Concluding summary

A simple model of the depositional environment and subsequent con­ solidation of clay sediments has been used in an investigation into the fabric changes associated with post-depositional effects. Much work has been required in the attempt to simulate the physical and chemical aspects of the model. New pieces of equipment have been built to assist in this investigation. A novel method of creating the correct saline conditions and the use of natural organic de- flocculants lend authenticity to the chemical aspects of the model.

Analysis of particle orientations has required the application of directional data statistical methods to evaluate the degree of particle anisotropy in natural and laboratory samples. The writer suggests using the mean resultant length (R) of the data, when plotted on a unit circle, to provide a measure of the anisotropy index. The value of R can be mathematically defined as:

r = (c2 + s2y

and S = sum of sines of angles divided by no of angles.

Statistical evidence for preferred orientation is given by comparing the value of R with its critical value at a given level of significance. The results from this study indicate that the critical values for proving random orientation could possibly be relaxed slightly. The writer suggests that at the 0.1% level of significance the critical value of R appropriate to defining random orientation is 0.45. Preferred orientation would be defined by a value of R greater than 0.7. Values in between would indicate a weak preferred orientation. The writer stresses that this suggestion is only appropriate to fabric studies of the type presented here.

8.2.1 Microstructural changes

With respect to the development of preferred particle orientation in mudrocks three important phases of sedimentation are recognised:

(i) colloidal phase (ii) interface phase (iii) sediment phase

The colloidal phase exists when the particle concentration in the water medium is very low. During this phase the particles form the structures defined by Val Olphen (1963) as shown in Fig. 4. Both single-plate cardhouse type structures and small domain-like packets are present. The stability of these structures will depend on the pH, Eh and salinity conditions of the water medium.

activity and chemical reactions of- an organic and inorganic nature help to create special conditions. This zone is important because clay particles normally will have a longer residence time here than the period spent settling through the water column. Reactions in which time spent in contact with the overlying water is important are more likely to occur in this zone. The important physical charac­ teristic of the interface phase is that there is a sudden increase in particle concentration. This results in the formation of more domain-type structures at the expense of a diminishing number of single particles. The microfabric tends to be of an open flocculated structure, particularly in the marine environment.

The sediment phase marks the change from the *soupground’ (Ekdale 1985) conditions of the interface zone, where the moisture content is around the liquid limit, to the more plastic conditions associated with a rigid structure. Particles may undergo rearrangement during this phase in response to overburden pressure.

With respect to microstructural changes, the writer suggests that during the colloidal and interface phases the overwhelming influence on preferred particle orientation is the chemistry of the environment. Once the sediment phase is entered this influence is negligible and compaction provides the means of furthering the degree of preferred orientation. The evidence from this study, discussed in section 7.5, indicates that most reorientation must occur at low pressure ( < 1000 kPa). It is concluded that primary fabric, where the particle orientation is parallel with the bedding, occurs very early in the sediment’s history. The hypothesis is supported by this

study and the experimental work of Krizek et al (1975).

8.2.2 Geotechnical properties

Test results have shown that the compressionspressure relationship, as defined by the compression index, does not alter significantly with variations in the chemistry of the depositional environment. This would suggest that initial soil structure has negligible effect on the compressibility of clay sediments. However, the coefficient of consolidation is shown to vary with changes in the chemistry and hence soil structure is significant to this parameter. These findings are in agreement with the experimental investigation of Woo et al (1977).

The writer considers it unwise to relate the absolute values of the geotechnical properties of his laboratory sediments to those of natural sediments, since the model neglects the influence of non-clay particles and time-dependent effects. It is thought acceptable, how­ ever, that the trends indicated by this work are a reflection of complex natural clay-water-electrolyte mixtures.

8.2.3 Particle orientation

The analysis of mudrock microstructure overwhelmingly points to a relationship between particle orientation and the environment of deposition. It would appear that strong preferred orientation is associated with anoxic environments.

In the laboratory investigations, organic matter has been shown to increase the parallelism of clay particles. The preferred orien­

tation in both cases is in accord with the direction of bedding. The writer concludes that fissility along the bedding planes is related to the environment of deposition and that it is primarily organic matter in this environment which is the cause of fissility under anoxic conditions. Since bioturbational effects were not modelled during this study it is not possible to form a conclusion on this aspect from the experimental evidence. The writer does accept that in the aerobic environment bioturbation will cause disturbance to the primary fabric.