Site Exploration, Core Logging, RQD & Rock Mass Classification Notes

Last reviewed 30 Sept 2026 · 8 min read

Why investigate the ground

Tunnel cost and risk depend on the ground more than on any other factor. A good investigation predicts the rock types, structure, strength, groundwater and stress along the alignment, so that the design and the contract can allow for them. The dangers of poor investigation — collapses, water inrush, squeezing, gas and delayed progress — are the leading causes of cost overruns in tunnelling.

Stages of exploration

FormulaA staged investigation

Desk study — topographical and geological maps, published reports, air photographs and satellite images, records of nearby tunnels, boreholes and wells, seismic and landslide history.
Field geological mapping — outcrops, rock types, dip and strike of bedding and joints, faults and shear zones, springs and seepages, weathering, landslides.
Geophysical survey — seismic refraction (rock velocity, depth to bedrock and weathering), electrical resistivity (water-bearing zones, fault zones, cavities), ground-penetrating radar, and borehole logging; cover long lengths cheaply and identify targets for drilling.
Boreholes — core drilling along the alignment (particularly at portals, shafts, faults, cover-critical spots and under valleys), typically NX (54 mm) or similar size cores, with water-pressure (Lugeon) tests, piezometers, in-situ stress measurement (hydraulic fracturing), and borehole televiewer logging.
Laboratory tests — UCS, tensile strength, modulus, density, porosity, mineralogy (petrography), swelling and slaking, abrasivity.
Pilot tunnel or adit for large or difficult projects; probing ahead during construction.

Spacing of boreholes follows the complexity of the geology; the borehole is taken below the invert level (at least one tunnel diameter below) to see the ground where the tunnel will be built.

Core logging

The core is laid out in core boxes, photographed and logged as soon as it is drilled. The log records:

Depth, run length and core recovery.
Rock type and description: colour, grain size, texture, mineralogy.
Weathering grade and strength grade.
Discontinuities: number, type, spacing, orientation (from oriented core or televiewer), roughness, aperture, infilling.
RQD (see below), fracture frequency (number of fractures per metre).
Water: losses, inflows, water level, Lugeon results.
Remarks: shear zones, voids, drilling problems.

Core recovery

Total core recovery (TCR) = length of core recovered ÷ length of the run × 100 %.
Solid core recovery (SCR) = length of full-diameter solid (cylindrical) core ÷ run length × 100 %.

Rock Quality Designation (RQD)

RQD is the most widely used quantitative index of rock quality:

RQD = \frac{\sum lengths of sound core pieces \geq 100 mm}{total length of the core run} \times 100 %

Only natural fractures count as breaks; mechanical breaks caused by drilling or handling are ignored (fitted together and measured as one piece).

RQD (%)	Rock quality
90–100	Excellent
75–90	Good
50–75	Fair
25–50	Poor
0–25	Very poor

RQD can also be estimated from joint frequency (joints per metre, $J_{v}$ ) on the tunnel face: $RQD \approx 115 - 3.3 J_{v}$ (for $J_{v}$ between about 4.5 and 35).

Worked ExampleExample — RQD

A core run of 1.50 m contains pieces of lengths (cm): 20, 15, 30, 25, 12, 18, 10, 8, 6 and 4, all natural breaks.

Sound pieces of 10 cm or more: 20 + 15 + 30 + 25 + 12 + 18 + 10 = 130 cm.

$RQD = \frac{130}{150} \times 100 = 86.7 %$ — "good" rock.

Rock Mass Rating (RMR, Bieniawski)

The RMR system (1973, revised 1989) assigns a rating to each of five parameters, adds them, then adjusts for the orientation of the discontinuities relative to the tunnel:

Parameter	Maximum rating
1. Strength of intact rock (UCS or point load)	15
2. RQD	20
3. Spacing of discontinuities	20
4. Condition of discontinuities (persistence, aperture, roughness, infilling, weathering)	30
5. Groundwater (inflow per 10 m of tunnel, or water pressure ratio, or general condition)	15
6. Adjustment for orientation of discontinuities	0 to −12 (tunnels)

The total (0–100) gives the rock class:

RMR	Class	Description
81–100	I	Very good rock
61–80	II	Good rock
41–60	III	Fair rock
21–40	IV	Poor rock
< 21	V	Very poor rock

Each class has average stand-up time, cohesion and friction angle of the mass, and recommended excavation and support: for example, in class I a full-face advance with little or no support; in class III a top heading and bench with systematic bolting and a layer of shotcrete; in class V multiple drifts, immediate support with steel ribs and shotcrete, and possibly forepoling.

Worked ExampleExample — RMR

A tunnel section has intact rock UCS 80 MPa (rating 7), RQD 75 % (rating 17), joint spacing 0.3 m (rating 10), slightly rough, slightly weathered joints with separation below 1 mm (rating 25) and damp conditions (rating 10).

$RMR_{ba s i c} = 7 + 17 + 10 + 25 + 10 = 69$ . An adjustment of −5 for a "fair" joint orientation gives $RMR = 64$ → Class II, good rock.

The Q-system (Barton, Lien and Lunde)

The Q-system (1974) is based on six parameters combined in three quotients:

Q = \frac{RQD}{J _{n}} \times \frac{J _{r}}{J _{a}} \times \frac{J _{w}}{SRF}

Symbol	Parameter	Meaning of the quotient
RQD	Rock quality designation	Block size (with $J_{n}$ ): $RQD / J_{n}$
$J_{n}$	Joint set number (1 for massive up to 20 for crushed rock)
$J_{r}$	Joint roughness number	Shear strength of joints: $J_{r} / J_{a}$
$J_{a}$	Joint alteration number
$J_{w}$	Joint water reduction factor	Effect of water and stress: $J_{w} / SRF$
SRF	Stress reduction factor (faulting, squeezing, swelling, rock burst)

$Q$ ranges from 0.001 (exceptionally poor) to 1000 (exceptionally good) on a logarithmic scale:

Q	Quality
400–1000	Exceptionally good
100–400	Extremely good
40–100	Very good
10–40	Good
4–10	Fair
1–4	Poor
0.1–1	Very poor
0.01–0.1	Extremely poor
0.001–0.01	Exceptionally poor

Support design with Q

The equivalent dimension of the excavation is

D_{e} = \frac{span or height}{ESR}

where ESR (Excavation Support Ratio) reflects the safety demanded of the opening: about 3–5 for temporary mine openings, 1.6 for permanent mine openings and water tunnels, 1.3 for minor road and railway tunnels and storage caverns, 1.0 for major road and railway tunnels and underground power stations, and 0.8 for underground stations and public facilities. The support chart gives the type of support (bolts, shotcrete with or without fibre, ribs) from $Q$ and $D_{e}$ . Bolt length in the roof is estimated as

L = 2 + \frac{0.15 B}{ESR} (m)

for a span $B$ ; the maximum unsupported span is $2 ESR Q^{0.4}$ .

Worked ExampleExample — bolt length and unsupported span

A road tunnel with a span of 10 m, ESR = 1.3 and $Q$ = 4 (poor rock):

Bolt length: $L = 2 + \frac{0.15 \times 10}{1.3} = 3.15 m$ .

Maximum unsupported span: $2 \times 1.3 \times 4^{0.4} = 2.6 \times 1.74 = 4.5 m$ — well below the 10 m span, so systematic support is needed.

Correlation between RMR and Q

A common empirical relationship is $RMR = 9 ln Q + 44$ . It is a rough guide only; each system should be applied independently and compared.

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