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Earth Observation

 

 

Earth Observation in the frame of EO-MINERS - Earth Observation methods

Geotechnical

Geotechnical methods encompass the fields of rock and soil mechanics. Of relevance are the properties of the materials as such and their behaviour in the field. Compressive, tensile and shear strength are the main parameters of interest that are determined inter alia by the mineralogical composition, cementation of grains, grain-size distribution, porosity (distribution), water retention capacity etc.

One can distinguish between methods that are applied in the field (in situ) and methods that are applied to disturbed or undisturbed rock and soil samples. Any sampling is likely to disturb some material properties temporarily or permanently. Thus the collection of undisturbed soil samples is a particular challenge, as their removal may alter their structure and water contents. Sampling of solid rocks, e.g. in form of drill cores, appears to be less invasive, though it will also lead to stress-redistribution and, hence, altered behaviour in laboratory tests.

The strength of rocks and soils depends on the strength of individual grains and the strength of the interaction between the grains. In soils the strength is determined by the friction between grains when subject to shear stress. While the porewater, as all liquids, cannot take up shear stresses, it needs to be squeezed out, when a soil subject to stress and this hydraulic resistance will contribute to the overall strength. On the other hand, in the field, the buoyancy of grains in water-saturated sediments will reduce the cohesive strength of these sediments.

Granulometry - Measurements of grain size distributions are only performed on incoherent materials, such as soils and sediments, and mudrocks that can be dispersed in aqueous solutions. Sediments and soils are dried and then sieved in sets of sieves of increasing fineness. Grain sizes are classed in a logarithmic scale, e.g. clay (Ø 0.002-0.002 mm), silt (Ø 0.002-0.2 mm), sand (Ø 0.2-2 mm), pebbles (Ø 2-63 mm). While the granulometry of silt- and above fractions is quite straightforward, the clay-sized fraction is not accessible to sieving. The traditional method for this fraction is measuring the change in density with time of a suspension of them. The evaluation requires assumptions on the grain densities and ideal spherical shape. Other techniques include the Coulter Counter, where a sample is passed through a small orifice that separated two reservoirs with electrolyte; each passing particle causes a change in resistance between the two reservoirs. Also the diffraction of a laser beam that depends on the grain sizes present in a suspension can be used. In a similar way the scattering of ultrasound signals can be measured. With all these techniques for the clay-fraction a fundamental procedural and interpretational problem is associated: the material needs to be perfectly dispersed, but this will destroy clay aggregates that may be relevant from a sedimentological or geomechanical point of view.

Set of sieves image grain size distribution curve  diagram
Set of sieves and grain size distribution curve

 

Gravimetry - Soil and rock samples that have been taken carefully so as to avoid water losses due to evaporation are weighed and then dried at 105°C for a prescribed period of time and then weighed again. The difference in weight is the natural water content per weight. Samples may also be taken with a defined volume to be able to determine the water content per volume.


Atterberg limits - The water content of coherent (clayey) soils determines their geotechnical behaviour. There is a number of laboratory tests, that aim to determine the plasticity and liquidity of soils. These semiquantitative tests evaluate the rheological and cohesive properties of samples at different water contents. The results are used in assessing the stability of slopes and to determine the need for adding water to soils used in civil engineering applications.

Equipment for determining Atterberg-limits
Equipment for determining Atterberg-limits (from www.gooch-eng.com)

 

Proctor compaction test - The compactability and hence maximum density of soils depend on their water content. Civil engineering applications may call for a high density as possible and these tests determine the optimum water content. They generally consist of compacting soil at known moisture content into a cylindrical mould of standard dimensions using a compactive effort of controlled magnitude.

Proctor test and equipment  diagram Proctor test equipment
Proctor test and equipment (from www.siteprepmag.com and www.test-llc.com)

 

Shear tests - A soil sample is placed into a cylinder consisting of two rings placed onto each other. A vertical, confining, load is applied that, for instance, is equivalent to what is to be expected in the field. The upper ring is then pulled sideways, either with a predefined force or a predefined rate. The ratio between load and deformation or strain and force is recorded. As the porewater content will contribute to the overall strength, these test are carried out under total confinement ('undrained') or allowing the porewater to seep out ('unconfined'). Similar tests can be carried out on drill cores cut from solid rocks.

Sketch of a shear box
Sketch of a shear box (from www.tonygraham.co.uk/).

 

Triaxial shear tests - The cohesive or non-cohesive samples are placed into cylindrical rubber sleeves, loads are applied via metal endplates and the rate of deformation and porewater pressures are recorded. The name 'triaxial' stems from the fact that a confining hydrostatic pressure is applied to the rubber sleeves. There are different test protocols: the sample may be allowed to consolidate, i.e. to settle and expel porewater, to settle but not expell porewater, or it is carried out so fast that neither settling nor porewater expulsion can happen. The different protocols give complementary information on the soil behaviour under load. Triaxial test are also carried out on rocks to determine their compressive strength.

Sketch of a triaxial apparatus
Sketch of a triaxial apparatus (from http://environment.uwe.ac.uk/)

 

Oedometer tests - These tests are similar in arrangement to triaxial shear tests, but the soil samples are confined in a strong steel cylinder. The cylinder has porous bottom and top covers to allow the drainage of porewaters. Typically a sample is subject to incremental load increases and the resulting compaction recorded.

Sketch of an oedometer
Sketch of an oedometer (from http://www.lwi.tu-bs.de/sfb/oedometer1.html)

 

Penetrometer tests - The principle consists of recording the resistance against a metal cone being pushed into the soil. Hand-operated penetrometers for shallow investigations, e.g. to determine soil compaction on agricultural fields, consist of a metal rod with cone (Ø 3 cm) of slightly larger diameter at the end that is pushed into the ground using a handlebar. The load is measured using a manometer spring-balance. Modern versions are equipped with digital dataloggers. In another variant (Standard Penetration Test), a weight of 63 kg is dropped from a height of 76 cm along the rod (Ø 5 cm) against a shoulder; the penetration depth for each drop or the number of drops needed for a specific depth is recorded. With these penetrometers, depths in the order of 2 m can be reached. In order to reach greater depths (10 m or more), larger diameter rods and heavier loads are needed. These are provided, for instance, by mounting the system in a 4x4 lorry and using hydraulic rams to push down the rod; the lorry itself acts as counterweight. Different protocols are possible, e.g. constant push rate or constant load, and the respective other variable is recorded. Penetrometer test allows recording the depth distribution of layers of different penetration resistance. The resistance is determined by the cohesiveness of the geological materials, their grainsize distribution etc. Very stiff materials or coarse gravel limit the applicability. The usefulness of the penetrometers has been widened by integrating pressure transducers to record porewater pressures and permeable frits to allow sampling porewaters for hydrochemical investigations. In the USA in more recent years these methods have become also known under the name 'direct-push' technology.

Variants of these penetrometers exist that have down-hole pneumatic vibrators to emit seismic signals that can be picked up by geophones (see also section on seismic).

Other variants allow recovering core samples. For a variety of geochemical and mineralogical investigations such cores are preferred over cores obtained from rotary drilling techniques as they are not contaminated by drilling fluids. In a light-duty variant the push rods have a groove machined in, in which sediment collects; the resulting profiles can be visually inspected and small samples for further analysis can be collected.

Cone penetrometer test
Cone penetrometer test

 

Plate-load tests - These are commonly carried out in civil engineering project to determine the natural load-bearing capacity of soils or as quality tests to ensure that a certain degree of compaction has been achieved. A plate of 20 to 30 cm diameter is pressed into the ground under a given load. This load is applied hydraulically using an available heavy construction plant or similar as a counterweight.

Plate-load test  image
Plate-load test (Source: http://www.gemtec.ca/gallery.html).

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