GeoE 436 Geol. Engineering Analysis and Design
SOIL AND ROCK ENGINEERING CLASSIFICATION
ENGINEERING CLASSIFICATION OF SOILS
Different Viewpoints of "Soil"
1. Agriculture -- granular, tillable material composed of minerals and organics, and which can support
2. Geology -- unconsolidated material that overlies bedrock; also known as overburden or regolith.
3. Engineering -- non-indurated natural material, derived from rock, and composed of discrete solid
particles and voids filled with gas (air) or liquid (water); contrast to "rock", a compact, indurated
natural material comprised of minerals and which requires brute force (drilling, blasting, etc.) to
Fines: soil particles that pass through a No. 200 sieve (mesh openings are 0.075 mm); includes silt
(nonplastic) and clay (plastic, i.e., the plasticity index > 4; see below).
Atterberg Limits for soil fraction finer than No. 40 sieve (0.425 mm):
a) plastic limit (PL) -- water content at which the soil shows a tendency to break apart and crumble
when manually rolled into 3-mm diam. threads (at the PL, the soil has a consistency of toothpaste or
b) liquid limit (LL) -- water content at which a specified trapezoidal groove, cut in moist soil in a
special shallow cup, will close after 25 standard taps (at the LL, the soil has a consistency of soft
c) plasticity index, PI = LL - PL : defines the range of water contents over which the soil is plastic
Note: the water content, w = Wt.water/Wt.solids (usually expressed as a percentage).
Sand: soil particles retained on a No. 200 sieve, but passing a No.4 sieve (0.075 to 4.75 mm);
fine sand -- No. 200 to No. 40 sieve (0.075 to 0.425 mm)
medium sand -- No. 40 to No. 10 sieve (0.425 to 2.00 mm)
coarse sand -- No. 10 to No.4 sieve (2.00 to 4.75 mm)
Gravel: soil particles retained on a No.4 sieve, but passing a 75-mm sieve (4.75 to 75 mm); 19-mm
size separates fine gravel from coarse gravel
Cobbles: particles from 75 to 300 mm
Boulders: particles exceeding 300 mm
GeoE 436 Geol. Engineering Analysis and Design Rock/Soil Engr.Classification 2
General Classification Procedures
Most engineers rely on the Unified Soil Classification System (USC), which has been formalized into
standard ASTM procedures, one based on laboratory analysis procedures (ASTM D2487 -85) and the
other based on visual/manual procedures (ASTM D2488-84). We will focus on the latter of these
systems for principal use in site investigation programs.
1. Particle size percentages -- select either coarse-grained (< 50% fines) or fine-grained (> 50%
fines) category, then note further breakdowns according to gravel/sand percentages (for coarse soils)
or exceedance of 70% fines (for fine soils).
2. For coarse soils, apply a gradation criteria and percentage of fines to identify the soil with an
alphanumeric symbol (e.g., GP, SW); use a dual symbol (hyphenated) to denote that soil has
properties of both categories (GW-GM is a well-graded gravel with silt); it may also be important to
note the following: percentages of grain sizes, maximum grain size, angularity/shape of grains,
cementation, structure & hardness of grains.
3. For fine soils, use percentages of coarse fractions and evaluate presence of organics to identify the
soil with an alphanumeric symbol (e.g., ML, OH); use a dual symbol (hyphenated) to denote that. the
soil has properties of both categories (CL-ML is a silty clay); use a borderline symbol (slash) to
denote that the soil has properties that do not distinctly place the soil in a specific group (CL/CH is a
clay that is neither lean nor fat); it may also be important to note the following: dry strength (based
on 12-mm diam. molded balls), dilatancy (appearance/disappearance of surface water during
shaking, indicating a volume change), toughness and plasticity (based on 3-mm diam. threads).
4. Other general descriptors:
a) insitu moisture condition
b) color when wetted.(damp)
c) odor (organic, petroleum, acidic)
d) reaction with HCI (indicates calcium carbonate cement)
f) consistency (based on thumb penetration into a molded ball)
g) apparent geologic origin -- parent rock type, glacial deposit, aeolian deposit, residual soil
(weathered in-place), colluvium, alluvium
Refer to ASTM publications for more details.
GeoE 436 Geol. Engineering Analysis and Design Rock/Soil Engr.Classification 3
ENGINEERING ROCK MASS CLASSIFICATION
Rock substance: solid, intact rock material that can be sanlpled and tested in the laboratory as a
Rock mass: insitu rock material that includes blocks, discontinuities, and weathered/altered zones
RQD (rock quality designation) is an indirect measure of fracture spacings observed in a drill-core
RQD i 1
where: Li = length of i-th drill-core piece longer than twice the core diameter,
LT = total length of drill run,
N = number of core pieces longer than twice the core diameter.
When drill core is not available, an RQD value can be estimated from line-scan measures obtained
from rock outcrops. First, estimate the fracture density as the number of fractures per meter (D).
Then, the following expressions can be used:
RQD = e-0.1D (1 + 0.1D) (100%) for any positive D
or RQD = (-3.68D +110.4)% for 5 < D < 20
Spacing descriptors for discontinuities (fractures):
Spacing (m) Descriptor
< .02 extremely close spacing
.02 -.06 very close spacing
.06 - .20 close spacing
.20 - .60 moderate spacing
.60 - 2.0 wide spacing
2.0 - 6.0 very wide spacing
> 6.0 extremely wide spacing
GeoE 436 Geol. Engineering Analysis and Design Rock/Soil Engr.Classification 4
Rock Mass Rating System (RMR)
This approach (initially developed by Z. T. Bieniawski, 1973, primarily for civil engineering tunnels in
sedimentary rock) provides results that allow the user to make preliminary recommendations for
tunnel support. It relies on six basic input terms, which are assigned ratings that sum to an overall
1. Uniaxial compressive strength (UCS) of the rock substance;
3. Spacings of discontinuities;
4. Physical condition of discontinuities;
5. Groundwater conditions;
6. Orientations of discontinuities.
The UCS can be estimated by:
a) uniaxial compression tests of laboratory specimens;
b) point load tests conducted in the field or laboratory;
c) Schmidt L-hammer (refer to conversion chart on separate handout).
RQD is estimated directly from drill core or indirectly from empirical equation(s) that relate measured
fracture density to RQD.
The RMR importance rating (weighting) assigned to the discontinuity spacing was based originally on
the mean spacing of three discontinuity sets in the rock mass. If fewer sets are present, then a
conservative assessment results; if more sets are present, the user should select the three sets
oriented most critically to the excavation from a design viewpoint. Likewise, when describing the
conditions of discontinuities, the user should concentrate on those sets deemed most critical to the
design of the excavation.
In addition to the adjustment for discontinuity orientations, other corrections may be appropriate for
hard-rock mining applications (see D.H. Laubscher, 1977, Geomechanics classification of jointed rock
masses -- mining applications, Transac.. of the Inst. of Mining and Met., London, v. 86).
GeoE 436 Geol. Engineering Analysis and Design Rock/Soil Engr.Classification 5
Given: UCS of 150 MPa (21,750 psi): high strength 12
RQD of 70% 13
Average discontin. spacing of 0.4 m 10
Slightly rough jts. with minor sep. and weathering 25
Generally wet conds. with some water inflow 7
A major joint set strikes perpen. to tunnel and dips
30° against the drive direction (unfavorable) -10
RMR rating: 57 (Fair Rock)
For a 5-m wide tunnel, the expected stand-up time is .02 x 103 hrs, or 42 days (unsupported case).
For permanent support of a 10-m wide, horseshoe-shaped tunnel, the following recommendations are
made for fair rock:
1. 3-m to 4-m long rock bolts on regular spacing of 2 m, with wire mesh;
2. 7 to 8 cm of shotcrete in the crown (roof) and as needed on the sides.
The RMR classification system is discussed in detail by:
Bieniawski, Z.T., 1988. The Rock Mass Rating (RMR) System (Geomechanics Classification) in
Engineering Practice; in Rock Classification Systems for Engineering Purposes, L. Kirkaldie, ed., ASTM
STP 984, p. 17-31.
Tunneling Quality, or Q-System
This approach (developed under the auspices of the Norwegian Geotech. Institute, NGI, by Barton,
Lien, and Lunde, 1974) provides results that allow the user to make preliminary recommendations for
tunnel support. It relies on six basic input terms, which are assigned ratings that become part of a
multiplicative equation leading to a tunneling quality index, or Q value:
2. Number of joint (discontinuity) sets (factor known as Jn);
3. Roughness of the least favorably oriented discontinuity set (factor known as J r);
4. Alteration of the least favorably oriented discontinuity set (factor known as Jn);
5. Joint water reduction factor (Jw);
6. Stress reduction factor (SRF) pertinent to stress and major structures.
Three ratios of these factors are formed to represent block size (RQD/J n), minimum interblock shear
strength (Jr / Ja), and active stress (Jw / SRF). The Q value is then calculated as:
GeoE 436 Geol. Engineering Analysis and Design Rock/Soil Engr.Classification 6
Values of Q range from approximately 0.001 to 1000; based upon numerous field case studies,
general rock mass conditions and preliminary tunnel support recommendations can be made given a
calculated Q value.
Descriptions of the factors and procedures for using the Q-system are presented in a separate class
handout. Additional input in regard to tunnel span and a so-called excavation support ratio is required
for support selection.
The basic steps for using the Q-system are as follows:
1. Inspect the rock mass and assign values to the six factors;
2. Calculate the Q value using the expression given previously;
3. Identify the span or diameter of the proposed excavation and its appropriate Excavation
Support Ratio (ESR) based on the type of excavation;
4. Calculate the Equivalent Dimension, De;
5. Use a log-log plot of De vs. Q to determine if rock support is required; if so, use Q, De,
(RQD/Jn), and (Jr/Ja) to specify support requirements using the printed table and comments
provided on the Q handout.
Given: RQD of 80% RQD = 80
Two joint sets plus random joints Jn = 6
Rough, undulating joints Jr = 3
Some clay coatings and fillings Ja = 4
Medium to large inflow of water Jw = 0.6
Competent rock, medium stress SRF = 1.0
Q = (80/6) (3/4) (0.6/1) = 6.0 Note: RQD/Jn = 13.3
De for 10-m wide permanent tunnel for transportation: De = 10/1.3 = 7.7
(ESR = 1.3)
Untensioned grouted dowels (length of 2-4 m) on a regular spacing of 1 m to 1.5 m.
The Q-system is discussed in detail by:
Barton, N., 1988. Rock Mass Classification and Tunnel Reinforcement Selection Using the Q-System;
in Rock Classification Systems for Engineering Purposes, L. Kirkaldie, ed., ASTM STP 984, p. 59-84.