks, rains raising the water table and soaking the clays again will cause them to expand so for many years after the building is completed it will be rising and sinking.
There are many different types of foundations that can be used when building a residential, industrial or commercial structure dependant on a number of different situations for example type of soil, drainage of the ground .
Residential structure
In normal situations for a residential structure a strip foundation would be suffice, a strip foundation consist of a continuous, longitudinal strip of concrete designed to spread the load from uniformly loaded walls of brick, masonry or concrete to a sufficient area of subsoil. The size of the strip depends on foundation loads and the bearing capacity and shear strength of the subsoil; it provides a level base on which walls may be built. The foundation transfers the loads of the building to the load bearing strata. (Emmitt, 2008, p71) (Diagram 2, page b) at back of assignment.
Industrial structure
Industrial buildings need a much stronger foundation than residential, they are normally a lot larger than residential buildings with people working from them, merchandise stored in them, large vehicles coming to and fro all adding more weight and movement onto the foundations, in this case pad foundations could be an option. The foundation to piers of brick, masonry and reinforced concrete and steel columns is often in the form of square or rectangular isolated pad of concrete to spread a concentrated load. (Emmitt, 2008.p73) (Diagram 3, page c) at back of assignment.
Commercial structure
Commercial buildings are normally larger than both residential and industrial buildings. In these building there are normally more people and merchandise inside, larger vehicle traffic coming and going so a stronger foundation is required than used for industrial. An option for this type of structure is pile foundations. The main function of pile is to transmit loads to lower levels of ground by a combination of friction of their sides and end bearing at the pile point or base. When piles are driven through weak soils such as alluvial or silty clay, peat layers or reclaimed land, the long -term settlement and consolidation of the ground can cause downdrag of the piles. (Glover, 2006, p207). (Diagram 4, page C) at back of assignment.
Task 5
The effects that water, chemicals and contamination can have on a design of:
Substructure
Ground water control
Troubles with water seepage and erosion occur mainly in soils. Internal erosion can result from ground water seeping into fractured sewers or culverts carrying with it fine soil particles. The consequent loss of ground from beneath foundations may lead to collapse of structures. Trouble of this kind is liable to occur in mining subsidence areas where sewers and water mains may be broken. It can also occur as a result of careless technique in deep excavations below the water table when soil particles are carried into the excavation by flowing ground water. Severe erosion can take place around the foundations of bridges or other structures in waterways subjected to heavy flood discharges. The required depths of such foundations can be obtained by hydraulic calculations and local observations.
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