Site Investigation is the gathering of information about the proposed location of the highway.
The reason for this can be twofold. Firstly to assist in the location of the highway and secondly to ascertain ground conditions.
Site investigation in the United Kingdom is concerned solely with the ground conditions.
Avoidance of towns, villages, historic buildings, areas of outstanding natural beauty, rail and river crossings and land severance all must be considered.
Only in extreme cases such as marshlands or landslip areas will ground conditions grossly affect the cost of construction.
The process of site investigation can be separated into the following areas:
The sequence of a site investigation is as follows:
- Desk Study
- Site Reconnaissance
- Preliminary report or feasibility study
- Preliminary Ground Investigation - Planning of main GI
- Preliminary report
- Main Ground Investigation
- Laboratory testing
- Final report
The principal objectives for a highway design Site Investigation are as follows:
- Suitability: Are the site and surroundings suitable for the highway?
- Design: Obtain all the design parameters necessary for the works.
- Construction: Are there any potential ground or ground water conditions that would affect the construction?
- Materials: Are there any materials available on site, what quantity and quality?
- Effect of changes: How will the design affect adjacent properties and the ground water?
- Identify Alternatives: Is this the best location?
In addition to these, it is necessary to investigate existing features such as slopes.
If there is a failure of such a feature then it is necessary to investigate the failure and suggest remedial works.
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The desk study is work taken up prior to commencing the work on site and the Ground Investigation.
It should always be the first stage of the Site Investigation and is used to plan the Ground Investigation.
The work involves researching the site to gain as much information as possible, both geological and historical.
- A good starting point is to use OS maps. These allow the site to pinpointed and an accurate grid reference to be obtained.
This permits much more accurate references on the geological maps used later. In addition to present maps, historical maps are used.
These are stored in the British museum and allow much information to be obtained such as former uses of the site; concealed mine workings;
infilled ponds; old clay, gravel and sand pits; disused quarries; changes in topography and drainage; changes in stream and river courses;
coastal erosion; changes in potential landslide areas.
- Geological Maps and memoirs are probably the most important source of information as these give an excellent indication of the sort of ground conditions likely to be encountered.
Dumbleton And West1 recommended the following maps. The 1:63,360 (1in./mile) series gives a good indication of the types of material and the structures occurring in the region.
More detailed information is contained in the 1:10,560 (6in./mile) series, which contains descriptive notes and location and detail of any existing boreholes.
In addition the handbooks published by the HMSO entitled British Regional Geology for England, Wales and Scotland are an extremely useful source of information.
- Ariel Photography is another extremely useful source of information, with almost all of the United Kingdom being covered at a 1:25,000 scale and much at a smaller scale.
These records can be obtained from one of several sources such as the Ordnance Survey, the Department of the Environment, local authorities and air-survey firms. Such records can be extremely useful in
ascertaining historical use of the site, hidden foundations, changes of river course and much other hidden data.
- Records of Previous SI reports are also helpful in a desk study. The many sources of SI data include previous company reports, BGS index of boreholes and the recently formed Association of Geotechnical data exchange for SI data.
- Services records are also an essential part of the desk study, necessary to locate hidden services such as electricity cables, sewers and telephone wires. This in formation is usually provided free of charge by the relevant service provider.
A suggested list of sources is: Local Authority; British Telecom; Electricity Company; British Gas; Water Companies.
- It is also essential to check for the location of former mine workings as these can considerably affect construction and lead to cost increases. The location of these mines may be difficult but help can be found from
the Divisional Plans Record Offices of the National Coal Board.
- The Director of Land Use Survey at King's College London provides a useful survey of land use, if other sources are not available.
- The Ministry of Agriculture, Fisheries and Food in London supply maps detailing the relative value of land for agricultural use. These can be useful when planning the route of a highway.
- In addition to these sources, other help may be found from University Engineering departments, Admiralty charts and local inhabitants.
It is essential when conducting a desk study that as much information as possible is obtained. Work at this stage of the Investigation saves much time later and vastly improves the planning and quality of the Investigation.
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The Site Reconnaissance phase of a site investigation is normally in the form of a walk over survey of the site. Important evidence to look for is:
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- Hydrogeology: Wet marshy ground, springs or seepage, ponds or streams and Wells.
- Slope Instability: Signs of slope instability include bent trees, hummocks on the ground and displaced fences or drains.
- Mining: The presence of mining is often signs of subsidence and possibly disused mine shafts. Open cast mining is indicated by diverted streams replaced or removed fence/hedge lines.
- Access: It is essential that access to the site can be easily obtained. Possible problems include low overhead cables and watercourses.
Dumbleton And West2 have discussed the planning and direction of site investigations. They state that "the main investigation is the full investigation of the site using boreholes and trial pits and includes
the preparation of the site-investigation report with revised plans and sections, interpretation and recommendations for design."
They consider that there are two aspects to the site investigation. The geological structure and character of the site and the testing of the soil both in the laboratory and in-situ. They suggest that the planning should consider the following questions.
Dumleton and West state that these questions should be asked for the whole route, and must be related to the proposed geometry of the road and it's structure. Investigations must be carried out to the depth at which ground conditions cease to affect the work. The more complex the situation, the more extensive the investigation will need to be.
The investigation must then be planned to answer the above questions and associated problems. As much information should be obtained from the points examined as each point is expensive. These are frequently used to clarify the interpretation of the site as a whole. Bridge sites, high embankments and deep cuttings are all points of engineering complexity and should be examined thoroughly.
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- Is the succession of strata known over the whole site and is there correlation across the whole site known? What measurements of depth dip and outcrop are required to clarify the stratigraphy? Should any key measurements, such as depth to bedrock or peat, be made?
- Are the different strata fairly homogeneous over the site or do local variations exist? Are there more complex areas of strata that require investigation or closer examination during construction? Will there be areas where the excavated material will be unsuitable for fill
and will need to be replaced? Are there areas where needs to be assessed to ascertain working methods?
- Are there ground structures that need closer examination - for example the extent of disturbed strata, the location and extent of natural cavities and mine workings, their liability to cause subsidence or movement, the extent and depth of surface movement and instability?
Are there other phenomenon likely to give rise to instability such as fractures and river crossings or alluvial areas that may have buried soft material or peat? Are there likely to be any undetected structures?
- Will any part of the route be subject to flooding? What contact will there be with water bearing strata and will ground water lowering methods be required during construction?
- Do requirements for the carrying out of special in-situ tests or the taking of undisturbed samples affect the conduct of the qualitative investigation? For example, with forethought a single trial pit may be made to serve both for examining ground materials and structure, and for the in-situ testing and the taking of block samples.
Ground investigation is taken to be that other than the information available from the walk over survey as discussed previously.
There are two principal methods of investigating the ground conditions, trial pits and boreholes. In addition, the reader should be aware of geophysical techniques such as seismic surveys, which are not discussed here.
Trial pits are shallow excavations going down to a depth no greater 6m. The trial pit as such is used extensively at the surface for block sampling and detection of services prior to borehole excavation.
||Wheeled Back Hoe
An important safety point to note is that ALL pits below a depth of 1.2m must be supported. In addition care should be taken as gases such as methane and carbon dioxide can build up in a trial pit. Breathing apparatus must therefore be used if no gas detection equipment is available.
Support for a trial pit generally takes one of three forms:
- Steel frames with hydraulic jacks
- Battered or tapered sides
Three types of sample can be taken from a trial pit:
- Disturbed Sample - Samples where the soils in-situ properties are not retained.
- Block Sample - A sample that is not undisturbed but retains some in-situ properies.
- Push in tube sample - Tube samples of the soil in a trial pit.
When preparing a trial pit log, the following information should be included. The location, orientation and size of the pit; sketches of faces; depth scale; root structur; water level; seepage.
In addidtion the weather at the time of sampling should be noted as many soils are weather dependant.
It is extremely important when finished to reinstate the trial pit as well as possible.
A borhole is used to determine the nature of the ground (usually below 6m depth) in a qualitative manner and then recover undisturbed samples for quantitative examination. Where this is not possible, for in gravelly soils below the water table, in-situ testing methods are used.
Obviously the information gained from a borehole is an extremely limited picture of the subsurface structure. It is therefore essential to compare the results obtained with those that could have been expected from the desk study. The greater the number of boreholes the more certain it is possible to be of the correlation and thus to trust in the results.
The two principal types of boring machine used for Site Investigation in the United Kingdom are light percussive and drilling machines.
Light Percussive is the process of making boreholes by striking the soil then removing it and the most common method is the shell and auger. This is a general term to dscribe various tools suspended from a triangular tripod incorporating a power winch. The tools are repeatedly dropped down the borehole while suspended by wire from the power winch.
The different tools used include:
- Clay Cutter - Used in cohesive materials and is raised and lowered, using it's own weight to cut into the material.
- Shell - Used for boring in silts and sands. Similar to the clay cutter, but has a trap door at the bottom to catch material.
- Chisel - Used for breaking up hard material such as boulders or rocks. Additional payment is required for chiselling as per the Bill of Quantities and permission is normally required from the Resident Engineer before work can start.
Drilling is the process of boring normally by using a combination of a rotating action and a hydraulic ram. There are many different types of rig depending on access and type of ground expected. Hollow drilling rods enable a flush of water, air, foam or mud which is used to carry the cuttings to the surface as well as lubricating and cooling the drill bit. The three main types of drill bit are:
- Double tube is where the outer tube rotates and allows for the removal of the cuttings while the inner tube is stationary and prevents the core from shearing. There are different designs of tube varying the location of the flush discharge so as to prevent sample erosion. It is necessary for the hole to be bigger than the tube and so the diamond bits are attached to the outside of the hole, thus allowing the flush to return to the surface.
- Triple tube in corporates a third tube to protect the core even further during extrusion and can have either a split tube, which is removed, or a plastic tube to provide longer term protection. A less effective alternative is to incorporate a nylon liner in a double tube.
- Retractable triple tube is a variation where the inner tube is attached to a retractor and can extend beyond the cutting edge. This gives complete protection to the core in softer rock whilst in harder rock where this is not necesary, it retracts to become a standard triple tube. This is used in alternating soft/hard rock, typical of a weathered profile.
Core bits are usually diamond tipped and are either surface set, where diamonds are mounted into a matrix, or impregnated where a fine diamond dust is used in the matrix. In softer rocks, the cuttings can clog up the matrix so the softer the rock, the larger the diamonds need to be. Tungsten carbide bits can also be used in the softer rocks.
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Sampling can be either undisturbed, of which in-situ testing is a form, or disturbed. The principal sampling methods used in boreholes are:
- SPT test: This is a dynamic test as described in BS1377 (Part 9) and is a measure of the density of the soil. The test incorporates a small diameter tube with a cutting shoe known as the 'split barrel sampler' of about 650mm length, 50mm external diameter and 35mm internal diameter. The sampler is forced into the soil dynamically using blows from a 63.5kg hammer dropped through 760mm. The sampler is forced 150mm into the soil then the number of blows required to lower the sampler each 75mm up to a depth of 300mm is recorded. This is known as the "N" value. For coarse gravels the split barrel is replaced by a 60 degree cone.
- Core Sample: Core samples must be sealed with parafin to maintain the water conditions and then end sealed to prevent physical interferance. The mpst common of these is the U100 (see below) although other sizes from 54mm to 100mm diameter are used. The standard U100 has a sample area ratio of 30% so large ammounts of soil are displaced. A thin walled Piston Sampler reduces this to 10%. The sample is pushed or jacked into the ground as opposed to a dynamic action.
- U100: This is a 450mm long, 100mm diameter undisturbed sample. The tube has a cutter at one end and the driving equipment at the other. Behind the cutter is a core catcher, incorporating 3 arms that go into the sample as it is withdrawn, to prevent the sample from falling out. Care should be taken to ensure that the cutting shoe is as clean and sharp as possible.
- Bulk Samples: Usually taken from trial pits or in soils where there is little or no cohesion. Often called block samples.
- WaterSamples: Water samples should be taken as soon as water is first struck and the depth recorded. After a suitable period of time (usually 10-15 mins) the depth should be re-recorded and a further sample taken. A final sample should be taken at the end of the borehole and the depth to water regularly recorded. The sample is taken using a device known as a bailer, made from teflon or plastic it incrporates a float to trap the water and should be cleaned after each sample.
The sampling procedure varies according to the type of strata in which the investigation takes place. A reccomended sampling procedure is listed below.
Clays: Normally need undisturbed samples
Sands & Gravels: Undisturbed samples are not practical due to the lack of cohesion.
- U100 every 1.5m or change of stratum. Blow count and penetration should be noted.
- If unable to obtain a U100 then bulk samples as above.
- If U100 does not full penetrate SPT test is required.
- SPT every 1m or change of stratum. Number of seating blows should also be recorded.
- Bulk samples to be taken between SPT's.
- Alternate SPT and U100 samples at 0.75m intervals
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- SPT on penetrating rock, every 1m and change of stratum where possible
- In softer rocks (Chalks, Marls) U100 may be possible
- Rock must be penetrated at least 1.5m to ensure it isn't a large cobble
- Obtain permission from Resident Engineer before pulling off-site
- If SPT refusal (>50 blows) record number of blows and penetration
The Site Investigation report for a highway design scheme should answer all the questions set out in the planning phase of the Investigation (recap). This should include an assesment of the viablilty of the proposed route and indication of any alternatives.
Included in the report should be a location of all the boreholes, trial pits, other excavations and their logs. These logs should give as much information as possible on the soil and rock structure as it is possible to obtain.
The soil and rock descriptions should be as defined in BS5930 and should contain the information described below:
Soil Description - Often remebered using the acronym MCCSSOW obviously!
Rock Descriptions - The acronym makers came up with CGTSWROS in a moment of inspiration
- Moisture Content - Dry, slightly moist, moist, very moist or wet. Not the measured value just the way it appears in the hand.
- Colour - This is an indicator of chemical and mineralogical content. Charts are available but not often used.
- Consistency - Loose or dense and other descriptions dependant on soil type. An approximate relationship can be made between stifness and undrained shear strentgh (Cu) and between density and the SPT 'N' Values.
||SPT 'N' Value
- Structure - Bedding, laminatesfissure, joints, fractures, shear zones etc.
- Soil Type - Given by particle sizes as described in BS5930 Table 6
Origin - Try and identify geological area and stratographic unit. This is difficult and often impossible
- GroundWater Conditions - Depth to groundwater and any other observations.
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- 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, amourphous, homogeneous and many more as described in BS5930 Table 9.
- Structure - Dependant on the type of rock, reference should be made to BS5930 Table 9. 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 Table 9.
- Other Stratographic information, geological period, presence of fossils or coral seams.
- Strength - Defined as below from field observations.
||Point Load Strength (MPa)
||Rocks Ring on hammer blows.
||Lumps only chip by heavy hammer blows. Dull ringing sound
||Lumps or core broken by heavy hammer blow
||Lump or core broken by light hammer blow
||Thin slabs broken by heavy hand pressure
||Thin slabs break easily in hand
||Crumbles in hand
||Can be indented by thumb nail
1) M. J. Dumbleton and G. West, Transport And Road Research Laboratory Report LR403, Preliminary Sources of Information for Site Investigation In Britain (Crowthorne, 1971)
2) M. J. Dumbleton and G. West, Transport And Road Research Laboratory Report LR403, Preliminary Sources of Information for Site Investigation In Britain (Crowthorne, 1971)
3) British Standard 5930 1981
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Last Updated: 25 February 1997