The strength and deformation properties of the Melbourne mudstone under uniaxial compression and Brazilian test conditions are examined for the range of weathering characteristics commonly encountered. The relevancy of these simply obtained properties to engineering design are considered with special reference to rock socketed piles. The resultant ratios of uniaxial compressive to tensile strength are examined to demonstrate that while the Brazilian test may provide a reasonable indirect estimation of uniaxial tensile strength for the slightly to moderately weathered mudstone, it may significantly underestimate the tensile strength of the more weathered mudstone.
Les proprietes de resistance et de deformation de l'argilite de la region de Melbourne soumise à la compression uniaxiale et à l'essai bresilien ont ete etudiees pour toute la gamme des caracteristiques de l'alteration dans des conditions normales. La pertinence des donnees ainsi simplement obtenues pour la conception des ouvrages de genie a ete consideree plus particulièrement en ce qui concerne les pieux emboites dans de la roche. Les rapports trouves entre la resistance à la compression uniaxiale et la resistance à la traction ont ete examines. Ils montrent que l'essai bresilien peut fournir une indicatlon lndirecte assez valable de la resistance uniaxiale à la traction de l'argilite peu ou moderement alteree, mais dans le cas de l'argillte davantage alteree ce même essai risque de sous-estimer sensiblement la resistance à la traction.
Die Festigkeits- und Verformungskenngrößen des Melbourner Tonsteins unter einachsiger Druckbelastung und unter Brasilianischen Testbedingungen werden fuer gewöhnlich anzutreffende Witterungsverhaltnisse untersucht. Die Relevanz dieser auf einfache Weise erhaltenen Kenngrößen fuer die ingenieurmaßige Bemessung insbesondere yon felsgegruendeten Pfahlen wird untersucht. Bei Auswertung der erhaltenen Quotienten yon einachsigen Zug- und Druckfestigkeiten zeigt sich, daß der Brasilianische Test in der Regel eine relativ gute Abschatzung der einachsigen Zugfestigkeit des leicht bis mittelmaßig verwitterten Tonsteins erlaubt, er jedoch die Zugfestigkeit des starker verwitterten Tonsteins erheblich unterschatzen kann.
In the Melbourne region, a large proportion of construction involves the rocks of the Silurian and Lower Devonian era which form the bedrock of the area. Although this material is predominantly a siltstone, it is known as the Melbourne mudstone and is usually encountered between a relatively hard and brittle slightly weathered condition (uniaxial compressive strength, σc = 8 MPa) and a softer more ductile highly weathered condition (σc = IMPa). For some time the authors have been involved in a detailed research programme aimed at establishing the engineering properties of the mudstone. A wide range of different forms of testing have been used and a full description of the various techniques, their results and interpretation may be found elsewhere (Chiu, 1981; Chiu et al, 1982; Johnston et al, 1980; Johnston and Chiu, 1981, 1982). This paper is however, limited to the uniaxial properties of the Melbourne mudstone; specifically the uniaxial compressive strength and deformation moduli, and the uniaxial tensile strength.
The uniaxial tests were applied to mudstone specimens representative of the range of weathering characteristics commonly encountered in construction. However, as the determination of the degree of weathering of anyone specimen is a somewhat subjective and qualitative exercise, sn alternative quantitative method was used. As discussed by Johnston and Chiu (1982), the saturated water content, w o, represents a simple and quantifiable parameter which reflects the degree of weathering. The range of water contents concerned in this investigation varied between about 4% (slightly weathered) and 20% (highly, weathered). The methods adopted for the uniaxial compression tests were generally in accordance with the ISRM Suggested Method (Brown, 1981), using specimens of 54mm diameter. Axial loads were measured with an electrical load cell and axial strain was determined from two diametrically opposed displacement transducers clamped to the loading stem of the frame and measuring the relative displacement between the stem and the base platen. The uniaxial compressive strength is simply the maximum axial stress sustainable by each specimen before failure. However, the axial deformation moduli for each specimen usually display some stress dependency and therefore a range of values could be quoted. Although the ISRM Suggested Method recommends specimen length to diameter ratios in the range of 2.5 to 3.0, it was considered that a softer rock would permit the use of a smaller ratio. This would enable the preparation of more specimens from the limited supply of high quality rock available for the investigation. According to the ISRM Suggested Method "load on the specimen shall be applied continuously at a constant stress rate such that failure will occur within 5–10 min of loading, alternatively the stress rate shall be within the limits of 0.5–1.0 MPa/s." While such loading rates may be acceptable for dry hard rocks, for a saturated soft rock such as the mudstone, load applications at the above rates could have a marked influence on induced pore water pressure.
