challenges of excavation by ripping works in weathered sedimentary zone

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

[email protected]

Seyed Vahid Alavi Nezhad Khaili Abad

Researcher, Department of Geotechnics and Transportation, Faculty of Civil

Engineering, Universiti Teknologi [email protected]

Rosli Saad

School of Physics, Universiti Sains Malaysia

[email protected]

http://www.ejge.com/Index.htm



Vol. 16 [2011], Bund. O 1338

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



Vol. 16 [2011], Bund. O 1339

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.



Vol. 16 [2011], Bund. O 1340

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.



Vol. 16 [2011], Bund. O 1341

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.



Vol. 16 [2011], Bund. O 1342

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.



Vol. 16 [2011], Bund. O 1343

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.



Vol. 16 [2011], Bund. O 1344

Figure 7: Horizontal layer

Figure 8: Vertical Layer

Ripper tooth

Ripping

Direction

Ripping

Direction



Vol. 16 [2011], Bund. O 1345

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



Vol. 16 [2011], Bund. O 1346

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



Vol. 16 [2011], Bund. O 1347

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



Vol. 16 [2011], Bund. O 1348

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



Vol. 16 [2011], Bund. O 1349

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



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,

Warta Geologi, Vol. 26, No. 3, May-Jun.

10. Thomas, M.F. (1994) Geomorphology in the Tropics- A Study of Weathering andDenudation in Low Latitudes. Chicester, John Wiley & Sons.

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