Geomechanical classifications — overview and common parameters¶
In geotechnical design in rock — both for the stability of a slope and for an underground work — detailed information on the strength and deformability characteristics of the rock mass is rarely available. The geomechanical classifications are empirical methods that, starting from the geostructural survey, return a rock-mass quality index and the characteristic parameters (cohesion, friction angle, deformation modulus) needed for the calculation.
Rock Mechanics NX implements six classifications, all fed by the same discontinuity survey (see Workflow):
| Classification | Index | Typical field of application | Page |
|---|---|---|---|
| Barton (1974, rev. 2002) | Q | Underground works, general characterisation | Barton |
| Bieniawski (1976) + Romana (1985) | RMR · SMR | Tunnels, foundations, slopes (SMR) | Bieniawski & Romana |
| Jašarević & Kovačević (1996) | n → RMR | Carbonate rock masses | Jašarević |
| Sen & Sadagah (2003) | continuous RMR | Simplified version of RMR | Sen |
| Robertson (1988) | SRMR | Slopes only in weak rock (RMR < 40) | Robertson |
| Singh & Göel (1999) | N → RMR · Q | Tunnels | Singh & Goel |
Many methods share the same input parameters. To avoid repetition, the common definitions and tables are collected here: the pages of the individual methods refer back to them.
Uniaxial compressive strength Su¶
The A1 parameter of all the methods derived from RMR is obtained from the uniaxial compressive strength \(S_u\) of the intact rock. It can be determined in three ways.
Point Load Test¶
Portable and field-based, it provides the point load index \(I_s\), correlated to \(S_u\) by:
The ISRM recommends \(K = 24\), but in practice the value varies widely. Palmström suggests relating \(K\) to \(I_s\):
| \(I_s\) (MPa) | K |
|---|---|
| < 3.5 | 14 |
| 3.5 – 6.0 | 16 |
| 6.0 – 10 | 20 |
| > 10 | 25 |
Schmidt hammer test (sclerometer)¶
A non-destructive test that measures the rebound hardness of the rock. From the rebound index \(R\), \(S_u\) is obtained with the relationship of Irfan and Dearman (1978):
ISRM standard (quick estimate)¶
In the preliminary phase, in the absence of tests, \(S_u\) is estimated from the response of the rock to a blow from the geologist's hammer:
| Response of the rock | \(S_u\) |
|---|---|
| Can be scratched with a fingernail or crumbled by hand | 0.25 – 1 MPa |
| Crumbles under blows of the pick; thin slabs break by hand | 1 – 5 MPa |
| The pick leaves shallow holes; thin slabs break under strong pressure | 5 – 25 MPa |
| The rock fractures with a single blow | 25 – 50 MPa |
| Fractures after two or three blows | 50 – 100 MPa |
| Only chips off | > 200 MPa |
Which method to use
If you have Point Load or Schmidt hammer tests, it is preferable to obtain A1 from the equations of the Bieniawski charts (given on the page of each method) rather than from the step tables: the result is continuous and less subjective.
RQD — Rock Quality Designation¶
The Rock Quality Designation measures the degree of fracturing of the rock mass. From a borehole it is computed as the recovery percentage referred to the core pieces of length \(\geq 100\) mm:
where \(L_c\) is the sum of the lengths of the core pieces > 100 mm and \(L_t\) is the total length of the measured run.
In the absence of borehole cores, RQD is obtained from the discontinuity survey with the relationship of Palmström (1982):
where \(J_v\) is the number of joints per cubic metre of rock (volumetric joint count).
Alternatively, from the formula of Priest and Hudson (1981):
with \(\lambda\) the mean number of joints per linear metre.
Minimum value
In the Barton and Singh & Goel classifications, if \(RQD < 10\) a value of \(RQD = 10\) is assumed anyway.
Discontinuity spacing¶
The mean spacing \(s\) is the mean distance between two adjacent discontinuities of the same set. It determines the A3 parameter of RMR (with method-specific equations) and enters the tables of Robertson and Jašarević.
Parameters of the Barton classification (Q)¶
The four parameters that follow (\(J_n\), \(J_r\), \(J_a\), \(J_w\)) and the SRF factor are common to Barton, Singh & Goel and, in derived form, to the Seismic collapse utility. The relevant pages refer back to the tables below.
Parameter J~n~ (Joint Set Number)¶
It depends on the number of joint sets present in the rock mass.
| Definition | \(J_n\) |
|---|---|
| Massive rock, no or rare discontinuities | 0.5 – 1 |
| One set of discontinuities | 2 |
| One set of discontinuities + random ones | 3 |
| Two sets of discontinuities | 4 |
| Two sets of discontinuities + random ones | 6 |
| Three sets of discontinuities | 9 |
| Three sets of discontinuities + random ones | 12 |
| Four or more sets of discontinuities | 15 |
| Completely disintegrated rock | 20 |
Tunnels
At a portal zone \(J_n\) must be doubled; at an intersection zone of two tunnels it must be tripled.
Parameter J~r~ (Joint Roughness Number)¶
It depends on the roughness of the most unfavourable set.
| Definition | \(J_r\) |
|---|---|
| Discontinuous joints | 4 |
| Rough or irregular, undulating joints | 3 |
| Smooth, undulating joints | 2 |
| Slickensided, undulating joints | 1.5 |
| Rough or irregular, planar joints | 1.5 |
| Smooth, planar joints | 1.0 |
| Slickensided, planar joints | 0.5 |
| Mineralised zones with clay minerals filling the discontinuity | 1.0 |
| Mineralised zones with sand, gravel, disintegrated zones | 1.0 |
The description refers to the small- and medium-scale characteristics. If the mean spacing of the main set exceeds 3 m, increase \(J_r\) by 1. For planar, slickensided joints with striations oriented in the most unfavourable direction, use 0.5.
Parameter J~a~ (Joint Alteration Number)¶
It depends on the degree of alteration of the fractures, on the thickness and the nature of the infilling, determined on the most unfavourable set.
Essentially closed joints (aperture 1–3 mm) with walls in contact:
| Definition | \(J_a\) |
|---|---|
| Sealed or mineralised joints | 0.75 |
| Unaltered joints or with slight oxidation | 1 |
| Slightly altered joints or with coatings of non-plastic material | 2 |
| Joints with silty coatings, limited non-plastic clay fraction | 3 |
| Coatings of low frictional-strength minerals (clays, mica, talc, graphite, chlorite, gypsum) | 4 |
Moderately open joints (< 5 mm), infilling that maintains contact between the walls in case of sliding:
| Definition | \(J_a\) |
|---|---|
| Sandy infilling | 4 |
| Non-plastic clayey infilling, heavily overconsolidated | 6 |
| Plastic clayey infilling, moderately overconsolidated | 8 |
| Swelling clayey infilling | 8 – 12 * |
Open joints (> 5 mm), no contact between the walls in case of sliding:
| Definition | \(J_a\) |
|---|---|
| Zones or bands of non-plastic silty or sandy clay | 5 |
| Zones or bands of disintegrated rock | 6 |
| Zones or bands of non-plastic clay | 6 |
| Zones or bands of swelling plastic clay | 8 |
| Zones or bands of swelling clay | 12 |
| Thick continuous zones of non-plastic clay | 10 |
| Thick continuous zones of non-swelling plastic clay | 13 |
| Thick continuous zones of swelling plastic clay | 13 – 20 * |
* The value depends on the percentage of the swelling clay fraction and on the possibility of it coming into contact with water.
Parameter J~w~ (Joint Water Number)¶
It depends on the hydrogeological conditions.
| Definition | \(J_w\) |
|---|---|
| Water absent or scarce, locally < 5 l/min | 1 |
| Medium inflow with occasional washout of the infilling | 0.66 |
| Strong or high-pressure inflow in compact rock with open discontinuities without infilling | 0.5 |
| Strong or high-pressure inflow with washout of the infilling | 0.33 |
| Exceptionally strong inflow immediately after excavation, decreasing over time | 0.2 – 0.1 |
| Exceptionally strong inflow immediately after excavation, constant over time | 0.1 – 0.05 |
In the last four cases, with effective drainage systems \(J_w\) should be brought back to 1 or 0.66. For a characterisation far from the influence of the excavation, with \(RQD/J_n\) sufficiently low (0.5 – 25), the values of \(J_w\) (1.0 – 0.66 – 0.5 – 0.33) can be assumed as a function of the overburden heights (0–5; 5–25; 25–250; > 250 m).
SRF factor (Stress Reduction Factor)¶
A function of the stress state in massive rock or of the tectonic disturbance. See the full table on the Barton page, which distinguishes its four contexts (weakness zones, competent rock mass, squeezing rock mass, swelling rock mass).
Comparison of the classifications¶
- Barton (Q) and Singh & Goel (N) start from the same joint indices; N is Q without the stress effect (SRF = 1).
- Bieniawski (RMR) is the reference from which Romana (SMR), Jašarević (n) and Sen are derived, linked by empirical correlations of the type \(RMR = f(\text{index})\).
- Romana (SMR) and Robertson (SRMR) are designed for slopes: the former adds to RMR the face/joint orientation factors, the latter is a scale dedicated to weak rock.
For the links between Q and RMR, the most widely used relationship of Bieniawski is:
Found an error on this page? Let us know.