INSITU TESTING

CIVL451 – Foundation Engineering Classroom Notes

Common Insitu Tests

• Standard Penetration Test

• Cone Penetration Test

• Pressuremeter Test

• Dilatometer Test

• Vane Shear test

• Plate Load Test

Introduction

• Insitu tests are carried out to aid determination of design parameters for foundation design which provide mostly indirect measurements of the design parameters.

• Published correlations are used to transform the indirect measurements to design parameters.

• A statistical approach is also required to study both spatial distribution of a measured parameter on site as well as its variability with depth / ground strata.

• It is essential that the limitations of the insitu tests and the conditions and ground type they can test is considered so as to avoid misinterpretation of the data obtained.

Standard Penetration Test (SPT)

• One of the most basic forms of testing yet quite powerful in terms of its applicability for a wide range of soils.

• It can be applied in exploratory holes and repeated at any depth required, mostly once per metre or 1.5m depth tending to be less frequent if within same geological unit.

• Applicable for most soils such as; gravels, sands, silts and clays or mixed soils.

Standard Penetration Test (SPT)

• The test method involves dropping a hammer of approximately 63.5kg onto a set of steel rods sunk into the exploratory hole from a standard drop height of 762mm.

• The first set of number of blows required for 150mm penetration of the tip of the test assembly is disregarded as the ‘seating blows’.

• The test is continued with measurement of two further sets of 150mm penetrations for which the number of blows are also measured. The total number of blows from these two latter sets of penetration (300mm) is called the SPT value (or simply N).

Standard Penetration Test (SPT)

• Test assembly;

Standard Penetration Test (SPT)

• An alternative way of carrying out the test after seating blows is to measure the number of blows required for three penetrations of 100mm, which may give more accurate results of stiff soils.

• Should the number of blows are equal to 50 and the aimed total penetration of 300mm is not achieved, the test is discontinued and the final penetration is reported. The test is called a ‘refusal’.

• The result is extrapolated for an equivalent 300mm penetration as; N x 300 / final penetration.

• One disadvantage of this method is when the ground contains large cobbles and boulders which may cause the tip of the test assembly to rebound as it penetrates into the ground leading to erroneous results.

Standard Penetration Test (SPT)

• Hammer efficiency;

• The standard practice now is to report the test results as N60, for which most of the correlations are produced.

Standard Penetration Test (SPT)

• Correlations for cohesive soil.

• Stroud and Butler (1974), Cu = f x N60 (kPa), where f is a factor varying in the range 4.5 – 6.5 with a tendency to be on the lower end of this range when PI 30.

• Hara et al. (1971), Cu = 29 x N60 (kPa)

• Szechy and Vargi (1978),

Standard Penetration Test (SPT)

• Correlations for granular soil.

• Correction of N value for overburden stress;

(N1)60 = CN x N60,

Where based on Liao and Whitman (1986)

• Kulhawy and Mayne (1990)

Pa = 100 kPa.

Standard Penetration Test (SPT)

• Hatanaka and Uchida (1996),

• Kulhawy and Mayne (1990),

Cone Penetration Test (CPT)

• Two types of cones;

– Mechanical cone.

– Electric cone.

• This test is carried out on its own from ground surface by penetrating the cone with a telescopic set of rods using a trucked or crawler mounted machine.

• The rate of penetration of the cone is constant and typically 20mm per second. This can also be reduced to a value of 10mm per second.

• Nowadays Electric Cone is the industry standard.

Cone Penetration Test (CPT)

Cone Penetration Test (CPT)

Cone Penetration Test (CPT)

• With the Electric Cone two important measurements are carried out;

– Cone tip resistance, qc,

– Cone sleeve friction, fc or fs.

• The friction ratio is calculated as;

• Fr is commonly used to help classification of the ground using correlations.

Cone Penetration Test (CPT) • Correlations for cohesive soils.

• Anagnostopoulos et al (2003), , from which the particle size corresponding to 50% smaller on the gradation curve can be backcalculated.

• Mayne and Kemper (1988), where; Nk can be assumed approximately as 15.

• Also for soil

classification;

Cone Penetration Test (CPT)

Cone Penetration Test (CPT)

• Correlations for granular soils.

• Kulhawy and Mayne (1990),

Pressuremeter Test (PMT) • This test is especially useful for obtaining lateral earth

pressure coefficient and shear modulus of soils.

• The test can be carried out in an exploratory hole as well as using a self-boring PMT rig.

Pressuremeter Test (PMT)

• The test involves sinking the test assembly into a borehole, which involves an inflatable flexible chamber.

• The flexible chamber is inflated to apply lateral pressure onto the walls of the borehole and the resulting cavity strain is measured.

• The plot of test results are then interpreted to calculate shear stiffness, G at varying shear strain levels.

• the cavity strain can also be converted to shear strain to obtain shear stiffness mobilisation (degradation) curve.

Pressuremeter Test (PMT)

• Correlations for cohesive soils.

• Baguelin et al (1978),

where,

and

Homework

Find information on the remaining three insitu tests.