Elaboración de Diagramas de la Metodología

4.2.1 Sample Site Selection

An essential part of any experimental study is the collection of appropriate samples, in this study sample sites were chosen on the basis of their accessibility, the opportunity to collect samples that had not been weathered and that were not mechanically damaged, and that had not undergone major changes to the pore fluid. Weathering and pore fluid chemistry changes are difficult to ascertain but chemical analyses on the oxidation of iron performed on identical samples of the Lichi melange

and Joe's River Formation by Fan (1994) indicates that weathering had been

Sites were chosen after detailed reconnaissance surveys of all available outcrops of the material. Visual examination of the nature of the material in situ was performed to

investigate the disruption of the sample by non-geological means. When the

optimum site was found, sampling was undertaken as below.

4.2.2 Sample Collection

It was essential to collect samples whose natural properties had not been altered. Soil samples have traditionally been collected by driving a U100 tube into the sediment (Chandler et a!., 1992). Thereafter, the sample core is sealed within the tube and may be stored. For harder sediments sampling is undertaken either by the collection of a large detached block of the material or by coring with a rotary core drill. After consultation with others (Leddra, Allison and Jones, pars comm,

1990), doubt was cast on the appropriateness of these methods to undisturbed mudrocks. In particular, the sensitivity of mudrock fabrics to mechanical disruption (section 4.1) suggests that these two sampling methods would fundamentally alter the behaviour of the materials. The insertion of a U100 tube Into a mudrock is accompanied by plastic deformation of the sediment around the tube (Chandler et ai,

1992). Such deformation could break any bonds within the sediment and reduce porosity in the vicinity of the tube. Reduction of pore volume in the vicinity of the

tube could induce an increase in the pore pressure, which may then be

dissipated throughout the sample, altering its effective stress state. Coring with a high speed, rotary core barrel may reduce the amount of plastic deformation as the drill cuts the sample, but may lead to a number of problems, which include.

• Alteration of the pore fluid chemistry as the drill lubrication fluid penetrates the sample.

• Cracking or micro-cracking of the sample due to vibration of the drill or sample. • Alterations of pore pressures due to vibration (representing dynamic loading) or

cracking.

These problems may be reduced with the use of a sharpened, well balanced and aligned drill with a lubrication medium of the same chemistry as the pore fluid. However it was felt that a better sampling technique was needed to collect high-quality samples.

For the all the outcrop samples a block sampling technique that reduced disturbance of the sample material to a minimum was adopted. The methodology following was used;

i. For convenience sampling was usually undertaken into a near vertical slope. The slope was excavated back to remove the upper zones of weathering. In the unweathered material a vertical face was excavated and cleaned to allow determination of stratigraphie variations In the material. If there were no such

variations in the excavated zone, sampling was undertaken. Such an

examination ensured that the samples collected were devoid of any major discontinuities, were homogeneous and had minimal weathering.

II. A slope was cut Into the face (figure 4.2.1) and, using a machete, two slots about 30 cm apart were excavated Into the slope to a distance of about 60 cm. A further slot was dug linking the rear of the two slots, leaving an undisturbed block Isolated within the slope. This block was gently trimmed with the machete to leave a cylindrical shape slightly smaller than the tin used to transport the material.

ill. A thin plastic layer was wrapped around the sample and surrounded with a layer of muslin.

iv. The muslin was coated in a layer of molten wax which, once cooled, provided an air-tight coating around the sample.

V. A clean 25 cm diameter cylindrical tin was placed over the sample and the date, time, sample type, number, depth and orientation was recorded on Its side.

vi. The base of the sample was sliced with a cheese-wire. The tin and sample was carefully inverted.

vii. A thin plastic layer was placed over the end of the sample and covered with a piece of muslin.

viii. The remainder of the tin was filled with wax which, when set, provided a waterproof coating and fixed the sample in place.

§

Face

_Sample

Figure 4.2.1 Diagrammatic representation of the proceedure for collecting a sample, a) Slope cut into face for sampling, b) Sample excavated by cutting trenches around it's perimeter.

By using this procedure, samples were collected that had undergone the minimum - possible fabric disruption and mechanical damage. The layer of waterproof wax around the sample prevented desiccation during transport. The sample was sealed in a robust tin with a wax 'shock absorber' around It to p‘ avide protection against mechanical damage. The thin plastic layer ensured that wax did not penetrate the fabric. Two sources of potential damage to the sample are evident with this technique. First the use of hot wax may induce desiccation or pore pressure changes. Second the sample is exposed to the atmosphere for much of the sampling period. To reduce the impact of these problems, the outermost 20 mm of the sample was discarded in the laboratory.

This sampling process is relatively laborious and time-consuming. Depending upon the depth of excavation needed to penetrate the weathering layer, collection of each sample took approximately 10 man hours. Sampling was usually undertaken by two people. All of the samples for each material were taken from the same location at the same depth, to ensure that samples were as similar as possible.

4.2.3 Sample transportation and storage

The mudrock samples were transported back to the laboratory and stored in an air

conditioned, constant temperature room. For each of the London Clay, Lichi

melange and Joe's River Formation samples, one tin was opened and examined for the effects of wax penetration. The sample was sliced and physically examined for signs of desiccation or thermal damage from the heated wax. Moisture content was

determined at various points within the sample to determine the amount of

desiccation. In all cases it was found that removal of the outer 2 cm of the sample would minimise fabric alteration during the sampling process. Periodically during storage, the wax seals of each tin were examined and replaced if any degradation was Indicated.

The degree of saturation of each sample was determined in the laboratory. Sampling was usually undertaken above the water table to prevent flooding of the excavation. For each material, the initial saturation was the same in all samples because they were taken from the same stratigraphie position. To ensure that all samples were fully saturated, an initial saturation phase was undertaken in the triaxial cell.

4.3 The Collection, Transportation and Storage of the North Sea