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7/28/2019 Challenges of Excavation by Ripping Works in Weathered Sedimentary Zone
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ABSTRACTRipping is a method of loosening rock during excavation using steel tynes attached to the rear
of bulldozers. The tynes are lowered into the ground as the bulldozer moves forward and soilor blocks of rock are displaced by the tynes. In tropical region where thick profile of weathered
zone can be encountered, ripping work is always accepted as the limit of mechanical breaking
before blasting works is preferred due to the economical reason. However, as we know, thenature of rock type and its weathering profile play a very significant role in evaluating the
excavation assessment. Great challenges in ripping work can be expected in sedimentary zone
where the occurrence of discontinuity such as bedding thickness, folding, foliation and theinhomogeneity of rock can greatly influences its excavatability. This paper aims to highlight
some of the problems that arise in weathered sedimentary area as what have been experienced
in Malaysia during the surface excavation works.
KEYWORDS: Ripping, Steel Tynes, Tropical Region, Sedimentary Zone, Bedding
Thinkness, Folding, Foliation, Inhomogeneity.
INTRODUCTION
Ripping is one of the main methods used for mechanical breaking or loosening the ground to
a degree sufficient enough for excavation and loading. It is done by dragging one or more steeltynes attached to a rigid frame to a bulldozer through the ground. The ripping of rock mass
materials has been done since the Romans era using oxen towed rippers. In modern times, the
ripper in its present form did not appear until around 1930s, when tractor-drawn rippers mounted
on steel wheels were introduced. Since then, the systematic development of ripping has
proceeded and the introduction of the tractor-mounted ripper has extended the capability of the
ripper by applying additional weight on the ripper tooth.
Challenges of Excavation by Ripping
Works in Weathered Sedimentary Zone
Edy Tonnizam Mohamad
Senior Lecturer, Faculty of Civil Engineering, Universiti Teknologi Malaysia
edy@utm.my
Seyed Vahid Alavi Nezhad Khaili Abad
Researcher, Department of Geotechnics and Transportation, Faculty of Civil
Engineering, Universiti Teknologi Malaysiav_alavi_59@yahoo.com
Rosli Saad
School of Physics, Universiti Sains Malaysia
rosli@usm.my
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Ripping performance is known to be influenced by the material strength and its discontinuity
characteristics (Pettifer et al., 1994). Ripping works can be efficient and economical in an area
where the hard material is homogeneous and the geology can be predicted. However, during site
study a few other factors such as presence of iron pan that surfaced the rock mass and the
stratigraphical characteristics in weathered sedimentary area (sandstone / shale dominated area)also plays important role to the performance of the excavation. On the other hand, initial
evaluation by using seismic velocity did not give very satisfied judgement on its excavatability as
what has been suggested by the ripper manufacturer. Generally, sedimentary zone with structural
and geological complexity will definitely cause greater challenges for the ripping works.
There are several methods that being used for assessing the surface excavation method,
namely seismic velocity, graphical and grading methods. The need for a reliable excavation
assessment method has been increased with developing technology. Some of the assessments are
based purely on P wave seismic velocity, rest of methods used combination of rock mass and
material properties. However due to interaction between these parameters, the assessment become
more complex and the material properties to be excavated are variables and not possible to
specify on what excavation method could be used in the planning stage. Furthermore, ripping isaffected not only to material properties but also depending on equipment selected. This paper
presents issues that have been experienced by writers during the rippability assessment in Bukit
Indah, Johor on sedimentary area.
Figure 1: Location of Studied Site
Bukit Indah
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GEOLOGY OF STUDIED AREA
The Bukit Indah Site includes mainly shale and immature sandstone, with some siltstone,
conglomerate, and volcanic layers. The presence of conglomerates, large-scale cross bedding inthe sandstone, and plant fossils, all imply deposition in shallow water. The Bukit Indah of the
Jurong Formation is characterized mainly by subdued topography. In accord with the regional
strike this feature swings from a north-northwest direction in the north to west-northwest in the
south. The ridge is composed mainly of argillaceous rocks and has been subjected to considerable
dissection.
Properties of Weathered Sandstone and Shale
Tropic country has sunny flux all the year (220-320 C), high moisture content in air and
underground, high quantity of rain (>1200 mm) and underground water of 280C (Thomas, 1994).
With these characters, climate has great influence to exogenic process especially to chemical
weathering process where the high intensity of rain and high temperature will accelerate theweathering process.
Several studies have been done to understand geotechnical properties of weathered
sedimentary rock in Peninsular Malaysia (e.g., Ibrahim, 1995). The results showed that material
properties deteriorate from the fresher material as more intense weathering taken place. The
weathered rock has lesser strength due to the presence of microfractures and the loosening of the
bonding between grains (Fookes et al, 1988). The weathering effect can take place up to 100m
down from the earth surface in tropical area (Ibrahim, 1995). IAEG (1981) classified the weak
rock will have uniaxial compressive strength from 1.5 – 50 Mpa.
Generally, sedimentary rock mass consists of more than a type of rock and always forms
alternate laminated because of natural forming process and also exposed to tectonic effect and pressure. The weak rock in grade III to V (Table 1) has always been the grey area in ripping and
excavation. This is because the layer where grade III to V is found to be interbedded or
sandwiched between different layers.
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Table 1: Description of the weathering zones of the in-situ rock (after Attewell, 1993)
Weathering
Zone (material
grade)
Descriptive
terms
Material description and likely engineering characteristics
Zone 6 (Grade
VI)
Residual soil Completely degraded to a soil; original rock fabric is
completely absent; exhibit large volume change; the soil has
not been significantly transported.
Stability on slopes relies upon vegetation rooting and
substantial erosion & local failures if preventive measures are
not taken
Zone 5 (Grade
V)
Completely
weathered
Rock is substantially discolored and has broken down to a
soil but with original fabric (mineral arrangement & relict
joints) still intact; the soil properties depend on the
composition of the parent rock.
Can be excavated by hand or ripped relatively easily. Not
suitable as foundation for large structures. May be unstable insteep cuttings and exposes surfaces will require erosion
protection.
Zone 4 (Grade
IV)
Highly
weathered
Rock is substantially discolored and more than 50% of the
material is in degraded soil condition; the original fabric near
to the discontinuity surfaces have been altered to a greater
depth; a deeply weathered, originally strong rock, may show
evidence of fresh rock as a discontinuous framework or as
corestone; an originally weak rock will have been
substantially altered, with perhaps small relict blocks but little
evidence of the original structure. Likely engineering
characteristics are as in Zone 5.
Zone 3 (GradeIII) Moderatelyweathered Rock is significantly discolored; discontinuities will tend to be opened by weathering process and discoloration have
penetrated inwards from the discontinuity surfaces;
Zone 2 (Grade
II)
Slightly
weathered
Some discoloration on and adjacent to discontinuity surfaces;
discolored rock is not significantly weaker than undiscoloredfresh rock; weak (soft) parent rock may show penetration of
discoloration.
Normally requires blasting or cutting for excavation; suitable
as a foundation rock but with open jointing will tend to be
very permeable.
Zone 1 (Grade I) Fresh No visible sign of rock material weathering; no internal
discoloration or disintegration. Normally requires blasting or
cutting for excavation; may require minimal reinforcement incut slope unless rock mass is closely jointed.
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Inhomogeneity of Strength
The difference in weathering grade between different rock materials although it forms in the
same rock mass can be a great challenge in ripping works. Sandstone behaves differently as
compared to shale to weathering agent because their genesis. Fresh sandstone which is well
cemented has minimal foliation and lamination as compared to shale and relatively difficult to rip
(Figure 2). Shale is always known to have lamination or fissile, which provide spaces for
weathering agents to be in contact. Furthermore, shale composed of clay size material that is
smaller than 0.062mm in size and some of clay types such as illite and montmorillonite may
absorb water aggressively and will degrade easily on exposure to weathering agents as compared
to sandstone.
Figure 2: Ripping performance is inconsistent when working on multi layered area
Md For et al. (2003) and Tajul et al. (2000) reported that, hard material has always become an
argument issues by contractors and clients if it cannot be classified as rock or soil. This statement
always refers to grade III (moderately weathered) to V (completely weathered) in the weathering
scale. Existing excavation assessments have always considered the strength factor to be one of its
major factors in deciding whether the material can be ripped or otherwise. However if strength is
the only parameter considered, overall results may be ambiguous especially if sandstone and
shale is evaluated separately as both materials may not have the same strength even though they
are in one massive rock body. The sandstone may be in Grade III but the shale may have further
deteriorated to grade V as shown in Figure 3. Shale, which is interbedded with sandstone, might
have lower in strength compared to the sandstone and their weathering grade might varies even
though exist in the same rock mass.
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Figure 3: Difference of weathering grade in sandstone and shale
In igneous origin area, we can expect abundant of boulders which may have similar strength,
but vary in the sizes. Small boulders can be excavated easily by normal digging, but the bigger
size would need different technique in excavating them (Figure 4).
Figure 4: Weathering in Granite
STRUCTURAL AND STRATIGRAPHYThe structural and stratigraphy of the sedimentary rocks play an important role in ripping
performance and should not be neglected in the excavation assessment study. Low strength
material which can be ripped easily if it stands independently might not be able to rip if it is
sandwiched between unripped materials. Figure 5 shows a significant volume of Grade V shale
lying under Grade IV sandstone which cannot be ripped. Stratigraphic of unripped rock mass on
B1-B3 are also presented to show how the softer material is sandwiched by harder materials.
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Figure 5: Significant amount of shale (Grade V) lying under Grade IV sandstone
Figure 6: Stratigraphy of B3 area, Bukit Indah
Ripping performance is also influenced by the structure of the rock material. Ripping on flat
horizontal bedding will depend in the properties of that particular rocktype (physical and
discontinuities). However, if ripping is done on the structurally vertical layers, the performance
will depend on the multilayer rock properties. Figure 7 and 8 show the scenarios.
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Figure 7: Horizontal layer
Figure 8: Vertical Layer
Ripper tooth
Ripping
Direction
Ripping
Direction
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Assessment by Seismic Velocity
Result of seismic velocity study at Bukit Indah, Johor has provided an ambigous indicator for
rippability assessment if seismic velocity is the only tools used. The seismic velocity data is
shown in Figure 5. The cross section shows that the upper layer with thickness about 5 meter has
velocity less than 2000 m/s. Caterpillar Performance Handbook (2001) recommended that CAT
D9 could rip this layer which consist of Grade IV sandstone. However, in reality CAT D9 could
not rip this whole part leaving protruding rocks as shown in Figure 9. Discontinuity analysis is
essential to evaluate whether the material is rippable or not. Weak material can be unrippable if
the discontinuity is widely spaced (more than 1m) and this will resist penetration of ripper shank.
Figure 9: Seismic velocity data for B1-B3 in Bukit Indah, Johor
B1
B3
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Figure 10: Location of B1-B3
Discontinuities and Ripping Direction
Small scale discontinuity refers to lamination, foliation, joints or any other discontinuity
which is usually less than 10 cm in size. Bedding and shearing are examples of bigger scale
discontinuity. These discontinuities should be properly addressed in the excavation assessment as
the smaller scale discontinuity will react differently to ripping as compared to bigger scale such
as bedding. The orientation and direction of big and small scale discontinuities need to be
assessed prior to the ripping works. Bedding and lamination for example may incline to certain
direction whereas the joints may scattered. Site experience in Bukit Indah, Johor showed that
ripping production varies if direction of ripping is towards discontinuity dominated prone
direction even if it is within the same rock mass.
Writers also experienced that softer material which is Grade IV sandstone that can be broken
by hand could not be ripped due to lack of discontinuities present. Not surprisingly, sometimes
very strong rock can be ripped easily by CAT D9 ripper due to closely spaced discontinuities(Figure 11). This shows that discontinuity play a significant role to material rippability.
Figure 11: Closely spaced joints help ripping process
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Figure 12: Unripped sandstone (Grade III)
Presence of infill material
Where chemical weathering is crucial in tropical climate, it will transport substance of
minerals in rock to be accumulated at joints opening. Accumulation of iron pan is a good example
of this secondary product (Figure 13). The iron pan can exist from few mm thick to more than
10cm thick. 3 cm thick of iron pan which blanket the surface of Grade IV (supposed can be
ripped) material is enough to resist the penetration of the ripper shank, hence disallowing the
ripping works. Other secondary mineral such as quartz also need to be assessed as it might have
higher strength than the host material.
Figure 13: Presence of iron pan (5cm thick) resisting penetration of ripper shank
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Effect of Moisture Content
Heavy rainfall will increase the moisture content of the rock material especially for those in
Grade IV and V materials. This is due to loose interaction between grains as weathering taken
place. Figure 14 shows the relation between moisture content and the point load test index Is50 for
several samples. It is noted that the strength of material gradually decrease with increase of
moisture content. This problem will lead to misjudgement during the rippability study as some
materials can be easily rip during wet condition but unrippable during dry weather. The ease of
excavating a highly moisturized rock could be easier compared to dried ones, even though it is
the same material. Although it can be interpreted easily by the strength of rock, the moisture
content parameter is also better to be counted in the assessment as it affects significantly during
dry and wet weather especially in tropical climate.
Figure 14: Graph Is50 vs Moisture Content
Figure 15: Highly friable sandstone (Grade IV) with high moisture content
0
0 . 5
1
1 . 5
2
2 . 5
5 . 6 8 6 . 3 9 7 . 3 9 8 . 6 6 9 . 4 5
M o i s t u r e C o n t e n t
I s
5
0
S h a l e G r a d e IV
F i n e S a n d s t o n e
G r a d e IV
M e d iu m
S a n d s t o n e G r a d e
II I
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CONCLUSION
In sedimentary rock, the occurrence of bedding, folding, foliation and inhomogeneity of
rocks are few distinctive differences compared to igneous rock. Shale which is interbedded withsandstone would have lower in strength compared to the sandstone and from the assessment;
shale could be excavated by different means of excavation techniques. However, due to its
existence in the rock mass which is interbedding between the dominancy of low or high strength
of rock, the excavation method could be differs from the assessment method. The small and
bigger scale of discontinuity that always present in the sedimentary rock such as thickness of
bedding, joints and foliation are not specified in the assessments but found to play significant role
in easing the excavation. The percentage of dominancy of low or high strength of rock need to be
assessed in advance as it may cause problems in ripping and during the preliminary excavation
assessments. A more specific approach for ripping assessment specially for sedimentary area isneeded as the assessments of material properties alone does not give accurate result to assess the
whole rock mass rippability.
ACKNOWLEDGEMENT
Authors would like to extent sincere gratitude and appreciation to Research Management
Centre, UTM and the Government of Malaysia for the research grant and making the study a
success.
REFERENCES1. Attewell, P.B. (1993), The Role of Engineering Geology in the Design of Surface and
Underground Structures, Comprehensive Rock Engineering, Hudson, J.D. (ed.),
Pergamon, Press, Oxford Vol.1 pg.111-154
2. Caterpillar Performance Handbook (2001). Edition 32, Caterpillar Inc., Peoria, Illinois,
U.S.A.
3. Fookes, P.G., Gourley, C.S. and Ohikere, C. (1988), Rock Weathering in Engineering
Time. Quart. Journal Engineering Geology 4: pg 139-185
4. IAEG (1981) Rock and Soil Description for Engineering Geological Mapping, Bulletin
Int. Assoc. Enging. Geology 24: 235-274
5. Ibrahim Komoo (1985) Engineering Properties of Weathered Rock Profile in Peninsular
Malaysia, Proc. 8th Southeast Asian Geotech Conference, Kuala Lumpur 1, 3-81 – 3-86
6. Ibrahim Komoo (1995), Geologi Kejuruteraan- Perspektif Rantau Tropika Lembap,
Syarahan Perdana, Universiti Kebangsaan Malaysia
7. Mohd For Bin Mohd Amin, (1995). Classification of Excavated Material Based on
Simple Laboratory Testings, Geological Society Malaysia, Bulletin 38, pp 179-190.
8. Pettifer G.S. & Fookes P.G., (1994). A revision of the graphical method for assessing the
excavatability of rock, Quarterly Journal of Engineering Geology, 27, pg 145-164
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Vol. 16 [2011], Bund. O 1350
9. Tajul Anuar Jamaluddin & Mogana Sundaram, (2000). Excavatability Assessment of
Weathered Rock Mass-Case Study From Ijok, Selangor and Kemaman, Terengganu,
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