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Introduction

Site Investigation is the collection of information about the proposed site for a project, such as a building, or a highway.

The purpose of a site investigation is the gathering of sufficient reliable subsurface information so that the most safe and cost-efficient design for the proposed structure can be determined. The site investigation should reveal sufficient subsurface information for the foundation to be stable, safe from both collapse and excessive movements (e.g. settlement, sliding, overturning of the structure, etc.).

A site investigation typically focuses on:

• the topography of the site and the surrounding area,
• the soil profile in the site, and
• the groundwater conditions (level of groundwater table, annual variations, etc.).

The four stages of a site investigation

Typically, a site investigation program comprises the following four stages:

1. desk study and site reconnaissance,
2. preliminary ground investigation,
3. detailed ground investigation, and
4. monitoring

Desk Study and Site Reconnaissance

The first stage of the site investigation process is the desk study. A desk study involves researching the site to gain as much information as possible, both geological and historical.

A good initial source of information are the Ordinance Survey maps, which can aid in the selection of the site by providing accurate grid reference over the maps. Old maps are available, apart from recent ones, to provide historical information, such as previous uses of the site, infilled ponds, old pits, concealed mine workings, disused quarries, changes in potential landslide areas, etc.

Similarly, geological information can be found at the website of the British Geological Survey. The website offers free access to available boreholes all over the UK. Also, comprehensive maps show the superficial and bedrock geology of the UK, allowing for a better understanding and prediction of the geological conditions in the selected site.

Sources of Information for a Desk Study

Stages

Stage 1: Desk study and reconnaissance

Typically, the desk study and reconnaissance aim to assess the feasibility of the planned project. Important information is gathered on site layout, surface and subsurface conditions, climate, potential hazards, etc. If the desk study shows that the site is appropriate for the structure, then preliminary investigation should follow.

Stage 2: Preliminary Investigation

The purpose of a preliminary investigation is to predict the geological conditions, soil profiles and the position of ground water table by geophysical methods or collecting results from a number of boreholes.

The preliminary investigation should give information on the ground conditions that require closer examination, such as the extent of disturbed strata, the location and extent of natural cavities and mine workings, fractures, river crossings, alluvial areas that may have buried soft material or peat and their potential to cause settlements, mass movements or instability of the over-structure. Also, information that is used in the design phase is collected such as indicators of the suitability of soil, ground water conditions, rainfall statistics, the probability of flooding events, etc.

Stage 3: Detailed Investigation

At this stage, further detailed testing takes place. Various in-situ and laboratory tests need to be conducted so that more reliable information is gathered. An additional number of boreholes might be required and their spacing depends on the findings of the preliminary investigation.

Soil exploration comprises the following three steps:

1. boring and in-situ testing,
2. sampling, and
3. laboratory testing.

Stage 4: Monitoring

Once the detailed investigation is finalised, the required information for the design is available. However, as the site conditions can change seasonally, or due to unpredicted extreme events, site monitoring is required not only throughout the realisation of the construction phase, but also during the maintenance period. Field observations allow for early diagnosis of problems that might be encountered during construction and their remediation. Measurements on settlement, displacement, deformation, inclination, and pore water pressure are conducted during the monitoring stage.

Boring

The most common method of subsurface exploration in the field is the collection of information through boreholes (typically referred to as ‘boring’). Boring is the procedure of collection of soil samples from a vertical hole in the ground. Depending on the boring method used, the samples can provide information on the soil composition, the thickness of each stratum, the existence of weak layers, etc.

Typical boring methods used are:

• hand/mechanical auger borings,
• wash borings,
• percussion drilling,
• rotary drilling, and
• core borings.

Hand/Mechanical Auger

An auger is a screw-like tool which is used to bore a hole. Augers can be operated by hand, or might be power operated. Hand augers are typically used for boreholes to a depth of about 6m. Power augers may be used for depth of 10m to 30m. Power auger set with a drill rig can be used to obtain samples from deeper strata. Some rigs can be used to drill a hole to 100 m depth. During the drilling procedure, the auger can collect soil samples. So, at various intervals, it may be lifted to remove the soil which can be used for field classification and laboratory testing. If augers are used, the sample can not be considered as an undisturbed soil sample.

Wash Boring

Wash boring is the procedure of simultaneous drilling and jetting action. A hole is bored through a casing using a drilling bit, while at the same time water is pumped downwards through the drilling bit in order to soften the soil. Samples taken using the wash boring method are disturbed samples.

Percussion Drilling

Percussion drilling is the process of boring by striking the soil and then removing it. The tools are repeatedly dropped down the borehole, while suspended by a wire from the power winch. Water is circulated to bring the soil cuttings to the ground surface.

Rotary Drilling

Rotary drilling uses rotation of a drill bit with the simultaneous application of pressure to advance through the hole. This method can be applied on soils and rocks and is the fastest one. Sample obtained from this method is less disturbed than samples obtained by the other methods.

Figure 1.1. (a) Construction drill auger, (b) skid-steer loader with an earth auger attachment, (c) multi-combination drilling rig

Figure 1.2. Schematic of an oil-drilling rig (with legend).

Boring Logs

Boring logs contain the information on a single borehole. Information such as the location of the borehole, the method used, and the findings of the method can be found.

Figure 1.3. Sample borehole (click to enlarge)

Sampling

Sampling refers to the procedure of collection of soil samples from boreholes. The sampling procedures vary depending on to the type of strata in which the investigation takes place. The sampling program should be consistent with the required accuracy of design and the scale of the structure.

There are two types of samples:

1. Disturbed samples: these samples are typically used for index properties of soil. Disturbed samples can be obtained from auger boring, core boring, split spoon sampler in standard penetration test (pit and trench, and some types of sampler such as thick-walled sampler, displacement sampler, and Beggemann sampler.

2. Undisturbed samples: these samples are needed to determine the engineering properties of soil, such as shear strength, dry density and consolidation coefficient, etc. Undisturbed samples are generally required during a detail subsurface exploration to provide specimens for laboratory testing.

Undisturbed samples are normally needed for clays every 1.5m or change of stratum. If undisturbed samples cannot be retrieved at a specific depth, then bulk samples should be taken. Undisturbed samples cannot be collected for sand and gravel, due to the lack of cohesion. Bulk samples need to be taken every 1m or every change of stratum, while alternate disturbed and undisturbed samples should be taken for silt layers at 0.75m intervals. Undisturbed samples can be collected from soft rocks, such as chalks and marls. Table 1.1 gives a guidance on the type of soil needed for typical geotechnical tests.

Table 1.1. Sample type selection for different types of laboratory tests.

Notes: This table assumes 10kg of sample obtained per bulk bag, therefore those highlighted in red require more than 1 bulk bag (e.g. 35kg).

The laboratory definitions of ’fine’ and ‘coarse’ soil differ from those used for engineering soil descriptions.

• Fine grained soil = not more than 10% >2 mm (includes clay, silt and sand)
• Medium grained soil = some >2 mm, not more than 10% >20mm (includes fine and medium gravel)
• Coarse grained soil = some >20 mm, not more than 10% >37.5% includes coarse gravel)

Soil types: C = CLAY/SILT, S =SAND, G = GRAVEL

Sample types: U = Undistirbed, B = Bulk distributed, D = Disturbed, M = samples in test specific moulds

Undisturbed Samples

The most common method of obtaining undisturbed samples is through a thin tube. The tube is mechanically driven into the soil and the part of the soil that was under the opening of the tube is trapped inside it. When the tube is removed from the ground, it the soil sample comes out with it virtually intact.

To ensure the quality of the sample, specific actions are required after its collection.

1. Immediately after the tube is brought to the surface, containing the sample, the ends of the tube need to be sealed with paraffin wax.
2. After sealing the tube, the following data should be attached to the sampling tube:
   1. project name,
   2. name of drilling operator,
   3. date of the sampling,
   4. borehole number and sample number, and
   5. depth of sample.
3. During the shipment and storage of the sample, attention needs to be paid so that the conditions do not become as such that would cause disturbance of the sample.
4. The sample needs to be stored in a room with conditions similar to those monitored in the field during its retrieval.
5. Visual inspection is required to ensure that:
   1. there has been no visible distortion of the strata in the sample,
   2. there is no opening or softening of the material,
   3. the specific recovery ratio ( $SSR=\frac{\text{Length of undisturbed sample}}{\text{Length of the tube}}$) is not less than 95%, and
   4. the area ratio ($A_r=\frac{D_{\text{ext}}^2-D_{\text{int}}^2}{D_{\text{int}}^2}\times 100\text{\%}$ , where, D_ext = external diameter and D_int = internal diameter) is not less than 15%.

In-situ Testing

Sometimes, it is preferable to conduct the test on site, instead of taking samples to the laboratory. This might be due to time constraints (e.g. concrete hardening), economic reasons (more expensive test procedure or excessive shipment and storage cost), or reliability of the tests (site conditions cannot be adequately replicated in the laboratory). For those reasons, part of the sampling program (or even in its total) might be planned to take place on the site (in situ).

There are various in-situ tests that are often conducted, such as the standard penetration test (SPT), the cone penetration test (CPT), the vane shear test (VST), the pressure meter test (PMT), and the flat dilatometer test (DMT).

Presently, most in-situ tests are standardised in the BS EN ISO 22476 series, while the previous standard, i.e. BS 1377-9:1990, is also partially current, since it covers some of the tests not included in the BS EN ISO 22476 series.

All aforementioned in-situ tests are described in the following sections.

Standard Penetration Test (SPT)

The standard penetration test (SPT) is a dynamic test aiming to define the type of soil at different locations on site so that a preliminary assessment can be performed. The test procedure is standardized in BS EN ISO 22476-3:2005+A1-2011.

A sampler of about 650mm length, 50mm external diameter, and 35mm internal diameter is inserted 150mm in the ground. Then, it is driven through the soil by repeated blows from a dropped hammer which weights 3.5kg and falls from a height of 765mm. This is done in 150mm increments, for at least 2 increments. The total number of blows needed for 300mm penetration is used.

The blow count (N) may be corrected by field conditions such as, energy losses due to frictional and other parasitic effects, variations in the test apparatus (length or diameter of the rods), the effect of the overburden, etc. (see BS EN ISO 22476-3:2005+A1-2011, Appendix A)

Cone Penetration Test (CPT)

The Cone Penetration Test (CPT) is a test used to define the properties of the soil on site and allow the mapping of the different strata. It is carried out by mechanically or hydraulically pushing a cone into the ground at a constant speed (20mm/sec), while measuring the tip resistance and friction. The parameters obtained from cone penetration test can be correlated with relative density, soil classification, and unconfined compression strength, sensitivity of clay, degree of over-consolidation, pile design parameter, bearing capacity and settlement. Other correlations relate the results of cone penetration test with the N value from Standard penetration test. The British standard for the Cone Penetration Test is BS EN ISO 22476-1:2012.

Vane Shear Test (VST)

The Vane Shear Test is an in-situ test commonly used to measure the shear strength and sensitivity of clays. It is conducted using a four-bladed rectangular vane, a rotating rod, and measuring device. It might be carried out either in a borehole or directly pushing the vane into the ground. The vane rod is rotated at a rate of 60RPM, while the torque is read at 30 second intervals. Once the maximum torque is reached, the vane is rotated at a higher rate to obtain the remolded strength of the soils. The vane shear test is usually conducted to estimate the sensitivity of a cohesive soil by repeating the test at the same point after remolding the sample by fully rotating the blade. The first maximum torque recorded represents the peak strength, while the second maximum torque represents the residual strength of the soil. Most clays have a sensitivity between 2 and 4. For ‘sensitive clays’ it ranges from 4 to 8. ‘Extra-sensitive clays’ have a sensitivity from 8 to 16, while clays with higher sensitivity are characterised as ‘quick clays’. The Vane Shear Test is standardised in BS 1377-9:1990.

Pressuremeter Test (PT)

The pressuremeter test is carried out to estimate the soil type and to measure its undrained shear strength (c_u), modulus of horizontal sub-grade reaction (E_m), and in-situ horizontal stress in the ground (s_ho). It requires a probe, a measuring unit, and a cable. The test takes place in a borehole, driving the probe into the ground and loading it horizontally until it reaches the limit pressure or the capacity of the device. There are three types of pressuremeters i.e. borehole pressuremeter (also referred to as ‘Ménard pressuremeter test’ in the literature), self-boring pressuremeter, and the full displacement pressure-meter. The type of soil, the rate of expansion, membrane stiffness and system compliance, and size of drilling hole can affect the results of the pressure-meter test. Pressure may also be corrected for the resistance of the probe with the pressure volumeter, and hydrostatic effects. The British standards for the pressuremeter test are:

• BS EN ISO 22476-4:2012 for the Ménard pressuremeter test
• BS EN ISO 22476-6:2018 for the self-boring pressuremeter test
• BS EN ISO 22476-8:2018 for the full displacement pressuremeter test

Flat Dilatometer Test

The flat dilatometer test is similar to the pressuremeter test, but the measurements take place through a blade-shaped steel probe with a thin expandable circular steel membrane mounted on one side of the blade. It is mainly applicable to clays, silts and sands, where particles are small compared to the size of the membrane The test is carried out by pushing or hammering a dilatometer blade into the soil at rate between 10 – 30mm/sec. The penetration resistance is measured, and gas pressure is used to expand the membrane approximately 1.1 mm into the soil. The results collected by the flat dilatometer tests are used to obtain information on soil stratigraphy, in-situ state of stress, deformation properties and shear strength. It is also used to detect slip surfaces in clays. The result can be affected by disturbance due to blade insertion, blade thickness, membrane stiffness and thickness, and the soil type. The flat dilatometer test is standardized in BS EN ISO 22476-11:2017.

Groundwater Observations

During site investigation it is important to collect information on groundwater. Groundwater can significantly affect the properties of the soil and, consequently, the performance of the structure, as the shear strength of a soil may be reduced below the groundwater table. Also, if the groundwater table rises seasonally, it can cause uplift of the foundations. In cases of groundwater table very close to the surface, dewatering might be required before the structure is constructed, as typically dry conditions are sought.

The location of groundwater table is typically determined by measuring the depth of water surface in a borehole. This takes place after a time lapse to allow for the water from to seep through the walls of the borehole. The amount of time needed to measure the depth of the groundwater table depends on the permeability of the soils. In granular soil with high permeability, such as sand and gravel, 24 hours is usually adequate for the groundwater level to stabilize. For silts and clays, which have low permeability, it might be several days before the groundwater level settles. The particular measurement can be made with a Tell Tale. However, if additional information is needed, different equipment might be required. For example, if the water pressure in a stratum is needed, then a piezometer is needed. Also, samples might be collected for chemical analyses, as some chemicals may attack the materials.

Groundwater sampling is standardized in the BS ISO 5667 series. Guidance on the design and installation of groundwater monitoring points is provided in BS ISO 5667-22:2010.

Laboratory Testing

While in-situ testing can provide valuable information, there is information that needs to be retrieved by laboratory testing. Disturbed samples are used to determine soil properties such as unit weight, specific gravity, dry density, particle size distribution, Atterberg limit, etc. Undisturbed samples are needed for more sophisticated tests such as the direct shear or shear box test, triaxial test, consolidation test, etc. Table 1.2 provides a list of different laboratory tests on soils and the relative British Standards.

Table 1.2. British Standards covering different laboratory tests on soils

Soil Exploration Report

At the end of a soil exploration program, a soil investigation report needs to be compiled, where the results of the in-situ and laboratory tests and their post-processing will be presented. This report is very important, as it is used to make decisions on the feasibility of the project, the most cost-efficient design approach and the analytical design, the construction methods, health and safety considerations, etc.

The typical layout of a soil investigation report is as follows:

1. Introduction

The introduction provides information on the project, its location, the main objective of the investigation conducted, the client, the investigator, etc.

2. Scope and Objectives

This section outlined the objectives of the geotechnical investigations. As the report can be used as a legal document, the investigator needs to clearly state any concerns related to the reliability of the results presented in the report, due to limited number of tests, or poor quality of geotechnical investigations.

3. Details of the Proposed Structure

Here, the proposed structure is described. The structure’s nature, its intended use and the location of the proposed structure in a site plan are highlighted.

4. Site Conditions

This section presents a brief description of the geomorphology of the area. Also, it provides information about the land use, historical details of the site, the existence of old structure(s), as well as any other noteworthy observations or findings during field visit(s).

5. Field Investigation

In this section, details of the field investigations are given. Detailed information on the date of execution, the investigator(s), the exploratory methods adopted, the types of tests conducted, specific observations of site conditions, the layout of test pits and borings, the type of samples collected is provided here. Specific comments and observations on the overall performance of the investigation are also made.

6. Laboratory Test Results

This section presents the results of the laboratory tests conducted. Information on the date of sampling, the method(s) of and conditions during sampling, the tools used, the method of transportation and preservation of the soil samples, the tests conducted, and the results yielded is included here. All test results are resented, typically in a tabular form. The results in this section are used for the design of the structure, the assessment of the soil, etc.

7. Soil Profile

Based on the findings of the in-situ and laboratory testing, the soil profile is defined. The soil strata and their thickness at each borehole are illustrated. The depth to the top of soft and hard rock should be reported in this section, along with the degree of weathering, as it can affect the decisions on the suitability of the selected location on the site, type and sizing of the foundation scheme, possible ground improvements required.

8. Allowable Bearing Capacity

Using the laboratory test results, the safe bearing capacity of the soil at various depths should be calculated and highlighted. This is used in the geotechnical design, can affect structural design and the assessment of structural performance.

9. Analysis and Interpretation of Results:

The results obtained from the field investigations, laboratory testing, and the safe bearing capacity calculations are shown in this section. The soil classification, its characteristics and their variation over the site are conclusively described. The anticipated design and construction problems are highlighted.

10. Assessment of Foundation Alternatives

The aforementioned information is used to design and appraise suitable foundation alternatives for the structure. Expected settlements of the structure under different foundation alternatives should also be calculated and presented. Possible problems that may be encountered during construction are pointed out. If more than one alternatives are deemed suitable for the structure, the individual merits and shortcomings are compared to allow the client to make the final decision.

11. Recommendations

In this section, all previous findings are used to make recommendations on:

• the most appropriate type of foundation and its characteristics,
• the soil characteristics to be used in the design of the structure,
• proposed ground improvement techniques,
• potential uses of the excavated soil,
• proposed schedule for construction of the foundation and the rest of the structure,
• locations that special attention is needed due to groundwater table being close to the surface, and
• anticipated problems and hazards during construction and mitigation measures for each one.

12. Limitations and Uncertainties of Soil Exploration:

In this section, the investigator specifies clearly the limitations and uncertainties of the soil exploration. Recommendations are made on further investigation required to improve the reliability of the results.

13. Annexes

All the data related to the geotechnical investigation cited in the report as well as the supporting calculations, maps, drawings, and photographs are enclosed in the annexes in a logical sequential order, with clear reference number as cited in the report.