Geotechnical investigations are performed by geotechnical engineers or
engineering geologists to obtain information on the physical properties
of soil and rock around a site to design earthworks and foundations for proposed
structures and for repair of distress to earthworks and structures caused by
subsurface conditions. This type of investigation is called a site
investigation. Additionally, geotechnical investigations are also
used to measure the thermal resistivity of soils or backfill materials required
for underground transmission lines, oil and gas pipelines, radioactive waste
disposal, and solar thermal storage facilities. A geotechnical investigation
will include surface exploration and subsurface exploration of a site.
Sometimes, geophysical methods are used to obtain data about sites. Subsurface
exploration usually involves soil sampling and laboratory tests of the
soil samples retrieved. Surface exploration can include geologic
mapping, geophysical methods, and photogrammetry, or it can be as simple
as a geotechnical professional walking around on the site to observe the
physical conditions at the site. To obtain information about the soil
conditions below the surface, some form of subsurface exploration is required.
Methods of observing the soils below the surface, obtaining samples, and
determining physical properties of the soils and rocks include test pits,
trenching, boring, and in situ tests. Soil sampling
Borings come in two main varieties, large-diameter and small-diameter.
Large-diameter borings are rarely used due to safety concerns and expense, but
are sometimes used to allow a geologist or engineer to visually and manually
examine the soil and rock stratigraphy in-situ. Small-diameter borings are
frequently used to allow a geologist or engineer to examine soil or rock
cuttings or to retrieve samples at depth using soil samplers, and to perform
in-place soil tests. Soil samples are often categorized as
being either "disturbed" or "undisturbed;" however, "undisturbed"
samples are not truly undisturbed. A disturbed sample is one in which the
structure of the soil has been changed sufficiently that tests of structural
properties of the soil will not be representative of in-situ conditions,
and only properties of the soil grains can be accurately determined. An
undisturbed sample is one where the condition of the soil in the sample is
close enough to the conditions of the soil in-situ to allow tests of
structural properties of the soil to be used to approximate the properties of
the soil in-situ. Offshore soil collection introduces many
difficult variables. In shallow water, work can be done off a barge. In deeper
water a ship will be required. Deepwater soil samplers are normally variants of
Kullenberg-type samplers, a modification on a basic gravity corer using a piston.
Seabed samplers are also available, which push the collection tube slowly
into the soil. = Soil samplers =
Soil samples are taken using a variety of samplers; some provide only disturbed
samples, while others can provide relatively undisturbed samples.
Shovel. Samples can be obtained by digging out soil from the site. Samples
taken this way are disturbed samples. Trial Pits are relatively small hand or
machine excavated tranches used to determine groundwater levels and take
disturbed samples from. Hand/Machine Driven Auger. This sampler
typically consists of a short cylinder with a cutting edge attached to a rod
and handle. The sampler is advanced by a combination of rotation and downward
force. Samples taken this way are disturbed samples.
Continuous Flight Auger. A method of sampling using an auger as a corkscrew.
The auger is screwed into the ground then lifted out. Soil is retained on the
blades of the auger and kept for testing. The soil sampled this way is
considered disturbed. Split-spoon / SPT Sampler. Utilized in
the 'Standard Test Method for Standard Penetration Test and Split-Barrel
Sampling of Soils'. This sampler is typically an 18"-30" long, 2.0" outside
diameter hollow tube split in half lengthwise. A hardened metal drive shoe
with a 1.375" opening is attached to the bottom end, and a one-way valve and
drill rod adapter at the sampler head. It is driven into the ground with a
140-pound hammer falling 30". The blow counts required to advance the sampler a
total of 18" are counted and reported. Generally used for non-cohesive soils,
samples taken this way are considered disturbed.
Modified California Sampler. Utilized in the 'Standard Practice for Thick Wall,
Ring-Lined, Split Barrel, Drive Sampling ofSoils1'. Similar in concept to the SPT
sampler, the sampler barrel has a larger diameter and is usually lined with metal
tubes to contain samples. Samples from the Modified California Sampler are
considered disturbed due to the large area ratio of the sampler.
Shelby Tube Sampler. Utilized in the 'Standard Practice for Thin-Walled Tube
Sampling of Soils for Geotechnical Purposes'. This sampler consists of a
thin-walled tube with a cutting edge at the toe. A sampler head attaches the
tube to the drill rod, and contains a check valve and pressure vents.
Generally used in cohesive soils, this sampler is advanced into the soil layer,
generally 6" less than the length of the tube. The vacuum created by the check
valve and cohesion of the sample in the tube cause the sample to be retained
when the tube is withdrawn. Standard ASTM dimensions are; 2" OD, 36" long, 18
gauge thickness; 3" OD, 36" long, 16 gauge thickness; and 5" OD, 54" long, 11
gauge thickness. It should be noted that ASTM allows other diameters as long as
they are proportional to the standardized tube designs, and tube
length is to be suited for field conditions. Soil sampled in this manner
is considered undisturbed. Piston samplers. These samplers are
thin-walled metal tubes which contain a piston at the tip. The samplers are
pushed into the bottom of a borehole, with the piston remaining at the surface
of the soil while the tube slides past it. These samplers will return
undisturbed samples in soft soils, but are difficult to advance in sands and
stiff clays, and can be damaged if gravel is encountered. The Livingstone
corer, developed by D. A. Livingstone, is a commonly used piston sampler. A
modification of the Livingstone corer with a serrated coring head allows it to
be rotated to cut through subsurface vegetable matter such as small roots or
buried twigs. Pitcher Barrel sampler. This sampler is
similar to piston samplers, except that there is no piston. There are
pressure-relief holes near the top of the sampler to prevent pressure buildup
of water or air above the soil sample. In-situ tests
A standard penetration test is an in-situ dynamic penetration test
designed to provide information on the properties of soil, while also
collecting a disturbed soil sample for grain-size analysis and soil
classification. A dynamic cone penetrometer is an insitu
test in which a weight is manually lifted and dropped on a cone which
penetrates the ground. the number of mm per hit are recorded and this is used to
estimate certain soil properties. This is a simple test method and usually
needs backing up with lab data to get a good correlation.
A cone penetration test is performed using an instrumented probe with a
conical tip, pushed into the soil hydraulically at a constant rate. A
basic CPT instrument reports tip resistance and shear resistance along
the cylindrical barrel. CPT data has been correlated to soil properties.
Sometimes instruments other than the basic CPT probe are used, including:
A piezocone penetrometer probe is advanced using the same equipment as a
regular CPT probe, but the probe has an additional instrument which measures the
groundwater pressure as the probe is advanced.
A seismic piezocone penetrometer probe is advanced using the same equipment as
a CPT or CPTu probe, but the probe is also equipped with either geophones or
accelerometers to detect shear waves and/or pressure waves produced by a
source at the surface. Full flow penetrometers probes are used
in extremely soft clay soils and are advanced in the same manner as the CPT.
As their names imply, the T-bar is a cylindrical bar attached at right angles
to the drill string forming what look likes a T, the ball is a large sphere,
and the plate is flat circular plate. In soft clays, soil flows around the probe
similar to a viscous fluid. The pressure due to overburden stress and pore water
pressure is equal on all sides of the probes, so no correction is necessary,
reducing a source of error and increasing accuracy. Especially desired
in soft soils due to the very low loads on the measuring sensors. Full flow
probes can also be cycled up and down to measure remolded soil resistance.
Ultimately the geotechnical professional can use the measured penetration
resistance to estimate undrained and remolded shear strengths.
Helical probe test soil exploration and compaction testing by the helical probe
test has become popular for providing a quick and accurate method of determining
soil properties at relatively shallow depths. The HPT test is attractive for
in-situ footing inspections because it is lightweight and can be conducted
quickly by one person. During testing, the probe is driven to the desired depth
and the torque required to turn the probe is used as a measure to determine
the soil's characteristics. Preliminary ASTM testing has determined that the HPT
method correlates well to standard penetration testing and cone penetration
testing with empirical calibration. A flat plate dilatometer test is a flat
plate probe often advanced using CPT rigs, but can also be advanced from
conventional drill rigs. A diaphragm on the plate applies a lateral force to the
soil materials and measures the strain induced for various levels of applied
stress at the desired depth interval. In-situ gas tests can be carried out in
the boreholes on completion and in probe holes made in the sides of the trial
pits as part of the site investigation. Testing is normally with a portable
meter, which measures the methane content as its percentage volume in air.
The corresponding oxygen and carbon dioxide concentrations are also
measured. A more accurate method used to monitor over the longer term, consists
of gas monitoring standpipes should be installed in boreholes. These typically
comprise slotted uPVC pipework surrounded by single sized gravel. The
top 0.5 m to 1.0 m of pipework is usually not slotted and is surrounded by
bentonite pellets to seal the borehole. Valves are fitted and the installations
protected by lockable stopcock covers normally fitted flush with the ground.
Monitoring is again with a portable meter and is usually done on a
fortnightly or monthly basis. Laboratory tests
A wide variety of laboratory tests can be performed on soils to measure a wide
variety of soil properties. Some soil properties are intrinsic to the
composition of the soil matrix and are not affected by sample disturbance,
while other properties depend on the structure of the soil as well as its
composition, and can only be effectively tested on relatively undisturbed
samples. Some soil tests measure direct properties of the soil, while others
measure "index properties" which provide useful information about the soil
without directly measuring the property desired.
Geophysical exploration Geophysical methods are used in
geotechnical investigations to evaluate a site's behavior in a seismic event. By
measuring a soil's shear wave velocity, the dynamic response of that soil can be
estimated. There are a number of methods used to determine a site's shear wave
velocity: Crosshole method
Downhole method Surface wave reflection or refraction
Suspension logging Spectral analysis of surface waves
Multichannel analysis of surface waves Refraction microtremor
Other methods: Electromagnetic
Optical/acoustic televiewer survey See also
Geotechnical engineering Engineering geology
Geotechnics Soil mechanics
Soil test Notes and references
External links UC Davis Video on typical drilling and
sampling methods in geotechnical engineering.