Geological Investigation of Cut Slope Failure in Contact Zone: a Case Study of Doi Tung, Thailand

GEOLOGICAL INVESTIGATION OF CUT SLOPE FAILURE IN CONTACT ZONE, A CASE STUDY OF DOI TUNG, THAILAND

SASIMA YOOCHAREON

A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF ENGINEERING (ENGINEERING TECHNOLOGY) SIRINDHORN INTERNATIONAL INSTITUTE OF TECHNOLOGY THAMMASAT UNIVERSITY ACADEMIC YEAR 2017

Ref. code: 25615922040406ZKO GEOLOGICAL INVESTIGATION OF CUT SLOPE FAILURE IN CONTACT ZONE, A CASE STUDY OF DOI TUNG, THAILAND

SASIMA YOOCHAREON

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Acknowledgements

First of all, I would like to express my gratitude to my advisors, Assoc. Prof. Dr. Alice Sharp and Assoc. Prof. Dr. Suttisak Soralump from Kasetsart University for helping me everything since the beginning. I have learned many things from both of you. I appreciate for your guiding, advices and support throughout a year. My sincere thanks go to Dr. Jessada Karnjana and Ms. Kasorn Galajit, the researchers from National Electronics and Computer Technology Center (NECTEC) for their advices, support, and setting the grant which derived from NECTEC for using in field study. My humble thanks go to the committee, Prof. Dr. Akihiro Takahashi from Tokyo Institute of Technology, Prof. Dr. Sandhya Babel and Asst. Prof. Dr. Amin Eisazadeh Otaghsaraei for valuable comments, suggestions, and their precious times on my thesis. Special thanks go to Asst. Prof. Dr. Burapa Phajuy from Chiang Mai University, who is my teacher from undergraduate degree, that always gladly provided equipment for me to use in field study. In addition, I am deeply indebted to TAIST-Tokyo Tech scholarship from corporation of Tokyo Institute of Technology, National Science and Technology Development Agency of Thailand (NSTDA), and Sirindhorn International Institute of Technology for giving me an opportunity to study in this program. Finally, I would like to thoughtful thanks to my friends and my family for helping me in field study and always supporting me. Without all of you, I cannot achieve this work.

Ref. code: 25615922040406ZKO Abstract

GEOLOGICAL INVESTIGATION OF CUT SLOPE FAILURE IN CONTACT ZONE, A CASE STUDY OF DOI TUNG

SASIMA YOOCHAREON

Bachelor of Science, Chiang Mai University, 2014. Master of Engineering (Engineering and Technology), Sirindhorn International Institute of Technology Thammasat University, 2017.

Wat Phra That Doi Tung is a famous temple in Chiang Rai province. High way 1390 is the main road that use for transportation to Wat Phra That Doi Tung. Last year, slope failures occurred at two locations along cut slope on highway 1390 by the influence of heavy and prolong rain fall. However, it was not the first time that the failure occurred. This study aim to investigate causes of failure in term of geology and geotechnical engineering and assess slope stability of cut slope of high way 1390. Four rock mass classification systems including Rock Mass Rating (RMR), Slope Mass Rating (RMR), Geological Strength Index (GSI), and Hazard Index (HI), and kinematic analysis were applied for slope stability assessment. General information of study area, field data and laboratory results were gathered to analyze slope stability and factors, which encourage the failure. Kinematic analysis, RMR, SMR, and HI were used for structurally controlled slope. Whereas, HI and GSI were applied for non-structurally controlled slope. Besides, HI can be applied with highly weathered rock slope. The results show that all slopes at both sites have very poor quality of rock and stability, which means the failure can happen easily from increasing of water content in slope. Geology of the area plays an important role on the stability as both failure occurred in contact zone between granite and host rocks. The intrusion created metamorphism, deformation, and discontinuities, which affect shear strength and rock quality of rock mass in the cut slope, and that make these zones have poor stability. In addition, wedge failure happened by influence of orientation of bedding with joint set faced with cut slope.

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Direction of road cutting and steep angle of slope also affect on stability of this highway. This results from this study can further apply to redesign or development of the highway 1390.

Keywords: Slope failure, Slope stability, Rock mass classification systems

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Table of Contents

Chapter Title Page

Signature Page i Acknowledgements ii Abstract iii Table of Contents v List of Figures ix List of Tables x

1 Introduction 1

1.1 Background 1 1.2 Objectives 3 1.3 Scopes of study 3

2 Literature review 4

2.1 Factor effecting slopr failure 4 2.1.1 Geological discontinuities 4 2.1.2 Geotechnical parameters 5 2.1.3 Water 6 2.1.4 Geometry of slope 6 2.1.5 Weathering and erosion 7

2.2 Rock slope stability 8 2.2.1 Kinematic analysis 8 2.2.2 Rock mass classification systems 8

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2.2.2.1 Rock Mass Rating (RMR) 9 2.2.2.2 Slope Mass Rating (SMR) 9 2.2.2.3 Geological Strength Index (GSI) 10 2.2.2.4 Hazard Index (HI) 10

2.3 Contact zone 12

3 Methodology 15

3.1. Field survey 15 3.1.1 Geological mapping and rock mass description 16 3.1.1.1 Rock and soil types 16 3.1.1.2 Types of discontinuity and geological structure 17 3.1.1.3 Orientations of discontinuities and face slope 17 3.1.1.4 Spacing of discontinuity 17 3.1.1.5 Persistence 18 3.1.1.6 Roughness 17 3.1.1.7 Intact rock strength 17 3.1.1.8 Weathering 19 3.1.1.9 Aperture 21 3.1.1.10 Infill or width 21 3.1.1.11 Seepage 21 3.1.2 Soil sampling 22

3.2 Laboratory test 22 3.2.1 Unified Soil Classification System (USCS) 22 3.2.2 Grain size analysis 22 3.2.3 Liquid limit test 25 3.2.4 Plastic limit test 26

3.3 Data analysis 26

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3.3.1 Kinematic analysis 27 3.3.2 Rock mass classification systems 28 3.3.2.1 Rock Mass Rating (RMR) 28 3.3.2.2 Slope Mass Rating (SMR) 29 3.3.2.3 Geological Strength Index (GSI) 31 3.3.2.4 Hazard Index (HI) 32

4 Results and Discission 38

4.1 Information of study area 38 4.1.1 Location and topography 38 4.1.2 Climate 38 4.1.3 Geology 39

4.2 Field data 41 4.2.1 Site 1 41 4.2.2 Site 2 42

4.2 Soil classification 44

4.3 Slope stability analysis 49 4.3.1 Site 1 49 4.3.2 Site 2 49

4.4 Factors effect on slope stability 53 4.4.1 Geology 53 4.4.2 Soil type 54 4.4.3 Engineering work 54

5 Conclusion and Reccommendation 56 5.1 Conclusion 56

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5.2 Recommendation 57

References 58

Appendix 62 Appendix A 62

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List of Tables

Tables Page 3.1 Intact rock strength by simple mean method 20 3.2 Weathering degree standard 20 3.3 Well-graded soil characteristics 24 3.4 Rock Mass Rating System 30 3.5 Rock mass classes and their engineering properties 31 3.6 Adjustment rating of F1, F2, F3, and F4 of SMR 32 3.7 Classification of rock slope according to SMR 32 3.8 Rating of sub-factors of normal condition 35 3.9 Apparent shear strength for an estimation friction angle 36 3.10 Rating criteria for fd 37 4.1 Orientation of discontinuities of S1 slope at site 2 48 4.2 Locations and types of soil samples under investigation 48 4.3 The results of LL, PL, and OI for fine grained soil samples 51 4.4 Results of GSI and HI methods for slope stability analysis of site 1 and 2 52 4.5 Result of RMR method for slope stability analysis of S1 slope of site 2 53 4.6 Result of SMR method for slope stability analysis of S1 slope of site 2 54

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List of Figures

Figures Page 2.1 Effect of orientation of discontinuity on type of failure 5 2.2 Relation between Factor of Safety and slope angle 7 2.3 Four common types of rock failure and typical pole plots of geological 11 conditions likely to lead to such failure 2.4 Contact metamorphism zone 13 2.5 Occurrence of fracture set from intrusion 14 3.1 Framework of the study 16 3.2 Unified Soil Classification System (USCS) 16 3.3 Description of roughness of discontinuities 19 3.4 USCS procedure 23 3.5 Grain size distribution graph 24 3.6 Example of liquid limit graph 25 3.7 Modified chart of the Geological Strength Index 29 3.8 Geological Strength Index for heterogeneous rocks 35 3.9 Stereoplot of wedge failure 36 4.1 Location of study sites on highway 1390 39 4.2 Example for stock work pattern of quartz vein 40 4.3 Tectonic setting of Chiang Rai during Late Permian to Early Jurassic 41 4.4 Circular slip in site 1 43 4.5 Mapping of cut slope site 1 44 4.6 Mapping of cut slope site 2 45 4.7 Wedge shape from slope failure in July, 2017 46 4.8 Fault plane and minor fold at S1 slope of site 2 46 4.9 Fault and bedding plane, joint 1, joint 2, and joint3 on S1unit at site 2 47 4.10 Slicken slide on fault plane 47 4.11 Circular slip in S2 of site 2 48 4.12 Grain size distribution of soil samples 50 4.13 Liquid limit chart of soil samples 51 4.14 Orientation of discontinuities on stereonet and potential failure caused 52

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from bedding plane and joint set 2

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Chapter 1 Introduction

1.1 Background

Several countries around the world, especially in mountainous region have been facing with instability of slope problem throughout our history. Slope failure occurs in both natural and man-made slopes due to changing of slope stability. In Thailand, most of slope failures have been founded in the northern and southern region. The frequency of failure event has been increasing significantly during the last decade starting from 1996 and the assumption of this issue is climate change and mismanagement of land use (Soralump, 2010). Besides the causes mentioned above, there is other long term trigger that is an interruption by excavation.

Slope cutting due to infrastructure constructions, such as road, and railway, is an important factor that make slope unstable. Slope failures happen before the life expectancy of infrastructures have been attribute to deterioration of rock and soil masses, which result from weathering and stress relief (Hack and Price, 1997). Newly exposed rock masses resulting from engineering works are subject to accelerated deterioration due to the release of confining pressure or stress relief, and general disruption of its equilibrium state that leads to intensified weathering right after excavation and cause failure before the life expectancy of infrastructures (Hack and Price, 1997; Niini et al., 2001). For example, many provinces in Northern Thailand such as Nan and Uttaradit have faced slope failure that causes damage particularly, to the road network in the hilly and mountainous terrains annually (Fowze et.al, 2012). In addition, there are many reports of slope failure occurrence caused on road-cut slopes correspond to the report from the Department of Highways of Thailand in 2007, which shows that the ratio of budget allocation for new highway construction to highway maintenance had altered to 50:50 from the earlier ratio of 60:40 due to the increased failures of roads and highway (Fowze et.al, 2012).

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An importance factor that control stability of the slope since the beginning is geological setting of the area. Geological setting is the determination of rock types, and geological structures, that consequences related to effect of weathering, orientation of bedding or discontinuities, intensity of deformation etc. which are the factors affect to stability of slope. Others than highly deformed area with fault and folds, contact zone of igneous rock intrusion through the country rocks has high potential to have unstable slope due to rock formation. Many slope failure have been occurred by influence of these ancient actions but did not much obtain an attention.

Instability problem has attracted researcher, therefore, many techniques for evaluation of slope stability were proposed. There are four basic methods for slope stability analysis, which are kinematic analysis, limit equilibrium, numerical modelling, and empirical method (Basahel and Mitri, 2017). Kinematic analysis employs stereonet projection technique using orientation of discontinuity and slope face to estimate potential structure failure mechanisms, i.e. wedge, and planar (Price and Cosgrove,1990). Limit equilibrium is a method that use driving and resisting forces to estimate safety factor (Coggan et al., 1998) and commonly used for studying slope structural stability. For complex failure mechanisms and slope geometries that limit equilibrium method cannot be applied, numerical modelling is proposed to handle with these complexities (Wyllie and Mah, 2004). Rock mass classification systems or empirical methods are widely used for preliminary assessment of rock mass (Duran and Douglas, 2000). This research focus on rock mass classification systems, which are Rock Mass Rating (RMR), Slope Mass Rating (SMR), and Hazard Index (HI), and kinematic analysis for slope stability assessment.

In July, 2017, there are two landslides occurred along highway 1390, which is the main route to Wat Phra That Doi Tung, due to continuous heavy rainfall by the influence of Talus storm. These events directly affect transportation and damage public properties. Luckily, there was no record of human injury and fatality. Moreover, at in the beginning of October, small landslide occurred again at the same location. It is obvious that rainfall was a trigger factor of slope instability of Highway 1390. According to the Department of Highway, this road was re-constructed resulting from

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highway restoration project after natural disasters in the last two years. Wat Phra That Doi Tung is famous tourist attraction in Chiang Rai province and has many visitors all the year. Accordingly, responsible agencies such as Department of Highway should be aware of the potential failure along this route. Due to repeating failures, this road has potential to have slope failure again in future especially in rainy season and may cause severe issues to safety of citizen, if there is no action taken. Consequently, this study aim to investigate geology of the area and how it relates with landslide and slope stability by using rock mass classification systems and kinematic analysis.

1.2 Objectives

The purpose of this research is to investigate slope stability of cut slope in contact zone and establish potential failure causes of cut slope, which was cut on contact zone, around failure sites along highway 1390, Doi Tung, Thailand for using as data for future development.

1.3 Scope of study

(1) Study area starts around kilometer 1+600 (landslide site 1) to kilometer 1+800 (landslide site 2) of highway 1390, Doi Tung, Chiang Rai, Thailand. (2) Slope stability assessments used in this research include Rock Mass Rating (RMR), Slope Mass Rating (SMR), and Hazard Index (HI) and Kinematic analysis. (3) Disturbed residual soils from the area will be collected and classify based on Unified Soil Classification System (USCS) and study the relationship between their types and landslide.

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Chapter 2 Literature Review

2.1 Factor affecting slope failure

Downward movements of earth materials, such as soil and rock, can be called landslide or slope failure, which results from of gravity and relation between shear stresses and shear strength. Several processes, i.e. pore pressure increasing, watering, swelling, etc., affect shear strength reduction in rock slope. While load increase at the top of slope, water and water pressure increase, thus, seismic activity, excavation, etc. may augment shear stress in rock slope and induce slope failure. Moreover, there are important factors which are erosion, climate, properties of rock mass, slope geometry, and state of stress that affect stability of slope. Although many factors affect stability of slope, only the relevant factors for this research will be discussed as below.

2.1.1 Geological discontinuities

Geological discontinuities play an important role on stability of rock slope, specially in slope that was excavated. Discontinuity, plane or surface is caused by changing in physical or chemical properties. In geology, discontinuities can be found in bedding plane, joint, fault, foliation, fracture, or schistosity. These structures and their properties, including orientation, roughness, infilling, and persistence significantly control stability and potential type of failure. Discontinuity occurrence can be only one plane or set. The orientations of discontinuities are measured as dip angle (the angle of inclination of the plane) and dip direction (the direction toward which the plane is inclined) and can be used to determine block shape of rock mass. Stable of slope not only depend on dip direction and dip angle of discontinuities, but it also relates with orientation of slope face. Even dip angle of discontinuities in Fig. 2.1a is lower than Fig. 2.1b, slope from Fig. 2.1b is more stable due to relation of orientation between slope face. Nevertheless, if dip angle of discontinuity increase as in Fig. 2.1c, the slope will have opportunity to cause toppling failure. Discontinuities in rock mass, whether

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they are joint, bedding, fracture etc. increase permeability and decrease shear strength in slope. Rock mass can have one, two or multiple joint sets. Containing more joint sets will be considered that rock mass has more fractured. Size and shape of blocks on slope are controlled from spacing of joint and these blocks also control structural failure mechanism. Close of spacing provides low cohesion of rock resulting to reducing shear strength in rock mass and that responsible for circular or even flow failure. Persistence of discontinuities is length or area that potential cause sliding. Moreover, a small area or rock from low persistence often has shear strength higher than shear stress providing positive influence on stability. Lastly, roughness of joint surface which measuring of existing unevenness and waviness of discontinuity surface relates with friction angle rock mass that is one of parameter controlling shear strength.

Fig. 2.1 Effect of orientation of discontinuity on type of failure (http://content.inflibnet.ac.in/data-server/eacharya- documents/53e0c6cbe413016f234436e8_INFIEP_3/2/ET/3-2-ET-V1- S1___causes_of_slope_failure.pdf)

2.1.2 Geotechnical parameters

Geological properties that involve with stability are shear strength, particle size distribution, permeability, density, moisture content, and slope angle. A very important factor affecting on stability is shear strength of rock mass. Shear strength of sliding surface depends on cohesion (�) and friction angle of material (�) (Coulomb, 1773). Bonding between surface of particles is called cohesion while resisting force between two surface is called friction. Both these parameters relate with many other factors, which are material properties, direction and magnitude of applied force and application rate, drainage condition, and magnitude of confining pressure. Equation 1

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by Mohr-Coulomb present the relationship between peak of shear strength (�) and normal stress (σ).

τ = c + σtanϕ (1)

where c is the cohesive strength and ϕ is the angle of friction

Rocks or materials which have rough texture have greater opposing high friction forces or shear strength to resist the movement. Stability of unconsolidated materials, which have no cement, have significantly lower than hard rock. Besides, loading, compaction and moisture content of the rock mass also affect slope stability.

2.1.3 Water

Two types of water that are considered to affect slope stability are groundwater and surface water. Groundwater is water under surface and surface water is rainwater that seeps and flows along the slope surface. Both types generate parameters that affect stability of slope, which are pore water pressure in groundwater and water pressure in surface water. Furthermore, precipitation, topography and geohydrology also relate to both pore water and water pressures (Sjöberg, 1999). Fractures or spaces are the place that water accumulated within the rock and it decreases stability of slope. When rainfall is absorbed water pressure within discontinuity will affect the effective normal stress by reducing shear strength. Increasing weight from water to slope increases pore water pressure. Slope containing high load due to heavy rainfall or continuous rainfall can lead to slope failure instantly. Fig. 2.2 (Hoek and Bray, 1977) shows the effect of water content from relationship between safety factor and slope dip angles.

2.1.4 Geometry of slope

Geometry of slope consists of two parameters that affecting stability, which are height and slope angle. Shear strength, density and bearing capacity of the slope foundation are the controllers of the critical height. Increasing of slope height reduces slope stability because newly weight added increase shear stress in toe. Shear stress in

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slope depends on weight of materials and slope angle, therefore, increase in slope angle will make amount of shear stress higher and that reduces slope stability.

Fig. 2.2 Relation between Factor of Safety and slope angle (Hoek and Bray, 1977)

2.1.5 Weathering and erosion

Erosion that affecting slope stability can be divided into two aspects which are large scale erosion and localized erosion. Large scale erosion, such as erosion from river that usually occurred at the bottom of slope, affects geometry of potentially failure rock slope. Decrease in confining pressure at toe of unstable slope results from large scale erosion by removal of material and that may affect stabilization on the slope. Another type of erosion is localized erosion by the influence of groundwater and run off. Localized caused in joint or weathering zone of rock generally reduce interlocking between adjacent rock blocks. Decreasing of interlocking also decreases shear strength of rock. Consequently, stable slope is interrupted and lead to failure. 2.2 Rock slope stability

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Slope stability is a major issue and need to be understood for geotechnical engineering. In order to understand the slope performances and their stability, and reliability and deformations, many techniques for slope stability assessment have been developed and can be categorized to four groups, which are kinematic analysis, limit equilibrium, numerical modelling, and empirical method. This research focus on kinematic analysis to determine and examine potential failure of rock slope and rock mass classification systems to primarily assessment of the cut-slope.

2.2.1 Kinematic analysis

Kinematic analysis is usually used to identify potential type of slope failure. Stereographic projections are convenient method by using orientation of discontinuities and slope face. This method is only suitable for structural failure, such as planar, wedge and toppling failures. Fig. 2.3 show main structural failure types and pole pattern on stereonet. After type of failure can identify, it can further be use to examine direction of sliding, and provide stability condition, which each type has different condition. This method is basic assessment in slope stability analysis. Soralump et al. (2013) used this method in to influence of discontinuity and potential failure to cut slope before further planning for investigation for The Study of Rock Slope Failure and Design Approach for Improving the Rock Slope Stability.

2.2.2 Rock mass classification systems

Rock mass classification is the process of placing a rock mass into groups or classes on defined relationships (Bieniawski, 1989) and describe to it based on similar properties/characteristics such that the behavior of the rock mass can be predicted (Abbas and Konietzky, 2017). Rock mass classification systems have been commonly utilized in the field of geotechnical engineering, especially for design purpose. They are widely used due to their simplicity and the limited need for detailed information (Duran and Douglas, 2000).

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2.2.2.1 Rock Mass Rating (RMR) Bieniawski developed RMR in 1973 and 1989 for evaluation the quality of rock masses for underground projects. The RMR system consists of five basic parameters that represent different conditions of the rock and the discontinuities. These parameters are: (1) uniaxial compressive strength (UCS) of intact rock, (2) rock quality designation (RQD), (3) spacing between discontinuities, (4) condition of discontinuities, and (5) groundwater and each parameter is valued which ranges between 0 and 100 (Bieniawski, 1973). An additional parameter was proposed by Bieniawski (1976) to account for the influence of the discontinuity orientation on the stability condition (correction factor). However, this parameter is introduced for tunnel and dam foundations but not for slopes (Aksoy, 2008). Therefore, Bieniawski (1989) implemented more descriptive details in the fourth parameter of the basic RMR (the condition of discontinuities). In the case of considering the effect of discontinuity orientation on the slope stability of a rock slope, he recommended the use of the SMR system proposed by Romana (1985).

RMS have been widely used for classification of rock mass several decades and mostly came with SMR. Many research and engineering projects applied RMR for rock classification and slope stability assessment. Soralump et al. (2010) applied RMR and RMR as geological investigation in order to qualify rock mass for Slope Stabilization of the Access Road of Mae Mao Dam project.

2.2.2.2 Slope Mass Rating (SMR)

Slope Mass Rating was proposed by Romana in 1985 for evaluation of slope stability, specially for cut slope or natural slope, and it is modified from Rock Mass Rating (Bieniawski, 1973) by subtracting adjustment factors of the joint slope relationship and adding a factor depending on method of excavation.

SMR is an important method using for assessment of slope stability for long time and it is usually used with RMR. Abad et al. (2011) worked on rock slope stability at Bandar Seri Alam, Johor by using kinematic analysis and SMR. According to the results, the results from these 2 systems were relevant.

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2.2.2.3 Geological Strength Index (GSI)

The Geological Strength Index (GSI) was established by Hoek in 1994 and had been modified by many authors (Cai et al., 2004, Hoek and Marinos, 2000; Hoek et.al., 1998; Sonmez and Ulusay, 1999; Marinos et al. 2005). This system has been developed in engineering rock mechanics to increase reliable input data, which related to rock mass properties as input for numerical analysis for tunnels, slopes, or foundations in rocks. Direct input, including the geological character of the rock mass by visual assessment, is used for selecting parameters to predict shear strength of rock mass and deformability. This system was created to support influence of geology on rock mechanism properties. There is method for characterizing difficult to describe rock masses. GSI resulted from combining observations of the rock mass conditions (Terzaghi’s descriptions) with the relationships and developed from RMR-system (Singh and Geol, 1999). GSI based on assessment of lithology, structure, and discontinuity condition from the exposed outcrop or surface of excavation such as road cut slope. The heart of the GSI classification is consideration in geology of the rock mass and can be used in both non-structurally controlled and differential weathering failure modes.

Singh and Tamrakar (2013) applied RMR and GSI for characterization of rock slope in Thopal-Malekhu. Consequently, the assessment of both systems show positive and good degree of correlation. However, GSI exhibits contrasting variation among the rock types compared to RMR.

Rahim and You (2017) evaluated slope stability by using GSI, kinematic analysis and rock mass properties to proposed slope design.

2.2.2.4 Hazard Index (HI)

In 2010, an alternative rock mass classification system had been established by Pantelidis. This new technique use to quantify the failure hazard of rock cutting. The failure hazard is classified with concerning of influence of drained

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Fig. 2.3 Four common types of rock failure and typical pole plots of geological conditions likely to lead to such failure. (a) plane failure; (b) wedge failure; (c) toppling failure; (d) circular failure in rock fill (Hoek and Bray, 1981).

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and climate condition on slope stability. The purpose of this system is to take account of failure hazard which consider climate condition as trigger factor. Seven failure types, including planar failure, wedge failure, individual block toppling, block and flexural toppling, failure from differential weathering of rock slope, and non-structurally controlled failure from excessive weathering. According to this technique, each failure type is separately examined, therefore only relevant parameters of each failure type is required (Pantelidis 2009). The input data of Hazard index are divided to two groups.

The first is normal condition of rock mass (fNC), which come from rating of sub-factors such as safety factor, orientation of dominant failure mechanism with slope face, Geological Strength Index, and volume of suspended rock mass per one meter of slope length, and trigger mechanism (fTM) which depended on surface and groundwater. Final score represents hazard level of rock slope failure.

Basahel and Mitri (2017) used an alternative rock mass classification or Hazard Index to assess slope stability both structurally and non-structurally controlled slopes comparing pros and cons with the results from SMR (only from structurally controlled slope). According to the comparison, they summarized that the HI method is as good as continuous SMR for the stability evaluation. However, the HI method is superior as it considers effect from water.

2.3 Contact zone

Contact zone in geology is the zone come from an intrusion from igneous rock through the host rock or called country rock (Fig 2.4 and 2.5). Intrusion causes contact zone, baked zone, and chilled margin. The metamorphism occurs in baked zone, where the properties of host rock is altered. Degree of metamorphism depended on amount of heat and pressure. However, in contact metamorphism, pressure was usually low. If the host rock is sedimentary rocks such as sandstone and shale, they will change to quartzite and hornfels. Nevertheless, if the temperature was not high enough to recrystallization, sandstone and shale will altered as meta-sandstone and meta-shale. As shown in Fig. 2.5 from an intrusion created fracture sets in area of occurrence. Fracture set 1 took place in baked zone, while complex fracture set 2

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occurred in igneous rock. Even host rock was not contact or metamorphose, fractures still appeared as set 3 (Senger et al, 2014). Accordingly, contact metamorphism structure provides instability of slope.

There are many slope failures occurred in Thailand and several of them occurred in contact metamorphism zone. Soralump et al. (2013) studied rock slope failure and design approach for improving rock slope stability in contact zone from granite interfering with limestone.

Fig. 2.4 Contact metamorphism zone (Plummer et al., 1999)

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Fig. 2.5 Occurrence of fracture set from intrusion (Senger et al., 2014)

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Chapter 3 Methodology

The aim of this research is to examine stability condition of back slope along highway 1390 by using rock mass classification system and kinematic analysis. The framework of this thesis is shown in Fig.3.1. The methodology starts with gathering basic information of study area such as geology, climate condition, and method of excavation. The second step is (3.1) field investigation and sample collection. Data collection in field survey include geological data (i.e. rock type, geological structure), information related with rock mass classification (i.e. intact rock strength, RQD, spacing of discontinuity, effect of water), and slope properties (slope height, dip angle, dip direction). Also, a visual assessment of the rock mass (i.e. weathering, formation characteristics) based on the Geological Strength Index (GSI) is collected for characterization and rating. In this study, GSI, and Hazard Index (HI), are important systems for assessment of non-structurally controlled slope. Especially Hazard Index, it can with both types of slope. The third step is (3.2) engineering property tests in laboratory that comprise of grain size analysis and liquid limit test for soil classification based on Unified Soil Classification System (USCS). The last step is (3.3) data analysis, which majorly includes rock mass classification and kinematic analysis. In this study, rock mass rating (RMR), Slope Mass Rating (SMR), and kinematic analysis are chosen to study structurally controlled slope while HI method is applied for highly weathered rock. In order to determine potential failure type and direction of slope along the road, the kinematic analysis is performed by stereonet technique according to relationship between discontinuities and face slope.

3.1 Field survey

The main works in field survey are geological mapping of cut-slope along both landslide sites, data collection required for data analysis, and sample collection. In order to performed data analysis, the parameters required for process must be

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determined before field survey. Most of the parameters were obtained by using Simple mean method (Hack and Huisman, 2002).

Fig. 3.1 Framework of the study

3.1.1 Geological mapping and rock mass description

Surface outcrop or existing cut slopes are observed and used to map and collect geology and engineering data required lists of parameters as below:

3.1.1.1 Rock and soil types

To define rock type (i.e. granite, sandstone), the origins of rock (i.e. sedimentary, metamorphic, and igneous), mineralogy, color, and grain-size are the parameters which use to consider and collect (Deere and Miller, 1966). Defining rock type is importance in engineering work because of different behaviors of rocks such as strength, weathering characteristics etc. Soil in the area is residual soil, which results from weathering process of rock. This research uses Unified Soil Classification System

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standard for soil classification, however, this standard requires laboratory test. Therefore, soil description in field survey uses only basic observation, such as color and texture.

3.1.1.2 Types of discontinuity and geological structure

In geology, discontinuities are categorized according to formed characteristics and comprise of six most common types, which are fault, bedding or lithology contact, foliation, joint, schistosity and cleavage. These structures are important for geotechnical engineering as they can be used in primary review of stability conditions (Wyllie and Mah, 1974)

3.1.1.3 Orientations of discontinuities and face slope

The orientations of discontinuities measure as dip and dip direction (or strike) of the surface. It is usually collected as geological information as it can tell direction and angle of plane and effect in engineering work. Stereonet projection of the orientation can used to analyze structural geology and slope stability.

3.1.1.4 Spacing of discontinuity

The discontinuity sets are measured in dip and dip direction. Discontinuities which have dip and dip direction will be grouped in one set. Following the British Standard, 1999, spacing between discontinuities should be measured perpendicular to the discontinuity. If there are variety of discontinuity set, the minimum spacing was considered according to Edelbro (2003). Spacing of discontinuity sets can define the size and shape of block and indicate structural failure mechanisms, such as toppling, planar, and wedge failure (Wyllie and Mah, 2004).

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3.1.1.5 Persistence

Persistence is measure of length or area of the discontinuity (Wyllie and Mah, 1974). The standard of persistence is divided to two ranges which are very high (> 20 m) and very low (< 1 m). This parameter control size of blocks and potential failure length that effect on stability of slope.

3.1.1.6 Roughness

The importance of the discontinuity surface roughness on the shear strength along the discontinuity planes depends on the stress configuration on the discontinuity plane and in the deformation characteristic of the discontinuity wall material and asperities (Hack, 1998). There are Lage-scale (Rl) and Small-sclae (Rs) measuring by assessing the wavelength and amplitudes of the discontinuity surface based on Fig. 3.3 (a) and (b) as reference. The large-scale roughness is determined in an area larger than 20 cm x 20 cm and smaller than 1 m x 1 m which comprises of five classes namely wavy, slightly wavy, curved, slightly curved and straight. Tactile roughness is classified as rough, smooth and polished as distinguished which feel by fingers in an area of 20 cm x 20 cm. The small- scale roughness is described as stepped, undulating and planar. Same as spacing of discontinuity, width of aperture is measured perpendicular with discontinuity.

3.1.1.7 Intact rock strength

Simple means method by Hack and Huisman (2002) is field test to determine intact rock strength classes as per the British Standard, 1981 that uses of hand pressure, geological hammer, etc. (Burnett, 1975). The simple means field tests following Table 1 are widely used in the field collection and it also able to calibrate with UCS test values as per proposed by Hack and Huisman (2002) shown in Table 3.1.

3.1.1.8 Weathering

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Reduction of rock strength due to weathering will reduce the shear strength of discontinuities. Weathering will also reduce the shear strength of the rock mass due to the diminished strength of the intact rock. Weathering categories range from fresh rock to residual soil as shown in Table 3.2 (ISRM, 1981) Weathering of rock takes the form of both disintegration and decomposition. Disintegration is the result of environmental conditions such as wetting and drying, freezing and thawing that break down the exposed surface layer. Disintegration is most prevalent in sedimentary rocks such as sandstones and shales, particularly if they contain swelling clays, and in metamorphic rocks with a high mica content. Decomposition weathering refers to changes in rock produced by chemical agents such as oxidation (e.g. yellow discoloration in rock containing iron), hydration (e.g. decomposition of feldspar in granite to kaolinite clay) and carbonation (e.g. solution of limestone) (Wyllie and Mah, 2004).

Fig. 3.3 Description of large-scale (Rl) and small-scale (Rs) roughness of discontinuities; (a) Rl is determined in 1 m x 1m area; (b) Rs is determined in 20 cm x 20 cm area of the discontinuity plane (Hack, 1998)

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Table 3.1 Intact rock strength by simple mean method (Hack and Huisman (2002)

Table 3.2 Weathering degree standard (Hack, 1998)

Degree Description

I No visible sign of rock material weathering; perhaps slight Fresh discoloration on major discontinuity surfaces II Discoloration indicates weathering of rock material and Slightly discontinuity surfaces. All rock material may be discolored by weathered weathering. III Less than half of the rock material is decomposed or disintegrated Moderately to a soil. Fresh or discolored rock is present either as a continuous weathered framework or as core stones. IV More than half of the rock material is decomposed or disintegrated Highly to a soil. Fresh or discolored rock is present either as a weathered discontinuous framework or as core stones. V Completely All rock material is decomposed and/or disintegrated to soil. The weathered original mass structure is still largely intact. VI All rock material is converted to soil. The mass structure and Residual material fabric is destroyed. There is a large change in volume, soil but the soil has not been significantly transported.

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3.1.1.9 Aperture

Aperture is the perpendicular distance separating the adjacent rock walls of an open discontinuity, in which the intervening space is air or water filled; categories of aperture range from cavernous (>1 m), to very tight (<0.1 mm). Aperture is thereby distinguished from the width of a filled discontinuity. It is important in predicting the likely behavior of the rock mass, such as hydraulic conductivity and deformation under stress changes, to understand the reason that open discontinuities develop.

3.1.1.10 Infill or width

Infilling is the term for material separating the adjacent walls of discontinuities, such as calcite or fault gouge; the perpendicular distance between the adjacent rock walls is termed the width of the filled discontinuity. A complete description of filling material is required to predict the behavior of the discontinuity include the following: mineralogy, particle size, over-consolidation ratio, water content or conductivity, wall roughness, width and fracturing/crushing of the wall rock. If the filling is likely to be a potential sliding surface in the slope, samples of the material should be collected for shear testing.

3.1.1.11 Seepage

The location of seepage from dis-continuities provides information on aperture because ground water flow is confined almost entirely in the discontinuities (secondary permeability); seepage categories range from very tight and dry to continuous flow that can scour infillings. These observations will also indicate the position of the water table, or water tables in the case of rock masses containing alternating layers of low and high conductivity rock such as shale and sandstone respectively. In dry climates, the evaporation rate may exceed the seepage rate and it may be difficult to observe seepage locations. The flow quantities will also help anticipate conditions during construction such as flooding and pumping requirements of excavations.

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3.1.2 Soil sampling

Soil samples were collected to represent residual soils which weathered form their parent rocks. Disturbed soil sampling is applied and will be used in grain size analysis test. Five samples of soil were taken at the bottom of landslide site 1 while in site 2, only one soil sample were collected as major material of the slope is rock. Amount of sample of each collecting point is around 2 kg.

3.2 Laboratory test

3.2.1 Unified Soil Classification System (USCS)

The USCS is the system using for soil classification based on their texture, and plasticity, which can refer to their behaviors. This system based on grouping They are usually found as mixtures with varying proportions of particles of different sizes; each component part contributes its characteristics to the soil mixture. The USCS is based on those characteristics of the soil that indicate how it will behave as an engineering construction material. Percentage of gravel, sand, silt, clay (fraction passing sieve no. 200), grading (shape of grain size distribution curve), and plasticity

3.2.2 Grain size analysis

Wet sieve analysis is used for testing grain size distribution. This procedure is performed to determine the percentage of different grain sizes contained in soil. Due to an assumption that the materials will have high amount of clay particles, wet sieving is used instead of dry sieve because high fraction of fine materials can make sieving difficult when they stick together as clumps and will provide an error. The samples will be separated by sieves depended on particle size. In this study, sieve no. 4, 10, 40, and 200 are used respectively. Retaining in each sieve after washing, will be dried and weighed to calculate amount of each grain size and plot in grain size distribution graph to classify soil type and characterize grading of the sample based on USCS.

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Fig. 3.4 USCS procedure (https://danielfgeotechnicaleng.wordpress.com/grain-size-distribution/)

After weighing the retaining of each sieve, use an equation 2 to calculate percentage of retaining soil in each sieve and then plot the result in grain size distribution diagram (Fig. 3.4).

% �� = ×100 (2)

Distribution curve can be used to indicate grading styles. For example, if the curve is very steep, it means the sample has poorly-graded. In addition, grading can be found from Coefficient of uniformity (Cu), which come from proportion between D60 and D10 as equation 3 and Coefficient of Curvature (CC), which can be found from equation 4

Well-graded soil will have Cu and Cc in range as show in Table 3.3.

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Fig. 3.5 Grain size distribution graph

� = (3)

where D60 = size of soil particle at 60% of passing,

D10 = size of soil particle at 10 % of passing

� = (4) ×

where D30 = size of soil particle at 30 % of passing

Table. 3.3 Well-graded soil characteristics

Type of soil Cu Cc Gravel More than 4 1-3 Sand More than 6 1-3

3.2.3 Liquid limit test

Soil sample, which has percent finer more than 50, is required to test for liquid limit test following USCS standard. The liquid limit (LL) is the moisture content

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that defines where the soil changes from a plastic to a viscous fluid state. The experiment use a pat of soil in a standard cup and cut by a standard dimension groove. When the soil flow together at the base of the groove for a distance of 13 mm (1/2 in.) within 25 shocks from the cup being dropped 10 mm in a standard liquid limit apparatus operated at a rate of two shocks per second. The liquid limit is the moisture content at which the groove, formed by a standard tool into the sample of soil taken in the standard cup, closes for 10 mm on being given 25 blows in a standard manner. This is the limiting moisture content at which the cohesive soil passes from liquid state to plastic state. The experiment should be measured in 4 ranges per sample including blows of 15-20, 20- 25, 25-30, and 30-35. The liquid limit can be found from the water content at which 25 blows of the slope gradient from measure of the 4 ranges an example shown as Fig. 3.5. The water content at which the soil closed can be calculated from equation 5.

Fig. 3.6 Example of liquid limit graph (http://www.basiccivilengineering.com/2015/07/atterberg-limit-test-soil- mechanics.html)

�. �. = � 100 (5)

3.2.4 Plastic limit test

Same as liquid limit test, plastic limit test was used for soil sample which has fine particles more than 50 percent. Plastic limit is the boundary of consistency between plastic state and semisolid state, which the changing of states is depended on

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the percentage of moisture content. The main point of the test is to find water content at which soil starts cracking during rolling of the sample into thread with 3 mm. in diameter on glass plate or any material, that has smooth surface and no absorption. The water content can be measured from subduction between weight of wet soil at the time that it started cracking and dry soil after baked. Plastic limit (PL) can be calculated from the equation 6.

�. �. = � 100 (6)

3.2.5 Calculation for Plasticity Index (PI)

The plasticity index (PI) is a measure of the plasticity of a soil by using range of water contents where the soil exhibits plastic properties. The PI is subtraction between the liquid limit and the plastic limit as shown in equation 7. High PI soil refer to clay, lower PI refer to silt, and PI of 0 (non-plastic) refer to little or no silt or clay. Soil descriptions based on PI including non-plastic (0), slightly plastic (< 7), medium plastic (7-17), and highly plastic (> 17)

�. �. = �. �. −�. � (7)

3.3 Data analysis

After gathering information of study area and field data, the data are used to input to each slope stability method. Four main methods are selected to assessment and that consist of kinematic analysis, Rock Mass Rating, Slope Mass Rating, and Hazard Index.

3.3.1 Kinematic analysis

This research uses steronet projection for kinematic analysis and aim to

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identify potential failure method from orientation of discontinuities and slope face. Some of the structural patterns can be seen in Fig. 2.3. In accordance with filed observation, potential of failure type of both landslide is wedge and planar. Thus, the method below will be focus on these failure types.

For planar failure, the pattern of the discontinuities may be comprised of a single discontinuity or a pair of discontinuities that intersect each other, or a combination of multiple discontinuities that are linked together to form a failure mode. The failure surfaces are usually structural discontinuities such as bedding planes, faults, joints or the interface between bedrock and an overlying layer of weathered rock. Block sliding along a single plane represents the simplest sliding mechanism. In case of a plane failure, at least one joint set strike approximately parallel to the slope strike and dips toward the excavation slope and the joint angle is less than the slope angle. The favorable conditions of plane failure are as follows:

(1) The dip direction of the planar discontinuity must be within (±20°) of the dip direction of the slope face,

(2) The dip of the planar discontinuity must be less than the dip of the slope face (Daylight)

(3) The dip of planar discontinuity must be greater than the angle of friction of surface.

Wedge failure of rock slope results when rock mass slides along two intersecting discontinuities, both of which dip out of the cut slope at an oblique angle to the cut face, thus forming a wedge-shaped block. Wedge failure can occur in rock mass with two or more sets of discontinuities whose lines of intersection are approximately perpendicular to the strike of the slope and dip towards the plane of the slope. This mode of failure requires that the dip angle of at least one joint intersect is greater than the friction angle of the joint surfaces and that the line of joint intersection intersects the plane of the slope. The necessary structural conditions for this failure are summarized as follows:

(1) The trend of the line of intersection must approximate the dip direction of the slope

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face.

(2) The plunge of the line of intersection must be less than the dip of the slope face. The line of intersection under this condition is said to daylight on the slope.

(3) The plunge of the line of intersection must be greater than the angle of friction of the surface.

3.3.2 Rock mass classification systems

In this study, four methods are used for slope stability assessment including RMR, SMR, GSI, and HI. RMR and SMR are suitable for rock slope that discontinuities can be measured. While GSI and HI are capable to apply in both structurally and non- structurally controlled failures. In addition, they can use with very high weathered rock mass.

3.3.2.1 Rock Mass Rating (RMR)

Six parameters including (1) Uniaxial Compressive Strength (UCS) of intact rock, (2) Rock Quality Designation (RQD), (3) spacing between discontinuities, (4) condition of discontinuities, and (5) groundwater condition, (6) orientation between discontinuities and slope face are used to evaluated for rating. The sum of rating is lie between 0 and 100, which low score refer to low rock quality whereas high score mean high rock quality (shown in Table 3.4 and 3.5). In accordance with RQD, this parameter is an index related to the degree of fracturing of core sample. This research does not have drill cores, hence RQD will be estimated from number of discontinuities per unit volume (or volumetric joint, Jv) for the exposure as suggested by Palmstrom (1982). In order to consider the effect of discontinuity orientation on rock slope stability, Bieniawski suggested the use of SMR system, which was developed by Romana (1985).

3.3.2.2 Slope Mass Rating (SMR)

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The Slope Mass Rating was proposed by Romana (1985) for rock slopes assessment and it is an extension of the RMR system. SMR uses to identifies different classes of slopes and their vulnerability to instability. The SMR score based on type of slope selected between wedge, planar, and toppling, adjustment factors related to strike and dip of discontinuities, and method of excavation as expressed in equation 6 and Table 3.6.

SMR = RMR + (F1xF2xF3) + F4 (6)

Where F1 is an adjustment factor depended on the parallelism between the joint strike (�j) and the slope face strike (�s). F2 refers to joint dip angle in the planar failure or the plunge of the line of intersection of wedge failure. F3 reflects the effect of the angle between the slope face dip (�s)and the joint dip (�j) or the plunge of the intersection. F4 is an adjustment factor that depends on the excavation method. The values are selected empirically as shown in Table 3.6.

The result from calculation can be classified classes as shown in Table 3.7, which describes rock quality and stability of slope. In addition, this system can provide prediction type of failure and engineering suggestion.

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Table 3.4 Rock Mass Rating system

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Table 3.5 Rock mass classes and their engineering properties

3.3.2.3 Geological Strength Index (GSI)

The main purpose of GSI is to classification rock mass based on geological observation. This system assumes that the rock mass behaves isotopically. Therefore, structurally controlled slope is not suitable. GSI can deal with very poor quality of rock, while RMR is difficult to apply. In this research, GSI is only used to characterize rock mass condition and evaluated to fulfil Hazard Index (HI). The quantification system is valid in the range i.e. 35 < GSI < 75. GSI evaluation can be measured by using chart as in Fig. 3.5, which is combination between geological structure and surface conditions. The extended GSI charts have been modified to accommodate the variety of rock masses, including extremely poor quality sheared rock masses of weak schistose materials (such as siltstones, clay shales, or phyllite) often interbedded with strong rock (such as sandstones, limestones, or quartzite). A GSI chart for heterogeneous lithological formation (Fig. 3.6) such as interbedded of sandstone and shale (Marinos and Hoek, 2001) is also applied for the cut slope.

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Table 3.6 Adjustment rating of F1, F2, F3, and F4 for SMR (Romana, 1985)

Table 3.7 Classification of rock slope according to SMR (Romana, 1985)

3.3.2.4 Hazard Index (HI)

Hazard index is an alternative rock mass classification system for rock slope proposed by Pantelidis (2010). This system used two functions which are normal condition (fNC) and triggering mechanism (fTM) to assessed. Sub-factors of two functions are used to assign the score for hazard level of a rock slope failure.

The quantitative attribution of the normal condition (fNC) (Table 3.8) consists of four sub-factors based on the failure mechanism, relationship between slope and joint orientations, the geological strength index and calculating the volume of suspended rock. Sub-factor f1 requires only for the structurally controlled failure such as planar, wedge and topples, and is also based on the apparent shear strength of the discontinuities. Sub-factor f2 is only used with planar and toppling failure which refers

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to the relative orientation of the dominant failure plane with slope face. Sub-factor f3 come from evaluation of Geological Strength Index (GSI) (Fig. 3.5 and 3.6) which can be used in case of non-structurally controlled and differential weathering (undercutting) failure modes. Sub-factor f4 is differential weathering which results from the volume of exposed rock mass per one meter length of slope. The rating scores for each sub-factor are 1, 3, 6 and 10, where higher score mean more unfavorable.

Fig. 3.7 Modified chart of the Geological Strength Index (Hoek et al., 1998)

In this study, only sub-factor f1 (safety factor for wedge) and sub-factor f3 (GSI) are applied to calculate. There are many different equations for calculation safety factors depended on type of failure such as planar and wedge. Safety factor wedge failure can be calculated from equation 7 by assuming that cohesive strength along plane A and B is zero (Hoek and Bray, 1981). A and B are intersecting failure planes

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and in the equation 7, they are dimensionless factors, which depend on the orientation of each plane (dip and dip direction) which come from the calculation of equation 8 and 9. Where � and � are the friction angle. In case of the friction angle are not tested, Table 3.9 is suggested to applied to estimate.

� = �� + �� (7)

.. � = (9) . .

.. � = (10) . .

ψA and ψB are the dips of planes A and B respectively and

ψ5 is the dip of the line of intersection

θ. can be measured from the streoplot example shown in Fig. 3.7

The triggering mechanism (fTM) come from an assessment of surface and groundwater influences or called drainage factor, and the ratio of the mean annual precipitation to the critical annual precipitation. The drainage factor fD can be examined by field inspection of the slope materials and structures of the rock cut or slope (see Table 3.10). The relationship between these 3 parameters show in equation 11.

� = � (11) where Im is the mean annual precipitation, Icr is the critical annual precipitation, and

fD is the drainage factor.

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Fig. 3.8 Geological Strength Index for heterogeneous rocks (Marinos and Hoek, 2001)

The final answer of HI Index can be calculated as equation 12, which is given on a scale of 1-10 defining 4 intervals, i.e. 1-4 (good), 6-8 poor), and 8-10 (very poor) (Basahel and Mitri, 2017).

/ �� = (��) (12)

Table 3.8 Rating of sub-factors of normal condition

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Table 3.9 Apparent shear strength for an estimation friction angle (Barton et. Al, 1974)

Fig. 3.9 Stereoplot of wedge failure (Hoek and Bray, 1974)

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Table 3.10 Rating criteria for fd

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Chapter 4 Results and Discussion

4.1 General information of study area

4.1.1 Location and topography

The study locations are on the back slope of highway 1390, where landslide occurred in 2017 (Fig. 4.1). Highway 1390 is main route for a famous temple of Chiang Rai known as Wat Phra That Doi Tung. In Thai language, “Doi” means mountain, accordingly, Doi Tung is a mountain area situated in the north of Chiang Rai province, which is the northernmost province of Thailand. The northern part of the mountain is in Mae Sai district, while the southern is in Mae Fa Luang district. In addition, the west part of the mountain shares the boundary with Myanmar. The topography of Doi Tung is typically rugged and most of mountain in northern of Thai generally oriented in North-South. The summit of Doi Tung is Wat Phra That Doi Tung with 1375 m of height, while the elevation of study area is around 1310 m.

4.1.2 Climate

General climate of the northern part of Thailand is under influence of tropical monsoons and was divided into 3 seasons as follows rainy or southwest monsoon season starts from mid-May to mid-October, winter or northeast monsoon season starts from mid-October to mid-February and summer or pre-monsoon season starts from mid-February to mid-May as per Thai Metrological Department (1994). According to Doi Tung Development Project (Maxwell, 2007), the average annual rainfall of 3 climates recorded stations at 550, 750, and 1200 m of elevation were 1925 mm, 2100 mm and 2500 mm respectively. The minimum temperature is around 13 ̊C in winter and the maximum is around 35 ̊C in summer at 1200 m elevation. In addition, the statistics from meteorology stations of Chiang Rai (2017) indicated that Doi Tung is the area, where has maximum of average annual rainfall of approximately 2,700 mm can be observed, and Chiang Rai has number of rainy days about 138 days.

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Fig. 4.1 The location of study sites on highway 1390, (Google Earth, 2018)

4.1.3 Geology

Based on geologic map of Chiang Rai province (Department of mineral and resource, 2007), the geology of study area is mainly characterized by sedimentary rocks including sandstone and shale deposited during Carboniferous to Permian period (CPk), and granite which intruded during Triassic period (Trgr). CPk or Kaeng Khachan group includes shale with intercalated sandstone and siltstone; shale with interbedded sandstone, mudstone, chert, feldspartic sandstone, tuffaceous sandstone, quartzose sandstone, pebbly shale and pebbly mudstone, dark grey, greenish gray, and brown, while Trgr or Triassic granite includes biotite granite, tourmaline granite, granodiorite, biotite-muscovite granite, muscovite-tourmaline granite, and biotite-tourmaline granite.

From The Geology of Thailand (Ridd and Barber, 2011), Lithological setting of Doi Tung consists of meta sedimentary rock (rock altered by metamorphic reconstitution but not recrystallized and without the development mineral orientation)

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from Devonian to Carboniferous and plutonic rock, which mainly is granite, quartz veins and stock work (Fig. 4.2) are significantly in granite and meta-shale of Doi Tung. In addition, gold mineral can be found in this area.

As per Geology of Thailand (Department of mineral and resource, 2007), in Carboniferous-Permian period (around 350-200 million years ago), marine environment and volcanic arc were significant and covered many part of Thailand by influence of plate tectonic. Therefore, limestone and shale resulting from marine deposition both shallow and deep sea, and tuff from eruption of volcanoes were spread widely in many provinces including Ching Rai. Plates of Sibumasu and Indochina continued to move closer until Paleo-Tethys closed during Triassic period (Fig. 4.3). Consequently, intrusion and metamorphism were caused by the impact of the regional tectonic.

Fig. 4.2 Example for stock work pattern of quartz vein (https://www.flickr.com/photos/thirnbeck/4297327056)

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Fig. 4.3 Tectonic setting of Chiang Rai during Late Permian to Early Jurassic (Gardiner et al., 2016)

4.2 Field data

4.2.1 Site 1

Study site 1 is located around kilometer 1+600 of highway 1390. Slope surface is significantly weathered to soil; however, rock material can be found inside the cover and the slump. Failure type of slope shows circular slip characteristic (Fig 4.4), which has usually found in soil slope failure, at the top of slope. From Fig. 4.5, red lines show the boundaries of landslide zone, while the yellow lines show the boundaries of rock units. Rock mass in this site can be categorized to 4 units (Fig. 4.4). First unit (S1) is sedimentary rocks group, which is the oldest group (country rock) comparing with 4 groups in this site and comprises of sandstone and shale, followed by S2 unit, which is granite intruded through the sedimentary rock group. Intrusion of igneous rock altered rock surrounding contact zone. The metamorphosed rock occurred in baked zone rely on the parent rock which found meta-sandstone and meta shale (S3). The youngest rock of this site is basalt (S4) intruded through granite (S2) as basaltic dike.

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S1 is on the right of Fig. 4.5 and consists of interbedded of fine sandstone and shale. Degree of weathering of S1 is around class 3 to 4 while S3 is stronger and has more resistance than S1 as it has class 3-4 of degree of weathering. Bedding of these two sedimentary rocks and the contact boundary between S3 and S1 are not obvious due to covered soil from weathering from rocks. The residual soil of S1 and S3 has reddish-brown color and high percentage of clay mineral.

S2 is felsic igneous rock group, which mainly is granite, as shown in Fig 4.5. Degree of weathering of this unit is class 5 that means very high weathered as rock is completely decomposed to soil, however, the rock structure can still be seen. Soil of this unit has yellow color and has high percentage of quartz. The texture of material is rough and consists of coarse-grained crystalline. Many quartz veins (stock pattern) were found infilled in fractures both of S1 and S2 rocks. Around the rim of contact between S2 and S3 at the bottom of cut slope, there is white soil which has clay texture and barely found quartz mixed in the soil, and it can be categorize as class 5 of weathering degree.

S4 is basaltic dike intrude in granite (S2). It is black in color and has aphanitic texture. The rock is not noticeably weathered and was classified to grade 2 of weathering degree.

4.2.2 Site 2

Study site 2 is located around kilometer 1+800 of highway 1390. Rock mass of this site can be categorized to 3 units (Fig. 4.6) that are sedimentary rock unit (S1), granite unit (S2), and metamorphosed rocks unit (S3). Similar with site 1, S1 is the oldest rock. The slope assessment was divided to 2 slopes as S1 slope and S2 slope. S1 composed of sandstone interbedded with shale located on the left side of Fig. 4.6. Rock mass on the slope is slightly weathered, which degree of weathering is class 2. Rock structures such as bedding, joint, fracture are obvious and their orientation and factors used for stability analysis can be collected. According to previous landslide, the rock slope was under wedge failure of structural control (Fig. 4.7). Fault and small fold (Fig.

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4.8) were found on the left rim of Fig. 4.9. Slicken slide (Fig. 4.10) was found on the fault plane and it is used to find the movement of fault, which is oblique normal fault. Orientation of fault plane and bedding plan is very close around 49/101 and 53/105 (dip/dip direction) respectively. There are 2 major joint sets: J1 – 53/308 and J2-76/6 show in Fig. 4.9. The slope face orientation is 74/52. All the orientation of discontinuities occurred of S1 unit shows on Table 4.1. There is no clear boundary between S1 and S3, then S3 was considered as S1 slope.

S2 of site 2 has felsic color and coarse-grained crystalline texture (phaneritic) same as S2 unit of site 1 but degree of weathering is around 3 or 4. The surface of slope covers with soil which has the same texture and color as S2 of site 1. In addition, there are many quartz vein found in stock-work pattern and dikes of felsic rock found in this rock mass. According to slope failure in July, 2017, characteristic of failure in S2 is circular failure shown in Fig. 4.11, which is different from S1.

Fig. 4.4 Circular slip in site 1

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Fig. 4.5 Mapping of cut slope site 1 (scale: 1.83 m of height)

4.2 Soil classification

Six soil samples were collected to classify based on USCS and the locations that the samples were taken shown in Fig. 4.6 and 4.7 as blue symbol and their co- ordinate and type of rock shows in Table 4.2. Five samples represent soil from site 1, which compose of sample 1 to 5 and sample 6 represents soil from site 2. Soil sample 1 and 2 came from S1 unit at slope site 1 (sandstone and shale). Both samples have reddish-brown color but sample 1 seem to have higher percent of clay mineral than sample 2. While, sample 3, which has white color, came from S2 unit of site 1 (granite), but this sample obviously had amount of clay particle more than sample 4 and 5. Resulting from high amount of clay in sample 3, it can refer to parent rock, which had low percent of quartz and light color such as syenite, monzonite or diorite. According to sieve analysis, three of them has fine grain particles more than 50 percent and S1 has the highest percentage of fine grain followed by S3 and S2 respectively (Fig. 4.13). Thereby, these three sample need to test to find liquid limit and plastic limit as per USCS. The average percent of water content at 25 blow of sample 1 is about 56,

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whereas the results from sample 2 and 3 are very close, which are 41 and 39 percent respectively (Fig. 4.14). For plastic limit, the average water content for sample 1 is around 34.32 percent, 30.16 percent for sample 2, and 27.68 percent for sample 3. The PI for 3 samples are 22.18, 10.84, and 11.32 respectively. All of them locate below A line as per standard (Fig. 3.4). The overall results from Atterberg’s limit are shown as Table 4.3. Consequently, sample 1 is grouped in MH (elastic silt) and sample 2 and 3 are in ML (clayed silt with slight plastic).

Soil sample no. 4, 5, and 6 have same characteristic such as yellow color, sandy texture and they also represent rock from S1 unit. Sample 4 and 5 were collected from slope site 1, while sample 6 was collected from slope site 2. However, the results from grain size analysis are very close as they have high percent of sand particles more than 75 percent and have around 15 to 25 percent of fine particles (silt and clay) that were classified to be SM or SC types.

Fig. 4.6 Mapping of cut slope site 2 (scale: 1.83 m of height)

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Fig. 4.7 Wedge shape from slope failure in July, 2017

Fig. 4.8 Fault plane and minor fold at S1 slope of site 2

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Fig. 4.9 Fault and bedding plane, joint 1, joint 2, and joint3 on S1unit at site 2, and

Fig. 4.10 Slicken slide on fault plane

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Fig. 4.11 Circular failure in S2 of site 2

Table 4.1 Orientation of siscontinuities of S1 slope at site 2 Discontinuity Orientation (dip/dip direction) Bedding 53/105 Joint set 1 53/308 Joint set 2 76/6 Joint set 3 5/230 Fault 49/101 Slope face 74/52

Table 4.2 Locations and types of soil samples under investigation Sample Co-ordinate Elevation Rock unit Soil type no. (UTM) (m) 1 47990 86494 1309 S1 MH

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2 47986 86498 1313 S1 ML 3 47977 86496 1315 S2 ML 4 47959 86514 1310 S2 SM-SC 5 47956 86519 1311 S2 SM-SC 6 47856 86692 1312 S2 SM-SC 4.3 Slope stability analysis

4.3.1 Site 1

Due to highly weathering of rock mass in this site, geological structures such as joint, bedding, etc. are not obvious, therefore, RMR, SMR, and Kinematic analysis are not suitable. However, GSI and HI methods are designed for both structurally and non-structurally controlled failure as mentioned in chapter 3. In this site, the assessment separated to 2 slopes including S1 slope and S2 slope according to different type of rocks. Both have dip direction around 20° and dip angle around 64°. Based on GSI assessment (Table 4.3), the slope stability of them are similar as they are very poor of rock mass quality. Whereas, the results from HI assessment are slightly different since S1 slope gets 8.94 of 10 meaning very poor stability and S2 slope gets 7.75 meaning poor stability of cut slope.

4.3.2 Site 2

In site 2, the cut slope is significantly different from site 1 as weathering rate seem to be lower than the other site, specially in S1 unit. As per Fig. 4.6, landslide occurred in S1 unit shows wedge shape characteristic. Based on kinematic analysis by plotting of the orientation of discontinuities, the stereonet (Fig. 4.11) shows the potential of wedge failure occurrence caused by combination of joint 2 and bedding (and fault plane) with the slope face and it corresponds with the actual failure. However, the stand-alone of each discontinuity does not much effect on slope stability. Because landslide in S1 slope caused from structural control, RMR and SMR methods can be applied for assessment.

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Based on field data of S1, required factors for RMR and SMR were filled and calculated as shown in Table 4.4 and 4.5. The results show 35% of RMR and 11% of SMR. According to RMR, rock mass is in class 4, which means it has poor quality, 100-200 kPa of cohesion, and 15-20 degree of friction angle. The result of SMR is 11 that is the slope has very bad quality of rock mass, completely unstable, high potential of causing large wedge failure and should be re-excavation. In the same way with HI result, the S1 slope has very poor condition for stability. Due to structurally controlled failure, normal condition (fNC) used safety factor of slope instead of GSI rating that was used with the other slopes. Nevertheless, GSI was evaluated as fair condition.

For S2 slope, only GSI and HI methods are capable for assessment. Comparing with S2 slope from site 1, this rock slope has lower weathering intensity. The surface of rock mass was weathered to soil; however, rock material can be found inside. The GSI is 15-30 but it is still providing poor condition. Unlikely HI result, there is no change between site 1 and site 2, which means poor for slope stability.

Fig. 4.12 Grain size distribution of soil samples

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Sample no. 1 50 Sample no. 2 40 Sample no. 3

30 Linear (Sample no. 1)

WATER CONTENT, W % Linear (Sample no. 2) 20 Linear (Sample no. 3) 10

0 10 100 NUMBER OF BLOW, N

Fig. 4.13 Liquid limit chart of soil samples

Table 4.3 The results of LL, PL, and PI for fine grained soil samples

Sample no. LL PL PI 1 56.5 34.32 22.18 2 41 30.16 10.84 3 39 27.68 11.32

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Table 4.4 The result of GSI and HI methods for slope stability analysis of site 1 and 2

Contents Site 1 Site 2 Rock unit S1 S2 S1 S2 Slope face (dip/ dip direction) 64/20 64/20 74/52 59/60

Table type Heterogeneous rock General Heterogeneous rock General

Structure Type IX Disintegrated Type VII Disintegrated GSI Surface condition Very poor Very poor Fair Poor

GSI Rating 5-20 5-20 30-45 15-30

Normal condition (fNC) 10 10 10 10

Trigger factor (fTM) 8 6 8 6 HI HI rating 8.94 7.75 8.94 7.75 Category Very poor Poor Very poor Poor

Fig. 4.14. shows orientation of discontinuities on stereonet and potential failure caused from bedding plane and joint set 2. 4.4 Factors effect on slope stability

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4.4.1 Geology

The area is in fluctuating zone of geology. The sedimentary rocks (S1), which are original rocks of the area, was interrupted and changed their properties by the intrusion of granite (S2). At the contact zone between host rocks (S1) and the intrusion (S2), heat from igneous rock metamorphosed the surrounding and changed sedimentary rocks around them to be metamorphic rock. The occurrence of igneous rock intrusion effected on shear strength of the rock mass, which was decreased by discontinuities and deformation that make the area became instability. Stock work pattern of quartz vein found in S2 is an evidence of a large number of fractures in rock mass. Not only in S2, quartz veins were also found in S1.

Many studies have been publicized that rocks types play an important role on intensity rate of weathering. In our study, Granite (S1) has weathering intensity slightly over than sedimentary rocks (S1). Material that has high weathering rating will also high rate of decreasing shear strength. However, in this study, weathering did not affect much on stability of cut slope. Rock slope at site 2 clearly has degree of weathering higher site 1 but slope failure still occurred.

Table 4.5 The result of RMR method for slope stability analysis of S1 slope of site 2 Item Value Rating Uniaxial compressive strength (UCS) 5-50 MPa 3 Rock quality designation (RQD) < 25% 3 Spacing of discontinuities 60-200 mm. 8 Discontinuity length (Persistence) 3-10 m. 2 Separation 1-5 mm. 1 Roughness Smooth 1 Infilling Hard filling < 5 mm. 4 Weathering Moderately weathering 3 Groundwater Damp 10 Total 35 Table 4.6 The result of SMR method for slope stability analysis of S1 slope of site 2

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αi: trend of line of intersection, αs: dip direction of slope, ßi: plunge of line of

intersection, and ßs: dip of slope Factors Rating Category Score

F1 | α i - α s| 27 Favorable 0.4

F2 |ßi| 50 Very unfavorable 1

F3 ßi - ßs -60 Very unfavorable -60

F4 Excavation method Mechanism - 0

SMR RMR+( F1 F2 F3) + F4 - - 11

4.4.2 Soil type

Soil samples from laboratory tests, which are soil from clayed sand or silty sand groups, usually have permeability higher than soil from clay group due to grain size particle, which bigger particles also have bigger pore space. Resulting from that, soil from sand group can drain water faster than silt or clay soil. In contrast, silt and clay have very slow drainage but have high potential to absorb water that also increase shear stress that lead to slope failure. However, if the slopes are influence from pore- water pressure, soil from clay can resist to slope failure better than soil from sand group.

4.4.3 Engineering work

Road cutting is one of factor effecting on the degradation of slope stability. Due to stress relief, weathering from new exposure is expanding and increasing and Design of construction also affect to stability. Specially in orientation of road cut slope both strike and dip with orientation of discontinuities, they can be used to determine potential failure. The study sites have steep dip angle of cut slopes around 60-75 degree and when face with discontinuities, some of discontinuities daylight from slope. If

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inclination of slope is gentle, type of failure may change or slope may have stable condition.

Chapter 5

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Conclusion and recommendation

5.1 Conclusion

The study was conducted for an assessment of slope stability by using four rock mass classification systems including Rock Mass Rating (RMR), Slope Mass Rating (RMR), Geological Strength Index (GSI), and Hazard Index (HI), and kinematic analysis by stereonet projection in order to investigate slope failure of cut slope in contact zone along high way 1390 in 2017

HI are mainly used for this study as it can be applied in several slope conditions such as structurally and non-structurally controlled slope, and highly weathered rock slope. RMR and SMR are only applicable with structural failure, while, GSI is not suitable for structurally controlled slope. The results from RMR, SMR, GSI, and HI show in the same direction in all slopes that both sites have very poor quality of rock and slopes are unstable. If trigger factors such as precipitation and seismic wave appear, the slopes will failure easily.

From kinematic analysis, the result relates with the actual failure characteristic, which is wedge failure resulting from influence of intersection line between bedding and vertical joint set and orientation of cut slope. Due to road cutting with the direction and steep inclination that match for wedge failure, potential of failure is increased.

Soil from granite is SM-SC and soil from sedimentary rock is MH-ML based on USCS. In addition, soil from igneous rock zone nearly contact boundary called chilled margin is ML group. SM-SC group has permeability better than CH-CL and water can drain faster. In contrast, MH-ML take the lime longer to drain water has high potential to absorb water. Consequently, MH-ML slope should have high potential to fail more than SC-SM resulting from higher shear stress when receives water. However, slope failure occurred in both slope of soil types in site 1 and ML-MH group in site 2.

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Geology of the area play an important role on the stability as both landslides occurred in contact zone between granite (S2 unit) and sedimentary rock (S2 unit), partially metamorphosed to meta-sandstone and meta-shale. The intrusion created metamorphism, deformation, and discontinuities, which effect shear strength and rock quality of rock mass in the cut slope, and that make these zones have poor stability and caused failure.

5.2 Recommendation

From the experience working on this thesis, in order to improve the analysis, there should be more test on soil such as shear strength and permeability. Instead of testing shear strength of rocks, simple mean method was used to estimate strength of rock. Resistivity can be applied to study more about groundwater and their effect on slope stability. An alternative rock mass classification (Hazard Index), which is new method, provides good results for structurally and non-structurally controlled slopes.

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Appendix

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Appendix A

Geologic map of Chiang Rai (Department of Mineral Resources, 2007)

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