Site investigation is the study of the local environment and ground conditions on and around a specific piece of land. This is used to ascertain the suitability of a site for a proposed building project: The condition & strength of the soil
Details of any man-made or natural hazards present on the site The economic viability of the site There may be three components, the walkover survey, the desk study and the ground investigation.
1. Walkover Survey
Purpose
A Walkover survey reports on the appearance and the characteristics of the site. It involves exploring the site for any obvious features that would affect the use of the site for a proposed building development. It is not possible to list every detail that should be recorded on a walkover survey, much of it is down to common sense.
For example, at a site on the side of a hill in Dorset it was found that fifty or so years ago it was used as a excavation, so a trial pit(whole in the ground) and deep bore holes were required. However the only way onto the site was on foot down steeply paved steps. The result was two-fold. The machinery to carry out the investigation was unable to enter the site, and secondly the construction company needed to buy an adjacent house and obtain permission to demolish it to build an access road. The company should have been made aware of these simple facts by the walkover survey before they committed themselves to the expense of a full ground investigation.
Local Knowledge
A brief history of the site can be found using a variety of methods. For example many local people may remember how the land has been used in the past. Also the name of local roads and towns give an in site into the surrounding landscape. A road known locally as ‘Watery Lane’ may be prone(flat) to flooding, and an estate known as ‘Brickfields’ may be built on the site of some filled in clay pits.
Topological Features
There are a number of topographical features, which allow for educated guesses about the soil. For instance large flat areas close to rivers are usually ‘river terraces’ comprising of layers of sand and gravel washed down by the river. Land areas close to river estuaries tend to be fine silts transported by the river and dumped in the estuary when the speed of the current slowed down.
Soil slope angles of greater than around 30 degrees are potentially unstable and liable to steady slippage or ’creep’. Trees with bent trunks, leaning posts and walls and bends in otherwise straight hedgerows, are evidence for the creeping movement in the surface soil layers.
Trees & Vegetation
It is important for the walkover survey report to include the position, height, spread, girth and species of all trees and large shrubs. Trees and shrubs extract minerals dissolved in groundwater. The watery solution is taken into the plant by the roots, up through the stem and branches, helping the tree to grow. The amount of water removed from the soil depends on the size of the plant. A large oak tree may release as much as 650 litres from the soil into the atmosphere each day!
In clay soils when water is taken out the clay shrinks, which can cause buildings close by to settle and crack. If a tree is cut down prior to construction the reverse happens and the soil takes up moisture and it swells. This process is called clay ‘heave’. This has often resulted in fractured foundations and drains and is a good reason for comparing old maps and photographs for the location of old wooded areas. The water demand by each species of trees is different. An oak will take up considerably more water than a pine, so the extent of the soil heave following removal varies with the different species.
Existing Buildings
If the site has existing buildings they will provide a good indication of suitability of the soil for construction purposes on the part of the site on which they are situated. Underpinning is indicative of previous foundation failure.
Buildings shown on old maps suggest that they no longer exist any more, or that they have become uninhabitable or derelict. The presence of old undetected foundations or basements may cause unplanned cost.
Watercourses
Surface water should be mapped so that the position can be checked with historic sources later. Once the project is under way serious problems can arise, if ponds or streams have not been located as local flooding can cause many problems with the new properties. Some old watercourses may been filled in naturally, with a mixture of topsoil and surface material, or artificially, usually with either household or construction rubbish. In either case the soil is unlikely to support a building and consideration should be given to either avoid that particular area if possible, or sinking the foundations through the infill to solid soil beneath with deeper foundations.
2. Desk Study
The preliminary information is gained by use of a desk study. As the name implies this does not necessarily demand a visit to the site, but it is better carried out in conjunction with a walkover survey. The desk study
will need you to research into existing documentation and maps of the site. Typical documents to be studied are:-
Aerial surveys where appropriate and available maps
Geological surveys and mining records
Local Authority records and documents
Local historical archives
Site investigation reports from adjacent building developments. Location and height of closest Ordnance Bench Marks (OBM) and contours, given on Ordnance Survey maps.
Information can be found from the above sources which could help you to discover any potential problems such as: –
Legal Requirements
The area around the site, as well as the site itself may be the subject of Local Authority restrictions. The site may contain listed buildings or trees with preservation orders. Ancient monuments are perhaps obvious, but those without an archaeological background, some burial mounds(hill) may look like piles of soil , which could easily be removed with a mechanical shovel. Part of the site may be a right-of-way or have restrictions imposed because of adjacent properties, possibly interfering with their right to light. The site may in the past have had approval for industrial buildings, which, if constructed, would cause problems to the prospective owners of adjacent houses. At many sites access provides no difficulties. At other, more secluded sites it may produce problems of ownership. Access to local maps and archives is essential in assessing ownership and legal requirements.
Geological Information
Information about the geology of the site may be found in geology maps and records which are produced by Ordnance Survey and geological institutions such as the Geological Sciences. There may also be available through the local Authority other soil data or information from previous local soil investigations.
Access to Services & Communications
Each site will require a supply of gas, electricity, telephone, optical cable, clean water, drainage system connected to the main sewers, and waste disposal. These together come under the heading of services, most of which are now privatised. Information should be required from these service authorities and your finding should form an appendix to the desk study report.
Certain aspects are common to all services suppliers and they are: 1 position/location
2 depth or height of cables or pipes
3 costs involved in installation
4 location of service authority local office
5 bye-law and other requirements of the authorities.
3. Ground Investigation
The ground supports all construction projects. Therefore the ground investigation is very important. The results from the ground investigation are mainly used to determine the strength of the soil and hence the size and depth of the proposed foundation. Other reasons for the ground investigation are to determine ground water levels which may effect the method of construction of the foundations. The correct foundation dimensions are necessary for two reasons. If the original estimate of the width of a strip foundation was too small due to incorrect calculations the building may punch through the soil and cause the building to crack and subside (collapse). If the foundation were too big there would be an excessive cost for the concrete and labour to construct it.
Approximately 50% of all projects over-run and losses of up to 35% of the entire original tender have been suffered. The primary reasons for these losses are either insufficiency of ground investigation or lack of understanding of the results.
Typically on-site problems fall into two categories:
1. Man made obstructions
(a) Foundations and services from previous projects on sites where only the superstructure has been demolished. (b) Methane pollution and soft ground on reclaimed land.
(c) Quarry and mine waste which will not support the proposed structure. (d) Recent removal of trees.
2. Natural obstructions
(a) Underground springs, which only appear as damp patches or a change in vegetation, cover in normal weather conditions. (b) High water table on a sloping site.
(c) Change in the sediments which is not visible at the surface, for example and in filled pond. (d) Chemical composition of ground water, e.g. concrete made from ordinary Portland cement is susceptible to chemical attack by sulphur.
That lack of suitable site investigation exists is indicated by the fact that the National House Building Council, an organisation that insures the structure of new domestic construction, pays out £3-6 million a year in claims related to geotechnical problems. The cost of site investigation varies according to the project, but is commonly less than 0.5% of the contract price and sometimes as low as 0.1.
The use of cut-price site investigation – or none at all – is a form of gambling. If the gamble pays off the company will have saved a small proportion of the costs. If not the additional on-going costs will inevitably exceed the price of competent site investigation report. The general view among civil engineers is that costs are generally increased by adequate or absent site investigation.
The extent of the ground investigation will be determined by the results from the desk study and walkover survey, and the type of buildings proposed for the site. Low-rise domestic construction investigation usually consists of a desk study and walkover survey, followed by in situ ground investigation using boreholes and/or a trial pit, and laboratory analysis of particle size distribution, plasticity and chemical composition.
Methods of Ground Investigation
The primary function of the ground investigation team is to describe the ground, not to make judgement concerning its suitability for any particular purpose. There are 3 basic methods depending on the size of the project:- Trial Pits
Hand Augers
Deep Shell & Auger
A trial pit, large enough for an operative to stand in, may be excavated to give easy access to the strata (soil layer), and take then to undisturbed soil samples for further investigation back at the laboratory.
The use of the hand auger or window sampler (up to a depth of 5m) has advantages where the soil is cohesive (but preferably not dense dry clay) in that: 1 It is cheaper than other methods
2 If carried out properly causes minimal disruption
A series of hand augered boreholes quickly provides an overview of the proposed site, and indicates possible further ground requirements. The soil samples taken are often disturbed i.e. chewed up by action of the auger, and are not very good for laboratory strength tests.
For deeper boreholes for large construction works such as bridges, tunnels and other civil engineering works a special bore hole rig with a mechanical winch is used. This is called a shell & auger borehole. This can go down to depths of soil in excess of 30m and retrieve soil samples for strength testing in the laboratory. Undisturbed soil samples can be retrieved for further investigation back at the laboratory with this type of bore hole.
Soil Description
A site investigation report should use language that can be understood by any construction technician or professional. An architect and a first year assistant site manager may both need to refer to the report. The method used for the field description of soils is therefore standardised and kept as simple as is possible. The type of deposit is written in capital letters so that it is highlighted on the borehole log. It is usually preceded by the colour, which can be arrived at by comparison with colour charts, or by a general impression. The soil on many beaches could therefore be described as ‘Orange SAND’. However many soils are a mixture of particle sizes so that gravel might contain sand, or a sandy clay. Sand may be further described by grain size. In the field a small board may be used with particles of known size glued to it for comparison. Otherwise, if the particles are difficult to see they are fine, if they are easy to see but they cannot be picked out individually by hand they are medium and if they are large enough to pick out they are coarse grained. In the field clay may be described by its strength. Table 1 below shows the relationship between strength and the application of the field tests.
Table 1 Soil Strength
Indicative DensityApplied field test
Very soft:A ball of clay is placed in the palm of the hand And the first squeezes out between the fingers.
Soft:A small ball of clay can be moulded with the fingers into a cube. Firm:Moulding is difficult and requires a great deal of
Pressure from the fingers.
Stiff:Clay can only be dented (smashed) by the fingers.
Very Stiff:Very firm finger pressure only produces slight indent. Hard:Can only be dented by a pencil or nail.
Unconsolidated soils can vary from loose, like a dune sand, to dense like a compacted gravel. The field test is simple. A 15mm steel reinforcing rod is pushed into the borehole to test each deposit. Table 2 shows the
relationship between indicative density and the appropriate field test.
Table 2 Soil Description
Indicative densityApplied field test
Loose:Easily penetrated by hand pressure by rod.
Firm:Driven in easily by 2.5 kg hammer, each blow driving
The rod in greater than 300 mm.
Dense:Each blow of the hammer drives the rod in less than
300 mm and more than 100 mm.
Very Dense:Penetrates less than 100 mm width each blow.
A more precise method is to use a penterometer. The mass falling over a fixed distance produces a constant force. The methods outlined are a combination of those used to produce a description of soils, which make up the strata, which is known as a borehole log. The altitude above sea level of the water table, the depth of each bed and the altitude of the surface of the borehole and all of the strata boundaries are recorded. The number of boreholes to be drilled and the interpretation of the ground between them is a matter of local and geotechnical experience and common sense.
Particle Size Distribution – Grading Tests
Sediments are formed of a variety of particle sizes. A glacial till often contains a broad spectrum. River sand is unlikely to contain very fine particles since they will stay in suspension until the velocity and density of the water changes at the estuary. In estuarine silts the range is therefore considerably narrower and towards the finer end of the spectrum. We can also look at the shape of the particles. The grains of sand produced by glaciers are the result of the grinding action and are therefore very angular. A desert sand will be blown across the surface and tend to be more spherical. Larger particles, which are moved without rolling, similar to those on a low energy beach, become discoid. It is therefore possible to get an idea of the depositional environment of a sedimentary deposit by looking at various features. The variation of grain sizes in a given sample is characterised statistically by separating the sample into classes based on particle diameter using a set of stacking sieves. The frequency of each class is represented as percentage mass and plotted against grain diameter. The simplest method is the histogram, which has the advantage of visual representation.
Shear Strength
Used to determine the maximum bearing capacity of the soil. Cohesive clay soils tested in a triaxial test machine, where a small cylinder of clay is squashed by a known force. A common site test is the Shear Vane Test used on exposed areas of cohesive soils found in trial pits. A sand box test is used for non-cohesive soils such as sands and gravels. Plasticity tests indicate the amount by which clays will shrink or swell as the water content varies inside the clay.
Site Investigation
Question Sheet: Read through the given materials then answer the following questions:
1. State the main objectives of a site investigation & ground appraisal.
2. What are the three main methods used in the site investigation & ground appraisal.
3. What do you think are the “main risks” that a developer needs to be aware of at the start of the project ?
4. Describe the purpose of the “walkover” (sometimes referred to as the reconnaissance of the site).
5. List the main items to be taken into account during the reconnaissance of the site.
6. Describe briefly why it is important to note down the type of vegetation on site particularly on clay soils.
7. Why could the presence of ground water on site be a problem both during the construction process and after the building has been built?
8. Why is it important to find out if the ground is contaminated?
9. What is the typical % cost spent on site investigation?
10. Describe the purpose of a ‘Desk Study’.
11. List the possible sources of information that may be relevant for the ‘Desk Study’.
12. Describe the purpose for undertaking trial pits and/or bore holes on the site.
13. What is the main disadvantage of hand-augered bore holes as compared to trail pits ?
14. What type of soil may swallow holes be found and why this be a problem ?
