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THE UNIVERSITY OF ADELAIDESCHOOL OF CIVIL AND ENVIRONMENTAL ENGINEERING

GEOTECHNICAL ENGINEERING II (C&ENVENG 2006)

M. B. Jaksa

LECTURE SERIES No. 10

SITE INVESTIGATIONS and DATA COLLECTION

References: Craig, R. F. (1997). Soil Mechanics, 6th ed., Chapman and Hall, 485p.Bowles, J. E. (1988). Foundation Analysis and Design, 4th ed., McGraw-Hill, New York, 1004p.Peck, R. B., Hanson, W. E. and Thornburn, T. H. (1974). Foundation Engineering, 2nd ed., JohnWiley & Sons, New York, 514p.Sowers, G. F. (1979). Introductory Soil Mechanics and Foundations: Geotechnical Engineering, 4thed., Collier Macmillan, New York, 621p.Standards Australia (1993). Geotechnical Site Investigations, AS 1726.Terzaghi, K. and Peck, R. B. (1967). Soil Mechanics in Engineering Practice, 2nd ed., John Wiley& Sons, New York, 729p.Whitlow, R. (1990). Basic Soil Mechanics, 2nd ed., Longman, 528p.

1. SITE INVESTIGATION

An essential part of geotechnical engineering is the site investigation where information regardingthe ground conditions of the site, both surface and subsurface, is obtained. A geotechnicalengineering design is only as good as the quality of the site investigation. Many geotechnical andstructural failures have been attributed to poor or scant site investigations. Sadly, only 0.1 to 3% ofa project's total budget is allocated to the site investigation. Money spent at this early stage canreduce or eliminate problems that may occur at a later stage. Unfortunately, ground conditions areoften taken for granted or even ignored.

The principal objectives of a site investigation are:� to determine the sequence, thickness and lateral extent of the soil strata, and where appropriate,

the level of bedrock;� to obtain representative samples of soil and/or rock for identification and classification and, if

necessary, for use in laboratory tests to determine relevant soil parameters;� to carry out any required in situ tests; and� to identify the groundwater conditions.

A site investigation is normally carried out in three phases:� Examination of existing information;� Visual inspection of the site; and� Subsurface evaluation.

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2. EXAMINATION OF EXISTING INFORMATION

Existing information for a given site could include the following:� Topographic maps: These may indicate marshes, springs and regions of past landslip.� Geological and soil maps: These give information on rock and soil types, geological features

such as faults and major boundaries.� Aerial photographs: These supplement topographic maps.� Existing site investigation data: The site may have been investigated or mined previously.

The data, however, may be unreliable.� Regional and local site investigation data: Other locations near or adjacent to the site, may

have been previously investigated. The data from these investigations may give valuableinsights into the ground conditions of the site in question.

3. VISUAL INSPECTION OF THE SITE

A visual inspection of the site and adjacent areas will reveal much valuable information. A visualinspection of the site often includes:� Topography: The location of the site in relation to topographic features, such as slopes and

valleys.� Drainage pattern: The location of the site in relation to existing drainage features, such as,

rivers, creeks, marshes, and springs.� Vegetation: The location, size and type of vegetation often reflect the underground conditions.� Outcrops and cuttings: Natural outcrops of rock, as well as cuttings exposed by creeks and

road and railway cuttings give enormous information regarding the physical and structuralproperties of the subsurface soil and rock, as well as stratigraphic information.

� Land use: The past and present use of the land. For example, whether the site was previouslyexcavated, quarried or built on.

� Adjacent structures and pavements: Inspection of adjacent structures and pavements reflectthe ground conditions.

� Fill: It is extremely important to ascertain whether the site has been previously filled, or used asa dump.

� Access: It is important to determine whether the equipment that will needed for the subsurfaceevaluation phase of the investigation, will have access to the site, both physically and legally.Will the terrain enable the equipment to get to the site?

� Local knowledge: Information obtained from locals who have lived in the area for some time,can provide data that can be of great value. For example, locals can provide informationregarding underground features such as cellars, mine shafts and pugholes; past land use andflooding history.

� Other: Erosion patterns, existing and past land use, also provide valuable information about thesite.

4. SUBSURFACE EVALUATION

The subsurface conditions of a site can be determined in a number of ways. These include:� Boring;� Excavation; and� Geophysical techniques.

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4.1 Boring

The subsurface conditions of a site are, in the majority of cases, evaluated by a series of boreholes.Borehole drilling, or boring, can be carried out using a number of techniques. These include:augering - hand, solid-flight and hollow-flight (Fig. 4.1(a),(b),(c)); wash-boring (Fig 4.1(d)),percussion; coring with a diamond or tungsten bit(Fig 4.1(e)); and static-push.

Boring enables the geotechnical engineer to determine the stratigraphy of the site, take samples ofsoil and rock for identification and testing, and to determine the groundwater conditions.

4.2 Excavation

For shallow depths, usually less than 2 to 3 metres, the subsurface conditions can be evaluated bymeans of excavating a test pit. This is usually done by the use of a back-hoe or an excavator. Thetype and thickness of the soil layers, as well as structural features, can be determined by inspectingthe walls of the pit.

4.3 Geophysical Techniques

Boring and excavation techniques can often be expensive, especially where large site investigationsare concerned. These techniques are prohibitive for investigations of pipelines and roadways, forexample. In these situations, geophysical techniques are often used. Examples of geophysicaltechniques include: refraction seismic; electrical resistivity; and gamma radiation logging downboreholes. These methods are well explained in Craig and Sowers.

4.4 Sampling

There are two types of sample: undisturbed and disturbed. As we saw in the first lecture series (§7)if a clay sample is disturbed much of its structure is destroyed and, as a result, tests carried out on adisturbed clay sample can be meaningless. Thus, when samples are required for laboratory testingof fine-grained soils, these samples must be undisturbed. This can also be true for tests carried outon coarse-grained soils, however, if the in situ void ratio is known, a coarse-grained soil can bereconstituted in the laboratory from a disturbed sample.

4.4.1 Undisturbed samples

An undisturbed sample is usually obtained from a borehole by means of pushing a steel tube into thebase of the hole (Fig. 4.2(a)). The tube is then sealed so that no moisture leaves the sample. Anundisturbed sample can also be obtained from a pit by carefully excavating a block of soil andsealing it in a box or can, as shown in Figure 4.2(b).

4.4.2 Disturbed samples

These are usually obtained from an auger or excavator, and placed in sealed plastic bags so that thein situ moisture content of the soil can be determined in the laboratory.

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(a)

(b) (c)

(d)

(e)

Figure 4.1 Examples of borehole drilling techniques (a) hand-auger, (b) solid-flight auger, (c)hollow-flight auger, (d) wash-boring, (e) coring.

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Figure 4.2 Undisturbed sampling.

4.5 Borelog

Once a site investigation has been completed, the results are summarised in the form of a boreholelog, or borelog. Usually a borelog has information regarding the client name, location of borehole,date of drilling, driller and loggers names, type of drilling method, reduced level of the groundsurface of the borehole, description and depths of soils and rocks encountered, groundwaterinformation, depth of borehole, depths and types of samples taken and additional remarks. Anexample of a borelog is shown in Figure 4.3.

5. IN SITU TESTING

Many geotechnical parameters can be determined in the field and on-site. It is often desirable toobtain geotechnical data in this way, because of errors associated with sampling and laboratorytesting. In the next sections, we will examine some of the more common in situ tests.

5.1 Standard Penetration Test (SPT)

Traditionally, this test has been, and is currently, used widely throughout the world. It consists of asplit-spoon sampler, as shown in Figure 5(a), which is placed at the base of a borehole. A 65 kghammer then freely falls a height of 760 mm and drives the sampler into the soil. Initially thesampler is driven 150 mm into the soil to seat the sampler and to by-pass any disturbed material atthe base of the hole. The sampler is then driven a further 300 mm and the number of blows requiredto achieve this is called the standard penetration resistance, N. The method for this test is outlinedin AS 1289.6.3.1.

The standard penetration resistance has been correlated with many geotechnical parametersincluding, �, su, RD (relative density), and E. The test is used in fine-grained soils, but it is normallyof most use in coarse-grained soils, particularly sands. It has the added advantage over other in situtests, in that a sample of the tested soil can be inspected.

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Figure 4.3 Example of a borelog.

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Figure 5.1 The standard penetration test.

Whilst it is widely used, the test has many limitations. The greatest disadvantage of the SPT is thelack of reproducibility of the test results.

5.2 Cone Penetration Test (CPT)

This test consists of driving a cone of standard dimensions (Fig. 5.2) which is fitted with two loadcells with associated strain gauges, one measuring the cone tip resistance, qc, and the othermeasuring the sleeve friction, fs . The advantage of this test is that it enables a continuous profile tobe obtained, as shown in Figure 5.3.

The cone tip resistance has been correlated with su, �, RD and E and the ratio of fs to qc , known asthe friction ratio, FR , can be used to give an indication of the soil profile, as shown in Figure 5.4.

The method for this test is outlined in AS 1289.6.5.1 and it gives good reproducibility of test results.

5.3 Screw Plate Load Test

The screw plate load test involves screwing a helical screw plate (Fig. 5.5(a)) into a pre-augeredborehole (Fig. 5.5(b)), applying increments of vertical load and recording the load-deflectionbehaviour of the soil. In this way, the Young’s modulus, drained and undrained, and cu can bedetermined.

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Figure 5.2 The electric cone penetrometer. Figure 5.3 Typical results from the CPT.

Figure 5.4 Profile interpretation from the CPT.

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Figure 5.5 Screw plate load test.

5.4 Self-Boring Pressuremeter Test

The self-boring pressuremeter test involves inserting the pressuremeter device into a pre-drilledborehole. The pressuremeter is then advanced into the hole to the required test depth by means ofthe self-boring pressuremeter's cutting tools. In this way, the pressuremeter is located at the testdepth with minimal soil disturbance. The cylindrical pressuremeter membrane is then inflated andthe pressure in the membrane is recorded. This test enables �, su, Eu and K0 to be determined. It is arelatively slow and expensive test.

5.5 Dilatometer Test

The flat-plate dilatometer test, shown in Figure 5.7, consists of driving the dilatometer to therequired test depth and, as for the self-boring pressuremeter test, applying pressure to themembrane, in this case, a flat plate. The applied pressure enables �, su, Eu and K0 to be determinedas well as the soil type.

5.6 Vane Shear Test

The vane shear test is generally used to estimate the in situ undrained shear strength of very soft,fine-grained soil deposits. The test involves the insertion of the vane, Figure 5.8, and applying atorque to failure. It can be shown that the undrained shear strength, cu, is related to the torque atfailure, T, by Equation (5.1).

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���

����

���

62

32 dhdcT u (5.1)

where d and h are the diameter and height of the vane respectively.

Figure 5.6 Self-boring pressuremeter test.

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Figure 5.7 Flat-plate dilatometer test.

Figure 5.8 Vane shear test.

Geotech2_LS10_Site Investigation.doc�2002, M. B. Jaksa