6166102A1φδ

University of Surrey

Rock Slope Engineering Assignment Geotechnical Engineering 2

Antonos A Mr (UG - Civil & Env. Eng.) 3-24-2015

Athanasios Antonos ENGM224

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Table of Contents 1. Introduction .................................................................................................................................... 2

2. Geology ........................................................................................................................................... 2

2.1 Rock Type and Condition ........................................................................................................ 2

2.2 Discontinuity data ................................................................................................................... 3

3. Division of road alignment in sections ............................................................................................ 4

4. Kinematic feasibility analysis .......................................................................................................... 5

4.1 Overlay Construction .............................................................................................................. 5

4.2 Plane Failure ............................................................................................................................ 5

Face 1 (North) ................................................................................................................................. 6

Face 2 (South) ................................................................................................................................. 6

4.3 Wedge failure .......................................................................................................................... 7

Face 1 (North) ................................................................................................................................. 7

Face 2 (South) ................................................................................................................................. 8

Single and Double Plane Sliding Check ........................................................................................... 9

4.4 Flexural & Block Toppling failure .......................................................................................... 10

Face 1 (North) ............................................................................................................................... 10

Face 2 (South) ............................................................................................................................... 10

5. Summary of Failure Mechanisms .................................................................................................. 11

6. Recommendations for Mitigation of Failure Mechanisms ........................................................... 12

7. Recommendations for Further Investigation ................................................................................ 12

8. Recommendations for Excavation Method, Reinforcement ad Protection ................................. 12

9. Slope maintenance ....................................................................................................................... 13

10. References ................................................................................................................................ 14


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1. Introduction The aim of this report is to propose a conceptual design for a road alignment in NE England, cutting

through sandstone and shale. Through this design, potential failure mechanisms will be identified as

well as recommendations will be made with regards to measures for reinforcement, protection and

maintenance of the cutting. To meet those aims, a preliminary design of the cut slopes will be

considered, accounting for the discontinuities that are present. These discontinuities identified from

the site investigation data will be ranked in order of importance along with the likely failure

mechanisms on each face, after dividing the alignment corridor into sections.

The scope of work is to analyze of the shear strength of the discontinuities followed by a kinematic

feasibility analysis. The later will take into account the different failure mechanisms (plane, wedge and

toppling) and any suitable mitigation measures. However this introduces certain limitations to the

work presented in this report. The first limitation is into account any water present in the slope which

could change significantly the failure mechanism analysis.

2. Geology

2.1 Rock Type and Condition

The investigation data points SANDSTONE and SHALE as two types of rocks present on the site. Further

analysis was conducted in order to describe the characteristics of those rocks, such as strength, degree

of cementation, slake durability index. The slake durability index is important for this road alignment,

as it measures the rock resistance to deterioration and breakdown when subjected to wetting and

drying cycles (Ayakwah, et al., 2009), and the cutting required will create exposure. Therefore the

long-term impact of weathering on the newly exposed faces should be considered and mitigated

through maintenance and protection of the slope.

Sandstone is described as moderately weak to moderately strong, moderately well cemented greyish

yellow calcareous SANDSTONE, with a slake durability index of 55%. Excavations nearby the proposed

site produced evidence which suggest that the material weathers relatively upon exposure and is

prone to frost attacks. Laboratory tests gave an average basic angle of friction of 32 degrees for the

sandstone joints. The shale is also described as moderately weak to moderately strong, greyish black

closely bedded SHALE with slake durability index of 40%. Additionally the shale has occasional nodules

of pyrite can compromise its strength, when exposed as it will be oxidized (Angelo, 2008). Laboratory

tests yielded an average basic angle of friction of 28 degrees for the shale joints.

As both rocks have been described as moderately weak to moderately strong, this description will be

used to determine their unconfined compressive strength from the Geological Society (1970) as seen

in the “A short course in geology for civil engineers” Figure 9.6( (Matthews, et al., 2008, p. 230). From

the description given, the range for the unconfined compressive strength is 5-50(MN/m2). This

strength range can be considered relatively large. Hence, the unconfined compressive strength of the

materials could be considered of sufficient resistance in the short-term but due to its susceptibility to

weathering, the long-term performance should be investigated. The bedding dip and bedding

direction are the same for both rocks.


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2.2 Discontinuity data

Geological mapping of the area has revealed two faults F1 and F2. Fault F1 has an average dip of 82

degrees towards 102 degrees and a throw of 40m (downthrow is on the West side). Fault F2 has a

average dip of 60 degrees towards 285 degrees and a throw of 30m (downthrow is on the East side).

The site investigation data includes 250 observations of the discontinuities that were made at different

locations in the sandstone along the proposed road alignment. These data have been analyzed

statistically using the Dimitrijevic counting net, however, there were insufficient exposures of the

shale to permit detailed observations of the discontinuities. A summary of the key characteristics of

the discontinuities is presented below:

The steeply dipping discontinuities have rough wavy walls, a high persistence and very close

to close spacing (D)

The low angle discontinuities with bedding which dip towards the South (A1) have relatively

smooth walls (JCR 2-4) and high to very high persistence and medium spacing. Especially

within the first 15m of the ground, there is evidence of clay infill/lining and the fractures are

open to up to 5mm. The joints that dip towards the West (A2) are rougher (JCR 3-7) and high

to very high persistence, whilst some of them are in filled with clay.

Joints dipping towards the North-East have a medium to low persistence and terminate in

other discontinuities. They have very rough walls (JCR12-14) and relatively tight apertures. (C)

Joints dipping towards the North have high persistence, medium spacing and relatively rough

clean walls (JCR 8-10) with apertures up to 2mm. (B)

The discontinuities dipping South continue to the shale and they are closely spaced. (A 2S)

Another set of bedding discontinuities was found in shale only that have generally high

persistence, close to medium spacing and relatively smooth walls (JCR 2-4) whilst they have

an average dip of 65° and a dip direction of 202°. (S)

The discontinuity criteria given where persistence, concentration, roughness and spacing, becoming

less critical in a descending order. The ranking will be done taking in to account those criteria with the

most dominant one being the persistence. In case of similarly persistent discontinuities, the

concentration will be taken into account if known. If absent, roughness will be taken into account. In

case all those are not enough to rank them the spacing will be taken into account. According to this

descriptions the discontinuities observed were where given acronyms noted in the end of their

description above.

The highest ranking discontinuities are F1 and then F2. The reason is that the first fault can be

considered as more persistent as it has a greater throw. The third one is A 1 because compared to A 2

and A 2S because it is smoother and therefore more critical. The fourth and fifth are A 2S an A 2

respectively because the A 2S is more closely spaced.

In the high persistence category, S does not have a concentration but is smooth and therefore being

more critical than B and D. Seventh on the list is D as it has greater concentration than B. Finally the

lower ranked discontinuity is C as it has the lowest persistence and concentration, while being

roughest and medium to widely space. The following table illustrates the tabulated version of this

ranking between the discontinuities.


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TABLE 1. RANKED IMPORTANCE OF DISCONTINUITIES

3. Division of road alignment in sections The following figure illustrates cross-section along the road alignment:

FIGURE 1: CROSS-SECTION ALONG THE CENTRELINE OF THE PROPOSED ROAD

The yellow area indicates the sandstone and the red the shale.

SET PERSISTENCE CONCENTRATION ROUGHNESS SPACING RANK

A 1 V.HIGH TO HIGH 13.5% SMOOTH MEDIUM 3

A 2 V.HIGH TO HIGH 13.5% SMOOTH TO ROUGH

MEDIUM 5

A 2S V.HIGH TO HIGH 13.5% SMOOTH TO ROUGH

CLOSE 4

B HIGH 10.8 ROUGH MEDIUM 8

C MEDIUM TO LOW

10.1 V. ROUGH MEDIUM TO WIDE

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D HIGH 12.2 ROUGH V. CLOSE TO CLOSE

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S HIGH - SMOOTH CLOSE TO MEDIUM

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F 1 V. HIGH - ROUGH 1

F 2 V. HIGH - ROUGH 2


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4. Kinematic feasibility analysis

4.1 Overlay Construction

For the overlay construction of all the failure mechanisms, the kinematic feasibility analysis was

performed for both shale (φ=28o) and sandstone (φ=32o). The dip of cut face to be considered from

60o to 80o, as the minimum allowable slope is 60 degrees and the maximum proposed slope should

not exceed 80 degrees. In the actual analysis of the dip of the cut faces ware checked for 60o, 70o

and 80o, which can give a better indication of the maximum dip that can be chosen in order to have

the minimum land intake and rock volume removal, without compromising the slope’s sustainability.

For all the failure mechanism overlays the first thing that should be chosen is the dip direction of the

face either due west or east. A line should be drawn for the chosen dip direction point, from the

middle towards the edge of the chosen dip direction (west or east) on the stereo-net. Then using the

angle(s) of friction, the dip of cut face and the individual criteria for each failure mechanism.

For the construction of the plane failure overlay specifically, the dip direction of the slip surface must

be within 20 degrees of the dip direction of the slope face. The overlays used for the shale and

sandstone are illustrated in Figure 2 and Figure 3.For the construction of the wedge failure overlay,

the dip of face should be indicated by measuring inwards. The wedge overlays for the two rocks are

illustrated on Figure 4 and Figure 5.

For the construction of the flexure toppling failure overlay, the dip direction of the discontinuities

must bewithin 180 ± 30 degrees from the dip direction of the slope face. Block toppling failure

requires the interaction of three sets of discontinuities One set should dip in the same direction as

the slope face (±20 degrees) and at an angle less than the angle of shearing resistance ( φ). One set

(basal set) should have a dip direction that is 180 ± 30 degrees from the dip direction of the slope

face and have a steep dip. Lateral release surfaces are represented by the third set. The total

toppling overlays for the two rocks are illustrated below:

4.2 Plane Failure

In order to from a plane failure several assupmtion are to be considered. The forces act through the

centre of graviy of the block so that no moment is developed. The sliding surface is planar, however

in reality the majority of surfaces are likely to be wavy and rough and also the shear is uniform along

the length of the sliding surface.


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Face 1 (North)

FIGURE 2: FACE 1 FOR THE SHALE (LEFT) AND SANDSTONE (RIGHT) FOR PLAIN FAILURE

For face 1 there is possibility of plain failure in all cut face angles. In case of the 60 degree cut face

the chance of failure is the very small as there are some poles in the enclosed area, however their

density is small. The likelihood of failure increases substantially from the 70 degree mark as the

density of poles in the enclosed area is significantly higher. This applies only in case of sandstone, as

the discontinuities were not found in shale. The lateral release surfaces are F1 and D are in place

therefore there is a considerable chance of plain failure.

Face 2 (South)

FIGURE 3: FACE 2 FOR THE SHALE (LEFT) AND SANDSTONE (RIGHT) FOR PLAIN FAILURE

For Face 2 plane failure is unlikely as no poles are within the enclosed area.


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4.3 Wedge failure

Face 1 (North)

FIGURE 4: FACE 1 FOR THE SHALE (LEFT) AND SANDSTONE (RIGHT) FOR WEDGE FAILURE

For face 1 there is possibility of plain failure in all cut face angles for sandstone:

In case of the 60 degree cut face, the potential wedges are CF2, CF1, and CD.

In case of the 70 degree cut face, the potential wedges are CF2, CF1, CD, BF2, BF1, BS, and

BD.

In case of the 80 degree cut face, the potential wedges are the same as above.

For face 1 there is no possibility of plain failure for shale, because discontinuities B and C are not

found in the shale.


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Face 2 (South)

For face 2 there is possibility of plain failure in all cut face angles for sandstone and shale:

In case of the 60 degree cut face, there are no potential wedges.

In case of the 70 degree cut face, the potential wedges are SF1 and SD.

In case of the 80 degree cut face, the potential wedges are the same as above.

FIGURE 5: FACE 2 FOR THE SHALE (LEFT) AND SANDSTONE (RIGHT) FOR WEDGE FAILUREK


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Single and Double Plane Sliding Check

For the Single or Double plane sliding test, the following diagram was drawn. In that diagram, the dip

direction of the planes and the lines intersection where drawn. To determine whether the plane

sliding will be single or double, the intersection lines (two lettered lines) should be checked against

their corresponding plane lines. If the intersection line is “within” the two plane lines then it is single

and if outside double.

Following that procedure, from the following figure it can be said that CF2, CF1, BF2, BF1, BD (face 1)

and SD, SF2 (face 2) will have potentially single sliding failure while the rest (CD, BC and BS) will

potentially fail by double sliding.

FIGURE 6: SINGLE OR DOUBLE PLANE SLIDING DIAGRAM


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4.4 Flexural & Block Toppling failure

Face 1 (North)

FIGURE 7: FACE 1 FOR THE SHALE (LEFT) AND SANDSTONE (RIGHT) FOR TOPPLING FAILURE

Face 1 for sandstone has no possibility of flexural toppling failure. For Shale there is a very small

chance of flexural toppling involving set S, since it is closely to medium spaced and falls in the low

risk area of the overlay. For Face 1 it is unlikely that block toppling failure will occur for both shale

and sandstone the 3 criteria are not simultaneously satisfied.

Face 2 (South)

FIGURE 8: FACE 2 FOR THE SHALE (LEFT) AND SANDSTONE (RIGHT) FOR TOPPLING FAILURE

Face 2 for shale has no possibility of flexural toppling failure. For Sandstone there is a possibility of

flexural toppling involving set B and C. Set B falls in the high risk area of the overlay and it is closely

to medium spaced, therefore the chance of flexural toppling failure is medium. Set C falls in the low

risk area of the overlay and it is medium to wide spaced, therefore the chance of flexural toppling


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failure is low. For face 2, block toppling failure is likely to occur. However the risk is very low as only

one set of data falls into that category.

5. Summary of Failure Mechanisms Having examined all possible failure mechanism the results of likelihood of occurrence of the failure

is tabulated in

Table 2.

TABLE 2 SUMMARY OF KINEMATIC FEASIBILITY ANALYSIS

It can be observed that the most critical failure is the wedge failure as there is a high possibility for

failure in face 1. There is also a chance of a lower risk failure possibility for flexural toppling. Another

worth mentioning thing is that a lot of failures would be eliminated if the angle of dip of the slope is

below 60 (-64) degrees.

Failure type Face 1 Face 2

Plane B (LOW) NONE

Wedge Single Plane CF2, CF1, BF2, BF1,BD(LOW) SD, SF2(LOW)

Wedge Double Plane CD,BC,BS(HIGH) NONE

Flexural toppling S (LOW) B (MEDIUM), C (LOW)

Block toppling NONE VERY LOW

Failure type Face 1 Face 2

Plane B (LOW) NONE

Wedge Single Plane CF2, CF1, BF2, BF1,BD(LOW) SD, SF2(LOW)

Wedge Double Plane CD,BC,BS(HIGH) NONE

Flexural toppling S (LOW) B (MEDIUM), C (LOW)

Block toppling NONE VERY LOW


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6. Recommendations for Mitigation of Failure Mechanisms The proposed cut slope design can be approached issue from conservative point of view in terms of

the stability. From the kinematic feasibility analysis it is apparent that the even the lowest slope

(60o) has chances of instability. Another issue is the vulnerability of both to weathering, as their low

slake durability index indicates. The mitigation measures that potentially could be used are many

and different in nature and targets. These can vary from a reinforced over lay to prevent weathering

and erosion to complete structural anchored solutions ( Shotcrete, 2012).

Should the failure mechanism occur due to shear failure the dowel bars or any similar reinforcement

method should be applied. The kinematic feasibility analysis revealed that the rock bolts or dowel

bars to enhance shear capacity and resistance might prove essential as there is a significant

possibility of wedge failure.

Seepage creates problem with sandstone, as it is prone to frost attacks. In addition to the

sustainability point of view, employing several rock slope sections could prove beneficial achieving a

‘best-fit’ line between rock removal and safety from failures. This can achieve steep slopes in

sections that can sustain themselves, and lower slopes in highly dangerous cases that even the

minimum of 60o slope ratio is hazardous. The best average cut slope based on the failure taking into

account the minimum rock removal should be around 65o.

7. Recommendations for Further Investigation The site investigation has provided many important data, however there some more information

that would improve the accuracy of the preliminary design. The exact rate of weathering should be

more accurately described and classified The seepage within discontinuities should be adressed on

volumetric basis (as per BE EN ISO14689-1:2003), as the drainage is essential for protecting slope

minimizing erosion and frost attack(sandstone mainly).

The shear resistance should be obtained on the site. The reason behind it is the infill, seepage, clay

etc. within the rocks as the site investigation revealed presence of clay and pyrite infill within the

discontinuities that can largely influence of the shear resistance. In addition the roughness of the

discontinuities can be a assessed in more realiably. This is proposed due to uncertainty of quality of

the samples that have been tested in laboratory conditions.

8. Recommendations for Excavation Method, Reinforcement ad

Protection The first stage before the excavation is the enabling works stage. In this stage, the slope is prepared

for excavation as well as trimming of the upper slope. Trimming is the removal of all overhanging

blocks with methods such as blasting or drilling. In case more loose material is present it can also be

scanned to remove it. Then the excavation will take place where the primary aim should be to

minimise the potential damage to the rock mass (Pettifer & Fookes, 1994).

A suitable that can be used method utilizes a tractor with an attached tooth that hits and drags the

rock to break it up. Considering that the rocks are actually to that it would be preferable to using

blasting means as they are cheaper and is less hazardous. Additionally it will not affect the wildlife and


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will produce less noise (U.S. Department of Transportation, 2015).Blasting and drilling on the other

hand offers a more familiar operation as well as great control of bench formation. Additionally it

provides a faster way to remove rocks when excavating and in some cases of excess rock removal it

should be considered as well.

Both excavation methods require a certain degree of protection. Obviously the explosions pose a

greater health hazard than an extractor, however the CDM co-coordinator should authorize its use

to suitable person to use them. Additionally the designer, contractor and client should follow the

CDM regulations in order to minimize the accidents on site and improve the overall welfare for

everyone involved.

After the excavations have accomplished the desirable results additional action should be taken to

ensure that the slope is protected and maintained. As briefly mentioned earlier there are certain

options to ensure stability. Apart from the drainage, shortcrete and rock bolts there are other

option. Netting and concrete may be appropriate choices especially if the exposed material is very

susceptible to weathering.

9. Slope maintenance An efficient design foresees the potential decrease in the stability due to weathering, corrosion etc.

so a robust maintenance plan should be in place, in order to prevent serious problems. The authority

who is responsible for the maintenance of the rock slope. This slope as part of a road can be considered

a public asset. Therefore the maintenance scheme should insure that the road is maintained routinely

to ensure public safety.

Inspection is a pre-stage to maintenance, which allows for data collection on the condition of the

slope. Similar to maintenance schemes, inspections can be routine, general or principal. As the level

of inspection increases the frequency decreases because of the time and allocation of resources

required. If these inspections indicate that the slope stability could be compromised then, then

emergency maintenance should take place, including immediate enforcement of the slope or other

protection measures.

Routine maintenance will take place at certain intervals and will include vegetation and debris

clearance, unblocking of drain pipes, repair of any drainage channels or netting etc. (Geotechnical

Engineering Office, 1995). Finally, proactive maintenance would focus at upgrading and increasing the

capacity of the rock slope and such maintenance works would include improving the drainage systems

and the surface protection systems. Therefore, a slope maintenance plan should precise with the

inspections as well as scheduled routine and proactivee maintenance works.


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10. References Shotcrete, 2012. Rock & Slope Stabilisation. [Ηλεκτρονικό]

Available at: http://www.shotcrete.co.uk/shotcrete_rock_slope_stabilisation.htm

[Πρόσβαση 23 03 2015].

Angelo, W., 2008. A Material Odor Mystery Over Foul-Smelling Drywall. [Ηλεκτρονικό]

Available at: http://enr.construction.com/

[Πρόσβαση 17 03 2015].

Ayakwah, G. F. και συν., 2009. EFFECTS OF WEATHERING AND ALTERATION ON POINT LOAD AND

SLAKE DURABILITY INDICES OF QUESTA MINE, Denver, CO : SME .

Matthews, M., Simons, N. & Menzies, B., 2008. A short course in geology for civil engineers.

[Ηλεκτρονικό]

Available at: http://app.knovel.com/hotlink/toc/id:kpSCGCE006/short-course-in-geology/short-

course-in-geology

[Πρόσβαση 21 03 2015].

Pettifer, G. & Fookes, P., 1994. A revision of the graphical method for assessing the excavatability of

rock. Quarterly Journal of Engineering Geology and Hydrogeology, 2(27).

U.S. Department of Transportation, 2015. Rock excavation Methods.. [Ηλεκτρονικό]

Available at:

http://www.cflhd.gov/programs/techDevelopment/geotech/css/05_chapter_3_rock_excavation_me

thods.pdf

[Πρόσβαση 24 03 2015].

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