Rock Mass & Intact Rock Properties, Discontinuities & Index Tests Notes

Last reviewed 30 Sept 2026 · 7 min read

Intact rock and rock mass

A tunnel is driven through a rock mass, not through a laboratory specimen. The distinction is the heart of rock engineering:

Intact rock — the solid rock material between discontinuities, as in a small core sample: its strength and stiffness are properties of the rock material (mineral grains and cement).
Discontinuities — joints, bedding planes, foliation, faults, shear zones and fissures: planes of weakness across which the rock has little or no tensile strength.
Rock mass — the intact rock plus the discontinuities: a jointed assembly of blocks. Its strength, deformability and permeability are controlled mostly by the discontinuities, and are much lower than those of the intact rock.

Rock mass behaviour = Intact rock + Discontinuities + Groundwater + In-situ stress

In hard, massive rock the intact strength governs; in jointed rock the failure occurs by sliding along joints and by the fall of blocks; in weak or weathered rock the mass behaves like a soil, and squeezing and swelling become problems.

Rock types

Group	Origin	Examples	Tunnelling character
Igneous	Cooling of magma	Granite, basalt, dolerite	Strong, massive; jointed by cooling and tectonic joints; abrasive
Sedimentary	Deposition and cementation	Sandstone, limestone, shale, conglomerate	Bedded; variable strength; shale may slake and swell; limestone has cavities (karst)
Metamorphic	Change by heat and pressure	Gneiss, schist, slate, quartzite, marble	Foliated; anisotropic; schist and slate can be weak along the foliation

Strength classification of intact rock

The uniaxial compressive strength (UCS, $σ_{c i}$ ) is the standard measure. A widely used classification (International Society for Rock Mechanics, ISRM):

Grade	Description	UCS (MPa)
R0	Extremely weak	0.25–1
R1	Very weak	1–5
R2	Weak	5–25
R3	Medium strong	25–50
R4	Strong	50–100
R5	Very strong	100–250
R6	Extremely strong	> 250

Main properties of intact rock

Density and porosity — most rocks are 2.4–2.9 t/m³; high porosity means low strength and durability.
Uniaxial compressive strength ( $σ_{c i}$ ) — from tests on cores with a length-to-diameter ratio of about 2.
Tensile strength ( $σ_{t}$ ) — usually 5–10 % of the UCS; measured by the Brazilian (indirect) test on a disc:

σ_{t} = \frac{2 P}{π D t}

$P$ = failure load, $D$ = diameter and $t$ = thickness of the disc.

Elastic modulus ( $E$ ) — from a few GPa in weak rock to over 100 GPa in strong rock; Poisson's ratio ( $ν$ ) typically 0.1–0.35.
Shear strength parameters — cohesion $c$ and friction angle $ϕ$ (of intact rock: $ϕ$ typically 30–60°).
Abrasivity — controls tool wear in TBM and drill-and-blast excavation; slake durability — weathering of shales.

Weathering

Weathering by water, temperature and chemical action weakens the rock and opens the joints. The usual weathering grades:

Grade	Term	Description
I	Fresh	No sign of weathering
II	Slightly weathered	Discoloured along joints
III	Moderately weathered	Less than half the rock decomposed
IV	Highly weathered	More than half decomposed; rock is a discontinuous framework
V	Completely weathered	Entirely decomposed to soil, but the fabric is still visible
VI	Residual soil	Soil with no rock fabric

Tunnels in grades IV–VI are effectively soft-ground tunnels.

Discontinuities and how they are described

Rock mass behaviour depends on the geometry and condition of the discontinuities. Each set is described by:

FormulaDiscontinuity descriptors

Orientation — dip (angle of the plane from horizontal) and dip direction (azimuth). Compared with the tunnel axis, it decides whether blocks can slide or fall.
Spacing — distance between adjacent discontinuities of a set: extremely close (< 20 mm) to extremely wide (> 6 m).
Persistence — the extent of the discontinuity in its plane; long, persistent joints make large, dangerous blocks.
Aperture — the gap between the walls.
Roughness — profile of the surface (measured by the Joint Roughness Coefficient, JRC); rough joints resist sliding.
Wall strength — strength of the rock at the surface (weathering).
Infilling — material in the joint (clay, calcite, sand); soft clay infilling gives low shear strength.
Seepage — water in the joint.
Number of sets — the more sets, the more blocky the mass.

Faults and shear zones are large discontinuities with crushed rock (gouge) — a common cause of collapse, water inflow and squeezing in tunnels.

The shear strength of a joint is often written as $τ = c + σ_{n} tan ϕ$ (Mohr–Coulomb) or, for rough joints, by Barton's criterion $τ = σ_{n} tan [ϕ_{r} + J R C lo g_{10} (J C S / σ_{n})]$ .

Index tests

Index tests are cheap and quick, and are used to estimate the strength when core tests are not possible.

Point load test: a piece of rock (a core, block or lump) is compressed between two conical platens; the point load index $I_{s (50)}$ is

I_{s} = \frac{P}{D _{e}^{2}}

( $D_{e}$ = equivalent diameter, corrected to a 50 mm reference). The UCS is estimated as $σ_{c i} \approx 20$ – $25 I_{s (50)}$ (the factor varies between rock types).

Schmidt (rebound) hammer: rebound number of a spring-loaded mass, related to the surface hardness and correlated with UCS with a chart.
Slake durability, abrasion, and cerchar tests for durability and tool wear.
Brazilian test for the tensile strength.

Worked ExampleExample — point-load and Brazilian tests

A rock core piece of equivalent diameter 50 mm fails at $P$ = 15 kN in a point-load test.

$I_{s (50)} = \frac{15 000}{5 0 ^{2}} = 6.0$ MPa → $σ_{c i} \approx 24 \times 6.0 = 144 MPa$ ("very strong", R5).

A Brazilian test on a disc of diameter 54 mm and thickness 27 mm fails at 25 kN:

$σ_{t} = \frac{2 \times 25 000}{π \times 54 \times 27} = \frac{50 000}{4580} = 10.9 MPa$ .

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