DEPARTMENT OF THE ARMY
U.S. Army Corps of Engineers
CECW-EG Washington, DC 20314-1000 ETL 1110-2-282
Technical Letter
No. 1110-2-282 30 June 1983
Engineering and Design
ROCK MASS CLASSIFICATION DATA REQUIREMENTS
FOR RIPPABILITY
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Title and Subtitle Contract Number
Engineering and Design: Rock Mass Classification Data
Requirements for Rippability Grant Number
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Department of the Army U.S. Army Corps of Engineers
Washington, DC 20314-1000
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8
DEPARTMENT OF THE ARMY ETL 1110-2-282
U. S. Army Corps of Engineers
DAEN-ECE-G Washington, D. C. 20314
Engineer Technical
Letter 1110-2-282 30 June 1983
Engineering and Design
ROCK MASS CLASSIFICATION DATA REQUIREMENTS
FOR RIPPABILITY
This ETL contains information on data required for rock
mass
1 -c asslflcation with respect to rippability.
This ETL is applicable to all field operating
actlvl
2 * les avlng civil works design responsibilities.
3. References and Bibliography.
a. References.
(1) EM 1110-1-1802, Geophysical Exploration.
(2) Bieniawski, Z. T., Tunnel Design by Rock Mass Classifications,
Technical Report GL 79-19, Available from U.S. Army Engineer
Waterways Experiment Station, P.6, Box 631, Vicksburg, MS 39180.
b. Bibliography.
(1) Weaver, J. M., Geological Factors Significant in the Assessment
of Rippability, The Civil Engineer in South Africa, Vol. 17, No. 12,
December 1975.
(2) Caterpillar Tractor Company, Handbook of Ripping, 6th Edition,
Caterpillar Tractor Company, Peoria, Illinois, June 19/8.
(3) Caterpillar Tractor Company, Caterpillar Performance Handbook,
Edition 13, Section 1-10, Caterpillar Tractor Company, Peoria, Illinois,
October 1982.
(4) Church, H. K., Two Exceptions to Seismic Principles, World
Construction, Vol. 27, No. 5, pp 26-32, 1974.
The ripper is a relatively narrow-profile implement (as
:;.W to a plow) which penetrates the earth and is pulled to loosen
soil or rock material for excavation. In the early days of our
technological development, tractor-drawn rippers were used to increase
the usefulness of scrapers. The advent of the tractor-mounted ripper
several decades ago offered increased possibilities for work in rock
because of the increased force on the ripper tooth. Since that time,
increases in tractor weight and horsepower, as well as improvements in
ETL 1110-2-282
30 Jun 83
ripper design, have further extended the capability of the ripper in
rock. Much rock, which was traditionally loosened for excavation by
drill and blast methods, is now rippable.
5. Rock mass parameters influencing rippability. Because relatively
harder and tighter rock IS now rlppable, a casual field observation
rippability assessment has become more difficult, A careful assessment
based on an evaluation of several rock mass parameters is often needed.
Such an evaluation frequently requires field data from core borings
and/or geophysical work (bibliography 3b(l)). Six geological factors
which are likely to influence the assessment of rippability are as
follows:
Rock type. Sedimentary rocks are usually the most easily ripped
due ?; part to their bedding characteristics. Common metamorphic rocks
such as gneiss, quartzite, schist, and slate are generally more difficult
but vary in rippability with their degree of lamination or cleavage.
Igneous rocks such as the granitic and basaltic types are the most
difficult to rip because they lack the stratification and ceavage planes
needed to rip hard rock.
b. Rock structure. Discontinuities in the form of fau ts, fractures,
joints, cleavages, schistocity, bedding, and laminations al act as planes
of weakness facilitating ripping. The continuity, spacing, and strike and
dip orientation of joints and fractures and the presence of gouge material
are of particular importance in assessing rippability.
c. Rock hardness. Softer rocks having lower unconfined compressive
strengths are more easily ripped.
d. Rock weathering. The greater the degree of weathering the more
easily the rock is ripped.
e. Rock fabric. Coarse grained rocks rip more easily than fine
grained rocks.
f. Seismic wave velocity. The velocity of a shock wave depends on
the density and degree of cementation of materials. Rock masses having
lower wave velocities are more easily ripped.
6. Rippabi lity Assessment: Seismic Wave Method. The seismic wave
veloclty method for rippabillty assessment was developed first during
1958 by the Caterpillar Tractor Company (bibliography 3b(Z) ). The
physical principal used for the determination of rippability is that
seismic waves travel faster through rock having a higher bulk density
than through rock less consolidated. The wave velocity (average) is
influenced by such geological factors as rock hardness, stratification,
degree of fracturing, and amount of decomposition or weathering, all of
which influence rippability. In general, a lower seismic wave velocity
2
ETL 1110-2-282
30 Jun 83
indicates material more easily rippable. However, the average velocity
of a seismic wave alone, does not correlate well with rippability. For
example, a weathered or badly fractured granite having a smiliar wave
velocity as a rippable siltstone may not be rippable. Caterpillar found
that a comparison of the wave velocities recorded with those obtained in
a similar material from previous experience gives a good indication of
ripper performance. They have published charts showing ripper
performance as related to seismic wave velocities for their equipment
(bibliography 3b(l) and (2)). A typical example of a ripper performance
chart is shown in Figure 1 (See Inclosure 1).
7. Use of Refraction Seismograph. The refraction seismograph can be
used to determine both he mass density and the thickness of the upper
layers. Its cost is low compared to closely spaced borings. This method
generally gives reliable results. However, several exceptions do exist.
In the case where a layer is underlain by one of lower velocity, (hidden
layer problem), interpretation is difficult and inaccuracy can be
expected (reference 3a(1) page 3-9). In areas where bedrock is covered
with large boulders or where the bedrock surface is highly uneven due to
solutioning or structural anomalies, seismic velocity data may be too
unreliable for evaluating rippability. Church (bibliography 3b(4))
suggested a method to compensate for this condition where it is suspected
which involves the lowering of velocity ranges for rippability in the
hard ripping classifications.
8. Rippability Assessment: Rock Mass Rating Method (RMR). It is
t)osslble to obtain an Indication of rlRuabllltv uslnq Bienfaski s
geomechanics classification system (reference ~a(l))~ Bieniawski
proposed the geomechanics classification system (RMR) to rate a rock mass
by assigning weighted numerical values to each of six rock mass
parameters. The final rating was the sum of the weighted parameters. An
inverse relationship exists between the classification description and
rippability, that is, a material classified as very poor rock for
tunneling would be considered easily rippable. Weaver (bibliography
3b(l)) proposed a rippability rating chart based upon a modification of
the geomechanics rating system; a similar chart is shown in Figure 2.
The user would determine a total rippability rating by adding the rating
for each of the rock mass parameters shown, resulting in a quantitative
determination of relative ripping difficulty. The lower ratings
correspond to easier ripping and the higher ratings correspond to more
difficult ripping or required blasting. Weavers system uses seismic
wave velocity as a very significant parameter and does not consider
groundwater inflow as used in the geomechanics rating system.
9. Correlation with Tractor Size. Rippability for a given tractor
selectlon 1s correlated with he total rippability rating in Figure 3.
This figure shows the corresponding seismic velocities for average
3
ETL 1110-2-282
30 Jun 83
conditions; the given velocity scale may be used where adverse conditions
such as unfavorable orientation of bedding planes or joints do not exist
(bibliography 3b(2)).
When data can be obtained on the parameters required for
10w
use o t e rock mass rating or other similar systems in rippability
assessment, their use will supplement an assessment using only seismic
data and rock type and should enhance overall engineering judgment. In
particular, t,he use of the rock mass rating system gives the user a means
of quantifying rippability assessments while taking into account a wide
spectrum of rock mass parameters. Although seismic wave velocity is a
good indicator of rippability, its use must be tempered by judgment. For
example, the predicted production from Caterpillars production estimating
charts (bibliography 3b(3)) is lowered where adverse conditions exist
such as thick bedding, vertical lamination, or any other factor which
would adversely affect production. Validation of the recommended
procedures in this ETL can only be made after the procedures have been
applied on construction contracts in differing geologic materials.
However, these procedures are not always applicable to rock excavation.
Even in marginally excavatable material, the maximum seismic velocity
applicable to the procedure is about 10,000 fps. Most crystalline,
unweathered and unfractured rock will exceed this velocity.
FOR THE CO~NDER:
d~? Tw
1 Incl WILLIAM N. McCORMICK, JR.
as Chief, Engineering Division
Directorate of Engineering and
Construction
4
ETL 1110-2-282
. 30 Jun 83
Figure 1.
D8L Ripper Performance Related to Estimated seismic wave vel~ity.
Multi or Slnglo Shank No. 8 Ripper from Caterpillar Performance Handbook, 1982.
!
Inc16sure 1
I1
CQo
WI
N
w Oescrigtfon Very good rock 6ood rock Fair rock Poor rock Very poor rock
&
c 1. Seistic velocity (ft/s) mm 7000-60W 6000-5000 5oal-m 4000-1500
H
m Rating 26 24 20 12 5
M
. 2. Rock herdness* Extr~ly hard Very hard Hard soft Very soft
z lm psi 1OOW-29OO psi 2-1450 PSi 1450-435 psi 435 psi
G
o- 3. Rock weathering Umseathered slightly Weathered Highly ~letely
*
w meathered wathered uea~
P.
G Rat ing 9 7 5 3 1
z 4. Joint Spuing (m) 3000 3OW-1OM IMO-300
H m 3C9-50 50
I I-t
M l-. Rating 30 25 20 10 5
I
s
[ 5. Joint cmtinuity Nmcmt i nuous Slightly Cmt inuous - no Cent inuous - Cent inuous -
G continuous 90W s- gouge ISi th gouge
&\
-: Rating
m 5 5 3 0 0
6. Joint gouge No separation S1i#lt aarat ion 6ouge 5 m Souge 5 m
2
:; separatim in
d
6 Rating 5 5 4 3 1
M
F 7. Strike md dip Very Unfavorable slightly Favorab 1e Very favorable
m
-
m orientatim unfavorable unfavorable
.
Rating 15 13 10 5 3
Correspmding ti unconfined c~ressive strength.
-.
ETL 1110-2-282
30 Jun 83
o
0
m
>...
o $;
N
:>>..
:3:
:::<.
:.::...
j. 0 i.
..
TRACTOR SELECTION
(Single
Shank Ripper)
Figure 3.
Tractor SelectIon Based on Rippabiiity Rating
from Caterpiiiar Performance Handbook, 1982.
I-3
