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Guidance Notes

Guidance notes for geotechnical and contaminated land investigation

Detailed below are explanations and reasons for the various tests we conduct during site investigations.

Site Investigation Procedure:

• General
• Ground investigation
• Borehole and trial pit records
• Shell and auger boring
• Groundwater
• Sampling
• Standard penetration tests
• Rotary drilling
• Permeability tests
• Trial pitting
• Gas monitoring
• Soil description
• Natural or in-situ moisture content
• Liquid and plastic limits
• Particle-size distribution
• Bulk density
• Permeability
• Consolidation characteristics
• Strength characteristics
• Compaction
• California bearing ratio test
• Chemical tests
• References

Contaminated Land Investigation Procedure

• General
• Review of contamination issues
• Preliminary Investigation
• Intrusive investigation
• Assessment of contamination

Site Investigation Procedure

General

The ground investigation is carried out in accordance with the requirements of BS5930 the Code of Practice for Site Investigation, and this appendix briefly describes the nature of the work carried out. It also gives a brief description of the more important tests which are made for engineering purposes on rocks and soils (see also BS1377).

By its very nature any ground investigation only samples a small percentage of the ground. Consequently changes in ground conditions and soil properties can occur between any two exploratory points, for example local features such as soft ground, pockets of contamination and faults. This is also true of the exploration of mineworkings and such features can extend beneath parts of the site not investigated. Unrecorded bell pits and shafts can also exist between exploratory points.

The ground investigation is designed to minimise such risks but they cannot be eliminated.

Ground investigation

1. Borehole and trial pit records - these illustrate the ground conditions only at the location of the particular borehole or trial pit. Correlation between boreholes is for guidance only and its accuracy cannot be guaranteed.
   
   
2. Shell and auger boring - this technique uses a tripod winch and an essentially percussive action using a variety of tools. Disturbed and undisturbed samples can be taken. This is the most suitable method for soft ground investigation, enabling the maximum amount of information to be obtained. However, minor changes in lithology may be overlooked unless continuous undisturbed sampling is used.
   
   
3. Groundwater - Groundwater levels vary seasonally and the details given on the borehole logs relate only to the dates and the conditions described in the borehole records. The rate of boring may not have allowed an equilibrium water level to be established and the use of casing may seal off certain seepages.
   
   
4. Sampling - Disturbed samples of soils are taken for identification and classification purposes. In cohesive soils 'undisturbed' samples 100mm in diameter are taken by open drive sampler for laboratory testing of strength, permeability and consolidation characteristics.
   
   
5. Standard penetration tests (SPT) - S.P.T tests are used in granular and cohesive materials and in soft or weathered rocks. Difficulties in obtaining true 'N' values means they must only be used as a guide and not as an absolute value in foundation design.
   
   
6. Rotary drilling - Two main types of rotary drilling are carried out in rock.
   
   Rock coring using diamond or tungsten carbide tipped core bits provides samples and information on rock types, fissuring and weathering.
   
   Open hole drilling only produces small particles for identification purposes and the information gained is therefore limited. The latter is, however, useful as a quick method for detecting major strata changes and for the location of coal seams and old workings. Water, air, foam or drilling muds may be used as the flushing medium in either case.
   
   
7. Permeability tests - These can be carried out in boreholes or trial pits and give a good indication of in-situ permeability.
   
   
8. Trial pitting - this enables soil conditions to be closely examined at any specific point and samples taken. It also gives useful information on the stability of excavations and ingress of water.
   
   
9. Gas monitoring - this is carried out in trial pits or probeholes to check for elevated levels of methane and carbon dioxide or oxygen deficiency.

Soil description

Samples from borings or trial pits are described as specified in the standard procedure outlined in BS5930. The description includes colour, consistency, structure, weathering, lithological type, inclusions and origin.

Natural or in-situ moisture content

The natural or in-situ moisture content of a soil is defined as the weight of water contained in the pore space, expressed as a percentage of the dry weight of solid matter present in the soil. Soil properties are greatly affected by the moisture content and the test can help to give an indication of likely engineering behaviour.

Liquid and plastic limits

Two simple classification tests are known as the liquid and plastic limits. If a cohesive soil is remoulded with increasing amounts of water, a point will be reached at which it ceases to behave as a plastic material and becomes essentially a viscous fluid. The moisture content corresponding to this change is arbitrarily determined by the liquid limit test. 'Fat' clays, which have high contents of colloidal particles, have high liquid limits; 'lean' clays, having low colloidal particle contents have correspondingly low liquid limits. An increase in the organic content of a clay is reflected by an increase in the liquid and plastic limits.

If a cohesive soil is allowed to dry progressively, a point is reached at which it ceases to behave as a plastic material which can be moulded in the fingers, and it becomes friable. The moisture content of the soil at this point is known as the 'plastic limit' of the soil.

The range of water content over which a cohesive soil behaves plastically, i.e. the range lying between the liquid and plastic limits, is defined as the plasticity index.

A cohesive soil with a natural water content towards its liquid limit will, in general, be an extremely soft material whereas a cohesive soil with a natural water content below its plastic limit will tend to be a stiff material.

Particle-size distribution

A knowledge of particle-size distribution is used to classify soils and to indicate likely engineering behaviour. BS5930 defines soils in relation to their particle-size.

Bulk density

The bulk density of a material is the weight of that material per unit volume and includes the effects of voids whether filled with air or water. The 'dry density' of a soil is defined as the weight of solids contained in a unit volume of the soil.

Permeability

The permeability of a material is defined as the rate at which water flows through it per unit area of soil under unit hydraulic gradient.

Consolidation characteristics

When subjected to pressure, a soil tends to consolidate as the air or water in the pore space is forced out and the grains assume a denser state of packing.

The decrease in volume per unit of pressure is defined as the 'compressibility' of the soil, and a measure of the rate at which consolidation proceeds is given by the 'coefficient of consolidation' of the soil.

These two characteristics Mv and Cv are determined in the consolidation test and the results are used to determine settlement of structures or earthworks.

Strength characteristics

The strength of geological materials is generally expressed as the maximum resistance which they offer to deformation or fracture by applied shear or compressive stress. The strength characteristics of geological materials depend to an important degree on their previous history and on the conditions under which they will be stressed in practice. Consequently it is necessary to simulate in the laboratory tests the conditions under which the material will be stressed in the field.

In general, the only test carried out on hard rocks is the determination of their compressive strength but consideration must be given to fissuring, jointing and bedding planes.

The tests at present in use for soils and soft rocks fall into two main categories. Firstly, those in which the material is stressed under conditions of no moisture content change, and secondly those in which full opportunity is permitted for moisture content changes under the applied stresses. Tests in the first category are known as undrained (immediate or quick) tests, while those in the second category are known as drained (slow or equilibrium) tests. The tests are normally carried out in the triaxial compression apparatus but granular materials may be tested in the shear box apparatus.

The undrained triaxial test gives the apparent cohesion C u and the angle of shearing resistance Ø u. In dry sands, C u = 0 and Ø u is equal to the angle of internal friction whereas with saturated non-fissured clays Ø u tends to 0 and the apparent cohesion C u is equal to one-half the unconfined compression strength qu. On site the vane test gives an approximate measure of shear strength.

For some stability problems use is made of a variant of the undrained triaxial test in which the specimen is allowed to consolidate fully under the hydrostatic pressure and is then tested to failure under conditions of no moisture content change. This is known as the consolidated undrained triaxial test. Pore water pressures may be measured during this test or a fully drained test may be carried out. In either case the effective shear strength parameters C' and Ø' can be obtained which can be used to calculate shear strength at any given pore water pressure.

Compaction

The density at which any soil can be placed in an earth dam, embankment or road depends on its moisture content and on the amount of work which is used in compaction. The influence of these two factors can be studied in compaction tests which can determine the maximum dry density (MDD) achievable at a certain optimum moisture content (OMC).

Calfornia bearing ratio test

In flexible pavement design a knowledge of the bearing capacity of the subgrade is necessary to enable the thickness of pavement for any particular combination of traffic and site conditions to be determined. The quality of the subgrade can be assessed by means of the California Bearing Ratio Test or approximately by the MEXE cone penetrometer.

Chemical tests

Knowledge of total soluble sulphate content and pH of soils and groundwater is important in determining the protection required for concrete or steel in contact with the ground. Other specialist tests may be carried out on sites suspected of being contaminated by toxic materials.

References

• BS5930 British Standard Code of Practice for Site Investigation 1999
• BS1377 British Standard on Methods of Test for Soils for Civil Engineering Purposes 1990
• Site Investigation Practice by Michael D Joyce. E & F N Spon 1982.

Contaminated Land Investigation Procedure

General

The desk study and intrusive ground investigation is typically carried out in accordance with the requirements of BS5930 and BS10175.

In relation to contamination the desk study is referred to as the preliminary investigation in BS10175 and the intrusive ground investigation is referred to as the Exploratory Investigation This appendix briefly describes the nature of the work carried out and explains the standards against which contamination data has been assessed.

The nature of any contamination investigation is such that only a small percentage of the ground, and therefore potential contamination, is sampled. Consequently variations in both ground conditions and contaminant levels can occur between any two sampling positions.

The contamination investigation is designed to minimise such risks, but they cannot be eliminated.

Review of contamination issues

Part IIA of the Environmental Protection Act 1990 created a new regime for the identification and remediation of contaminated land. It introduced a definition of contaminated land described in Section 78A(2) of the Act of:

"any land which appears to the local authority in whose area it is situated to be in such a condition, by reason of substances in, on or under the land, that significant harm is being caused or there is a significant possibility of such harm being caused; or significant pollution of controlled waters is being caused or there is a significant possibility of such pollution being caused"

Both Part IIA and the planning regime embrace the "suitable for use" approach. In the context of Part IIA, action is necessary only where there are unacceptable risks to health or to the environment, taking into account the current use of the land and its environmental setting.

For humans, significant harm is defined as "death, disease, serious injury". Specifically, disease is taken to mean an unhealthy condition of the body or part of it. "Significant possibility of significant harm" is described as health effects arising from the intake of contaminant or other direct bodily contact with the contaminant, where the intake or exposure is unacceptable. The assessment should also take into account the total intake from all sources, the relative contribution of the pollutant linkage in question, and the duration of intake or exposure. The various statutory definitions are given overleaf.

The presence of unnatural substances does not automatically constitute a risk unless there is a link or pathway between the contamination (the hazard) and the receptor (the target) be it humans, the environment or property. Therefore the assessment needs to determine whether a hazard is present and whether the necessary pathway exists the so-called "pollution linkage".

The effect of any hazard on a site depends primarily on the site use and groundwater conditions since these determine who and what may be at risk and the routes by which they may be exposed to the hazard. Site uses can include allotments, domestic gardens on residential developments, amenity and recreational areas, public open space and industrial and commercial buildings. On any site, the potential contaminants have to be identified together with the potential receptors. The pathway for that contaminant to reach its target has then to be considered. This forms the so called 'Concept Site Model'.

Examples of pathways and effects from land contamination (after PPS 23) are given below:

Human Health

Uptake of contaminants by food plants grown in contaminated soil - Uptake will depend on concentration in soil, its chemical form, soil pH, plant species and prominence in diet.

Ingestion and inhalation - Substances may be ingested directly by young children playing on contaminated soil, by eating plants which have absorbed metals or are contaminated with soil or dust. Ingestion may also occur via contaminated water supplies. Metals, some organic materials and radioactive substances may be inhaled from dusts and soils.

Skin contact - Soil containing tars, oils and corrosive substances may cause irritation to the skin through direct contact. Some substances (e.g. phenols) may be absorbed into the body through the skin or through cuts and abrasions.

Irradiation - As well as being inhaled and absorbed through the skin, radioactive materials emitting gamma rays can cause a radiation response.

Fire and explosion - Materials such as coal, coke particles, oil, tar, pitch, rubber, plastic and domestic waste are all combustible. Both underground fires and biodegradation of organic materials may produce toxic or flammable gases. Methane and other gases may explode if allowed to accumulate in confined spaces.

Buildings

Fire and explosion - Underground fires may cause ground subsidence and cause structural damage. Accumulations of flammable gases in confined space leads to a risk of explosion. Underground fires may damage services.

Chemical attack on building materials and services - Sulphates may attack concrete structures. Acids, oils and tarry substances may accelerate corrosion of metals or attack plastics, rubber and other polymeric materials used in pipework and service conduits or as jointing seals and protective coatings to concrete and metals.

Physical - Blast-furnace and steel-making slag (and some natural materials) may expand. Degradation of fills may cause settlement and voids in buried tanks and drums may collapse as corrosion occurs or under loading.

Natural Environment

Phytotoxicity (prevention/inhibition of plant growth) - Some metals essential for plant growth at low levels are phytotoxic at higher concentrations. Methane and other gases may give rise to phytotoxic effects.

Contamination of water resources - Soil has a limited capacity to absorb, degrade or attenuate the effects of pollutants. When this is exceeded, polluting substances may enter into surface and groundwaters.

Ecotoxological effects - Contaminants in soil may affect microbial, animal and plant populations. Ecosystems or individual species on the site, in surface waters or areas affected by migration from the site may be affected.

For each of contaminant source identified, judgement has been used regarding the probability of a pollution linkage occurring and the potential consequences of that linkage. Based on the probability and likely consequences, the overall risk (significance) can be established. The probability of a hazard, combined with its consequences, can be used to assess risk. This follows the so-called "Conceptual Site Model'.

The preliminary Phase I Geoenvironmental Assessment (desk study) report

Findings are based on information obtained and described during the desk study and site inspection without intrusive ground investigation. It is possible that further information exists. The absence of indicators of impairment does not mean that such impairment does not exist. Additional investigation including intrusive methods can reduce the risks but cannot eliminate them and may not be cost effective. We can advise on the additional research opportunities, their cost and their possible impact on mitigating risk. Recommendations are normally given based on the proposed redevelopment of the site.

Intrusive investigation

BS10175 describes this as an exploratory investigation. Intrusive ground investigation is described in Standard Appendix A. During the investigation representative or indicative samples are obtained for testing by an accredited laboratory. The aim is to determine (with a degree of confidence appropriate to the objectives), the presence, concentration and distribution of contaminants.

Assessment of contamination

The assessment of contaminated land under the terms of Part II A of the Environmental Protection Act 1990 is based upon pollution linkage (source - pathway - receptor model).

The Contaminated Land Report (CLR) series of documents have been produced by the Department for Environment, Food and Rural Affairs (DEFRA) and the Environment Agency, to provide regulators with "relevant, appropriate, authoritative and scientifically based information and advice on the assessment of risk from contamination in soils".

As part of the guidance they have issued a number of Soil Guideline Values (SGVs), which whilst non-binding, may be used as guidance in assessment of land and in setting remediation targets. They should only be applied to human health assessments.

The SGVs have been derived using the Contaminated Land Exposure Assessment Model (CLEA) and are based on assumptions relating to soil conditions, pollutant type and behaviour, land use patterns and the availability of receptors. SGVs are also subject to statistical assessment. The CLR documentation requires that the results of laboratory testing are subject to statistical analysis to remove uncertainty over a so-called 'averaging area'.

The CLR documentation states that should any contaminant levels exceed the SGV, this would mean that the soil was contaminated, there would be a "significant risk of significant harm", and the site, or part of the site, should be deemed contaminated land. Alternatively further investigation would be necessary. The existing SGVs are given below.

To date only selective SGVs have been issued for the following land-uses:

• Residential with and without plant uptake
• Commerical/Industrial

The Interdepartmental Committee on the Redevelopment of Contaminated Land (ICRCL) originally set guidance levels for "contamination" based on the proposed end use of the site. The levels depend upon the degree by which the end use was affected by the "contamination" but have now been withdrawn.

Geoenvironmental Risk Assessment