Engineering Description or Classification of Weathered Rocks

Introduction

The description and Classification of Weathered rocks brought together a series of important studies by leading geologist and engineering researchers due to the countless difficulties encountered by engineers in weathered rock areas, how it affects site exploration, plan and evaluation steps during projects. In the 1950s there has been a considerable amount of literature published on the characterisation of weathered rocks and engineering properties of weathered materials, but there was little or no standardisation of descriptive terminology during this period, the use of these published data for the prediction of the engineering behaviour away from the original study areas is limited. Thereafter the publications by the Geological Society Engineering Group Working Party; the International Society of Rock Mechanics, the British Standards Institution, the International Association of Engineering Geologist and Dearman have rationalised the terminology by the use of standard descriptive indices and simple index testing. These schemes enable the geologist or Civil engineer to describe the materials and rock structure in engineering terms, and how it has been adopted for general practice worldwide. However the choice of which scheme to use is generally dependent on the preference of the engineer responsible.

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Background Analysis

The earliest attempts to classify weathered rocks in such a manner that will prove useful to engineering goes back to the early 1950s when weathering classification of granite was introduced by Moye in the Snowy Mountains of Australia. Moye proposed the scheme for the promotion and identification of engineering properties of weathered rock materials and how it would help to formulate the weathering terms of granite when they are used during site investigation by various group of persons.

Six classes of rock materials were described based on the concept that logging of cores would be done on the basis of recognition of weathering types irrespective of order; surface or downwards and without any attempt fitting the initial results of the logging into a formatted or general weathering profile with subsequent zones established in the rock mass. Little (1967), discussing laterites, suggested that the Moye classification could be applied to “other lateritic-type soils” and would be more successful for the purpose, than the usual temperate soil classification into clay, silts etc. The interesting feature was that Little applied grade numbers to the various degrees of weathering recognised by Moye (1955); grade I was fresh rock and grade VI was rock weathered to a residual soil.

Then the classifications of weathered rock came to a stage where a group of specialist came together in the UK and published a report on the first formalised weathering classification of rocks under the name of Geological Society Engineering Group Working Party Report on The Logging of Rock Cores for Engineering Purposes (Anon, 1970 in (Dearman, 1995). It’s evident that this was also mainly based on the classification of (Moye, 1955) scheme. However, few amendments were made; the Moye’s granitic soil term was changed to residual soil thereby increasing the number of categories from six to seven classes. Thus the new scheme was suggested to apply for a broader range of rocks while it was initially devised to granite. Another point of interest was that of the Working Party scheme which relied mostly on general descriptions and observations, which includes friability while that of Moye was based on classification on specific index test.

To pave the way for the mass scheme recommendation in BS5930: 1981 (Anon, 1995), thus the latter scheme grades were reduced back to 6 as it was originally done by (Moye, 1955) and the same terminology was used with a complete different meaning. The argument over using the description of the materials or the mass scales for a classification came on. However, the required different approaches was advocated and grading the materials recommended to be; decomposed, disintegrated, fresh and discoloured. Duncan (1969) also proposed a scheme based on texture, structure, composition and classification (calcareous or non-calcareous), colour and grain size.

Following the publication of BS5930: 1981, attempts have been made at developing classification schemes which allow the degree of weathering to be defined for different lithologies, (Table.2) (Anon 1970, Anon 1977, BS 5930 1981). The early schemes (Anon 1970, for instance) were based on the chemical weathering of granite rocks and represented a hybrid material grade and zone scheme. In 1977, the working party of the Engineering Group of Geological Society on the Description of Rock Masses (Geological Society of London 1977) clearly separated the description of weathering on a rock mass scale. This scheme, like the earlier ones, placed great emphasis on the weathering profiles developed on granitic rocks in tropical and sub-tropical environments, although, little guidance was given for the description of weathering.

The British Standard proposed that weathered rock materials may be described or graded using four terms: decomposes, disintegrated, fresh and discoloured, but they did not provide any guidance for determining and describing the degree or weathering. Attempts to use these schemes in the description of rock materials have met with difficulty. It is the opinion of the authors that any reference to the degree of weathering should be omitted from the description unless it is known with some certainty on the basis of experience and knowledge of the typical weathering profile for that rock type. For rock weathering in conditions where physical disintegration dominates, it is unlikely that the degree of weathering may be determined from examination of rock materials alone.

The descriptive scheme for weathering was later criticized as being too restrictive in scope and not easily applicable to a wide range of rock types and structural situation, though this was the original intention. As it’s indicated by (Cragg and Ingman, 1995) a number of problems may arise when (BS5930: 1981) is used for major projects. At the initial stage it is difficult to extrapolate the weathering pattern in two or three dimensions of rock cores. In some occasions this question cannot be fully answered unless closely spaced boreholes are cored, logged and then mass zones are ascribed after the complete set has been logged. In addition, drill-hole may be insufficiently dense for a derivation of a mass tract from material core logs.

The critics of (BS5930: 1981) do not always have more objective alternative schemes of classifications. For some the type of rock alternative schemes may be preferred over the (BS5930: 1981), including (Chandler, 1969) for Merica Mudstone and (Chandler, 1972) for Upper Lisa Clay and etc. In a situation where weathering dominates many aspects of geotechnics, the complementary classifications of (Moye, 1955) and (Ruxton and Berry, 1957) were essentially adopted as standards by the Hong Kong Government for engineering descriptions in 1979 (Anon, 1979) in (Anon, 1995), however, it is clear that all these alternatives are site or region specified. They are generally rock mass schemes based and gradational or depth controlled.

The IAEG (Anon, 1981c) in (Anon, 1995) recommended a presumably factual scale of percentages of weathering with no guidance for how this scale should be applied. According to the scheme, the degree of weathering can be expressed quantitatively by laboratory study. But these approaches remain liable to lead to misunderstanding and disputes concerning descriptions.

As far as the problems and difficulties prolonged in all engineering projects in the weathered rocks, the demand for such classification increased. Weathered rocks can cause particular difficulties especially in site investigations. They are often open textured and weakly bonded; they can be very sensitive to disturbance during sampling. Also their profile are often complex and variability cannot be predicted with standard geological interpolation or extrapolation.

The confusion, inapplicability of the suggested classification schemes combined with the lack of agreement between professionals working in the field has led to various bodies producing their own classifications, e.g. (Anon, 1988b). Similarly, many have turned back to the (Anon, 1970) and (Anon, 1972). Some others keep using formation specified schemes like that of (Chandler, 1969).

The Engineering Group of the Geological Society in an attempt to make critics see the ambiguity in its thesis has commissioned a Working Party to study the description and classification of weathered rocks for engineering purposes. The Report of Working Party (1995) provides a scheme for describing the state of weathering for uniform rock materials which are moderately strong or stronger in the fresh state which shows a clear gradation in engineering properties during weathering. The proposed classification scheme requires the use of appropriate index tests such as the point load text and slaking tests.

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The most logical approach to the problem of classifying degree of weathering is to describe the rock material without attempting to provide a statement on how weathered it may be, apart from commenting on the presence of discolouration, decomposition, voids and softening. Once sufficient descriptive data on the rock material and the rock mass has been acquired to establish the mechanisms and stages of weathering present, a site specific weathering classification can be developed to provide a consistent means of describing both the rock material and, more importantly the rock mass.

According to (Anon, 1995); “The Working Party this time preferred to make recommendations rather than attempting to deal with all aspects of weathering”. In addition to circulating the draft at various stages among many correspondents and adding valued contribution to it, a three day meeting was held at the Leeds University, UK, in April 1994 for a more elaborate discussion. The Working Party report was then used during a day in the field and employed in the description of the weathered rocks in the laboratory (Anon, 1995).

Conclusion

Apart from the potential communication problems, there is a strong argument for the need to devise a single modified version of the weathered rock classification proposed that might find more general acceptance and be more readily applicable in field assessments, both rock material and rock mass.