CIVL2410 Chap.15 Site investigation and in situ testing
Site investigation and in situ testing
Every parameter this unit has used arrived from a site investigation, and the quality of any prediction is capped by the quality of that investigation. This chapter covers the three phases of an investigation, what a borehole delivers, why sample quality decides which tests a specimen may be asked for, the two standard in situ penetration tests, and the geophysical methods used to decide where the expensive holes should go.
What this chapter covers
- 01
Desktop, preliminary and detailed phases, and why the order matters
- 02
The four purposes of a borehole, and how they trade against each other
- 03
Disturbed and undisturbed samples, and what each may legitimately be used for
- 04
Why no sample is truly undisturbed, and what that does to a measured strength
- 05
The standard penetration test: hammer, drop, sampler and the three intervals
- 06
Why the seating drive is discarded, and how the N value is formed
- 07
Refusal criteria, and why refusal is a result rather than a failure
- 08
N value against sand density and clay consistency
- 09
Empirical stiffness correlations, and the caution the unit attaches to them
- 10
The cone penetration test, its standard dimensions and its three recorded parameters
- 11
Soil behaviour type against grain size, and why the two are not the same
- 12
Seismic, resistivity and radar surveys, and how they guide drilling
- 13
Remote sensing, from digital elevation models to millimetre scale monitoring
An N value and what may be built on it
- +1The first interval is the seating drive through material disturbed by drilling, so its blows are discarded and N = 7 + 11 = 18.
- +1An N value of 18 falls in the 10 to 30 band, so the sand is of medium density.
- +1The sand correlation gives a drained modulus of roughly 2600 to 2900 times N, that is about 47 to 52 MPa.
- +1Report it as an order of magnitude check rather than a design value, because the correlation is empirical, the scatter is large and the delivered hammer energy is unknown.
Key terms
- Disturbed sample
- Soil recovered without preserving its fabric, suitable for classification, index properties and testing on remoulded specimens, but not for strength or consolidation parameters.
- Standard penetration test
- A dynamic test in which a split spoon sampler is driven by a 63.5 kg hammer falling 760 mm through three 150 mm intervals, with the blows for the last two summed to give the N value.
- Refusal
- The condition at which a penetration test is stopped, defined by 50 blows in one 150 mm interval, 100 blows across the counted intervals, or ten successive blows with no advance.
- Cone penetration test
- A test in which a standard cone is pushed hydraulically at a constant rate, recording cone resistance, sleeve friction and pore pressure continuously with depth.
- Soil behaviour type
- The classification a cone returns, based on mechanical response rather than on particle size. A cemented sand may respond like a stiff clay and be classified accordingly.
- Shear wave velocity
- The propagation speed of a shear wave through soil, related to the small strain shear modulus and the density, and therefore the quantity a seismic survey converts into a stiffness profile.
Site investigation and in situ testing FAQ
Why can a disturbed sample not give a strength?
Because strength depends on the fabric and the stress history that the disturbance destroyed. Classification and index properties survive, since the Atterberg tests begin by remoulding the soil in any case and a grading curve does not care how the particles were arranged.
Strength and consolidation parameters do not: a structured natural clay that has been remoulded reports a far lower strength and a different compressibility, and the error is not conservative in any useful sense, it is simply wrong. Whether undisturbed samples are taken is decided at the drilling stage and cannot be recovered afterwards.
Should I use a cone or a borehole?
Usually both, in that order. A cone gives a continuous profile quickly and objectively, and a soft layer announces itself in all three of its traces at once, so it is the right tool for finding where the problems are across a site. It brings nothing back, so it cannot supply a compression index or a coefficient of consolidation, and it classifies by mechanical response rather than by grading.
Boreholes are then placed at the locations the cone identifies as worst, to take undisturbed samples for the laboratory tests that a design actually needs.
What is geophysics for if it does not give design parameters?
For deciding where to spend the drilling budget. Geophysical methods are non destructive, cover area rather than points, and are quick and cheap, so they are used early to map the depth to rock, locate the groundwater table, find cavities or contamination, and identify anomalies. They are qualitative, need space and expertise, and rarely deliver a design parameter directly.
The standard workflow is therefore geophysics then drilling: survey to plan the borehole locations, target the anomalies, interpolate between the holes, and validate the interpretation.
Exam move
Read this chapter as the answer to a question the rest of the unit assumed: where do the numbers come from. The examinable skill is matching a method to a parameter, so practise proposing an investigation for a described site and justifying each element by what it delivers rather than by what it costs.
Learn the standard penetration test dimensions and the counting rule precisely, because they are the kind of fact a short item asks for directly, and pair every correlation you quote with the caution the unit attaches to it. Remember that sample quality is decided during drilling and cannot be improved later, which is why a plan that asks a disturbed sample for a strength is a planning error rather than a laboratory one.
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