ACTIVITY

WEEK 10

Lecture (3 hours)

Week 10 : (3H) Coverage : Methods of soil and rock

sampling

LEARNING OUTCOMES

Learning outcomes:

At the end of this lecture/week the students would

be able to:

discuss the methods for soil and rock sampling

Soil and Rock sampling, Rock mass

rating (RMR), Q system

OUTLINE of PRESENTATION

IN SITU TESTING & ANALYSIS

IN SITU TESTING & ANALYSIS

IN SITU TESTING & ANALYSIS

IN SITU TESTING & ANALYSIS

Site Investigation (ECG513) ARM –2008

IN SITU TESTING & ANALYSIS

IN SITU TESTING & ANALYSIS

IN SITU TESTING & ANALYSIS

IN SITU TESTING & ANALYSIS

Intact rock element to heavily jointed rock mass (Hoek &Brown, 1980)

Rock mass contains fractures rendering its structure discontinuous (loss of

intactness/homogeneity)).

Rock material (element) vs rock mass.

Rock material is the intact rock element in between discontinuities. Rock

mass is the total or multi-phase medium containing both intact elements and

fractures (bedding planes, joints, faults, etc.)

Discontinuities in rock mass

Characterisation & classification

• Rock “material” — strong, stiff, brittle

– Weak rock > Strong concrete

– Strong in compression, weak in tension

– Postpeak strength is low unless confined

• Rock “mass” — behavior controlled by discontinuities

– Rock mass strength is 1/2 to 1/10 of rock material strength

• Discontinuities give rock masses scale effects

The term discontinuity is used collectively referring to all fractures in rock such

as joints, faults, bedding plane, etc.

Rock is a natural material, created, modified and recreated over millions of

years. Geotechnical engineers must design works utilizing the properties that

nature provides (what the rock mass has inherited).

Conforming to this concept, discontinuities that weaken rock mass have

direct impact on any design & construction in rock so much so that their

various characters must be recognized. In addition, to ease an otherwise

complex system suitable simplified classification system of the rock mass is

drawn. Without formulating accurate model of the rock mass in conducting

our design and construction, our engineering efforts will be wasted.

Massive rock

– Rock masses with few discontinuities, or

– Excavation dimension < discontinuity spacing

Jointed or “blocky” rock

– Rock masses with

moderate number of

discontinuities

– Excavation dimension

> discontinuity spacing

Heavily jointed rock

– Rock masses with a large

number of discontinuities

– Excavation dimension >>

discontinuity spacing

• Discontinuities in rock mass have 2 phases of defects: fabric & structural

• Fabric defects are those related to formation process (e.g. bedding plane in sedimentary

rock; cleavages, schistosities (in metamorphic rock)

• Structural defects related to historical tectonic or external forces (e.g. faults, folding,

joints, sheared, pre-existing slide surface, slickenside)

Types & Characterisations of discontinuity

• Bedding plane: inherited in sedimentary rocks as stratification. Preferred orientation in depositional process may give rise to planes of weakness parallel to the bedding

• Folds are structures changing the attitudes of beds by tectonic forces. May alter bed orientations & shear strength.

• Faults are fractured zones along which appreciable shear displacement has taken place.

• Shear zones are bands in which local shear failure has taken place. Surface may be slickenside or coated with low-friction material.

• Joints are breaks of geological origin along which there has been no visible displacement. A group of parallel joints is called a joint set. Joints may be open or filled.

• Cleavage planes are generated under tensile stresses which determine splitting of rock along definite parallel plane.

• Schistosity are foliations in coarse-grained metamorphic rocks.

Geologic

structures

• Orientation: defined by dip & dip direction (strike) and recorded as e.g. 290/90 i.e. azimuth of 290 deg. inclined vertically.

• Spacing is perpendicular distance between 2 discnt. Among others, mass permeability is influenced by it. Has its own classification. Table 3.1

• Rock Quality Designation (RQD) is the percentage of individual lengths > 0.1m to total length of drill run. Related to discnt. Spacing. See Fig. 3.8

• Persistence is the extent of discnt. within a plane. Fig. 3.9.

• Roughness is a measure of surface unevenness. Fig. 3.11

• Aperture is the opening size of discnt. Fig. 3.11

• Filling is the material occupying in the aperture. It has major influence on shear strength of discnt.

Mapping of outcrop exposures and discontinuities

• Start with study of regional geology

• Geological mapping of surface & undergrd.

exposures & logging of BHs

• Where undergrd. access is not possible, surface

exposure must be utilised to obtain info on rock

mass. Such as from natural outcrops or faces of

exposed excavation – by scanline survey. This,

however, must be validated by endergrd exposure

Scanline is a measuring tape fixed on

to the rock wall/face. Photograph with

tape graduation.

D: Dist. Along scanline

L: Length of discnt above line (set line @ base)

T: Nature of termination pt. (e.g. A means at another discnt; i= in rock material; O= obscured

C: curvature or waviness o scale of 1 to 5

R: roughness on scale of 1 to 5

Comments: nature of infilling, aperture, seepage, etc.

Scanline survey logging sheet

Locate scanline in each zone of rock mass

Avoid bias on scanline position. If

necessary carry out in two octhogonal

directions

ROCK MASS CLASSIFICATION

• Behaviour of rock mass can be so complex that it cannot be predicted by engineering analysis. Thus design decision sometimes has be based on previous experience gained

• Previous successful project experiences have been correlated with the nature (behaviour and properties) of the rock mass or simply the classified nature: thus the rock mass classification

• A number of rock mass classification schemes have been developed. Each seek to assign numerical values to selected features of the rock mass (likely to influence the mass behaviour) such as spacing of joint, strength of intact rock, etc.

• The combined numerical values into classification rating is correlated with rock mass behaviour such as stable span of unsupported excavation, stable pit slope, etc.

• Two widely used schemes are the NGI tunnelling quality index (Q) by Burton (1974) and CSIR geomechanics or Rock Mass Rating (RMR) by Bieniawski (1973,1976)

• The schemes have shortcomings so do not fully evaluate important aspect of a problem

• The schemes give reliable results only for rock masses and under circumstances from which the guidelines were originated

• RMR has been developed based on data obtained mainly for civil engineering excavations in sedimentary rocks in South Africa.

• RMR uses 5 classification parameters: intact rock strength, RQD, joint spacing, condition of joint and grd water conditions.

Geomechanics classification of jointed rock mass (RMR)-Bieniawski 1976