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-1INTRODUCTION Urgency of the topic Vietnam’s Central Highlands is the Southwest mountainous area of the country, including the provinces: Lam Dong, Dak Nong, Dak Lak, Gia Lai, KonTum The Central Highlands is a land with enormous potentials for development, has a key strategic location for politics, economy, culture and national defense of the whole country The industrialization - modernization of the country in general and of the Central Highlands in particular requires construction of more roads through the provinces, such as: - National Highway no.14 running from KonTum to Gia Lai, Dak Lak, Dak Nong, Binh Phuoc to Ho Chi Minh City - National Highway no 24 connecting KonTum with Ba To (Quang Ngai Province) - National Highway no 25 connecting from Pleiku (Gia Lai) to Tuy Hoa (Phu Yen Province) - National Highway no 26 connecting Dak Lak (Buon Me Thuot Province) with Nha Trang City (Khanh Hoa Province) - National Highway no 27 connecting Dalat City (Lam Dong Province) with DakLak (Buon Me Thuot Province) - National Highway no.28 connecting Dalat (LamDong Province) with DakNong - National Highway no.19 connecting Pleiku (GiaLai Province) with QuyNhon City - National Highway no 40 connecting with Xayden-Antoum (Laos) Po Y Border gate with the National Highway no.14 - Especially, the Ho Chi Minh Trail passing the provinces in the Central Highlands, this is a key route which not only bears the strategic meaning in the cause of industrialization – modernization, socio-economic development and assurance of national security for the Central area and the Central Highlands but also is historic route, associated with the liberation of the country (Truong Son road) - Moreover, many routes connecting townships with districts and remote areas where many ethnic people are living, there are a lot of traffic routes serving the construction of hydraulic works, hydroelectric power plants and tourist operation in the Central Highlands provinces The motorways running along hill foots or high mountainous passes are formed by various types of soils with different originations -2In the rainy season, after long periods of heavy rains, it often occurs the creep of earth mounds on roadside, causing traffic jams and requiring long period and cost for remedy One of the reason causing the above mentioned incident is mainly due to long periods of rain resulting in the variation of durability of roadside earth mass, causing large displacement leading to landslide Therefore, the selected topic is: STUDY ON THE CHANGE OF PHYSICAL PROPERTIES OF SOILS BUTTS BODY IN THE CENTRAL HIGHLANDS AREA AFTER PROLONGED FLOODING THAT AFFECTS THE STABILITY OF THE SLOPE NEXT TO THE MOTORWAY Purpose, object and scope of the study Purpose of the study: Study on variation features of durability of residual soil – deluvial deposit in the Central Highlands in dry condition (in dry season) and in water absorption saturation condition (in rainy season); those are the basics for evaluation of the stability of roadside hill soil and provision of necessary data for reference of the readers in case of building traffic routes in the Central Highlands - Object of the study: The variation of physico-mechanical properties of residual soil – deluvial soil mainly found in the Central Highlands relates to the stability of earth slope The stability of the slopes closing to the traffic roads is also affected by vehicles on the roads Within the scope of this thesis, only the reduction of durability of soil due to long periods of rain affecting the factor of stability safety of the slope is studied without taking into consideration of the impact of vibration caused by the vehicles on the roads Scientific meanings and realities of the topic a) Testing research determines variation features of natural density and shearing parameters (, C) according to humidity (W) from the dry (W season to the rainy season of four types of residual soil – deluvial soil commonly found in the Central Highlands They are types of residual soil – deluvial soil belonging to weathering crust in Basalt rocks, Granite intrusion rocks, terrigenous sedimentary rocks and metamorphic rocks b) Calculate, compare and identify stability factor against sliding for the same slope calculated by Bishop circular method (via Geo software - Slope International Ltd Canada) and by enhanced circular method of M.Н Голbдштейн and Г.Ц Тер-cтепанян giving approximately same values The research student has selected the enhanced circular method of M.N.Gônxtên to calculate the limited elevation of the slope (h) according to the falling gradient (1:m) of the slope basing on an estimated stability factor K -3c) Using the data studied in item a, apply the calculation method in item b, with the safety factor according to regulated safety factor of k=1.40, the research student has calculated the limited elevation (h) according to the falling gradient (1:m) and different humidity (W) of soil in the slope for four types of residual soil – deluvial soil studied in the Central Highlands d) The study results provide necessary data for the reference of the readers in designing or reviewing the stability of actual slopes with various elevations (h) and the falling gradients (1:m) according to the dry and rainy seasons of four types of soils commonly found in the Central Highlands Methods of study - Study theories relating to the calculation of the stability of the slope and the testing method identifying physico-mechanical characteristics of soil - Experimental study: Select survey locations for different types of soil in dry and rainy seasons in many years, perform taking samples of undistributed soil for testing to identify the physico-mechanical characteristics of soil in different seasons In addition, collect factual data for supplement - Report the studying results on scientific and seminar magazines; liaise with various Surveyors, Designers and Contractors in the Central Highlands for obtaining facts; discuss with the management agencies, such as the Department of Science and Technology, Department of Transport, Department of Environment and Natural Resources, Department of Agriculture and Rural Development in the Central Highlands provinces to identify requirements to be studied as well as actual experience of the local Structure of the thesis The thesis comprises sections: Explanation and Appendices The Explanation section includes 103 pages; beside the introduction, the thesis comprises 04 chapters and the conclusion at the end of the thesis At the end of the explanation section, there are pages listing the reference documents of local and international authors and page listing the articles of the research student relating to the contents of the thesis Appendices: 28 pages, including: Appendix to Chapter III: 17 pages Appendix to Chapter IV: 11 pages -4CHAPTER I NATURAL CONDITIONS, ENGINEERING GEOLOGICAL FEATURES IN THE CENTRAL HIGHLANDS AREA, LANDSIDE OF SLOPES ALONG THE MOTORWAYS IN THE CENTRAL HIGHLANDS 1.1 OVERVIEW OF NATURAL CONDITIONS OF THE STUDIED AREA 1.1.1 Topographic and geomorphological features The studied area comprises the provinces KonTum, Gia Lai, DakLak, Lam Dong, a part of Quang Nam Province, Binh Phuoc and is mainly distributed at the West of Truong Son The topography comprises the following types [9]: - Block mountains (Ngoc Linh, Mon Ray, Kon Ka Kinh, Dong Con Cho Ro, Chu Yang Sin, Dong Don Duong, Tay Bao Lam, Nam Di Linh, etc.) - Binh Son Nguyen erosion (Chu Pong – Chu Gau Ngo, Chu Ro Rang, Xnaro, Dalat, etc.) - Plateau basalts (Kon Ha Nung, Pleiku, Buon Ma Thuot, Dak Rlap, Bao Loc, Dinh Van) - Accumulated denudation valleys (Po Ko, KonTum, Dak To, Song Ba, Krong Ana, ect.) 1.1.2 Meteorological features 1.1.2.1 Characteristics of rivers and streams: The studied area gets the crest line of Truong Son Range as the datum line, dividing the area into two main basins, i.e the basin of rivers flowing into the East sea, including the rivers Ba, Da Rang, Dong Nai, Be, Saigon, Vam Co, etc And the basin of rivers flowing into the Mekong River (in the West), including the rivers SeRePok, PoCo, Se San, etc Basic characteristics of river and stream system in the region are short, narrow, falling and there are many water falls Rivers and streams in this region commonly have sections with specific characteristics, i.e a section crossing hills and mountains, a section crossing the highlands and the other crossing the plains In reality, the section crossing the hills and mountains has very little of sediment Only when flowing into the highlands, the plains or the valleys, can the rivers expand and create large but not thick sediment 1.1.2.2 Characteristics of rain: Rainy season in the Central Highlands often lasts from May to October, the rainfall during this period occupies about 75% of the annual rainfall The average annual rainfall in the region is about 1200mm to 3000mm -5In which: Medium high mountainous area – Ngoc Linh: from 2500mm to 3000mm Pleiku plateau: from 2600mm to 2800mm PoCo valley, Mandrak plateau: from 2000mm to 2500mm Cheo Reo, An Khe, Krong Buk valleys: from 1200mm to 1400mm In the South Central Coast region, the rainy season lasts from September to December The average annual rainfall is from 1100mm to 1300mm In the Southeast region, the rainy season lasts from August to November The average annual rainfall is from 1400mm to 2000mm 1.1.2.3 Characteristics of wind: In the Central Highlands, the Southwest monsoon prevails from May to September, the average wind speed is from 4.1 to 5.2m/s From November to April of the following year, there is mainly Northeast monsoon The Central Highlands is less directly affected by storm from the East sea but the storm can cause heavy rains on wide region, leading to floods, affecting the production and daily activities of the people; especially, the floods can cause damages to hydraulic works and traffic routes, etc In the South Central Coast, the Southwest monsoon prevails from May to September, the Northeast monsoon prevails from October to April of the following year In addition, this region is also affected by storms and sweeping floods in the period from August to October annually In the Southeast region, the Southwest and Southeast winds are equable throughout the year 1.1.3 Characteristics of weather and climate The studied area is located in the monsoon tropical region with two specific seasons, the rainy season and the dry season; the dry season starts from January to May and the rainy season starts from June to December The annual average temperature in the Central Highlands (Cheo Reo) is 25.5oC, in the Southern Central Coast (Nha Trang) is 26.4oC, in the Southeast region (Binh Duong) is 26.5oC The annual average humidity in the Central Highlands is from 74% to 90%, in the Southern Central Coast and the Southeast region is from 75% to 80% The amount of radiation is abundant (on an average of about 140Kcal/cm2/year) but there are differences according to seasons In the dry season, the solar radiation is high, the period with high radiation is in April and May -6(reaching 400 - 500 Kcal/cm2/day) In the rainy season, the solar radiation is lower, the highest radiation intensity reaches 300-400 cal/cm2/day In months of dry season, because the evaporation exceeds the rainfall, such as in Pleiku plateau, Cheo Reo – Phu Tuc region, making the soil exhaustedly dried, grass weathered, the weather hot, and the underground water level deeply dropped, etc The characteristics of weather, climate, hydrographic at the studied area are very severe, the dry season is much different from the rainy season, seriously affecting the construction conditions and the quality of construction works Figure 1.1 Map of the studied area -71.2 ENGINEERING GEOLOGICAL FEATURES IN THE REGION In the document [27] – an overview about the Engineering geological conditions in the regions from Quang Nam – Da Nang to the Southeast region introduces “the Map of engineering geology in the Central Highlands” (Figure 1.2) Basing on tectonic regime, there are different geodetic formations and geological complexes noted on the map Figure 1.2 Map of engineering geology in the Central Highlands -81.2.1 Characteristics of geological structure According to the studying results of Nguyen Viet Ky and Nguyen Van Tuan [9] on geological strata, this region is popular with seven groups of rocks, i.e.: Kainozoi unconsolidated deposit group originated from rivers, ponds, marshes of Neogene period, distributing mainly along river valleys creating river terraces, flat plains or filling fault-blocks under the form of weak consolidation Sedimentary rock group is mainly distributed in the Southern Central Coast, including sedimentary rocks of the early – middle Jara period, some of them belonging to Permi period with the strata system Chu Minh (Permi period); Ban Don type (the early – middle age of Jara period) with strata systems Dak Bung, Dray Linh, La Nga, Ea Sup; the strata system Dak Rium (the late Creta period) Metamorphic rock group with the age from the pre-Cambri period to the early Paleozi period, distributing mainly in the Northwest, the North and the East of the Central Highlands and including the following strata systems: Kon Cot, Xalamco, Dak Lo, Ki Son, Re River, Tak Co, Vu Mountain, Tien An, Dak Ui, Dak Long and Chu Se which are distributed under the form of high, sharp and strong cleavage mountain Intrusive acid - neutral rock group includes rocks with age from Paleozoi and Mezozoi periods, belonging to the complex Dien Binh, Ben Giang – Que Son, Hai Van, Van Canh, Dinh Quan, Ca Mountain Pass, Ankroet, Ba Na, etc., creating high mountain ranges Volcanic acid and neutral rock group includes rocks from Andezit (Dak Lin strata system with the age from Cacbon – Permi period and Bao Loc mountain pass system with the age from late Jara period – early Creta) to Ryolit, Felsit (Mang Yang, Chu Prong, Nha Trang, Don Duong strata systems); these rocks create high and sharp mountains with strong differentiation Mafic and super Mafic intrusion rock group occupies a very small area of the studied region; they exit under the form of small blocks Mafic volcanic rock group includes Basalt, the types with the age from Neogen to the Troskysit period with the strata systems Tuc Trung, Dai Nga and Xuan Loc This is the rock group with large distribution area, occupying up to ¼ of the Central Highland area About the tectonic features, the Central Highlands is totally located in large tectonic belts, i.e KonTum and Dalat belts (Nguyen Xuan Ba and nnk, 2000) The boundary between these two belts is the fault system Ea Sup - Krong Pach Each tectonic belt has different characteristics of components, structure and their -9geological features are really different On each tectonic belt, many fault systems are developed, such as Po Co, Ho sea – Chu Ho Drong, Mang Yang – An Trung Mountain pass, Dak Min - Madagui, Đắk Min - Krong Bong, Ba River, the fault systems Batơ - Kontum, Bien Hoa – Tuy Hoa, Da Nhim – Tanh Linh At the studied area, there’s the sign of new tectonic activities, this place develops horizontal and vertical movements The forms of geological catastrophes with endogenous origin are often associated with these activities 1.2.2 Weathering crust in the Central Highlands There are different types of weathering, such as: chemical weathering, physical weathering, biological weathering, etc In the Central Highlands, due to favorable conditions of the climate, the chemical weathering mainly occurs in this region The agents of the chemical weathering mainly are water, oxide, carbonic acid, organic acid and other acids dissolved in water The chemical weathering has very complicate features Different processes can be happen at the same time, such as dissolution, oxidation, ion exchange and hydrolysis The dominant of any process depends on the compositions and properties of rock itself, ambient conditions, weathering time, depth, laying status of rock 1.2.2.1 Weathering crust in intrusion rocks: Distributed into two large strips: one strip at the edge of the East, lasting continuously from Tu Mo Rong to Krong Pa, Chu Yang Sin; the other strip locating at the West of Truong Son, from DakGlie to Chu Prong, turning to Krong Pa in Southeast direction This region is popular with the weathering crust in intrusive acid rocks with thickness form to 10m, the largest weathering crust of 50m – 80m in Granite – Migmatite rock locates at ManDen region, belonging to Chu Lai geological complex, the smallest weathering crust of 0.5m-2.5m locates at the slope The top crust is totally weathered becoming clay and clay loam 1.2.2.2 Weathering crust in volcanic rocks: a) Weathering crust in Basalt volcanic rocks: Distribute widely and cover most of large Basalt plateaus: Kon Ha Nung, Pleiku, Buon Ma Thuot, Dak Nong and Di Linh They include two following groups: Weathering crust in Basalt Pliocen – early Pleistocen (βN2-QI1): Distribution: occupy most of the area of large plateaus, except for the central parts of Pleiku, Buon Ma Thuot, Dak Nong -10 Its thickness is from 10-20cm, the thickest part is at the plateau arc Kon Ha Nung, Dak Nong with thickness of 32 - 82.5m on Granite-migmatit rocks, Chu Lai geological complex, the thinnest part is at the edge of the plateau with thickness of only 3m- 5m The specific characteristic of this type of weathering crust in Basalt volcanic rocks is laterite crust, the cross section from the top to the bottom includes zones: pedology, laterite, clay and weak metamorphic zones Pedology zone is from 0.1-1m thick, mainly of clay mixing with tree roots and some pieces of laterite Laterite zone is from 0.5-12.3m thick under the form of gravels, grits, sticks, bones, and pores with rather rigid structure Clay zone is from 2-70.2m thick This is argillaceous alteration under the form of spherical remnant; this zone still remains the structure of mother rocks Weak metamorphic zone of 1-5m thick is Basalt fracture forming crushed stones, block stones; the minerals are mainly primary b) Weathering crust in Basalt Pleistocene volcanic rocks (βQ12): Distribution: develop at the centre of the plateau arcs Pleiku, Buon Ho, Krong Ana, Dak Min, Duc Trong Its thickness is from 15 - 20m, the thickest part at the plateau arcs Kon Ha Nung, Dak Nong reaches 50 – 70m at Pleiku plateau arc, the thinnest part at Krong Ana area is only 3m – 10m The specific characteristic of this type of weathering crust in Basalt volcanic rocks is the crust of argillaceous alteration, the cross section from the top to the bottom includes zones: pedology, clay and weak metamorphic zones The pedological zone of - 0.5m: mainly comprises clay slurry with tree roots Clay zone of - 10m is red brown clay transferred to spotted gray brown color; it still remains the structure of mother rocks The weak metamorphic zone of 1-3m is Basalt fracture forming crushed stones, block stones; the minerals are mainly primary c) Weathering crust in neutral volcanic rocks: Distribution: develop in Andesite volcanic rocks in Ban Don, Bao Loc mountainous pass, at the southeast of Di Linh, Da Dang Its thickness is from to 5m; the thickest part of 10-12m is at Dak Lin, in Pleiku plateau arc; the thinnest part at Bao Loc mountainous pass is only 05 – 1m The thickest zone on the top is the clay zone d) Weathering crust in acid volcanic rocks: -93According to the formulas (2-7) and (4-1) and the data in the table 2-2, we can easily calculate the stability coefficients K of the slope and the limit height (h) of the slope when the stability coefficient K is predetermined The research student has applied the improved circular arc method of M Н Голbдштейн and the formula (4-1) to define the limit height (h) of the slope with the grade (1:m) according to predetermined safety coefficient K Table 4.1 Summary of anti-sliding safety coefficient of some types of soils in the Central Highlands calculated by different methods The physico-mechanical criteria of soils Types of soils Residual - deluvial soils in ancient basalt rocks Residual - deluvial soils in Granite intrusion rocks Residual - deluvial soils in Terrigenous sedimentary rocks Residual -deluvial soils in Metamorphic rocks Characteristics slopes C T/m Height h, (m) Grade of slope (1:m) 16o30 2.0 5.0 1:2 1.73 18o00 1.7 5.0 1:2 1.50 1.82 19o20 3.7 10.0 1:2 1.51 1.83 18o00 3.7 10.0 1:2 W, % c , T/m3 w , T/m3 49.0 1.16 1.73 27.0 1.36 21.0 21.0 Anti-sliding safety coefficient The method Fp The method Gônxtên Ter-xtêpanian The method Bishop Residual - deluvial soils in ancient basalt rocks 1.050 2.500 2.570 Residual - deluvial soils in Granite intrusion rocks 1.043 2,360 2.420 Residual - deluvial soils in Terrigenous sedimentary rocks 1.100 2.490 2.460 Residual -deluvial soils in Metamorphic rocks 1.054 2.404 2.456 Types of soils -944.2 CALCULATION FOR DETERMINATION OF LIMIT HEIGHT (h) CORRELATIVE TO SAFETY COEFFICIENT K, ACCORDING TO GRADE (1:m) OF SLOPE ON SOME WEATHERING CRUSTS IN THE CENTRAL HIGHLANDS WITH VARIABLE HUMIDITY (W) 4.2.1 Calculation method: The application of the sliding circular arc method improved and simplified by M Н Голbдштейн and Г.Ц Тер-cтепанян is presented in the item 2-2 According to this method, if the stability coefficient K is determined according to the standard and the formula (2-7), we can determine the limit height (h) of the slope according to the formula (4-1) h = CB/( K – f.A) In which, the friction coefficient values of soil f = tg; C, are physicomechanical characteristics of soils, A & B are the coefficients depending on geometrical size of falling wedge If the grade (1:m) of the slope is given in advance, we can determine the values A, B according to the table 2-2 (Chapter 2) The values , ; C vary according to the humidity (W) of soils; therefore, we can determine the limit height (h), correlative to the safety coefficient (K) under the grade (1:m) and the humidity (W) of soils 4.2.2 Selection of anti-sliding safety coefficient K: The anti-sliding safety coefficient K of the slope is evaluated by the antisliding stability coefficient K According to Н.А Цытович (N.A Xưtôvich) [53], normally, it’s concluded that when the safety coefficient value K >1.1 1.5, the slope is considered as stable Stipulating allowable anti-sliding safety coefficient [Kcp] depends on the professional sector and the grade of the project In Water resources sector, the stability coefficient of dam face shall not be smaller than the allowable safety coefficient [Kcp] on stability of the dam face according to the grade of the project and the working condition of the dam, as stipulated in table 4.2 Table 4.2 Minimum stable safety coefficient of dam face [Kcp] Grade of dam Working condition (Applied force combination) I II III IV V Normal (basic) 1.40 1.30 1.20 1.15 1.10 Abnormal (special) 1.20 1.15 1.10 1.05 1.00 Basic 1.30 1.25 1.20 1.15 1.15 China Standard (reference) Special 1.20 1.15 1.10 1.05 1.05 Basic US Standard Not according to grade of project, according to (reference) working condition from 1.2 to 1.5 Special -95According to the requirements of the standard 22TCN 262 - 2000 of the Ministry of Transport: When applying the method for calculation of stability according to traditional fragmentation method with sliding circular surface drilled deeply into soft soil ground, the minimum stability coefficient is Kmin = 1.20 When applying Bishop method for calculation, the minimum stability coefficient is Kmin = 1.40 To calculate the stability of the slopes closing to the motorways, the research student has selected the stable safety coefficient K = 1.4 4.2.3 Physico-mechanical characteristics used in calculation: Basing on the test results on studying the variation of natural density (W) and shearing parameters (, C) of four types of soils in the Central Highlands under different conditions of natural humidity (W) of soils as presented in chapter 3, the research student has selected the characteristics W, , C according to variable humidity of four types of soils for calculation The characteristics of four types of soils used in calculation are listed in table 4-3 Table 4.3 Physico-mechanical characteristics of soils used in calculation of limit height The physico-mechanical criteria of soils Types of soils 60.0 70.0 80.0 90.0 100.0 55.0 70.0 80.0 95.0 W, T/m3 1.52 1.57 1.62 1.68 1.75 1.63 1.68 1.76 1.80 , degree 25o 22o 19o 17o30 16o 24o 21o30 16o30 14o30 C, T/m2 4.8 4.2 2.8 2.2 2.0 3.0 1.9 1.3 1.2 20.0 70.0 1.81 20o20 4.0 25.0 80.0 1.86 16o30 3.0 30.0 98.0 1.94 15o20 2.7 20.0 25.0 30.0 70.0 80.0 99.0 1.80 1.88 1.94 19o30 16o 15o30 4.2 2.5 1.5 Case Soils in weathering crust in ancient basaltic rocks (Lâm Đồng) Soils in weathering crust in Granite intrusion rocks (Gia lai) Soils in weathering crust in Terrigenous sedimentary rocks (Daklak) Soils in weathering crust in Metamorphic rocks (KonTum) W, % G, % 30.0 35.0 40.0 45.0 50.0 20.0 25.0 30.0 35.0 -964.2.4 Calculation results The sliding surfaces on positive talus slopes along the traffic routes in the Central Highlands often go through the lower edge of the side slopes on motorway pavement Applying the formula (4-1) with the sliding stability coefficient K = 1.4, the physicomechanical characteristics of each type of soils given as in table (4-3), the parameters A and B are subject to the grade (1:m) of the slope, referred to the table (2-2) to calculate and determine the limit height (h) according to the grade (1:m) and the humidity (W) of each type of soils The calculation results are presented in data sheets and represented by graphics in drawings for each type of soils as follows: - Soils in weathering crust in ancient basaltic rocks: table 4-4 and figure 4-5 - Soils in weathering crust in Granite intrusion rocks: table 4-5 and figure 4-6 - Soils in weathering crust in Terrigenous sedimentary rocks: table 4-6 and figure 4-7 - Soils in weathering crust in Metamorphic rocks: table 4-7 and figure 4-8 Because the physico-mechanical properties of each type of soils are different, the variation of the limit height (h) of the slopes with the same grade (1:m) under the humidity (W) is different For example: Considering the side slopes of four types of soils having the same grade of 1:3 in the rainy season and different humidity (W) while the saturation degree is G ≥ 95% (considered as saturated), the limit height (h) is different according to the type of soils (table 4-8) Table 4.4 Limit height (h) of slopes with the same grade (1:3) in the rainy season of different types of soils Grade Types of soils of slope (1:m) Soils in weathering crust in ancient basaltic rocks Soils in weathering crust in Granite intrusion rocks Soils in weathering crust in Terrigenous sedimentary rocks Soils in weathering crust in Metamorphic rocks Humidity W, % Saturation degree G, % Height (h), (m) 1:3 50 100 27,55 1:3 35 95 12,54 1:3 30 98 29,53 1:3 30 99 17,00 -97Table 4.5 Limit height (h) correlative to safety coefficient (K=1.4) according to the grade (1:m) of the slope in weathering crust in Basalt rocks with different characteristics w, , , C Grade of slope l:m Sliding surfaces go through the lower edge of the side slopes 1:1.00 1:1.25 1:1.50 1:1.75 1:2.00 1:2.25 1:2.50 1:2.75 1:3.00 A 2.34 2.64 2.64 2.87 3.03 3.19 3.53 3.59 3.59 B 5.79 6.05 6.50 6.58 6.70 7.27 7.30 8.02 8.91 h = CB/( K – f.A) Case Case Case Case Case 59.07 112.54 120.91 - 35.92 52.71 56.63 83.10 123.12 - 16.82 21.26 22.84 27.56 32.38 41.52 67.95 83.99 93.31 11.44 13.94 14.98 17.37 19.69 24.09 33.19 39.02 43.35 9.08 10.76 11.57 13.05 14.44 17.15 21.56 24.79 27.55 130 120 110 Chiều cao giới hạn (h) 100 90 80 70 60 50 40 30 20 10 1:1.00 1:1.25 1:1.50 1:1.75 1:2.00 1:2.25 1:2.50 1:2.75 1:3.00 Độ dốc (1:m) W = 30% (G = 60%) W = 35% (G = 70%) W = 45% (G = 90%) W = 40% (G = 80%) W = 50% (G = 100%) Figure 4.5 Limit height (h) according to the grade (1:m) of earth slope on basalt rock weathering crust -98Table 4.6 Limit height (h) correlative to safety coefficient (K=1.40) according to the grade (1:m) of the slope in weathering crust in Granite intrusion rocks with different characteristics w, , C Sliding surfaces go through the lower edge of the side slopes slope l:m Case h = CB/( K – f.A) Grade of 1:1.00 1:1.25 1:1.50 1:1.75 1:2.00 1:2.25 1:2.50 1:2.75 1:3.00 A 2.34 2.64 2.64 2.87 3.03 3.19 3.53 3.59 3.59 B 5.79 6.05 6.50 6.58 6.70 7.27 7.30 8.02 8.91 Case 29.71 49.44 53.12 98.58 - Case 13.70 19.01 20.43 27.64 36.75 57.44 - 6.05 7.22 7.76 8.83 9.84 11.78 15.18 17.56 19.51 Case 4.85 5.61 6.03 6.65 7.22 8.40 9.95 11.29 12.54 100 90 Chiều cao giới hạn (h) 80 70 60 50 40 30 20 10 1:1.00 1:1.25 1:1.50 1:1.75 1:2.00 1:2.25 1:2.50 1:2.75 1:3.00 Độ dốc (1:m) W = 20% (G = 55%) W = 25% (G = 70%) W = 30% (G = 80%) W = 35% (G = 95%) Figure 4.6 Limit height (h) according to the grade (1:m) of earth slope on Granite intrusion rocks weathering crust -99Table 4.7 Limit height (h) correlative to safety coefficient (K=1.40) according to the grade (1:m) of the slope in weathering crust in terrigenous sedimentary rocks with different characteristics w, , , C Grade of slope l:m 1:1.00 1:1.25 1:1.50 1:1.75 1:2.00 1:2.25 1:2.50 1:2.75 1:3.00 Sliding surfaces go through the lower edge of the side slopes A 2.34 2.64 2.64 2.87 3.03 3.19 3.53 3.59 3.59 B 5.79 6.05 6.50 6.58 6.70 7.27 7.30 8.02 8.91 h = CB/( K – f.A) Case 24.06 31.79 34.16 43.38 53.67 74.21 - Case Case 13.20 15.78 16.95 19.28 21.48 25.73 33.16 38.34 42.60 10.59 12.40 13.32 14.85 16.28 19.12 23.29 26.58 29.53 100 90 Chiều cao giới hạn (h) 80 70 60 50 40 30 20 10 1:1.00 1:1.25 1:1.50 1:1.75 1:2.00 1:2.25 1:2.50 1:2.75 1:3.00 Độ dốc (1:m) W = 20% (G = 70%) W = 25% (G = 80%) W = 30% (G = 98%) Figure 4.7 Limit height (h) according to the grade (1:m) of earth slope on terrigenous sedimentary rocks weathering crust -100Table 4.8 Limit height (h) correlative to safety coefficient (K=1.40) according to the grade (1:m) of the slope in weathering crust in metamorphic rocks with different characteristics w, C Grade of slope l:m 1:1.00 1:1.25 1:1.50 1:1.75 1:2.00 1:2.25 1:2.50 1:2.75 1:3.00 Sliding surfaces go through the lower edge of the side slopes A 2.34 2.64 2.64 2.87 3.03 3.19 3.53 3.59 3.59 B 5.79 6.05 6.50 6.58 6.70 7.27 7.30 8.02 8.91 h = CB/g( K – f.A) Case Case 23.63 30.33 32.59 39.98 47.75 62.66 - Case 10.57 12.53 13.46 15.18 16.80 19.95 25.09 28.85 32.05 5.95 7.00 7.52 8.41 9.24 10.89 13.37 15.29 16.99 100 90 Chiều cao giới hạn (h) 80 70 60 50 40 30 20 10 1:1.00 1:1.25 1:1.50 1:1.75 1:2.00 1:2.25 1:2.50 1:2.75 1:3.00 Độ dốc (1:m) W = 20% (G = 70%) W = 25% (G = 80%) W = 30% (G = 99%) Figure 4.8 Limit height (h) according to the grade (1:m) of earth slope on metamorphic rocks weathering crust -1014.2.5 Calculation and check of stable safety coefficient of some slopes according to Bishop circular arc method: The results calculated by the improved circular arc method of M Н Голbдштейн, with the safety coefficient K= 1.40 determining the limit height (h) according to the grade (1:m) of some slopes on some types of soils with variable humidity (W) in the Central Highlands are presented in figures and by graphics in item 4.2.4 In this section, the research student has selected some slopes with the same grade of 1:m = 1:2.5, the limit height (h) varies depending on the humidity condition (W) of soils on the slopes; Bishop circular arc method (via Geo-Slope software) is used to calculate and determine the anti-sliding stability coefficient (K) of the slope Applicable data and calculation results are presented in the appendices in chapter and summarized in table 4.9 The data in the table 4.9 show that the antisliding coefficient of the slope calculated according to Bishop circular arc method gives the same value K 1.4, appropriate to the selection of K=1.40 to determine the limit height (h) according to the grade (1:m) and different cases of humidity (W) of soils calculated by the improved circular arc method of M Н Голbдштейн Table 4.9 Anti-sliding stability coefficient of slopes with the same grade 1:m, different heights (h) of slopes calculated according to Bishop circular arc method (via Geo-Slope software) Types of soils Grade of slope (l:m) Height (h), m (1) 1:2,5 (2) The physico-mechanical criteria of soils W, % , T/m3 , degree C, T/m2 The safety coefficient K 21,5 50,0 1,75 16o00 2,0 1.358 1:2,5 10,0 35,0 1,80 14o30 1,2 1.401 (3) 1:2,5 23,5 30,0 1,94 15o20 2,7 1.395 (4) 1:2,5 13,5 30,0 1,94 15o30 1,5 1.387 Ghi Chú (1) Soils in weathering crust in ancient basaltic rocks (2) Soils in weathering crust in Granite intrusion rocks (3) Soils in weathering crust in Terrigenous sedimentary rocks (4) Soils in weathering crust in Metamorphic rocks -1024.2.6 Sliding process on actual slopes Item 4.2.4 presents calculation results determining the limit height (h), correlative to the safety coefficient K=1.40, according the grade (1:m) of the slopes on some weathering crusts in the Central Highlands with variable humidity (W) In calculation, the research student has assumed that the soil mass on the slope is uniform without seepage flow In reality, very few cases have uniform slope Depending on the environment, type of soils forming the slope, strength and duration of the rainfall, storm water enters into the slope with different degrees according to the depth of the side slope Therefore, in reality, depending on each location, the sliding phenomenon may occur at area closing to the top of the slope; at some places, sliding occurs at the foot of the slope while in other cases, the sliding occurs at the halfway up the slope The sliding phenomenon does not end in one rainy season, it continuously occurs in many years, following the rainy and flood seasons In the rainy season in 2008, after a prolong rain lasting for nearly days and nights, the basalt earth berm on the left side of the National road no 28 from DakNong to Dalat, at the section Km172 which is 6km far from Gia Nghia Town, was eroded The erosion hole is 5-:-6m deep After the occurrence of such landslide, the research student together with the collaborators have performed measurement, geological survey at the remaining natural hill edge closing to the erosion hole, the survey results are presented in the appendix 4-2 The measurement results show that the side slope is h=20m high, the average grade is 1:1.25 The upper edge of the erosion hole is h=11.0m high The test results are presented in tables and of the appendix 4-2 When performing survey drilling, there’s no water in all boreholes It means that there is no seepage flow in the soil mass except for the filtration of storm water The test data in table of the appendix 4-2 show that: - The humidity (W) and the water saturation degree (G) of the soil sample decrease in accordance with the depth of the borehole The soil samples closing to the slope surface with the humidity (W) greater than that of the soil samples at deeper location On the top of the slope (H1), water enters more deeply than in the boreholes H2, H3 and H4 on the slope surface - Due to the effects of the humidity (W) decrease, the natural density (w) of soils also decreases gradually and the shearing parameters (, C) are in the tendency of increasing gradually according to the depth from the slope surface - Contour of water saturation degree (G) falls down according to the depth from the slope surface The farther the surface slope of approximately 5m, the water saturation degree (G) reduces about 70% -103It is the decrease of the humidity (W) and of the water saturation degree (G) according to the grade of the slope, the landslide will occur when the height of the slope exceed the allowable limit height, subject to the status of each type of soils and the grade of the slope According to the calculation results, the limit height (h) presented in table 4-5 and represented by graphic in figure 4-5, for the red Basaltic soils with the humidity W=45%, the saturation degree G= 90%, the grade of 1:1.25, the limit height shall be h=14m (calculated with the safety coefficient K=1.40) According to the dual analysis results of Slope/W and Vadose/W introduced in figure of the appendix 4-2, it’s shown that the limit height appears correspondent sliding surface from 13m to 14m (with the safety coefficient FSw =1.2) The actual sliding surface firstly appears at the height between the slope h=11m As a result, the calculation and measurement data mentioned above are quite consistent with each other (There is an variation of some meters among the data due to selection of different sliding stability coefficients) After the rainy seasons in the following years, water has continuously entered into soil and the slope has been eroded again After the local builds a gravity concrete wall at the slope foot to prevent the landslide from falling on the traffic roads, soils accumulate at the slope foot forming a pressure division system for prevention of landslide On the other hand, at some places, the depth of the erosion hole has reached the source rocks, the slope has been stable CONCLUSION DRAWN FROM CHAPTER 4.3.1 The calculation results on anti-sliding safety coefficient (K) of the slopes with different height (h) and grade (1:m) for various types of soils in the Central Highlands show that: - The anti-sliding safety coefficient calculated according the improved and simplified method on the basis of the circular cylindrical sliding surface method recommended by Professor M.Н Голbдштейн and Professor Г.Ц Тер-cтепанян and the anti-sliding safety coefficient calculated according to Bishop circular arc method by using the Geo-Slope software have the approximate values This means that, it’s possible to apply the improved method of Professor M.Н Голbдштейн and Professor Г.Ц Тер-cтепанян for calculation of slope stability in case the application of GeoSlope software for calculation according to Bishop circular arc method is not available From the formula for calculation of the anti-sliding safety coefficient according to the improved method of Professor M.Н Голbдштейн and Professor Г.Ц Терcтепанян , using the formula (2-7), it’s possible to easily determine the limit height (h) of the slope when the safety coefficient (K) is predetermined -1044.3.2 Using the test results on studying the variation of physico-mechanical characteristics according to the year-round humidity of four main types of soils on zone and zone in the weathering crust in the Central Highlands (quoted in table 43), applying the formula (4-1) with the safety coefficient K=1.40, determining the limit height (h) according to different slope (1:m) and humidity (W) of soils on the slopes, the calculation results show that: - With the same humidity (W), the limit height (h) increases according to the reduction of the grade (1:m) of the slope - With the same grade (1:m), the limit height (h) decreases according to the increase of the humidity of soils on the side slope 4.3.3 In the rainy season, the slopes on the basalt weathering crust and the Terrigenous sedimentary rock weathering crust have higher stability than on Granite intrusion rock weathering crust and on Metamorphic rock weathering crust - The residual-deluvial soils in Granite rocks and in Metamorphic rocks have stronger swelling nature than that of basalt soils, they are less stable in the rainy and flood season 4.3.4 It’s possible to refer to and use the results on calculation of the limit height (h) according to the grade (1:m) and the humidity (W) of different types of soils introduced in item 4.2.4 to check and evaluate the stability of the slopes closing to the motorways in weathering crusts in the Central Highlands -105GENERAL CONCLUSIONS AND RECOMMENDATIONS CONCLUSION From the study results presented in the chapters 1, 2, and of the thesis, it’s possible to draw the following general conclusions: 1.1 The weathering crusts in the Central Highlands exit in different types, the most common ones are: - Weathering crust in ancient basalt volcanic rocks; - Weathering crust in Granite intrusion rocks; - Weathering crust in Terrigenous sedimentary rocks; - Weathering crust in Metamorphic rocks; The zones in weathering crusts in the Central Highlands have the following general characteristics: - The top zone is the soil zone; - The 2nd zone is the weathering clay zone, rocks weathered totally becoming clay, loam - The 3rd zone is the weak metamorphic zone which is not totally weathered; this zone mainly comprises soils mixing with gravels, blocks, fissured rocks Thickness of the zones is different, subject to the weathering degree of the source rocks, the terrain at each location, the soil layer is eroded, the second zone exposes on the ground 1.2 The traffic routes in the Central Highlands are often located at the second zone The roadside slopes mainly comprise clay-type soils The clay-type soils are very sensitive to changes when contacting with water Entrance of water into soil only occurs in the rainy season, it has the effect of increasing the humidity of soil Prolonged flooding may cause soil saturated However, in general, there’s not enough water to form a water bearing stratum in the slope The types of landslides comprise the collapse of cliffs, slide of side slopes in weathering crusts under the form of block landslides, flat landslides, and combined landslides 1.3 There are many reasons causing landslides in weathering crust, but the main reason is the effects of water on soil during prolonged flooding season The results on site investigation, follow-up and taking samples seasonally for testing show that when prolonged flooding occurs, the humidity (W) in soils increases, the natural density (w) increases considerably, the shearing parameters (, C) of soils reduce Those are disadvantageous elements for the stability of the slopes 1.4 It’s possible to apply the improved method of Professor M.Н Голbдштейн and Professor Г.Ц Тер-cтепанян for calculation of slope stability in case the application -106of Geo-Slope software for calculation according to Bishop circular arc method is not available From the formula for calculation of the anti-sliding stability coefficient according to the improved method of the Professor M.Н Голbдштейн (formula 2-7), it’s possible to easily determine the limit height (h) of the slope when the safety coefficient K is predetermined (formula 4-1) 1.5 Using the test results on studying the variation of physico-mechanical characteristics according to the year-round humidity of four main types of soils on zone and zone in the weathering crust in the Central Highlands, applying the formula (4-1) with the safety coefficient K=1.40, determining the limit height (h) according to different slope (1:m) and humidity (W) of soils on the slopes: - With the same humidity (W), the limit height (h) increases according to the reduction of the grade (1:m) of the slope - With the same grade (1:m), the limit height (h) decreases according to the increase of the humidity of soils on the side slope - In the rainy season, the slopes in the basalt weathering crust and the Terrigenous sedimentary rock weathering crust have higher stability than on Granite intrusion rock weathering crust and on Metamorphic rock weathering crust - The residual-deluvial soils in Granite rocks and in Metamorphic rocks have stronger swelling nature than that of basalt soils, they are less stable in the rainy and flood season II RECOMMENDATIONS 2.1 For important works, directly contacting with rain and flood, it’s required to use the test data of soil samples in totally saturated condition for calculation of slope design For temporary works, or those provided with covering solutions and well waterproof, it’s possible to use the soil testing documents according to actual conditions of the works for slope design 2.2 The traffic routes in the Central Highlands run through various weathering crusts on different source rocks Therefore, it’s necessary to use the soil testing data according to each section of the routes for calculation of slope design It’s not advised to use formed design for overall routes In case there is water bearing formation at special location in the slope and appearance of water flow, it’s required to identify clearly the characteristics of the flow and take into consideration of hydrodynamic pressure in calculation of the slope stability 2.3 It’s possible to use the results on calculation of the limit height (h) of the slope according to different gentle degree (1:m) of the slope according to the variation of -107the humidity (W) (introduced in chapter 4) to check, evaluate the stability of actual slopes in the Central Highlands and propose appropriate solutions for prevention of landslides 2.4 The problem on prevention of landslide for slopes in complex geological terrains with a lot of natural disasters and flooding along the traffic routes in the Central Highlands particularly and the mountainous areas generally, it’s required to perform monitoring and testing the solutions applied in reality and those which have just applied abroad to draw some appropriate solutions according to topographic, geological, and hydrological conditions in the Central Highlands Within in the scope of the thesis, the research student has not had enough condition to study and compare the appropriate solutions for prevention of landslides but only introduces some general methods in the overview section (chapter 1) ... 0.3m/ years Very slow: 0.3m/ years – 1.5m/year Slow: 1.5m/year – 1.5m/month Medium: 1.5m/month – 1.5m/day Fast: 1.5m/day – 0.3m/minute Very fast: 0.3m/minute – 3m/second Extremely fast: >3m/second... 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