PSZ19:16 P ( ind. 1/07)
UNIVERSITI TEKNOLOG I MALAYSIA
DECLARATION OF THESIS / UNDE RGRADUATE PROJ ECT PAPER AND COPYRIGHT
Author’s full name :
LIU HUI
Date of birth
:
31 OCTOBER 1987
Title
:
SLOPE REMEDIAL WORK AT SRI PLENTONG
Ac a demic Session:
2009/2010
I dec lare that thisthesisis c lassified a s :
C ONFIDENT IAL
(Contains confidential information under the Offic ial Sec ret Act 1972)*
RES TRICTED
(Contains restricted information as spec ified by the organisation where researc h wa s done)*
OPEN AC C ESS
I agree that my thesis to be published as online open ac c ess (full text)
I ac knowledged that Universiti Teknologi Malaysia reserves the right as follows : 1. The thesis is the prop erty of Universiti Teknologi Ma laysia. 2. The Library of Universiti Teknologi Ma laysia has the right to make copies for the purpose of research only. 3. The Library has the right to make cop ies of the thesis for a ca demic excha nge.
C ertified by :
SIGNATURE
SIGNATURE OF SUPERVISOR
871031-13-5308
DR. NAZRI BIN ALI
(NEWIC NO. / PASS PORTNO.)
NAME OF SUPERVISOR
Date :
NOTES :
*
15 APRIL 2010
Date :
15 APRIL 2010
If the thesis is CO NFIDENTIAL or RESTRICTED, please a ttac h with the letter from the orga nisation with period and rea sons for confidentiality or restriction.
“I hereby declare that I have read this project report and in my opinion this project report is sufficient in terms of scope and quality for the award of the Degree of Bachelor of Civil Engineering”
Signature
:
………………………………….
Name of Supervisor
:
DR. NAZRI BIN ALI
Date
:
15 APRIL 2010
SLOPE REMEDIAL WORK AT SRI PLENTONG
LIU HUI
A report submitted in partial fulfilment of the requirements for the award of the degree of Bachelor of Civil Engineering
Faculty of Civil Engineering Universiti Teknologi Malaysia
APRIL, 2010
ii
I declare that this thesis entitled “Slope Remedial Work at Sri Plentong” is the result of my own research except as cited in the references. The thesis has not been accepted for any degree and is not concurrently submitted in candidate of any other degree.
Signature
:
…………………………….
Name
:
LIU HUI
Date
:
15 APRIL 2010
iii
To my supervisor, Dr. Nazri Bin Ali, Thousand of thanks for the guidance and advice. To my beloved mother and father, Who always be there for me. To my caring sisters and brother, Your support is my motivator. To my dear course-mates and friends, Thanks for the encouragement and helping hands.
iv
ABSTRACT
Slopes that have failed should be repaired with appropriate remedial measures to prevent them from progressive failure. The slope remedial methods commonly applied in engineering field are the use of retaining wall, gabion wall, reinforced earth, soil nailing and geometry alteration. This study involves a slope that has failed in Sri Plentong, Johor. Soil nailing has been justified as the remedial solution as this slope is very high and steep. In addition, the top face of slope is very close to the Public Utility Board (PUB) pipeline. The remedial solution is considered safe if the Factor of Safety (FOS) of slope is at least 1.40. To obtain the FOS, the slope without and with soil nails are analysed by using finite element software, Plaxis. The length of soil nails adopted start from five meter and increase one meter if the previous length failed to reach FOS of 1.40. From the results of analysis, reinforcement with seven-meter nails yields FOS of 1.44, which is greater than the recommended FOS. Therefore, seven-meter nails should be used to provide sufficient reinforcement strength to the slope.
v
ABSTRAK
Cerun yang telah mengalami kegagalan seharusnya diperbaiki dengan cara remedi yang sesuai untuk mengelakkan kegagalan yang seterusnya. Cara-cara remedi cerun yang biasa digunakan di bidang kejuruteraan adalah seperti penggunaan dinding penahan, dinding Gabion, pengukuhan tanah, paku tanah, dan perubahan geometri. Kajian ini melibatkan sebuah cerun yang telah mengalami kegagalan di Sri Plentong, Johor. Oleh sebab cerun ini sangat tinggi dan curam, kaedah paku tanah telah dijustifikasikan sebagai kaedah remedi yang paling sesuai. Selain itu, muka cerun adalah sangat dekat dengan paip Public Utility Board (PUB). Kaedah remedi adalah selamat digunakan sekiranya faktor keselamatan mempunyai nilai sekurang-kurangnya 1.40. Untuk mendapatkan nilai faktor keselamatan, cerun tanpa paku tanah dan cerun diperkukuhkan dengan paku tanah telah dianalisiskan dengan menggunakan perisian unsur terhingga, Plaxis. Panjang paku tanah yang digunakan dalam analisis bermula dari 5 meter dan ditambahkan 1 meter sekiranya panjang yang sebelumnya gagal mencapai faktor keselamatan 1.40. Daripada keputusan analisis, pengukuhan dengan paku tanah 7 meter menghasilkan faktor keselamatan yang bernilai 1.44. Nilai ini adalah lebih besar daripada nilai yang telah dicadangkan. Oleh itu, pengukuhan dengan menggunakan paku tanah 7 meter seharusnya digunakan untuk memberikan daya pengukuhan yang mencukupi kepada cerun berkenaan.
vi
TABLE OF CONTENTS
CHAPTER
1
TITLE
PAGE
DECLARATION
ii
DEDICATION
iii
ABSTRACT
iv
ABSTRAK
v
TABLE OF CONTENTS
vi
LIST OF TABLES
ix
LIST OF FIGURES
x
LIST OF SYMBOLS
xiv
INTRODUCTION
1
1.1
Background of the Study
1
1.2
Statement of the Problem
3
1.3
Objectives of the Study
4
1.4
Significance of the Study
5
1.5
Scope of the Study
5
vii 2
LITERATURE REVIEW
6
2.1
Justification
6
2.2
History of Soil Nailing
8
2.3
Application of Soil Nailing in Slope Stabilization and Deep Excavation 2.3.1
Slope Remediation at a Riverbank Slope of a Water
2.3.2
Treatment Plant Cut Slope at Kuala Lumpur
2.3.3
Stabilization of Slipped Slopes that Caused the Collapse of
10 12
a Pylon
15
2.3.4
30m High Soil Nailed Slope in Kuala Lumpur
17
2.3.5
Soil Nail Strengthening Work over Uncontrolled Fill for a 14.5m Deep Excavation
2.3.6
3
10
19
40m High Nailed Excavation in Gneissic Residual Soil at Brazil
22
METHODOLOGY
24
3.1
Theoretical Background
24
3.1.1
Definition
24
3.1.2
Reinforcement Concept of Soil Nailing
25
3.1.3
Components of Soil Nailing Reinforcement
26
3.1.3.1 Nail Element
26
3.1.3.2 Facing Element
27
3.1.3.3 Surface and Subsurface Drainage
27
3.1.4
Suitability of Soil Nailing With Respect to Soil Types
28
3.1.5
Comparison between Soil Nailing and Reinforced Earth Wall
29
3.1.6
Design Methods of Soil Nailing
30
3.1.7
Stability Assessment
31
viii
4
5
3.1.8
Serviceability Assessment
33
3.1.9
Construction Sequence
33
3.1.9.1 Initial excavation
33
3.1.9.2 Drilling of holes
34
3.1.9.3 Insertion of nail reinforcement and grouting
34
3.1.10 Advantages and Disadvantages of Soil Nailing
35
3.2 3.3
Introduction of the Methodology Problem Identification
36 36
3.4
Case Study
38
3.5
Literature Review
38
3.6
Theoretical Background
39
3.7
Application of Plaxis Software
40
3.8
Results and Discussions
41
3.9
Conclusion
41
RESULTS AND DISCUSSIONS
42
4.1
Models
42
4.2
Parameters
44
4.3
Slope without Soil Nailing
45
4.4
Slope with Five-Meter-Nail Reinforcement
48
4.5
Slope with Six-Meter-Nail Reinforcement
51
4.6
Slope with Seven-Meter-Nail Reinforcement
55
4.7
Summary of the Results for Analysis of Various Nail Lengths
58
CONCLUSION
REFERENCES
59
60
ix
LIST OF TABLES
TABLE NO.
TITLE
PAGE
4.1
Geometry models and descriptions
42
4.2
Soil parameters used in analysis
44
4.3
The x- and y- coordinate of nodes (five-meter nails)
49
4.4
The x- and y- coordinate of nodes (six-meter nails)
52
4.5
The x- and y- coordinate of nodes (seven-meter nails)
56
4.6
The factor of safety of slope reinforced with various nail length
58
x
LIST OF FIGURES
FIGURE NO.
TITLE
2.1
The eroded slope surface (Chen and Lim, 2005)
2.2
Typical remedial design for the unstable riverbank slope
PAGE
10
(Chen and Lim, 2005)
11
2.3
Site condition after the remedial work (Chen and Lim, 2005)
12
2.4
Front view of the failed slope (Liew, 2004)
13
2.5
Soil nailing strengthening work (Liew, 2004)
14
2.6
Completed soil nailed slope (Liew, 2004)
14
2.7
The site condition after the collapse of pylon 4A (Phan and Tan, 2005)
15
2.8
Slope stabilized by soil nails (Phan and Tan, 2005)
16
2.9
The soil nailed wall at pylon 4C (Phan and Tan, 2005)
17
xi
2.10
The 30m high soil nail wall under construction (Tan and Chow, 2007)
18
2.11
The completed 30m high soil nailed wall (Tan and Chow, 2007)
19
2.12
The locations of project site and excavation (Liew and Khoo, 2006)
20
2.13
Soil nailing strengthening work (Liew and Khoo, 2006)
21
2.14
3D view of soil nailing design showing excavation stages (Sayao et al., 2005)
2.15
22
The nearly-completed 40m high nailed excavation (Sayao et al., 2005)
23
3.1
Effect of soil nail in reinforcing the soil mass (Liew, 2005)
25
3.2
Construction sequence for soil nailing and reinforced earth wall (Bruce and Jewell, 1986)
3.3
30
Typical types of failure mechanism in soil nailed slope (Liew, 2005)
32
3.4
Flowchart of the study process
37
4.1
The geometry model with dimensions
43
4.2
The geometry model of slope without soil nailing
45
xii 4.3
The finite element mesh of slope without soil nailing
46
4.4
The deformed mesh of slope without soil nailing
46
4.5
The slip failure of slope without soil nailing
47
4.6
The graph of factor of safety of slope without soil nailing
47
4.7
The geometry model of slope reinforced with five-meter nails
48
4.8
The finite element mesh of slope reinforced with five-meter nails 49
4.9
The deformed mesh of slope reinforced with five-meter nails
50
4.10
The slip failure of slope reinforced with five-meter nails
50
4.11
The graph of factor of safety of slope reinforced with five-meter nails
51
4.12
The geometry of slope reinforced with six-meter nails
52
4.13
The finite element mesh of slope reinforced with six-meter nails 53
4.14
The deformed mesh of slope reinforced with six-meter nails
53
4.15
The slip failure of slope reinforced with six-meter nails
54
4.16
The graph of factor of safety of slope reinforced with six-meter
4.17
nails
54
The geometry model of slope reinforced with seven-meter nails
55
xiii 4.18
The finite element mesh of slope reinforced with seven-meter nails 56
4.19
The deformed mesh of slope reinforce with seven-meter nails
57
4.20
The slip failure of slope reinforced with seven-meter nails
57
4.21
The graph of factor of safety for slope reinforced with seven-meter nails 58
xiv
LIST OF SYMBOLS
γ dry
-
Dry soil weight
γ wet
-
Wet soil weight
kx
-
Horizontal permeability
ky
-
Vertical permeability
Eref
-
Young’s modulus
υ cref
-
Poisson’s ratio Cohesion
φ
-
Friction angle
ψ
-
Dilatancy angle
EA
-
Stiffness
FHWA
-
Federal Highway Administration
FOS
-
Factor of Safety
PUB
-
Public Utility Board
CHAPTER 1
INTRODUCTION
1.1
Background of the Study
In Malaysia, slope failures are no longer new issues as their occurrence becoming frequent. Landslide is one of the forms of slope failure that is significant and is a recurring hazard to Malaysia especially during the raining seasons. According to Bromhead (1992), the causes of slope failure can be simplified into three broad categories which are the weak subsoil, the increase in pore water pressure and the external influence such as seismic forces, scouring and undercutting at the toe of slope.
The most impressive landslide incident in Malaysia should be the collapse of Highland Towers Condominium (Ooi, 2008) on December 11, 1993 at 1.30p.m. in Taman Hillview, Ulu Klang, Selangor. The whole nation went into a state of shock. This tragedy which involving the collapse of one out of three blocks had grabbed 48 lives and caused the other two blocks unoccupied till today. The continuous rainfall for
2 ten days led to this incident after the retaining wall of Tower’s car park failed. The landslip occurred beneath the entire rail pile foundation brought down the Block 1 Condominium within minutes of the landslide occurrence.
On November 20, 2002 at 4.30 a.m., a sudden landslide flattened a double-storey bungalow owned by retired general Affin Bank Berhad’s chairman Tan Sri Ismail Omar. Eight of his family members were perished in the incident. The bungalow was located 300 m away from the Highland Towers Condominium.
The previous landslides failed to warn the authorities to improve the precaution measures against the subsequent occurences. The landslide happened on December 6, 2008 at about 4 a.m. in Bukit Antarabangsa, Ulu Klang, Selangor must be still fresh in everyone’s mind. This time five people were killed while fourteen bungalows were destroyed in Taman Bukit Mewah and Taman Bukit Utama. A senior engineer, Siraj Akhbar Ali, who has fifteen years of experience in the field of slope development, judged that the collapse of the slope was due to excessive water retention, probably due to poor drainage. He judged this based on the eyewitness of some victims that there was an explosion like a bomb before the disaster happened.
It cannot be denied that the landslide and other slope failures could impose a large amount of direct and indirect costs to society. Direct costs include the loss of life and the damage of properties whereas indirect costs include business disruption, loss of property’s value, and loss of productivity.
Prevention is better than cure. Since the slope failure may bring so much large negative impacts to the society, the remedial works should be carried out for unstable slopes to prevent the occurrence of unwanted tragedy and then unnecessary loss.
3 1.2
Statement of the Problem
There are many methods being practiced in geotechnical engineering for the purpose of slope remedial work or slope stabilization. These methods may be adopted singly or in combination.
Generally, common adopted remedial measures can be
grouped into three main categories (Broms and Wong, 1985): a. Geometrical method This method is usually simple and cost effective. The stability of a slope can be increased by changing the slope geometry from a steep to a gentler slope. This method can be done by cutting the slope, removing external load on top of the slope or backfilling the toe of the slope. However, this method requires sufficient space to be applied.
b. Drainage method Building up of pore water pressure inside subsoil is one of the slope failure factors. The chances for pore water pressure to build up can be minimized if proper drainage system is provided.
Normally this method is used in
combination with other methods.
c. Retaining structure method Due to its flexibility in a constrained site, this method is the most commonly adopted method. However, it is more costly. The principle of this method is to use a retaining structure to resist the downward forces of the soil mass. The retaining structures include gravity type of retaining wall, cantilever wall, contiguous bored piles, caisson, steel sheet piles, etc.
The analysis of these alternative remedial measures for soil slope problems requires experience and sound judgment on the part of the engineer. In evaluating the
4 alternatives, the engineer will be influenced by a few factors such as the nature of failure, ground and groundwater conditions, site topography, environmental impact, availability of materials, labor and equipment, design life and maintenance requirements, adjacent and underground structures, confidence in design and construction, time constraints, and costs (Oliphant et al., 2000).
Sri Plentong Industrial Park, Johor Bahru is situated on hilly and undulating terrain. A 20m high steep cut slope with slope angle of approximately 700 is formed by cutting the hill where a number of industrial premises and buildings are located at the toe of the slope. The top face of the slope is very close to the boundary of the Public Utility Board (PUB) pipeline (Department of Geotechnics and Transportation, 2009).
The
stability of this slope is concerned as it is high and steep. The remedial works must be carried out on this slope to prevent erosion of the slope surface which may be followed by massive progressive slope failure. Therefore, this study is conducted to analyze and determine the best slope remedial method for the concerned slope.
1.3
Objectives of the Study
The objectives of this study are: a. To determine the most appropriate remedial method for a cut slope at Sri Plentong Industrial Park. b. To analyze and obtain the factor of safety and design parameters for the remediation method chosen.
5 1.4
Significance of the Study
Although currently there has no similar slope stability case at which the slope is located near the PUB pipelines involving another country, Singapore, the findings of this study are important to help the authorities to identify the most suitable remedial method to be applied on the slope of concern. By considering all of the important factors such as the PUB pipelines and territory of Singapore, the most appropriate method can be identified and then analyzed by the Plaxis software. Thus, an accurate and appropriate solution for remediation of this slope can be obtained. The stability method to be applied on the slope can prevent the progressive failure of the slope in future, for instance, the occurrence of landslide. The analysis, results and findings of this study could also be used as a reference for solution to the similar case in future whereby the slope has similar geometry and properties or there involves the properties of others which should be taken into consideration. With the information obtained, it is hoped that more extensive studies could be planned in the future for the similar slope.
1.5
Scope of the Study
This research is designed to determine the most appropriate slope remediation method by considering the geometry and location of the slope which is close to the PUB pipelines. The analysis and results are obtained by using Plaxis simulation and the parameters required will be obtained from the Department of Geotechnics and Transportation, Faculty of Civil Engineering, University of Technology, Malaysia.
6
CHAPTER 2
LITERATURE REVIEW
2.1
Justification
The most appropriate remedial measure for the cut slope in this case study has to be justified first. The slope involved in this study is a special case where the top face of the slope is very close to the boundary of the Public Utility Board (PUB) pipeline which belongs to Singapore. The available remedial measures for slope such as geometrical method, gabion wall, reinforced earth, retaining wall and soil nailing will be discussed below and decided the most suitable one for this case study.
By altering the geometry of a steep slope to a gentler slope either flatten the slope or backfill at the toe of slope, the stability of a slope can be increased. This method is simple and most cost effective. However, it cannot be applied for the slope in this case study. This is because part of this slope is claimed as territory of Singapore. So, the land of slope cannot be altered much.
7 Gabion wall is also a simple and easy-to-construct remedial method.
However,
it is not suitable for this steep and critical slope with height of 20 m. It is not safe enough to apply on this slope which requires high safety measure.
Besides these, reinforced earth is one of the slope remedial measures. Due to the consideration of the land belongs to Singapore, this method is not suitable to be implemented on this slope. This is because this measure requires excavating the earth from the slope and placing the reinforcement into the slope and filling the earth again. This will disturb the territory of Singapore.
The retaining wall is an effective method to stabilize and remedial the failed slope. It is adaptable to many site and soil conditions. However, it is not economic to apply in this case study because the concerned slope is high, which is 20 m. If retaining wall is constructed, it is as tall as a five-storey building. This consumes a high cost.
Soil nailing is a remedial method that only has to excavate a small amount of earth to install the reinforcement steel bars. The excavation is done step by step after completing the drilling, installing of nails and facing for previous step. It is cheaper if comparing with retaining wall. It is also effective in stabilize and repair a failed slope. Therefore, it is the most suitable remedial method to be applied for the cut slope in this case study.
Hence, soil nailing is chosen as the remedial method for the cut slope in this case study since it is the most suitable method to be applied after considering a few critical factors.
However, the soil nails installed should not exceed 7m of length to avoid
encroaching into the territory of Singapore. The following topics will all discuss on the topics of soil nailing.
8 2.2
History of Soil Nailing
The technique of slope stabilization and remediation by using soil nailing has been used since four decades ago. The New Austrian Tunnelling method (Bruce and Jewell, 1986) which is evolved in the early 1960s primarily as a hard rock tunneling system using a combination of shotcrete and fully bonded steel inclusions to provide early, efficient excavation stability. The French contractor Bouygues gained experience in France with the New Austrian Tunnelling Method and saw that similar techniques could be applied for the temporary support of soft rock and soil slopes. In conjuction with this, Bouygues co-operated with the specialist contractor, Soletanche to work on a 700 cut slope for a railway widening project near Versailles in year 1972. This is the first recorded application of soil nailing where a total of 12,000m 2 of face was stabilized by over 25,000 steel bars grouted into predrilled holes up to 6m long.
Since then, soil nailing as a technique to stabilize slopes and deep excavations grew in popularity in France. Numerous researches on soil nailing had been carried out in France. The CERMES (Bruce and Jewell, 1986), a research group under supervision of Dr. Juran has carried out model tests and finite element analysis of soil nailing. Besides this, the French government started a national research project named as Clouterre (Clouterre, 1991) by which clou means nail and terre means soil. This study was conducted from 1986 until 1990 with total budget of 22 million francs (equivalent to RM 16.7 million). Results from this study were later compiled under Soil Nailing Recommendations 91.
This had allowed large development of soil nailing and its
application in permanent structure in geotechnical engineering.
Technique of soil nailing was started to use in West Germany and North America in mid 1970s. Development in West Germany has been led by the specialist contractor Karl Bauer AG in association with the Institut fur Bodenmechanik and Felsmechanik
9 (IBF) of the University of Karlsruhe. According to Gassler and Gudehus (1981), over twenty projects had been successfully completed by 1981, confirming that “the technique has again and again been found to be safe and economic”.
The first recorded application of soil nailing in USA is the case of foundation excavation for the extension to the Good Samaritan Hospital, Portland, Oregon (ENR, 1976). 2140m2 of nailed wall was provided around three sides of a foundation excavation of maximum depth 13.7m. The longest wall was 76.3m and up to 11.3m deep. It was noted that the work was conducted in 50 to 70% of the period required for conventional support and at about 85% of the cost. The overall support and wall system costs were claimed to have been reduced by about 30%.
According to Wong (2001), many of the soil cuts in Hong Kong engineered in late 1970s and 1980s involved trimming back to a safe gradient based on theoretical stability analyses. Since 1990s, the trend has been gradually towards the more extensive use of soil nails. Under Hong Kong Government’s Landslip Preservative Measures (LPM) Programme, soil nailing has been extensively use to upgrade the old soil cut slopes as this method requires minimum change to the existing ground profile (Pang and Wong, 1997). To effectively upgrade a large number of old man-made slopes, a simple and reliable approach named as Application of Prescriptive Measures to Soil Cut Slops (Wong and Pang, 1996) has been developed for the design of soil nails, without the need of detailed ground investigation, laboratory testing and stability analysis.
Due to the ease of construction and relatively maintenance free advantages, soil nailing has gained popularity in Malaysia as stabilization measure for distressed slopes and for new steep cut slopes for highway and also hillside development projects. Chow and Tan (2006) has recommended a design method to be adopted for Malaysian practice to ensure safe and economical soil nail slope design in line with international practice.
10 2.3
Application of Soil Nailing in Slope Stabilization and Deep Excavation
2.3.1
Slope Remediation at a Riverbank Slope of a Water Treatment Plant (Chen and Lim, 2005)
The riverbank slope of a water treatment plant had been continuously eroded and undercut at the toe of the slope as shown in Figure 2.1. Because of this, retrogressive slips occurred very often. It is necessary to carry out the slope remedial work since a water treatment plant and an accessing bridge are located beside the slope. From the site investigation, the subsoil is mainly consisted of medium stiff to stiff silty and clayey residual soil.
Figure 2.1
The eroded slope surface, after Chen and Lim (2005).
11 Initially, retaining wall was considered to stabilize the slope.
However, the
construction of retaining wall required temporary excavation which may affect the operation of treatment plant. Moreover, the water level of river fluctuated very fast especially after the rain and this could affect the excavation and construction.
After further study, soil nailing was decided to be applied to repair the unstable slope. The advantages of using soil nailing for this slope are the installation of nails is fast and the required equipment are light. The site is located in a remote area, so there will not have problems to transport the machinery and equipment to site if soil nailing is adopted.
The slope surface was covered by a layer of shotcrete to prevent further
erosion. In addition, horizontal drains were installed to avoid pore pressure to build up behind the facing. Riprap was placed at the toe of slope to prevent undermining.
Figure 2.2
Typical remedial design for the unstable riverbank slope, after Chen and Lim (2005).
12
Figure 2.3
2.3.2
Site condition after the remedial works, after Chen and Lim (2005).
Cut Slope at Kuala Lumpur (Liew, 2004)
This cut slope is located in Kuala Lumpur. It was formed in 1990s and believed to have been carried out by the adjacent developer during the earthwork stage.
It
consists of six berms with slope gradient varies from 1V:1.72H (lowest berm) to 1V:1H (highest berm). Berm drains were constructed together with the slope formation. Three layers of gabion walls of about 3m high were placed at the toe of the slope. The site is underlain by granite formation which texture varies from fine to coarse grained biotite granite.
13 Slope movement was detected in November 2002 and clearly visible tension cracks were then observed at the lowest three berms especially a crack extending from Berm 3 to Berm 1 as shown in Figure 2.4. The depth of crack generally varies from 150 to 300mm. Berm drains were damaged due to the ground movement. Since there are residential structures at the toe of the distressed cut slope, a sound remedial measure should be carried out for the safety purpose.
Figure 2.4
Front view of the failed slope, after Liew (2004).
Due to the site constraint, it is not suitable to trim the existing slope to a gentler slope. Thus, soil nailing has been decided to apply as the remedial measure for this cut slope. All nails were designed to nail the failed mass through the shear failure surface into the intact slope material as illustrated in Figure 2.5. Furthermore, horizontal drains were installed and repairing of drains was carried out to improve both the subsoil and surface drainage system and minimize surface erosion. results do not show any significant movement.
After the work, monitoring
14
Figure 2.5
Figure 2.6
Soil nailing strengthening work, after Liew (2004).
Completed soil nailed slope, after Liew (2004).
15 2.3.3
Stabilization of Slipped Slopes that Caused the Collapse of a Pylon (Phan and Tan, 2005)
A landslide occurred in early March, 2004 near a hilltop at Segari, Perak after days of heavy rains. The debris flow of the landslide caused the collapse of a power transmission pylon, known as Pylon 4A which is locating at the down slope. This landslide had also undermined the footings of another two pylons, Pylon 3B and 4C on the top of the slide area. Pylon 3A was badly affected and was demolished. The site condition after the incident is as shown in Figure 2.7. The site was covered by residual soils deriving from the weathering of granite parent rocks.
Pylon 3B
Pylon here collapsed
Figure 2.7
The site condition after the collapse of pylon 4A, after Phan and Tan (2005).
16 The selection of suitable stabilization method for these slopes was highly restricted due to the difficult access to site and the steep topography. Soil nailing is adopted after comparing with grouting and hand-dug caisson methods since it does not require heavy equipment and excessive construction materials on site. Soil nails were installed at horizontal spacing of 2m by 2m with working capacity of 140kN per nail. All the nails were installed to a maximum length of 18m or anchored in granite bedrock just in case the slip surface is deeper than expected. The soil nailed slopes were protected by the shotcrete facing. Besides these, horizontal drains and weep holes were introduced to reduce the possibilities of the excessive raise in groundwater table.
Figure 2.8
Slope stabilized by soil nails, after Phan and Tan (2005).
17
Figure 2.9
2.3.4
The soil nailed wall at pylon 4C, after Phan and Tan (2005).
30m High Soil Nailed Slope in Kuala Lumpur (Tan and Chow, 2007)
A permanent soil nailed slope had been adopted in place of diaphragm walls and contiguous bored pile walls for a mixed commercial development project in Kuala Lumpur. The site condition is composed of residual soils overlying metasedimentary formation.
Tan and Chow (2007) stated that this project posed significant challenges as the existing residential houses are located very close to the soil nail slope where around 3m
18 from a brick wall boundary fencing and about 6m from the building (as shown in Figure 2.10). The design of this 30m high slope also challenging because in Malaysia, soil nail has been used mainly for slope stabilization with heights ranging from 5m to 15m and rarely for high cut slopes.
After completion, the monitoring results have shown encouraging results so far with deflection less than 10mm for this 30m high soil nailed slope.
Figure 2.10
The 30m high soil nail wall under construction, after Tan and Chow (2007).
19
Figure 2.11
2.3.5
The completed 30m high soil nailed wall, after Tan and Chow (2007).
Soil Nail Strengthening Work over Uncontrolled Fill for a 14.5m Deep Excavation (Liew and Khoo, 2006)
This slope was involving in the construction of a high-rise mixed development with a five-and-half storey basement car park adjacent to an existing commercial development as illustrated in Figure 2.12. The entire excavation face was about 250m long and the depths vary from 7m to maximum of 14.5m. The site was overlying an uncontrolled dumped backfill primarily made up of loose silty sand and sandy silt. It is formerly a natural valley with natural stream and without proper land clearing.
20
Figure 2.12
The locations of project site and excavation, after Liew and Khoo (2006).
The saturated loose fill had insufficient strength to stabilize itself after the initial steep excavation and tension cracks were found.
The extreme weather condition of
monsoon season and the existence of uncontrolled dumped material within the excavation had aggravated the distresses.
To solve, soil nailing was chosen as the remedial method to replace the initial steep open cut excavation in view of the tight space constraint at site. The strengthening strategy was to reinforce the saturated loose fill with closely spaced soil nails, i.e. 1.25 to 1.5m with length ranging from 6 to 12m. The excavation surface was finished with shotcrete facing. At the valley area which was seated over the 2m thick srcinal soft deposits, 12m long sheet pile wall with two rows of 18m long soil nails and reinforced concrete props were used to enhance the passive resistance of the retained ground. In addition, horizontal drains and weephole drains were installed to control the
21 groundwater and prevent pore water pressure to build up behind the facing respectively. The strengthening work is as shown in Figure 2.13.
Figure 2.13
Soil nailing strengthening work, after Liew and Khoo (2006).
After the completion of this 14.5m high soil nailed excavation, the monitoring results proved that soil nailing strengthening technique is suitable to apply in saturated loose fill as long as substantial care is taken during implementation.
22 2.3.6
40m High Nailed Excavation in Gneissic Residual Soil at Brazil (Sayao et al., 2005)
This excavation was required for the construction of an eight-storey high rise building in a waterfront area in the city of Niteroi, Rio de Janeiro State, Brazil. The geological condition at the site is quite complicated, with a thick layer of residual soil from gneissic rock which is inherently heterogeneous.
The geometry of the nailed excavation is as illustrated in Figure 2.14. The top two stages of the slope were protected only by grass while the third and forth excavation stages were reinforced by soil nails. The main nailed wall is about 40m high. Due to the excavation was in U-shape, lateral nailing was also required for this case.
Figure 2.14
3D view of soil nailing design, after Sayao et al. (2005).
23 All nails used were made of corrosion-protected steel bars, 25mm in diameter and were inserted in 75mm boreholes and then cement-grouted. The nails were installed in thirteen rows, with both vertical and horizontal spacing of 2m. The upper nails had longest length of 24m in order to reach the active soil zone beyond the potential failure surface. As the excavation progressed, nails used were getting shorter. The nails at the bottom were only 15m long. The shotcrete facing was about 18mm thick, with a double mesh of steel wire. Drainage was provided by weep outlet holes and deep perforated pipes which were wrapped in filter fabric for clogging protection. This 40m high nailed excavation was completed in about seven months, from January to July, 2004.
Figure 2.15
The 40m high nailed excavation, after Sayao et al. (2005).
24
CHAPTER 3
METHODOLOGY
3.1
Theoretical Background
3.1.1
Definition
Soil nailing consists of reinforcing the soil mass by the introduction of a series of thin elements called nails to resist tension, bending and shear forces. The reinforcing elements are made of steel round cross-section bars. Nails are installed sub-horizontally into the soil mass in a pre-bored hole, which is grouted (Ortigao and Palmeira, 2004).
25 3.1.2
Reinforcement Concept of Soil Nailing
Soil nail is basically a steel bar encapsulated in a cementitious grout to transfer tensile load from less stable active zone of restrained soil mass to the more stable passive zone. Typically, soil nails are spaced at close spacing ranging from 1m to 2.5m in either horizontal or vertical directions to achieve massive soil-nail interaction within the soil mass for its reinforcement effect. Tensile, flexural and shearing stresses are typical modes of reinforcing actions by soil nails (as shown in Figure 3.1) whereby tensile stress has relatively more contribution to the reinforcing effect. The efficiency of reinforcement effect is related to the inclination of the nail with respect to the rupture surface and the stiffness of the nail element.
Figure 3.1
Effect of soil nail in reinforcing the soil mass (Liew, 2005)
The fundamental reinforcement of nail takes part in partially increasing the normal stress on the sliding surface, hence improving the shear strength. Besides this, the nails are directly reducing the destabilizing force of the reinforced soil mass, which is mostly due to the practically horizontal or sub-horizontal nail inclination. As a passive system, the mentioned actions require deformation of the soil mass to mobilize the nail strength. There are two modes of soil nail mobilization in relation to the ground movements. Firstly, this can be achieved by the alternate top down sequence between excavation and nail installation with stress relief predominantly in horizontal direction.
26 Secondly, the on-going ground movements of a marginally stabilized ground can also mobilize soil nail without any stress relief from excavation. The first mode will have earlier mobilization at the upper nails and undermobilized lower nails whereas the later mode have more uniform mobilization of the installed nails as the nails are mobilized at the same time with the ground movement.
3.1.3
Components of Soil Nailing Reinforcement
3.1.3.1 Nail Element
The soil nail element plays the major role in providing support to the slope mass. Corrosion protection can be achieved by adequate grout cover, galvanization and encapsulation. However, fissured cracks may develop within the grout body when nail is subjected to tension.
Therefore, for permanent application, galvanization and
encapsulation shall be considered.
Centralizers are important elements to ensure achieving full nail capacity and adequate grout cover for durability. If the nail reinforcement is not centralized and when the nail reinforcement is stretched during mobilization of nail force, flexural stresses will exist within the nail causing cracking and shattering of grout.
For nail element, there are three aspects controlling the nail resistance, namely grout-soil strength, nail head strength and structural strength of nail reinforcement.
27 3.1.3.2 Facing Element
Proper design of the soil nail facing is important as it affects the development of bond resistance along the nails. An inadequately designed facing will reduce the Factor of Safety (FOS) in addition to potential face failure. For slopes with gradient ranging 0
0
from 45 to 30 with only moderate heights, the active pressure acting on the shotcrete facing is insignificant. However, for soil nail slopes with gradient up to 76 0 and great heights, the active pressure acting on the shotcrete is significant and cannot be ignored (Tan and Chow, 2007). Type of facing used is also an issue on environments (Kotake and Tayama, 1996). Facing that having some degree of rigidity while allowing planting or vegetation can be used for natural landscape preservation.
3.1.3.3 Surface and Subsurface Drainage
For most slope strengthening works, it is vital to control the groundwater as it has significant impact on the safety factor.
For efficient control of groundwater,
horizontal subsoil drains are usually installed at certain horizontal and vertical spacing to proactively lower the groundwater profile and depressurize excess pore pressure within the slope mass. For rock mass where fractures and water seepage are observed, subsoil drains should be installed at these locations. If shotcrete or gunite is used as slope facing, it is important to have sufficient weephole drains to prevent buildup of water pressure immediately behind the shotcrete or gunite facing.
28 3.1.4
Suitability of Soil Nailing With Respect to Soil Types
As soil nail construction requires temporary stability in the staged excavation and also the drilled-hole stability, any soil with sufficient temporary self-support of about 2m sub-vertical height for minimum of 1 to 2 days and hole stability for minimum four hours are considered suitable ground for soil nailing. The following soil types would be suitable for soil nailing:
Stiff fine or cohesive soils
Cemented granular soils
Well graded granular soils with sufficient apparent cohesion of minimum 5kPa as maintained by capillary suction with appropriate moisture content
Most residual soils and weathered rock mass without adverse geological settings (such as weak day-lighting discontinuities, highly fractured rock mass, etc) exposed during the staged excavation
Ground profile above groundwater level
The major impacts to soil nailing works in the unsuitable ground condition are mostly:
Loss of grout through the fractured rock mass, open joints and cavities
Collapse of drilled-hole
Poor nail-to-soil interface resistance due to disturbance of drilled-hole
Localized face stability
Soil nailing can still be considered suitable for certain soil types or ground conditions if proper drilling equipment and flushing agent are carefully selected.
29 3.1.5
Comparison between Soil Nailing and Reinforced Earth Wall
The main similarities are:
The reinforcement is placed in the soil unstressed, hence the reinforcement forces are mobilized by subsequent deformation of the soil.
The reinforcement forces are sustained by frictional bond between the soil and the reinforcing element.
The main dissimilarities are:
Soil nailing is constructed by staged excavations from top to down while reinforced earth is constructed from bottom to top as illustrated in Figure 3.2. This has an important influence on the distribution of the forces which develop in the reinforcement, particularly during the construction period.
Soil nailing is an in situ soil reinforcement technique exploiting natural ground whereas the properties of fills for reinforced earth can be preselected and controlled.
In soil nailing, grouting techniques are usually used to bond the reinforcement to the surrounding ground where load is transferred along the grout to soil interface. In reinforced earth, friction is generated directly along the strip to soil interface
30
Figure 3.2
Construction sequence for soil nailing and reinforced earth wall (Bruce and Jewell, 1986)
3.1.6
Design Methods of Soil Nailing
The following documents have been widely referred by designers in designing the soil nailing strengthening work (Chow and Tan, 2006): a. British Standard BS8006: 1995, Code of Practice for Strengthened or Reinforced Soils and Other Fills. b. HA 68/94, Design Methods for the Reinforcement for Highway Slopes by Reinforced Soil and Soil Nailing Techniques. c. U.S. Department of Transportation, Federal Highway Administration (FHWA, 1998), Manual for Design and Construction Monitoring of Soil Nail Walls.
31 Based on the opinion of Chow and Tan (2006), FHWA method with some modifications is suitable to adopt for Malaysian practice as the method is complete and it provides a rational approach towards soil nail design inclusive of design aspects for shotcrete, soil nail head, etc. Another advantage of the FHWA method is the assumption of slip surface limiting equilibrium failure mechanism which considers all internal, external and mixed potential slip surfaces for the wall and evaluates global stability for each. Besides this, slip surface limiting equilibrium approach is also more convenient and accurate for heterogeneous geometries, soil types, and surcharge loadings than other methods. FHWA can easily to adopt in practical application as various commercial slope stability analysis software are available to carry out such analysis.
3.1.7
Stability Assessment
For all soil nail strengthening work, the primary objective is to improve the safety factor to the design requirement. A designed soil nail structure must be stable both internally and externally. Figure 3.3 shows the typical types of failure mechanism of a soil nailed slope. External failure mechanisms are developed due to insufficient stability of the complete structure block. External stability must be evaluated to ensure that failure would not occur because of one or more of the sliding, overturning of the reinforced structure, bearing capacity failure and rotation of the complete structure block. The external failure mechanism is not due to the reinforcement of structure itself.
For a soil nailed structure to be internally stable, the reinforcements must be able to carry the tensile stresses transferred by the soil without rupture. Moreover, sufficient bond between the reinforcements and soil is required so that the soil mass will not be
32 pulled out by the reinforcement.
The purpose of analyzing the internal stability of
reinforced soil structure is to evaluate the required spacing and length of reinforcements.
Figure 3.3
Typical types of failure mechanism in soil nailed slope (Liew, 2005)
Slope stability program by either limit equilibrium method or strength reduction method in finite element analysis will normally be used to assess the srcinal safety factor and the improvement after strengthening. The extent of such stability assessment should be carried out, at least, at areas where there is impact to human being if slope instability occurs. If there is any surcharge loading, it should be considered in the stability assessment. Both global stability and local stability shall be carried out.
In limit equilibrium stability assessment, the individual nail load should be adjusted for every slip surface in order to obtain a correct safety factor. Therefore, it is important to carry out iterative process to adjust individual nail load based on the intercepts for the critical slip surface until the safety factor converges. If finite element analysis is used to assess the safety factor for the nailed slope, strength reduction method on slope material strength can be adopted. The required safety factor of the soil nail slope assessment can refer to the recommendation by Geotechnical Engineering Office.
33 3.1.8
Serviceability Assessment
Generally, the lateral ground deformation for an adequately reinforced soil nailed slope or excavation typically ranges from 0.2% to 0.5% of the slope height or retained height. Finite element analysis can provide useful predictive magnitude and trend of the deformation profile. If any measured deformation exceeds the limiting range, caution should be taken to timely implement the contingency plan to prevent disastrous failure.
3.1.9
Construction Sequence
3.1.9.1 Initial excavation
This initial excavation is carried out by trimming the srcinal ground profile to the working platform level where the first row of soil nails can be practically installed. The pre-requisite of this temporary excavation shall be in such a way that the trimmed surface must be able to self support till completion of nail installation. Sometimes, sectional excavation can be carried out for soil with short self support time. If shotcrete or gunite is designed as facing element, the condition of the trimmed surface shall be of the satisfactory quality to receive the shotcrete.
34 3.1.9.2 Drilling of holes
Drilling can be done by either air-flushed percussion drilling, augering or rotary wash boring drilling depending on ground condition. Typically, the hole’s size can range from 100mm to 150mm. In order to contain the grout, the typical inclination of 0
the drill hole is normally tilted at 15 downward from horizontal. Flushing with air or water before nail insertion is necessary in order to remove any possible collapsed materials, which can potentially reduce the grout-soil interface resistance.
3.1.9.3 Insertion of nail reinforcement and grouting
The nail shall be prepared with adequate centralizers at appropriate spacing and for proper grout cover for first defense of corrosion protection. In addition to this, galvanization and pre-grouted nail encapsulated with corrugated pipe can be considered for durability. A grouting pipe is normally attached with the nail reinforcement during inserting the nail into the drilled hole. The grouting is from bottom up until fresh grout return is observed from the hole. The normal range of water/cement ratio of the typical grout mix is from 0.45 to 0.5.
35 3.1.10 Advantages and Disadvantages of Soil Nailing
The advantages of applying soil nailing as a mean of soil strengthening method are as following: a. Allow in-situ strengthening on existing slope surface with minimum excavation and
backfilling,
particularly
very
suitable
for
uphill
widening,
thus
environmental friendly. b. Allow excellent working space in front of the excavation face. c. Sub-vertical cut surface reducing loss of space. d. Avoid unnecessary temporary works. e. Only requires light machinery and equipment. f.
Flexible at constraint site and excavation shape.
g. Can be used for strengthening of weather natural slope, natural or man-made cut slopes. h. Thinner facing requirement. i.
Robust i.e. less sensitive to unforeseen weak geological materials and adverse groundwater condition.
j.
Lower cost and rapid construction.
The disadvantages of soil nailing are: a. Nail encroachment to retained ground rendering unusable underground space. b. Generally larger lateral soil strain during removal of lateral support and ground surface cracking may appear. c. Tendency of high ground loss due to drilling technique, particularly at course grained soil. d. Less suitable for course grained soil and soft clayey soil, which have short self support time, and soil prone to creeping. e. Lower mobilized nail strength at lower rows of nailing. f.
Suitable only for excavation above groundwater.
36 3.2
Introduction of the Methodology
This chapter explains how the study is carried out in order to achieve the objectives mentioned in Chapter 1. The objectives that have to be accomplished in this study are to identify the most appropriate slope remedial measure for a cut slope. Then, the factor of safety is determined by using Plaxis simulation. The flow chart of this study process is as illustrated in Figure 3.4.
3.3
Problem Identification
At this stage, the problem which leads to the implementation of this study will be identified. In order to stabilize or remedy either a new or failed slope, various remedial measures are available. The available methods are identified and briefly discussed. To choose a suitable remediation, an engineer should equip himself or herself with abundant knowledge and valuable experience in the field of concern, so that accurate judgment can be made.
37
Problem Identification
Case Study
Literature Review
Theoretical Background
Application of Plaxis Software
Results and Discussions
Conclusion
Figure 3.4
Flowchart of the study process
38 3.4
Case Study
A high and steep cut slope which is located at Sri Plentong Industrial Park has failed, so an appropriate soil slope remedial work should be carried out for it. For information, Sri Plentong Industrial Park is situated on hilly and undulating terrain. The slope is formed by cutting the hill where a number of industrial premises and buildings are located at the toe of slope. The special condition of this case study is that the top face of the slope is very close to the boundary of the Public Utility Board (PUB) pipeline, which is separated by a lined interceptor drain. Therefore, this condition should be taken into consideration when deciding the remedial measure.
From the data collected, the slope angle is approximately 700 with respect to horizontal. The height of the slope varies with the maximum height of about 20m. According to the results of laboratory tests on the soil samples from site, the soil within the area is generally categorized as gravelly sand with the presence of silt. The important soil parameters obtained are unit weight, γ = 19 kN/m3, cohesion, c’ = 12kN/m2 and friction angle, Φ’ = 430.
3.5
Literature Review
At the beginning of this stage, justification for the appropriate remedial method is carried out. The potential remedial alternatives are discussed one by one and the reason why it is not adopted is stated. Then, the most suitable remedial measure for this case study is justified.
39 After justification, the information for the method chosen, which is soil nailing, is searched from the journals, proceedings, technical papers, guidelines, textbooks and undergraduate thesis through internet or resources from library. The history of the development of soil nailing is studied. Besides this, the main part of literature review for this study is to review the previous cases which are applying soil nailing in their soil strengthening work.
Six cases are presented with five of them are application in
Malaysia. The site conditions are different for each case and so for the properties of the slopes.
3.6
Theoretical Background
At this stage, the understanding of soil nailing technique is greatly improved by searching and studying the theory part of this soil strengthening method. The sub-topics included in this section are the definition, reinforcement concept of soil nailing, components of soil nailing reinforcement, suitability of soil nailing with respect to soil types, comparison between soil nailing and reinforced earth wall, design methods of soil nailing, stability assessment, serviceability assessment, construction sequence, and advantages and disadvantages of soil nailing.
40 3.7
Application of Plaxis Software
At this stage, the soil parameters and other pre-determined variables are used as inputs to run the slope stability analysis by using Plaxis software. The stability analysis is divided into two phases with respect to the slope conditions, which are the slope without soil nailing and the slope with soil nailing. The expected outputs from this analysis are the factor of safety and the deformation behavior of the concerned slope.
PLAXIS is a finite element package which is specifically intended for the analysis of deformation and stability in geotechnical engineering projects. Geotechnical applications require advanced constitutive models for the simulation of the non-linear and time-dependant behavior of soils. Furthermore, special procedures are required to deal with the hydrostatic and non-hydrostatic pore pressure in the soil since soil is a multi-phase material. The modeling of the soil itself is an important issue. However, many geotechnical engineering projects involve the modeling of structures and the interaction between the structures and the soil. Plaxis is equipped with special features to deal with many aspects of complex geotechnical structure.
Plaxis offers a variety of soil models to adapt the different material properties for soils. Mohr-Coulomb model is a robust and simple non-linear model which is based on soil parameters that are known in most practical situations. Cam-Clay type model is available to analyze accurately the logarithmic compression behavior of normally consolidated soft soils.
Besides these, an elastoplastic type of hyperbolic model is
available for stiffer soils, such as over consolidated clays and sand.
Plaxis produces useful output such as the exact values of displacements, stresses (effective stresses, total stresses, pore pressures and excess pore pressures) and structural
41 forces (axial forces, shear forces, hoop forces and bending moments). The graphs for all types of stress and displacement on any desired cross section of geometry can also be created easily. In addition, a special tool is available for drawing load-displacement curves, stress paths and stress-strain diagrams. The visualization of stress paths can provide a valuable insight into local soil behavior and enables a detailed analysis of results of a Plaxis calculation.
3.8
Results and Discussions
A detailed data analysis on the results obtained from the Plaxis simulation will be done. The findings from the Plaxis analysis will be discussed further too.
3.9
Conclusion
A conclusion will be drawn for this study based on all of the stages undergone during this study.
42
CHAPTER 4
RESULTS AND DISCUSSIONS
4.1
Models
In order to determine the most suitable nail length in analysis by using PLAXIS, four geometry models are formed for this study. The description of each model is as shown in Table 4.1. In addition, the important dimension of the model is as shown in Figure 4.1.
Table 4.1 : Geometry models and descriptions Model
Description
1
Slope without reinforcement
2
Slope reinforced with five-meter nails
3
Slope reinforced with six-meter nails
4
Slope reinforced with seven-meter nails
43
20 m
20 m
70
40 m
60 m
Figure 4.1
The geometry model with dimensions
The safety analysis is executed for all of these four models to acquire their factor of safety (FOS). It is executed through a process in PLAXIS known as phi-c reduction. During this process, the strength parameters of soil are reduced. Number of additional steps used for this calculation is 40.
Taking into consideration of FOS recommended by the Geotechnical Control Office of Hong Kong as a guideline, the appropriate FOS for slope with high risk of economic and life loss is 1.40 (Liew, 2005).
44 4.2
Parameters
The values of various soil parameters used in analysis are as listed in Table 4.2.
Table 4.2 : Soil parameters used in analysis
(Department of Geotechnics and Transportation, 2009) Parameter
Symbol
Sand
Unit
Material model
Model
MC
-
Type of behavior
Type
Drained
-
Dry soil weight
γ dry
19
kN/m3
Wet soil weight
γ wet
19
kN/m3
Horizontal permeability
kx
1
m/day
Vertical permeability Young’s modulus
ky Eref
1 30000
m/day kN/m2
Poisson’s ratio
υ
0.33
-
Cohesion
cref
12
kN/m2
Friction angle
φ
43
0
(Degree)
13
0
(Degree)
Dilatancy angle
ψ
The nails used in this study are steel bars with 32mm diameter and inclining at 0
15 to the horizontal surface. Therefore, the stiffness, EA of each nail is 1.6 x 10 5 kN and the material type is elastic.
The nails are installed at vertical spacing of 5 meters and horizontal spacing of 5 meters. The close spacing is avoided because the soil type in this case study is gravelly sand. The closely-installed nails may decrease the strength of the sandy soil.
45 4.3
The Slope without Soil Nailing
The first geometry model is designed to analyze the stability of slope without any reinforcement. The geometry model is as shown in Figure 4.2. The finite element mesh with medium coarseness of element distribution is generated, which is as illustrated in Figure 4.3.
The deformed mesh of slope without soil nailing is shown in Figure 4.4. In addition, Figure 4.5 shows the slip failure of the slope. From the graph of factor of safety (Figure 4.6) obtained from the analysis, the factor of safety in this case is 0.77. This proves that the unreinforced slope is very unstable and remedial work should be done to increase the stability of this slope.
Figure 4.2
The geometry model of slope without soil nailing
46
Figure 4.3
Figure 4.4
The finite element mesh of slope without soil nailing
The deformed mesh of slope without soil nailing
47
Figure 4.5
Figure 4.6
The slip failure of slope without soil nailing
The graph of factor of safety for slope without soil nailing
48 4.4
Slope with Five-Meter-Nail Reinforcement
To determine an appropriate remedial solution for the unstable slope, the analysis is started by using the five-meter nails. The geometry model is shown in Figure 4.7 while the x- and y- coordinate for each node are listed in Table 4.3. The finite element mesh generated is also in medium coarseness as illustrated in Figure 4.8. The deformed mesh and the slip failure are shown in Figure 4.9 and Figure 4.10 respectively.
From Figure 4.11, which is the graph of factor of safety, the FOS for slope reinforced with five-meter nails is 1.17, which does not meet the recommended FOS value. The five-meter nails have insufficient strength to reinforce the slope.
Figure 4.7
The geometry model of slope reinforced with five-meter nails
49 Table 4.3 : The x- and y- coordinate of nodes (five-meter nails) Node No.
X-Coordinate
Y-Coordinate
0
0
0
1
0
40
2
20
40
3
27.3
20
4
60
20
5
60
0
6
22.5
18.7
7
25.5
25
8
20.7
23.7
9
23.7
30
10
18.9
28.7
11
21.9
35
12
17.1
33.7
13
15.2
38.7
Figure 4.8
The finite element mesh of slope reinforced with five-meter nails
50
Figure 4.9
Figure 4.10
The deformed mesh of slope reinforced with five-meter nails
The slip failure of slope reinforced with five-meter nails
51
Figure 4.11
4.5
The graph of factor of safety for slope reinforced with five-meter nails
Slope with Six-Meter-Nail Reinforcement
The analysis is continued with six-meter nails as the reinforcement with fivemeter nails cannot meet the required factor of safety. The geometry model for this analysis is shown in Figure 4.12 while the x- and y- coordinate are listed in Table 4.4. The medium coarseness element distribution mesh, the deformed mesh and the slip failure are as shown in Figure 4.13, Figure 4.14 and Figure 4.15 respectively.
From the graph of factor of safety plotted (Figure 4.16), the factor of safety for slope reinforced with six-meter nails is 1.35, which does not achieve the recommendation by the Geotechnical Control Office of Hong Kong.
52
Figure 4.12
The geometry model of slope reinforced with six-meter nails
Table 4.4 : The x- and y-coordinate of nodes (six-meter nails) Node No.
X-Coordinate
Y-Coordinate
0
0
0
1
0
40
2
20
40
3
27.3
20
4
60
20
5
60
0
6
21.5
18.4
7
25.5
25
8
19.7
23.4
9
23.7
30
10
17.9
28.4
11
21.9
35
12
16.1
33.4
13
14.2
38.4
53
Figure 4.13
Figure 4.14
The finite element mesh of slope reinforced with six-meter nails
The deformed mesh of slope reinforced with six-meter nails
54
Figure 4.15
Figure 4.16
The slip failure of slope reinforced with six-meter nails
The graph of factor of safety for slope reinforced with six-meter nails
55 4.6
Slope with Seven-Meter-Nail Reinforcement
The analysis is proceed with seven-meter nails as the previous attempts to use five-meter and six-meter nails are fail to comply with the recommended FOS. The geometry model is shown in Figure 4.17 while the x- and y- coordinate for each node are listed in Table 4.5. The meshing is also medium coarseness of element distribution as illustrated in Figure 4.18. The deformed mesh and slip failure are shown in Figure 4.19 and Figure 4.20 respectively.
From the analysis by PLAXIS, the factor of safety for slope with seven-meternail reinforcement is 1.44. The graph of factor of safety is shown in Figure 4.21. Since the factor of safety by using seven-meter nails is greater than the recommended FOS, reinforcement by using seven-meter nails can be adopted to remedy the slope. Seven meter is also the maximum nail length that can be applied to this slope due to the installation of PUB pipelines behind the slope.
Figure 4.17
The geometry model of slope reinforced with seven-meter nails
56 Table 4.5 : The x- and y- coordinate of nodes (seven-meter nails) Node No.
X-Coordinate
Y-Coordinate
0
0
0
1
0
40
2
20
40
3
27.3
20
4
60
20
5
60
0
6
20.5
18.2
7
25.5
25
8
18.7
23.2
9
23.7
30
10
16.9
28.2
11
21.9
35
12
15.1
33.2
13
13.2
38.2
Figure 4.18
The finite element mesh of slope reinforced with seven-meter nails
57
Figure 4.19
Figure 4.20
The deformed mesh of slope reinforced with seven-meter nails
The slip failure of slope reinforced with seven-meter nails
58
Figure 4.21
4.7
The graph of factor of safety for slope reinforced with seven-meter nails
Summary of the Results for Analysis of Various Nail Lengths
Table 4.6 : The factor of safety of slope reinforced with various nail length Nail Length (m)
Factor of Safety (FOS)
None
0.77
5 6
1.17 1.35
7
1.44
59
CHAPTER 5
CONCLUSION
A steep and high slope located in Sri Plentong has failed and require remedial work. This study is carried out to determine an appropriate remedial measure for this slope. Taking into consideration the special condition at where the top face of the slope is very close to the PUB pipelines, the soil nailing has been justified as the most suitable remedial method for the slope. PLAXIS software is used to do the analysis for slope reinforced with various nail lengths in order to obtain the factor of safety for slope. The following conclusions can be drawn based on the results of the study: 1. Soil nailing is the most appropriate remedial method for the slope in this case study after considering the geometry of slope and the location of PUB pipeline. 2. The seven-meter nails should be applied to provide the sufficient reinforcement strength to the failed slope.
Seven-meter-nail reinforcement gives factor of
safety of 1.44 to the slope. 3. The installation of seven-meter nails has greatly increased the factor of safety of slope from 0.77 to 1.44.
60
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