Engineering Geology of Melbourne, Peck, Neilson, Olds & Seddon (eds) 1992 Balkema, Rotterdam. ISBN 90 5410 083 4

Silurian and Lower Devonian engineering properties I. W Johnston Department of Civil Engineering, Monash University, Clayton, Vic., Australia

INTRODUCTION

Materials of the Silurian and Lower Devonian periods, generally referred to as the Melbourne mudstone, consist of interbedded clay stones, siltstones and sandstones in which the siltstones clearly predominate.

In engineering works, the mudstone can be encountered in a range of weathered states. At the stronger end, the mudstone approaches its fresh state where it is usually of a dark blue colour and has a uniaxial compressive strength of around 10 MPa or more. As the degree of weathering increases, the mudstone changes its colour through pale grey, pale grey/brown mottled, to yellow brown and occasionally pink as the high degree of weathering is approached. Typically, a highly weathered mudstone has a uniaxial compressive strength of about 1 MPa. In the extremely weathered state, the mudstone is typically a yellow brown silty or sandy clay of a stiff to hard consistency.

Although there are areas where significant folding and faulting have taken place to produce a highly fractured mudstone mass with rough, irregular and closely spaced clay filled joints, the majority of the mudstone has a comparatively large defect spacing with generally clean, tight and planar joints.

The mudstone falls into a category of geotechnical materials which may be referred to as soft or weak rock. These materials are intermediate to the traditional materials which are considered in the related disciplines of soil mechanics and rock mechanics. Although the intact mudstone is harder, more brittle and more dilatant than soils, its engineering behaviour still bears many similarities with established clay technology. Furthermore, although the behaviour of the soft rock mass may not be as markedly different from the intact material as may be the case in rock mechanics, the presence of

discontinuities can still exert an influence which is more in the keeping with the principles of rock mechanics. It follows, therefore, that a comprehensive understanding of the engineering performance of the Melbourne mudstone requires a working knowledge of both soil and rock mechanics (Johnston, 1989).

For any engineering application, it is important that the properties derived for engineering analysis are relevant to the development under consideration. This requires a careful assessment of the overall approach adopted during site investigation. There are two basic means of obtaining engineering properties; these being through in-situ tests and through laboratory tests. However, a consideration of the relative merits of these two approaches is beyond the scope of this contribution. Suffice it to say that a combined approach may be more relevant with an emphasis on smaller scale laboratory testing in situations where the mudstone mass behaviour is not dominated by the presence of discontinuities. Where discontinuities are more dominant, then there may be a greater emphasis placed on larger insitu forms of test which are capable of modelling the influence of these important features.

FUNDAMENTAL CHARACTERISTICS When carrying out tests to derive relevant engineering properties, it is important to apply boundary conditions which are appropriate to the likely full-scale performance of the material concerned. One of the most critical aspects of this performance is the drainage conditions which are applied to a laboratory test or to the analysis of an in-situ test.

One might be tempted to assume that because the coefficient of permeability, k, of the intact mudstone is similar to that of a clay, undrained conditions

should apply just as would normally be assumed for geotechnical design in clay. However, it should be emphasised that it is not the permeability which is of fundamental concern, but the coefficient of consolidation, ev, which controls the rate at which porewater pressures dissipate.

An approximate relationship between these two parameter is given by :

where, in addition to k and cv, mv is the coefficient of volume compressibility and w is the unit weight of water.

While the mudstone may have the same order of permeability as a clay, its compressibility, as reflected by the value of mv, can be at least two orders of magnitude less. It follows from Eq. (1) that for mudstone, the value of cv can be at least two orders of magnitude greater and typically at least 100 m2/year as compared to perhaps 1 m2/year for clay. It follows that the time to achieve a high degree of porewater pressure dissipation in the mudstone can be very much smaller than for a clay. This may be smaller still when considering the presence of much more permeable discontinuities within the rock mass. Therefore, on the basis of the above, it is suggested that, unless there are specific reasons to the contrary, drained conditions should generally apply to testing and analysis for the Melbourne mudstone. A more detailed consideration of the above properties and characteristics may be found in Chiu and Johnston (1980), Johnston and Chiu (1981), Chiu (1981), Johnston (1985a). In the paragraphs that follow, all engineering properties presented are based on full drainage occurring.

One of the major problems that exists in a geological formation as extensive and variable as the Melbourne mudstone is the definition of a means of classification which can reliably indicate engineering properties. While descriptions involving a visual assessment of features such as colour, hardness, friability, weathering characteristics can be very useful, these terms are subjective and may only give a broad indication of engineering properties. There would be great advantages in the determination of a simple objective, and preferably numerical, description which could be reliably correlated with engineering properties and which could be determined with minimal expense and effort.

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It would appear that the water content fulfils this role admirably as was originally suggested by Parry in 1958 in connection with the investigations for the King Street Bridge project, although not published until much later (Parry, 1970). The reason for this strong correlation of water content with engineering properties is that the mineralogy of the mudstone is reasonably constant across the various degrees of weathering (Cole, 1979). Therefore the water content reflects the void ratio or material density which has such a strong influence on properties. It should be emphasised, however, that the saturated water content, w, should be used for these correlations, as partially saturated values would not reflect the true void ratio.

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TEST METHODS Laboratory Testing

As noted previously, the laboratory test approach for property determination is frequently adopted during site investigations. There are a range of properties which can be determined, but those relating to geotechnical design are usually concerned with strength and deformation characteristics. The exact form of these properties for use in engineering applications will depend on a number of factors including code requirements, local practice and available design methods.

The strength parameters may include the following (a) uniaxial compressive strength, qu (b) peak cohesion, cp (c) peak angle of friction, p (d) residual angle of friction, r (e) point load strength, IS(50) (f) uniaxial tensile strength, t (g) plane strain fracture toughness, KIC

The deformation parameters which normally require determination are:

(h) Young's modulus, E(i) Poisson's ratio,

Triaxial testing

The properties (a), (b), (c), (h) and occasionally (i)

mudstone may necessitate confining pressures well in excess of 10 MPa. This requires specialist equipment which must be safe for the operator. Furthermore, the measuring systems used in conjunction with this equipment must be suitable for the accurate measurement of axial displacements, axial loads, volume changes and porewater pressures as may be required. Details of such equipment may be found in Chiu, Johnston and Donald (1983).

are normally established by means of triaxial equipment wherein cylindrical specimens, normally obtained from drill core, are confined within a pressure cell and loaded to failure by means of a deviator stress applied at an appropriate rate. It is vitally important that these tests are conducted in a manner that is appropriate to the characteristics of the mudstone so that meaningful results are produced. It may also be worth noting that the residual angle of friction cannot normally be estimated from triaxial tests because of limitations in the shear displacement which can be achieved.

The general principles of triaxial testing are well known to soils engineers (Bishop and Henkel, 1962). However, because of the mudstone's greater strength and smaller compressibility, some additional test procedures require careful consideration. One of the most important is that to adequately define cohesion and angle of friction, the confining pressures required for testing can be very much greater than for soils testing. Whereas pressures of up to 1 MPa are normally adequatc for soils, triaxial testing for

The specimens used in this form of testing should be prepared in such a way that the results obtained are not influenced by procedure. It would appear that the stringent specimen preparation procedures suggested for triaxial and uniaxial testing in the ISRM Suggested Methods (Brown, 1981) are not necessary for soft rocks such as the Melbourne mudstone (Pells and Ferry, 1983). Indeed, it would appear that end preparation can be satisfactorily achieved by turning the specimen ends in a lathe (Chiu, Johnston and Donald, 1983). It is important for soft rocks, however, that test specimens are not permitted to dry out prior to testing or major errors can be introduced. Indeed, since much of the mudstone encountered in engineering applications is below the water table, it could be argued that testing should be conducted on fully saturated specimens. The overall geometry of the test specimens should be such that the length to diameter ratio is at least 2 with a suggested minimum diameter of about 50 mm (N size core).

One of the most critical aspects of triaxial testing of soft rocks such as the mudstone is the rate at which load is applied to the test specimen. According to ISRM Suggested Methods (Brown, 1981), the load on the specimen should be applied at a constant rate such that failure occurs within 5 to 15 minutes of loading. Alternatively, the stress rate can be applied within the limits of 0.5 to 1.0 MPa/sec. If the first rate were adopted, then the rate of strain application would approach 10-2 strain/min. If the alternative rate were selected, then the rate of strain application would be significantly higher with the test completed in a matter of seconds. According to the ISRM Suggested Methods, either of these rates would be considered acceptable as the mudstone is a rock, albeit a soft rock. These rates may be acceptable to relatively hard rocks where induced porewater pressures are not generally significant. However, if these rates were applied to the mudstone, significant errors could be introduced.

Fig. 1 shows the results of a series of triaxial tests conducted on moderately weathered specimens of

mudstone. Each specimen was tested under identical conditions of one-way drainage at a confining pressure of 3.56 MPa. The only differences in the tests was that the rates of strain application were varied to cover the range of very slow (about 10-5 strain/min) to relatively fast (10-2 strain/min). The figure shows the effect of this change of applied strain rate on the measured deviator stress to cause failure. The reason for these changes is reflected in the excess porewater pressures measured at the undrained ends of the specimens as shown in Fig. 1. Clearly then, had this same material been tested according to ISRM Suggested Methods, serious error would have resulted.

It is suggested that the well established methods of Gibson and Henkel (1954) should be applied to triaxial tests conducted on the mudstone, so that an appropriate test rate can be selected without the development of significant excess porewater pressures (Chiu, Johnston and Donald, 1983). If the above procedures cannot be followed, then it is suggested that triaxial strain rates should be about 2 x 10-5 strain/min for one way drainage in specimens of the dimensions discussed above. Where drainage is permitted at both ends, then strain rates may be four times greater. Irrespective of the means of strain rate selection, it is suggested that porewater pressures should be periodically monitored during testing to ensure that excessive values are not being generated.

From a series of such tests under different confining pressures, it is possible to construct Mohr circle plots from which representative values of peak cohesion and peak angle of friction can be derived. It may be worth noting that since fitting a representative envelope to a number of Mohr circles can be very difficult, consideration should be given to the presentation of test results on a principal stress plot (i.e. a plot of l against 3 at failure). This plot would give specific points rather than tangential points, thereby allowing the application of statistical fitting techniques for a more confident estimation of a failure envelope. This envelope can then be simply transformed to give cohesion and angle of friction for the construction of the failure envelope on a Mohr circle plot.

The uniaxial compressive strength is obtained by means of techniques which are very similar to the triaxial test but considerably simplified since confining pressures are not required. However, there may be some advantages in surrounding the test specimen with a water filled membrane so that the specimen does not dry during testing (Chiu and

Johnston, 1983). As with the triaxial tests, one of themost important factors influencing the results obtained is the rate of strain application. Just as was discussed in connection with the triaxial tests, the ISRM Suggested Methods do not appear entirely appropriate to the uniaxial testing of soft rock. On the basis of the recommendations of Chiu and Johnston (1983), it is suggested that a strain application rate of not more than 3 x 10-4 strain/min should be acceptable. The reason for this greater strain rate is that for uniaxial testing, drainage is permitted over the curved outer specimen surface.

As noted above, Young's modulus can also be derived from uniaxial compressive and triaxial tests. However, in view of comments already made with regard to the importance of maintaining natural water contents if not full saturation, the use of strain gauges applied directly to test specimens, as is common in rock mechanics testing, is not feasible. However, as described by Chiu, Johnston and Donald (1983), it would appear that overall specimen deformation measurements are sufficiently accurate for modulus determination. The determination of Poisson's ratio is not a simple matter, particularly under confined conditions, and therefore, it is rarely attempted. However, as the value does not appear to vary a great deal, it is normally sufficiently accurate to assume a value from previously published data, as presented below.

Direct shear testing

The residual angle of friction is normally obtained by means of direct shear testing equipment, often with a reversing facility to obtain sufficiently large shear displacements to reach the residual condition. Except in the case of the highly and completely weathered mudstone it may not be possible to obtain peak strength parameters with this equipment because of equipment load capacity limitations. However, it should be noted that there are a range of high capacity specialist direct shear machines which are capable of testing large intact mudstone specimens or testing rough joint specimens for which both peak and residual strength parameters are required (e.g. Johnston, Lam and Williams, 1987).

It would appear that the ISRM Suggested Method (Brown, 1981) for the laboratory direct shear test is reasonably applicable to mudstone. However, in addition, it would seem reasonable that the test specimen should not be permitted to dry out during testing. Therefore, testing saturated specimens under

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water would seem appropriate. Furthermore, to ensure that the drained shear strength parameters are being measured, testing should be carried out at a rate of shear load application not more than 0.05 mm/min (Lam, 1983; Johnston and Lam, 1984). This rate appears to be applicable to test specimens of up to approximately 150 mm x 75 mm shear area.

Point load testing

The test to determine the point load strength is very easy to perform and is often used for classification purposes. Details of its application may be found in Broch and Franklin (1972). However, although there is a strong temptation to use the results of these simple tests to predict the results of more complex and costly tests, in particular the prediction of the uniaxial compressive strength, some considerable restraint should be exercised as will be detailed below.

Uniaxial tensile testing

Because of the strongly dilatant characteristics of the mudstone, the importance of tensile strength is becoming recognised as an important property. Examples of this influence may be found in foundation and pile performance (Johnston and Choi, 1985) and in pressuremeter interpretation (Haberfield and Johnston, 1986). The uniaxial tensile strength can be determined with relatively simple techniques such as the Brazilian test, the ring test and the direct uniaxial tensile test. However, because of practical difficulties with specimen preparation and premature failure due to stress concentrations in the ring and direct tests, the Brazilian test is normally preferred. Full details of the Brazilian test may be found in the ISRM Suggested Methods (Brown, 1981), and details concerning its application to a soft rock may be found in Choi, Haberfield and Johnston (1988).

Plane strain fracture toughness testing Although this particular parameter has not received much attention in the design of engineering construction, it is likely that it may be required eventually as the importance of tensile stress distributions, cracking and crack propagation on the performance of engineering construction is recognised. Whereas the tensile strength of the mudstone can be used to describe the stress at which a crack will form, the fracture toughness is required

to describe the way in which a crack will propagate. Details of the types of test which are appropriate for the determination of this strength parameter, as well as sample preparation, test procedures and interpretation may be found in Haberfield and Johnston (1990a).

In-situ Testing

The alternative approach to property determination during site investigations involves the use of in-situ tests. These tests can be very useful when there is the likelihood of significant disturbance on sampling or when the discontinuities within the rock mass make the results of smaller laboratory tests less relevant to full scale mass behaviour. Of course, when neither of these situations exist, then in-situ testing can be a useful means of confirming some of the results obtained in the laboratory.

For soft or weak rocks, there are essentially three forms of in-situ test which have a real influence on the derivation of design parameters (Johnston, 1987). These will be discussed briefly in the following sections.

Pressuremeter testing

The use of the pressuremeter for establishing engineering properties, particularly the shear and Young's modulus, of sands and clays has been recognised for many years. However, within the last decade, much more robust instruments, capable of considerably greater strain resolution, have been developed for use in soft and weak rocks such as the mudstone (e.g. Hughes and Ervin, 1980; Clarke, Newman and Allan, 1989).

To arrive at estimations of modulus and, in certain cases, shear strength, it has been necessary to make some assumptions concerning the behaviour of the soft rock. It may appear that since the permeability of the mudstone is of the same order as clays, a pressuremeter test conducted in mudstone will reflect an undrained Young's modulus and an undrained cohesion with an angle of friction of zero. For reasons given above and expanded in more detail for pressuremeters in Ameratunga (1986) and Johnston (1987), the response of the pressuremeter in mudstone is much more likely to reflect fully drained conditions. Therefore, it is the drained modulus and the drained shear strength parameters which should be derived. Unfortunately, because the response of a pressuremeter in a soft rock at relatively large radial strains is a function of initial

horizontal stress, dilation angle and radial crack propagation as well as cohesion and angle of friction (Haberfield, 1987: Haberfield and Johnston, 1990b), a meaningful derivation of shear strength parameters from pressuremeter tests in soft rock is not yet possible.

The Young's modulus of mudstone, however, can be derived reasonably from pressuremeter tests. In actual fact, it is the shear modulus of the rock which is produced from the resulting pressure-radial strain curve, but, by making a reasonable assumption regarding the relevant Poisson's ratio, a drained Young's modulus can be estimated. At this stage it should be emphasised that there is compelling evidence to suggest that the modulus estimated from a pressuremeter test in soft rock can be significantly influenced by radial cracking (Haberfield and Johnston, 1986,1989a). The result can be further influenced by the location of these cracks relative to the strain measuring points within the pressuremeter (Haberfield and Johnston, 1990c). At this stage of understanding, it would seem appropriate that the modulus is established from an unload-reload portion of the response curve made at a relatively high radial strain. Also, by averaging a number of radial strain readings, it is more likely that the result will become less influenced by crack location.

In-situ shear testing

This form of testing can be very useful in the evaluation of the shear strength parameters of a plane of weakness, such as a joint or a bedding plane, within the rock mass. Such a test is very useful when scale effects are likely to reduce the relevancy of small laboratory shear tests as may occur when the plane of weakness contains roughness characteristics of greater wavelength than the size of these latter small tests. The general recommendations of ISRM Suggested Methods (Brown, 1981) are considered relevant to the mudstone, although for near surface investigations, normal loads may need to be provided by alternative means such as dead loading or by jacking against anchored beams.

Load tests

When dealing with a reasonably intact rock mass, the smaller laboratory forms of testing are likely to produce material properties which may be relevant to design. The Young's modulus derived by these means can he used with the appropriate analytical

methods to make estimations of deformations. However, although reliable estimates of the strength properties of the rock may be obtained from laboratory tests, at this stage in the development of soft rock mechanics, it is not always possible to apply these properties to the calculation of load capacity of the rock mass. It may be necessary, therefore, to conduct some form of loading tests to check that there is an adequate factor of safety against loading failure.

Alternatively, for more discontinuous rock, small scale laboratory tests may not be able to yield representative design data at all. For these situations it may become necessary to conduct much larger insitu load tests to evaluate deformations and factors of safety.

Where these above situations exist, load tests may require consideration if an effective economic design is to be achieved. However, the scale of such a test should be carefully evaluated so that the influence of any defects are reflected in the results obtained. It is a general rule that the larger the in-situ test, the more relevant the result generally becomes.

Unfortunately, as these tests become larger, the cost of carrying them out tends to increase dramatically.

From the results of these tests it is possible, by means of making some relevant assumptions, to arrive at a representative Young's modulus. There are many interpretational methods available and Pells (1983) provides a useful summary for a range of different load conditions. With regard to load capacity, because of the lack of reliable analytical methods, it is not always possible to back-calculate the strength parameters from loading tests. However, provided the load test is considered to be representative of likely prototype conditions, then it may be possible to check that the working load is acceptable by loading to at least 1.5 to 2 times the anticipated working stresses.

Apart from the more conventional load tests such as plate bearing tests and borehole jack tests, a wide range of specialist tests have been used for the assessment of the performance of the mudstone such as is described by Johnston, Donald, Bennet and Edwards (1980), and Williams (1980).

ENGINEERING PROPERTIES

The properties presented in the following sections have been derived principally for the siltstone and claystone components of the mudstone. While some

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(a) and shearing (b). The axial splitting mode may be caused by platen end effects which leads to the development of tensile stresses within the test specimen. These conditions lead to a failure which is dominated by the tensile strength of the material. Since the tensile strength is considerably less than the compressive strength of these materials, the strength recorded can be significantly less than would have been recorded had the shearing mode occurred. This effect has been demonstrated by Choi (1984) and discussed further in Johnston (1985b).

results will have been influenced by the relatively small sandstone component, they do not necessarily apply to this coarser material in general.

Uniaxial Compressive Strength

The uniaxial (or unconfined) compressive strength of Melbourne mudstone is arguably one of the most commonly derived strength parameters and determinations have been made for a great number of locations throughout the region. When these are plotted against water content, it is not uncommon to find a very wide scatter. The reason for this scatter may be due to measured water contents that are not saturated. This may lead to two possible opposing effects. For specimens tested at water contents less than saturation, the strength measured is likely to underestimate the strength typical of that water content had it been representative of saturated conditions, (i.e. a saturated specimen of 12% water content may have dried to 6%. Its measured strength would reflect the 12% value, and this would be significantly less than the strength of a specimen with a saturated water content of 6%). Conversely, as a mudstone dries out, its strength can increase significantly (Choi, 1984), thereby giving a general overestimate of strength.

Another factor which can influence test results but is often overlooked is the mode of failure during uniaxial testing. As discussed by Johnston (1985b), there are two possible extreme modes of failure for these tests with a range of combinations in between. Figure 2 shows these two extremes as axial splitting

As a consequence of these various influences, it is perhaps not surprising to find that even for such a relatively simple test, there is a wide scatter of results. To remove some of these influences, Chiu and Johnston (1983) produced a set of results obtained from specimens which were saturated and displayed the shearing mode of failure. These and some other results are shown in Fig. 3 and it is believed that this gives a much clearer indication of the variations of uniaxial compressive strength with saturated water content.

Confined Compressive Strength

As discussed previously, it is common practice to use triaxial testing techniques to define the compressive strength characteristics of geotechnical materials in the form of the well known Mohr-Coulomb failure envelope. However, as was pointed out previously, the testing techniques required for this detailed work to be applied to Melbourne mudstone may be well beyond the capabilities of the normal operational geotechnical testing laboratory. A detailed investigation of the confined compressive strength of the mudstone for a number of sampling sites was carried out by Johnston, Williams and Chiu (1980). A large number of triaxial tests were conducted on mudstone specimens using the test techniques described above and for confining pressures up to about 5 MPa. This confining pressure was selected on the basis of it being considered a reasonable estimate of the maximum confining pressure normally experienced by the mudstone during engineering works. The results obtained were then divided into groups according to the saturated water contents. For each water content group, best fit Mohr-Coulomb envelopes were established using linear regression techniques. The results of these analyses are shown in Fig. 4. It should be noted that each point shown in Fig. 4 is an average value and is representative of a number of actual test results.

(a) (b)Figure 2: UCS failure modes; a) Axial splitting

b) Shearing

To compare the confined strength parameters given in Fig.4 with the unconfined results given in Fig. 3, the following relationship may be used :-

A comparison of the uniaxial compressive strengths obtained directly with those derived from values of cp and p is shown in Fig. 3. It would appear that within the scatter obtained, both sets of independently derived results are in very good agreement.

It should be noted, however, that while the use of a linear Mohr-Coulomb envelope is clearly a great advantage in design and analysis, it is nonetheless a simplification of reality. Like all materials which have experienced various degrees of over consolidation and/or cementation, failure envelopes can be quite markedly non-linear. Therefore, in order to examine this phenomenon, an alternative strength criterion for the mudstone will be presented later.

The foregoing discussion has concentrated on the intact mudstone. In many situations where jointing is clean, tight and relatively infrequent, it may be argued that these intact parameters are reasonably applicable to design. However, there are areas where the defects can have a significant influence on mass strength. Unfortunately, soft rock technology has simply not developed sufficiently to permit a simple assessment of the likely mass strength, and each situation must be judged on its own merits. However, some guidance may be obtained from local experience such as that offered by Williams and Ervin (1980).

Residual Angle of Friction

On the basis of a limited number of drained direct shear tests conducted on mudstone specimens representing a range of different saturated water contents (Williams, 1980; Chiu, 1981; Johnston and Lam, 1984), Fig. 5 presents the variation of the residual angle of friction. It would appear that for the range of mudstone specimens investigated, the residual angle of friction is reasonably constant and generally in the range 20 to 26o with mean of about 23o. This result is not unexpected because the residual friction angle is considered to be function of particle size distribution, mineralogy and the unbonded rock flour characteristics (Kenny, 1977), all of which appear to be reasonably constant for the mudstone across a

range of weathering zones. This should be compared

with the peak angle of friction discussed in the previous section which is considered to be additionally a function of structure. This leads to the result that the angle of dilation may be approximated as the difference between the peak friction angle and the residual friction angle (Haberfield, 1987; Haberfield and Johnston, 1990b).

Point Load Strength

many instances where this strength has been used as the basis of design, it must be remembered that the result produced is only an index and does not normally attempt to model likely prototype performance.

One of the problems with the point load strength is that there appears to be a temptation to extrapolate from the results obtained to give more relevant and/or more complex test data for use in design. This would seem to be a very dangerous practice. For example, it was suggested in the original ISRM Suggested Method (1972) for the point load test, that the uniaxial compressive strength is about 24 times the point load strength. In the new edition of the same test (ISRM, 1985), this relationship appears in more guarded form but still states that, on average, the uniaxial strength is 20-25 times the point load strength.

While there may be some basis for this relationship for hard rocks as has been demonstrated by Pells (1975), there is also ample evidence to suggest that there can be large variations in this ratio (e.g. Read, Thornton and Regan, 1980). It must be emphasised that in the works that have investigated the relationship between the uniaxial compressive and point load strength, the rocks concerned have almost always had uniaxial strengths well in excess of 25 MPa. The mudstone in engineering applications is rarely encountered with a strength greater than about 10 MPa. Therefore, considerable care should be exercised in extrapolating general results for hard rocks to soft rocks.

There is some evidence which suggests that for rocks with uniaxial strengths considerably less than 25 MPa, this ratio is considerably less than 24. Indeed, in some limited studies involving a synthetic mudstone of uniaxial compressive strength of about 10 MPa, it was found that the ratio was about 12. While further studies will be necessary to confirm these preliminary findings, some additional support for these lower values has been provided from a non-linear strength criterion which will be discussed later.

As noted above, the point load test may be considered a reasonable test technique for use with the mudstone. Just as it is used with more competent rocks, the point load test represents a classification test which can be used for a rapid comparative assessment of strength across a given location. However, although there are

Uniaxial Tensile Strength On the basis of a range of Brazilian tests conducted on saturated samples of the mudstone (Williams 1980; Chiu, 1981; Johnston and Chiu, 1984), it would appear that the variation of uniaxial tensile strength with saturated water content is as illustrated in Fig. 6

toughness will also show a strong correlation. Further

discussion of this topic may be found in Haberfield (1987) and Haberfield Johnston (1989b). Young's Modules When dealing with an intact rock, uniaxial and triaxial forms of testing can often display quite marked non-linear response in the resulting stressstrain plots. It follows that a range of Young's moduli can be derived from these curves depending on the points used for calculation. For example, the initial tangent modules of such a curve can be considerably larger than secant moduli determined for various stages of loading. A more rational approach would be to adopt a definition of modules which would reasonably reflect the deformation of the rock under design construction stresses, while maintaining some degree of conservatism to ensure that allowable deformations are not exceeded. It would appear that the secant modules at half the peak deviator stress at failure is such a parameter.

With regard to this secant modules derived from uniaxial compression testing, Chiu and Johnston (1983) have presented the saturated water content correlation shown in Fig. 8. This figure shows that, as expected, the Young's modules decreases with greater weathering characteristics as indicated by increasing water contents. As discussed by Chin and Johnston, there would appear to be two groups of mudstone giving two slightly different correlations with a noticeable step at a water content of about 12%. The reason for this minor anomaly was attributed to the observation that for water contents of less than about 12%, the mudstone was generally blue-grey in colour and therefore representative of a reducing weathering environment. For water contents greater than about 12%, the mudstone was generally yellow-brown or pink and therefore, representative of an oxidizing weathering environment. The principal difference between the two forms of mudstone is that the oxidized mudstone contains varying amounts of cementing agents such as iron oxides. It follows that the oxidized mudstone may show a stiffer response to loading than the reduced mudstone, hence explaining the step in the property variations with water content.

Although Fig. 8 provides an indication of the variations in uniaxial compressive modules, deformation of the mudstone may occur under confined conditions. Therefore it would seem appropriate to investigate the influence of confining

Plane Strain Fracture Toughness Although there have been very few plane strain fracture toughness determinations made for the Melbourne mudstone, a comparatively greater number have been made on a synthetic soft rock (made largely from the mudstone) which has a range of engineering properties very close to those recorded for the mudstone. These results along with those of a range of other geotechnical materials are shown in Fig. 7 in the form of a plot of plane strain fracture toughness against uniaxial tensile strength (Haberfield and Johnston, 1989b). It would appear that the mudstone and the synthetic rock have very similar values and fall into a distinctive pattern with the results obtained from the various other geotechnical materials. This would seem to indicate chat since the uniaxial tensile strength shows a strong correlation with saturated water content as shown in Fig. 6, then it is likely that the plane strain fracture

If these results are plotted together with the uniaxial results of Fig. 8, then Fig. 11 is produced which gives the variation of the secant Young's modules of the mudstone against the saturated water content at the start of the application of the deviator stress, which is the same water content as after consolidation. Since the uniaxial tests are not confined, the water content both before and after consolidation must be the same. A study of Fig. 11 will show that the two sets of results are effectively the same. It follows, therefore, that the modules which applies during the application of a deviator stress is characterised by the saturated water content at the start of loading.

The explanation of this phenomenon is not immediately obvious. However, as the mineralogy of the mudstone is reasonably constant across various weathering zones, it is possible that the only effect of a confining pressure is a decrease in void ratio and consequently in saturated water content. It follows then that an application of confining pressure merely recompresses the mudstone to an apparently lower degree of weathering and the secant modules which applies is characterised by the decreased saturated water content.

An indication of secant modules for applications where the confining pressure is zero or very low may be obtained directly from Fig. 11 without any correction applied to the saturated water content. However, where confining pressures are significant, the water content which applies to the estimation of modules may be calculated from a knowledge of the initial saturated water content and the confining pressure which is to be applied. Fig. 12 (after Johnston, Williams and Chiu, 1980) gives an indication of the decrease in water content which may occur on the application of a confining pressure. It may be of interest to note that the reduction in water contents given in Fig. 12 are consistent with the consolidation properties presented in Johnston and Chiu (1981).

In order to give an indication of the relevancy of the modules values given in Fig. 11, Johnston, Williams and Chiu (1980) back-calculated the Young's moduli from pile loading tests conducted

at a number of sites in the Melbourne area where the mudstone is massive. This was achieved using standard elastic solutions for the various test conditions involving side resistance only piles, end resistance only piles and complete piles. It should be noted that several of these pile tests were conducted at or close to the surface and therefore are

pressures on the Young's modules. Figure 9 shows the variation of the drained secant Young's modules with confining pressures to in excess of 30 MPa (Chiu, 1981) for specimens of 10% and 14% water content. It will be seen that confining pressure can significantly increase the modules and therefore its influence should be included in design. As discussed by Johnston, Williams and Chiu (1980), the influence of confining pressure on modules can be accounted for in a relatively simple manner. From a large number of triaxial test determinations of Young's modules, rather than plotting modules against the saturated water content at the stage of preparing the test specimen, the modules was plotted against the saturated water content after the confining pressure had caused each mudstone specimen to consolidate. The result is as shown in Fig. 10. It may be worth noting that, as discussed above, there appears to be some differences in the trends displayed by the oxidized and reduced mudstone.

106

equivalent to the performance of shallow foundations. These moduli are presented in Fig. 13 against the representative values of saturated water content determined at the test locations. Also shown in this figure are the results of some pressuremeter tests conducted in the mudstone. A comparison of ibis figure with the results shown in Fig. 11 will show that the Young's moduli variations obtained from the two very different sources are in quite reasonable agreement.

It may be worth noting, however, that the Young's moduli discussed above are concerned with compressive loading. There would seem to be some evidence to suggest that for tensile and bending stresses, Young's modulus may be significantly different (Haberfield and Johnston, 1988).

With regard to mudstone which contains a significant number of discontinuities, it is clear that the rock mass modulus is likely to be much less than the intact modulus. Moreover, the mass modulus will be strongly influenced by not only the quantity

of discontinuities but also their weathering characteristics, orientation, roughness, in-fill materials and a number of other features. Unfortunately, with all these variables, there is simply insufficient data to allow any generalisations concerning the likely mass moduli that may apply to a given location. It is suggested shat in-situ testing may be necessary if reasonable estimations of moduli are to be made. Particular consideration should be given to pressuremeter tests and loading tests.

Poissons Ratio

A STRENGTH CRITERION FOR THE MELBOURNE MUDSTONE In an earlier section, the strength of the Melbourne mudstone over a range of weathering characteristics was investigated in terms of the linear Mohr-Coulomb criterion to derive values of cohesion and angle of friction. Although this criterion is very commonly used in geotechnical engineering and has obvious computational attractions, it is nonetheless only an approximation to the clearly non-linear response of the mudstone. It follows that there may be some advantages in developing a more realistic criterion to describe the strength of the mudstone over a wide range of confining pressures and for a range of test techniques.

A suitable criterion, originally developed specifically for the intact mudstone, was suggested by Johnston and Chiu (1984) and was subsequently shown to apply to all intact geomechanical materials by Johnston (1985b). This criterion has the major advantage that it is equally applicable to materials such as normally consolidated soils which show well defined linear strength envelopes as well as to very hard rocks which have extremely non-linear envelopes. It also applies to all grades of material in-between.

Variations of Poisson's ratio with saturated water content are presented in Fig. 14 for tests carried out under uniaxial compressive conditions (Chiu and Johnston, 1983). As may be seen there appears to be a reasonable scatter of results, although values in the range of 0.2 to 0.3 would seem to be reasonable average estimates.

As a further development of Eq. (4), it may be instructive to consider the ratio of uniaxial compressive strength to point load strength.

It follows that the ratio of predicts the ratio ofuniaxial compressive to tensile strength.

From the analysis of the Brazilian test method, the uniaxial tensile strength is given by :-

where D is the specimen diameter, t is its thickness and P is the applied load at failure. Since for the Brazilian test, t should be approximately equal to the radius of the specimen, it follows that Eq. (5) can be approximated by :-

For the point load strength test, it is suggested that the value of I s(50) is given by :-

with the appropriate scale correction.

It follows from Eqs. (6) and (7) that

This confirms the physical similarity between the tests which implies that the point load test is a form of indirect tensile test. From the relationship given by Eq. (4), the above strength criterion predicts a ratio of uniaxial compressive strength to point load strength of

A general view of the degree of fit of the criterion to approximately 150 individual triaxial test results representing all weathering zones is given in Fig.15 Where these zones were divided into much more limited water content increments, an even better was achieved. Details of the analytical methods used, the B and M values which were derived and their variations with water content may be found in Johnston and Chiu(1984).

The criterion for intact materials is given by :

where (1n and 3n are the major and minor principalstresses, normalised by the uniaxial compressive strength. B and M are two material constants. By placing 3n = 0, the uniaxial compressive strength is correctly modelled with the right-hand side of Eq. (3) becoming unity.

By placing s1n = 0, the uniaxial tensile strength is given by :-

(3)

(4)

which possess greater values of M and smaller values of B (Johnston, 1985b), the ratio of uniaxial compressive to point load strength would be much greater than for the mudstone and values in the range 20 to 25 would be very common.

It would appear then that the proposed strength criterion can reasonably predict the strength of the mudstone for a wide range of weathering zones and test conditions. Perhaps this same criterion will prove more appropriate to performance modelling in the years to come.

To compare the predictions made by this criterion, three weathering zones and their typical saturated water contents have been chosen and displayed in Table 1. Typical values of B and M extracted from Johnston and Chiu (1984) are also presented. From Eqs. (4) and (9), predictions have been made of the ratios of uniaxial compressive to tensile strength and uniaxial compressive to point load strength. A study of Figs. 3 and 6 will reveal that the predictions of uniaxial compressive to tensile strength are quite reasonable.

With regard to the ratios of uniaxial compressive to point load strength, for the slightly weathered mudstone a value approaching 12 is predicted. This agrees very well with the limited test data discussed previously. A reduction in this ratio for more weathered mudstone cannot be substantiated yet, but on the basis of the tensile strength results and comments made above on the nature of the point load test, a reduction should perhaps be expected. It may be of interest to note that for much harder rocks

CONCLUSIONS

The Silurian and Lower Devonian formations of the Melbourne region are more commonly referred to as the Melbourne mudstone. This soft rock falls into a category of geotechnical materials which is intermediate to the more traditionally accepted materials of soil mechanics and rock mechanics. As a result of this position in the geotechnical spectrum, an understanding of the engineering performance of the mudstone requires a working knowledge of both soil and rock mechanics.

This contribution has examined some of the fundamental characteristics of the mudstone, and on the basis of these characteristics, has considered the test methods, both in the laboratory and in-situ, most suited to the determination of engineering properties. The properties themselves have been considered in some detail and their variations against an indicator in the form of the saturated water content have been presented.

Finally, because of the intermediate behaviour of the mudstone between soils and rocks, it is suggested that a more representative criterion should be adopted to describe the strength of the mudstone over a wide range of testing conditions.