16Rock quality classification Having tests that give an idea of the quality of the rock mass is essential for engineering purposes
17What are we calling a rock? GradeDescriptionLithologyExcavationFoundationsVISoilSome organic content, no original structureMay need to save and re-useUnsuitableVCompletely weatheredDecomposed soil, some remnant structureScrapeAssess by soil testingIVHighly weatheredPartly changed to soil, soil > rockScrape NB corestonesVariable and unreliableIIIModerately weatheredPartly changes to soil, rock > soilRipGood for most small structuresIISlightly weatheredIncreased fractures and mineral stainingBlastGood for anything except large damsIFresh rockClean rockSoundEngineering classification of weathered rock
18Rock Mass StrengthStrength depends on the density, nature and extent of the fractures within it
19Rock fractures and their characterization Typically carried out using 3 orthogonal scanlinesorientationspacinglengthroughnessaperturefillingblock size
20Rock Quality Designation (RQD) Quantitative estimate of rock mass quality from drill core logs% intact core pieces >10cm in total length of coreDeere et al., 1967
22RQD RQD Very poor 0 – 25 Poor 25 – 50 Fair 50 – 75 Good 75 – 90 Excellent
23Terzaghi’s Rock Mass Classification (1946) Rock Mass DescriptionsIntactStratifiedModerately jointedBlocky and SeamyCrushedSqueezingSwelling
24Terzaghi’s Rock Mass Classification (1946) Intact rock contains neither joints nor hair cracks. Hence, if it breaks, it breaks across sound rock.Stratified rock consists of individual strata with little or no resistance against separation along the boundaries between the strata. The strata may or may not be weakened by transverse joints. In such rock the spalling condition is quite common.Moderately jointed rock contains joints and hair cracks, but the blocks between joints are locally grown together or so intimately interlocked that vertical walls do not require lateral support. In rocks of this type, both spalling and popping conditions may be encountered.
25Terzaghi’s Rock Mass Classification (1946) Blocky and seamy rock consists of chemically intact or almost intact rock fragments which are entirely separated from each other and imperfectly interlocked. In such rock, vertical walls may require lateral support.Crushed but chemically intact rock has the character of crusher run. If most or all of the fragments are as small as fine sand grains and no recementation has taken place, crushed rock below the water table exhibits the properties of a water-bearing sand.Squeezing rock slowly advances into the tunnel without perceptible volume increase. A prerequisite for squeeze is a high percentage of microscopic and sub-microscopic particles of micaceous minerals or clay minerals with a low swelling capacity.Swelling rock advances into the tunnel chiefly on account of expansion. The capacity to swell seems to be limited to those rocks that contain clay minerals such as montmorillonite, with a high swelling capacity.
26RMR and Q Rock classification systems Primary use of RQD is as a parameter in more widely usedRMR (Bieniawski, 1976) andQ System (Barton et al., 1974)classification systems
27Rock Mass Rating (RMR), Bieniawski (1976, 1989) Classifies rock according to 6 parameters:UCSRQDSpacing of discontinuitiesCondition of discontinuitiesGroundwater conditionsDiscontinuity orientation
37In the case of the residual strength, the cohesion c has dropped to zero and the previous relationship can be represented by:
38The basic friction angle b is a quantity that is fundamental to the understanding of the shear strength of discontinuity surfaces. This is approximately equal to the residual friction angle r but it is generally measured by testing sawn or ground rock surfaces. These tests, which can be carried out on surfaces as small as 50 mm *50 mm, will produce a straight line plot defined by the equation:
41Shear strength of rough surfaces Patton (1966) demonstrated this influence by means of an experiment in which he carried out shear tests on 'saw-tooth' specimens such as the one illustrated in Figure 4.
43Barton’s estimate of shear strength While Patton’s approach has the merit of being very simple, it does not reflect the reality that changes in shear strength with increasing normal stress are gradual rather than abrupt. Barton (1973, 1976) studied the behaviour of natural rock joints and proposed that equation (4) could be re- written as:
44Barton developed his first non-linear strength criterion for rock joints (using the basic friction angle):where r is the Schmidt rebound number wet and weathered fracture surfaces and R is the Schmidt rebound number on dry unweathered sawn surfaces.
50Consider an element of rock at a depth of 1,000 m below the surface Consider an element of rock at a depth of 1,000 m below the surface. The weight of the vertical column of rock resting on this element is the product of the depth and the unit weight of the overlying rock mass (typically about 2.7 tonnes/m3 or MN/m3). Hence the vertical stress on the element is 2,700 tonnes/m2 or 27 MPa. This stress is estimated from the simple relationship:
52The horizontal stresses acting on an element of rock at a depth z below the surface are much more difficult to estimate than the vertical stresses. Normally, the ratio of the average horizontal stress to the vertical stress is denoted by the letter k such that:
53Terzaghi and Richart (1952) suggested that, for a gravitationally loaded rock mass in which no lateral strain was permitted during formation of the overlying strata, the value of k is independent of depth and is given by k = v /(1 − v) , where v is the Poisson's ratio of the rock mass.
54Sheorey (1994) developed an elasto-static thermal stress model of the earth. where z (m) is the depth below surface and Eh (GPa) is the average deformation modulus of the upper part of the earth’s crust measured in a horizontal direction. This direction of measurement is important particularly in layered sedimentary rocks, in which the deformation modulus may be significantly different in different directions.