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 designparameters for foundation design which provide mostlyindirect measurements of the design parameters.

• Published correlations are used to transform the indirectmeasurements to design parameters.

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

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

Standard Penetration Test (SPT)

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

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

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

Standard Penetration Test (SPT)

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

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

• The test is continued with measurement of two further sets of150mm penetrations for which the number of blows are alsomeasured. The total number of blows from these two lattersets of penetration (300mm) is called the SPT value (or simplyN).

Standard Penetration Test (SPT)

• Test assembly;

Standard Penetration Test (SPT)

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

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

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

• One disadvantage of this method is when the ground containslarge cobbles and boulders which may cause the tip of the testassembly to rebound as it penetrates into the ground leading toerroneous results.

Standard Penetration Test (SPT)

• Hammer efficiency;

• The standard practice now is toreport the test results as N60, forwhich most of the correlationsare 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 = 2.9 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 bypenetrating the cone with a telescopic set of rods using atrucked or crawler mounted machine.

• The rate of penetration of the cone is constant andtypically 20mm per second. This can also be reduced to avalue 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),

Typical results from a CPT test, including pore water pressure

measurements

An example of interpretation of results for ground classification and

engineering parameters

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 ontothe walls of the borehole and the resulting cavity strain ismeasured.

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

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

Pressuremeter Test (PMT)

• Correlations for cohesive soils.

• Baguelin et al (1978),

where,

and

Plate Load Test (PLT)

Plate Load Test (PLT)Modulus of subgrade reaction, subgrade modulus, k

An example of effect of foundation size to settlement.

Homework

Find information on the remaining insitu tests.