Southwest Environmental Limited

Geotechnical Site Investigation (GSI)

You may have need for geotechnical ground data, by using traditional sampling methods, such as trial pitting, shell and auger drilling in softer soils, and rotary / pedant systems in hard soils / rock, Southwest Environmental can provide the data that you need.

In depth: Subsurface Geotechnical Investigations or Surface Geotechnical Investigations

Reporting

Reporting can be factual, or interpretative, depending on you requirements.

Factual Geotechnical Report

A factual geotechnical investigation report will include un-interpreted data from field measurements, and borehole / trial pit logs. A factual report is normally sufficient where you might pass on the report to an architect who will undertake foundation design themselves. It may also be suitable for passing on to piling contractors.

Interpretive Geotechnical Report

An interpretive geotechnical report will contain the above information and also recommendations on foundation design. Some client might specify a foundation type when commissioning, for example on contaminated land the client may specify piled foundations so as to arise disposal costs for strip foundation arising's.

An interpretive geotechnical report will included geotechnical engineering recommendations, for example for piled foundations it will specify pile diameters, pile lengths, likely bearing capacities.

Samples Taken During Geotechnical Investigation

Specification & Standards used when conducting a Geotechnical Investigation

Thomas Telford published a "Specification for Ground Investigation" in 1993, which is very thorough.

BS3950 is the standard for site investigation issued by British Standards.

Testing

Testing is carried out by a third party UCAS accredited laboratory.

Geotechnical reports can be Factual or Interpretive.

Please feel free to telephone the office for an informal discussion regarding any of the services we provide. Detailed written quotations can be provided upon request. Read About an Example Phase 2 Investigation

What is a Surface Geotechnical Investigation?

They are typically utilized in larger scale projects such as damn or road building.

Surface investigations consist of surface mappings. Surface mapping is generally initiated with the help of aerial photographs or satellite imageries. Aerial photographs and satellite imageries are very useful tools in the hands of a geologist since they give a bird's eye view of a vast project area on a small photograph.

With their help, geologist can map lithology and the structure of outcrop in considerable details. Sedimentary, igneous and metamorphic rocks can often be recognized by aerial photographs. Similarly the structure of the rocks, faults, joints, fractures, Folds, etc., are also frequently recognized. When detailed surface geological maps are required, the geological mapping is done on contour maps with the help of a compass or by telescopic alidade.

Surface geological mapping provides the stratigraphic, structural geological data of the area. The stratigraphic study gives the idea of the age of the various rock units, together with succession and its detailed lithological characteristics. The details of the lithology are picked up and studied in the field and samples of the rock are collected for laboratory testing. Along with lithological and stratigraphic mapping, geological structures are also picked up and plotted on a contour map to prepare a geological map. Structural mapping generally includes the mapping of the attitude of the beds, joints, folds, faults, shear zones and cleavage planes etc? The weathering and the slope inclinations are marked on the surface geological map. The surface geological map provides the basic data of the lithology and structure, with the help of which the geological cross sections can be drawn

Works related to surface investigations may include field trips reconnaissance study of aerial photograph topographic maps, contour maps, visual study of geological outcrops, exposed vertical faces of the rock, identification of rock units, types over burden material, location of borrow areas etc. etc. Where necessary, a few auger holes may be drill and a few pits and trenches over the rock slopes may be dug. Exposed rocks faces may be cleared for bet inspection of the rock. A few rock and over burden samples may be collected for rock testing.

The instruments used in preparing surface maps are a geologist hammer, a telescopic alidade, compass an other minor instruments.

Sub Surface Geotechnical Site Investigations

On the basis of this investigation, local sections of the sites are drawn. Sub-surface investigations include investigations on rocks ,sub-soils. These investigations are described below.

Geotechnical Investigations in Soil

The purpose of sub-surface investigations is to determine the extent of vertical and areal variations of soil strata, in the region in which the infrastructure or the buildings are expected to be located. Where the material for the construction of the embankment is not available from the excavation of the canal prism, the sub- surface investigations help in the selection of borrow areas and in the estimation of the quantity of the suitable material which can be borrowed from their.

Furthermore, by collecting representative Samples, the engineering properties of soil under various service conditions are determined both in the field and in the laboratory, before adequate and economical designs for the dam and canals can be developed.

Where uniform soil conditions exist or where sufficient data about soils is available, only minor exploration may be needed. In variable soil conditions, extensive soil investigations will be required. This may involve drilling auger holes, excavating test pits and collecting representative samples for both field as well as laboratory testing. The soil profiles, which are obtained by the preliminary exploration, are Oude but the information is sufficient to prepare preliminary designs and estimates. However, for the preparation of specifications and designs, detailed exploration will be required.

Geotechnical Investigations in Rock

In the light of information obtained from the surface geological mapping, preliminary geological sections are drawn and sub-surface explorations planned accordingly. Sub-surface explorations generally comprise excavating pits, trenches, adits and shafts, the digging of a few auger holes together with drilling deep bore holes with the help of percussion and rotary drills.

These explorations are at times supplemented by geophysical surveys to get a better idea of the sub-surface geology. The quantum and the extent of these sub-surface explorations depend on the nature, size of the structure, and the geology of the project area. The sub-surface explorations provide better information about the stratigraphy, geologic structure and physical and chemical properties of various rock units and surface deposits exposed at the project site. It also provides information about the condition of the foundation rock and its physical and chemical characteristics.

Some of the properties of the foundation rocks, such as permeability, seepage characteristics and bearing capacity etc., can be determined by analyzing the sub-surface exploration data. Cores and samples obtained from drill- ales, pits, adits and shafts, etc, are obtained and tested for various physical and chemical properties the laboratory to get better information about the foundation conditions. The information of the surface exploration and laboratory testing provides useful data to the designer for developing a safe and economical design for the project features.

Thomas Telford: Specification for Ground Investigation

Below are some short extracts from the Thomas Telford specification, the specification offers practical advice and targets, some of which may be a little optimistic . . .

With regards to Core Recovery (recovery is usual entirely dependent of rock type):

"Rotary core drilling shall produce cores of not less than the required diameter (Schedule 1) throughout the core length, and 100% core recovery in any single run should normally be obtained. Core recovery less than 90%in any drill run will not normally be acceptable unless the Engineer is satisfied that more than 90% recovery is impracticable under the prevailing conditions. If in the opinion of the Engineer more than 90% recovery can be achieved, the Contractor, after consultation with the Engineer, shall take measures to improve the core recovery."

With regards to Core Labeling (this is good advice the inexperienced operator can get a muddle if labeling is not done well):

1. All operations entailed in recovering the cores from the ground after labeling of liners completion of drilling shall be carried out in a manner such as to minimize disturbance to the cores.
2. Core barrels shall be held horizontally while the innermost liner containing the core is removed without vibration and in a manner to prevent disturbance to the core. The core should be rigidly supported at all times while it is being extruded and during subsequent handling, and the liner containing the core must not be allowed to flex.
3. Immediately after removing the liner the top and bottom shall be marked in indelible ink. The ends of liners shall be capped and sealed using adhesive tape. Liners shall be cut to the length of the enclosed core.
4. Where the length of core recovered from any single core run is such that it cannot be accommodated in one channel of the core box, the liner shall be cut to coincide, if possible, with existing fractures. The liner either side of the cut shall be marked 'cut' and the ends capped as above.

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Geotechnical Site Investigations

Bristol - 01173 270 092

Exeter - 01398 331 258

London - 02076 920 670