Site Investigation

Introduction

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:


Objectives of Site Investigation

The principal objectives for a highway design Site Investigation are as follows:

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.

Return to top of page.

Desk Study

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.

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.

Return to top of page.

Site Reconnaissance

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:

Return to top of page.

Planning A Site Investigation

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.

  1. 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?

  2. 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?

  3. 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?

  4. 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?

  5. 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.

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.

Return to top of page.

Ground Investigation

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

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.

Depth Excavation Method
0-2m By Hand
2-4m Wheeled Back Hoe
4-6m Hydraulic Excavator

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:

Three types of sample can be taken from 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.


Boreholes

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:

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:

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.

Return to top of page

Sampling

Sampling can be either undisturbed, of which in-situ testing is a form, or disturbed. The principal sampling methods used in boreholes are:

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. Silts: Rock

Return to top of page.

Reporting

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

Return to top of page.

References

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

Return to top of page.


e-mail: D.G.Toll@Durham.ac.uk.

Last Updated: 25 February 1997