ANALYSIS, DESIGN & COST COMPARISON BETWEEN COUNTERFORT RETAINING WALL & MECHANICALLY STABILIZED EARTH WALL

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1 International Journal of Civil Engineering and Technology (IJCIET) Volume 9, Issue 7, July 2018, pp , Article ID: IJCIET_09_07_110 Available online at ISSN Print: and ISSN Online: IAEME Publication Scopus Indexed ANALYSIS, DESIGN & COST COMPARISON BETWEEN COUNTERFORT RETAINING WALL & MECHANICALLY STABILIZED EARTH WALL Assistant Professor, Department of Civil Engineering, BTLIT&M, Bengaluru, India ABSTRACT The transportation sector involves a number of activities which are mainly depending on the financial aspect. In the planning and design of highways there is increasing need for analysis to indicate justification of the expenditure required and the comparative worth of proposed improvements, particularly when various alternatives are being compared. In roadway or railway sector the alignment will be passing through cutting or filling. In this portion a number of structures like masonry retaining wall, RCC retaining wall, Mechanically Stabilized Earth Wall etc. will be provided to retain the earth, depending upon the site condition and funds availability. Based on cross evaluation among various alternatives, the structure considered for retaining the earth is Counterfort Retaining Wall and Mechanically Stabilized Earth Wall. These two structures are designed, analyzed and detailed cost estimation is done, thus arriving at the economic structure for the same site condition. The site selected for the present study is in Badanguppe-Kellamballi Industrial area, in Chamarajanagara district, Karnataka. Counterfort Retaining wall and Mechanically Stabilized Earth Wall is designed for existing valley portion in Road No.1 between chainage 300m and 540m. Analysis and design for both structures is done based on the geotechnical investigation and critical height of embankment. Detailed cost estimation is done in accordance with Schedule of Rates , PWP & IWTD, Mysore circle. The cost varies with every particular item of work for both the structures. The cost of Counterfort Retaining Wall is found to be more than that of Mechanically Stabilized Earth Wall. In case of Mechanically Stabilized Earth Wall nearly 47 % of cost saving has been achieved in the present study. Key words: Counterfort Retaining Wall, Mechanically Stabilized Earth Wall, Safe bearing capacity, Cost analysis and Cost comparison. Cite this Article:, Analysis, Design & Cost Comparison Between Counterfort Retaining Wall & Mechanically Stabilized Earth Wall, International Journal of Civil Engineering and Technology, 9(7), 2018, pp editor@iaeme.com

2 Analysis, Design & Cost Comparison Between Counterfort Retaining Wall & Mechanically Stabilized Earth Wall 1. INTRODUCTION 1.1. General The most commonly adopted earth retaining structure is RCC retaining wall or mechanically stabilized earth wall. The design and cost comparison between counterfort retaining wall and mechanically stabilized earth wall is considered in the present study. The counterfort retaining wall is purely a RCC structure. This structure is constructed when the height of fill is >5m. It is an externally resistant structure. The mechanically stabilized earth wall is an internally resistant structure. The reinforcements will be provided in the backfill soil for resisting the external load Objective of the Study Structural design of retaining wall and mechanically stabilized earth wall. To prepare a total cost estimate for retaining wall as well as mechanically stabilized earth wall and Cost comparison between the same. 2. METHODOLOGY 2.1. Reconnaissance Survey In present study, the project site is in Badanguppe Kellamballi Industrial Area, Chamarajanagara District. There is a valley portion at Road No.1 between Ch. 280 m and 540 m with approximate depth of about 7.3 m i.e. difference between peak point and valley point from OGL and FRL. Five types of Stepped Retaining walls are constructed at this section Detailed Survey Table 1 Reduced levels of upstream and downstream side of gorge portion Chainage Downstream side Upstream side (m) OGL FRL Depth OGL FRL Depth Geotechnical.Investigation Table 2 Consolidated results of laboratory tests conducted for foundation soil and backfill soil Sl. No. Soil Properties Foundation soil Backfill soil 1 Specific Gravity Wet sieve analysis 2 Gravel % % % Sand % % % Silt & Clay % % % 3 Atterberg Limits Liquid limit (LL) % % editor@iaeme.com

3 Plastic limit (PL) Plasticity index (PI) % Soil Classification CL CL Compaction properties 5 OMC % MDD g/cc 7 Shear parameters Angle of internal friction (ϕ) Cohesion (c) % % g/cc kn/m 2 0 kn/m Determination of Bearing Capacity of Foundation Soil IS code method (IS: ) for computing bearing capacity of the soil (collected at 1.5 m depth below the RW) Angle of shearing resistance of the soil (ϕ) = 29.50, Cohesion (c) = 2.95 kn/m 2 Average bulk density of foundation soil (ρ) = g/cc Average unit weight of the foundation soil (γ) = kn/m 3. Average bulk density of backfill soil (ρ 1 ) = g/cc Average unit weight of backfill soil (γ 1 ) = kn/m 3. Assume breadth of footing (B) = 2.5 m Depth of footing below the ground level (D) = 1.5 m. Effective surcharge at base level of the foundation (q) = γ*d + 1.2*γ 1 = kn/cm 2 Since 28 < ϕ < 36, we cannot adopt both general failure as well as local shear condition. For mixed zone, = *( + = From Table 1, IS: Bearing capacity factors obtained is: For ϕ' = 20.76, N c = , N q = 8.105, N γ = 7.6. For Non-cohesive soil (ϕ > 0; c = 0) & for String footing: Shape factors, s c, s q, s γ = 1, Inclination factors, i c, i q, i γ = 1 = = 2.94 Depth factors, d c = * + = * + = d q = d y = * + = * + = Net ultimate bearing capacity (q nf ) = kn/m 2. Assume Factor of safety (F) = 2.5. Therefore Safe Bearing Capacity, q s = 266 kn/m Structural Design First step is to know what all types of loads that arrives on the structure and then the loads are calculated. Here two types of loads are arriving on the structure, first one is overturning load and the other is resisting load. Overturning load is determined using Coulomb s theory. The overturning load has two components; backfill force and surcharge force. The backfill force acts as per Coulomb s theory. Overturning load at the top is zero and at bottom it is Ka*γ*H (i.e. uniformly varying load). Surcharge load mainly depends on type of vehicle moment. For design purpose, 70(R) ton load vehicle is usually considered. Some of the international journals have published that the surcharge load is equivalent to (1.2 m backfill height * backfill soil density) for new earth retaining structures where it is impractical to calculate the editor@iaeme.com

4 Analysis, Design & Cost Comparison Between Counterfort Retaining Wall & Mechanically Stabilized Earth Wall surcharge load and the Surcharge load is (Ka*γ*1.2m backfill height) at the top and (Ka*γ*1.2m backfill height) at the bottom (i.e. uniformly distributed load). One more load acting on the structure is resisting load. Counterfort Retaining.Wall The Counterfort R.W is adopted when the height of fill in greater than 5m. The stem and base slab are monolithically casted. The key purpose of providing counterforts is to minimize the shear force and bending moment that is generated in the stem, toe slab and heel slab. The counterforts are tied both to the stem and heel slab. The counterfort is designed as T-beam which supports both stem and heel slab. The vertical stem acts as a continuous slab which is supported on counterforts. Heel slab acts similar to that of stem. The toe slab acts as a cantilever portion. The vertical stem, heel slab, toe slab, counterfort are designed and checked against sliding and overturning failures. Drawings of cross-section near counter, cross-section through counterfort, plan, and pressure diagram are drawn and bar bending schedule is prepared. The minimum depth of foundation is decided by considering SBC of the foundation soil. The width and thickness of the base slab is decided based on stem height. Similarly considering the height of the valley portion at different chainages, stepped counterfort R.W is designed. Finally the cost estimate for different quantities is done in accordance with Schedule of Rates , PWP & IWTD, Mysore circle. Mechanically Stabilized Earth Wall At present, mechanically stabilized earth wall is undoubtedly the most used, particularly in road work where deep cuts are there. These structures eliminate the necessity for using natural slopes. Components materials and their specifications: Foundation levelling pad: M-15 grade PCC levelling pad of dimension 0.35m X 0.15m has been adopted in the present study. Facing unit: Cruciform shaped RCC precast panels (M-35 grade) of dimension 1m X 1m and 0.18m thickness has been adopted for the present study. Figure 1. A typical cruciform shaped RCC precast panel Reinforcement: Galvanized smooth steel strips has been considered for the present study with a corrosion rate of 0.025mm/yr. Backfill material: As per MORT&H-5 specification and IRC SP , it is mandatory that the angle of internal friction of the earth fill material should be greater than 26 and cohesion should be zero. Plasticity index should be lower than 20. The Compacted Thickness Of Each Layer Of Soil Should Not Exceed 200mm And No Compaction Equipment Should Be To Run Directly On The Reinforcement editor@iaeme.com

5 Figure 2. Longitudinal section of upstream side with stepped retaining wall Figure 3 Longitudinal section of downstream side with stepped retaining wall Design of Counterfort Retaining Wall (R.W) Table 3 Consolidated data for design of Counterfort Retaining Wall Sl. No. RW1 RW2 RW3 RW4 Height of RW above the ground level 7.3 m 5.9 m 4.9 m 2.9 m Base slab width 6 m 4.6 m 4 m 2.8 m Spacing of Counterforts 3 m 3 m 3 m 3 m Toe projection 1.5 m 1.15 m 1 m 0.7 m Base slab thickness 0.59 m 0.45 m 0.4 m 0.3 m Height of the steam 8.21 m 6.55 m 5.6 m 3.7 m Bending moment KN-m KN-m KN-m KN-m Ultimate BM KN-m KN-m KN-m KN-m Heel slab projection 4 m 2.95 m 2.55 m 1.75 m TOTAL EARTH PRESSURE P KN 161 KN KN KN P KN 29.4 KN KN KN P H KN KN KN KN Design of Steam Main bars #12@180mm c/c #12@180mm c/c #12@200mm c/c #12@260mm c/c Distribution bars #10@130mm c/c #10@130mm c/c #10@140mm c/c #10@180mm c/c editor@iaeme.com

6 Analysis, Design & Cost Comparison Between Counterfort Retaining Wall & Mechanically Stabilized Earth Wall Main bars Distribution bars Main bars Design of Horizontal Links Vertical Stirrups Design of Heel slab c/c c/c c/c c/c c/c c/c c/c c/c Design of Counterfort 7 Nos. #25mm 5 Nos. #25mm 4 Nos. #25mm 3 Nos. #25mm bars bars bars bars #10@140mm cc #10@170mm c/c #10@190mm c/c #10@200mm c/c #12@100mm c/c #12@110mm c/c #12@100mm c/c #12@160mm c/c Figure 4. Cross section through counterfort 2.5. Design of Mechanically Stabilized Earth Wall Overall height of structure, H = 7.65m Angle of internal friction of backfill soil (ϕ 1 ) = 32.9 Unit weight of backfill soil (γ 1 ) = kn/m 3 Reinforcement used: metallic strips/galvanized steel strips. Assume width of strip, w = 100 mm = 0.1m Dimensions of precast facia panels = 1m X 1m, 0.18m thickness & M-35 grade Therefore horizontal spacing of reinforcement strips, S h = 1m Vertical spacing of reinforcement strips, Sv = 1m Yield strength of reinforcement strips, f y = 250 Mpa = kn/m 2 Soil-Tie friction angle, ϕ µ = 0.7*ϕ 1 = 0.7*32.9 = 23 Factor of safety against tie breaking, FS (B) = 1.5 Factor of safety against tie pullout, FS (P) = 1.5 Rate of corrosion of galvanized steel strips = mm/yr. Life span of the structure = 100 years Check for Internal Stability: Active earth pressure coefficient, K a = = = Maximum Rankine active earth pressure, σa (max) = (0.296*22.317*7.65) + (0.296*22.317*1.2) = kn/m 2 ( ) editor@iaeme.com

7 Therefore thickness of tie (t), required to resist the tie breakout, (58.461*1.5)/(0.1*250000)= m = 3.5 mm. For life span of 100 years and rate of corrosion of mm/yr., actual thickness, t = = (0.025*100) = 6.0 mm Tie length (L) Calculation: tan (45 + ϕ 1 /2) = tan {45 + (32.9 /2)}= 1.838, tan (ϕ µ ) = Table 4 Tie length calculation at different depth (Z) for Earth retaining structure z (m) Tie length, L (m) So use tie length, L = 9.2m Check for External Stability: Check for Overturning: W 1 = γ 1 *H*L = *7.65*9.2 = kn/m, x 1 = 4.6 m Resisting Moment, M R = W 1 *x 1 = kn-m P 1 = 0.5*(γ 1 *Ka*H)*H = kn/m, z 1 = H/3 = 2.55 m P 2 = 1.2*γ1*Ka*H = kn/m, z 2 = H/2 = m Active force, P a = P1 + P2 = kn/m Overturning moment, Mo = (P 1 *z 1 ) + (P 2 *z 2 ) = kn-m FS (overturning) = M R /Mo = / = > 3; Hence ok. Check for Sliding: = 2.49 > 1.5; Hence OK. Check for Bearing Capacity: Angle of internal friction of foundation soil (ϕ 2 ) = 29.5 Unit weight of foundation soil (γ 2 ) = kn/m 3, Cohesion, c = 2.95 kn/m 2 Bearing capacity factors: N c = 29, N q = 17.42, N γ = Depth of earth retaining structure below ground level, D = 0.45m q = γ 2 *D + 1.2*γ 1 = kn/m 2 Ultimate bearing capacity, q ult = c*n c + q*(n q -1) + 0.5*γ 2 *L*N γ = kn/m 2 σ'o (H) = γ 1 *H + 1.2*γ 1 = kn/m editor@iaeme.com

8 Analysis, Design & Cost Comparison Between Counterfort Retaining Wall & Mechanically Stabilized Earth Wall = / = > 5; Hence OK Cost Analysis & Cost Comparison Table 5 Abstract of detailed estimation for Counterfort retaining wall Sl. No. Particular Item name Unit Quantity Rate in Rs Amount 1 Earth excavation Cum ,35, Earth filling in foundation up to plinth Cum ,24, Embankment filling Cum ,78, PCC (M15 grade) Cum , ,24, M25 Concrete Cum , ,41,97, Shuttering for foundation Sq , Shuttering for wall Sq ,21, Steel Reinforcement (Fe-500) Tonnes , ,28,96, Weep holes Nos , Filter media Cum , ,00, M-25 concrete Crash Barrier Cum , ,27, Steel Reinforcement (Fe-500) for crash barrier Tonnes , ,97, Additional WMM required Cum 228 1, ,11, Additional prime coat required above WMM Sq , Additional tack coat required above prime coat Sq , Additional DBM required Cum , ,28, Additional tack coat required above DBM Sq , Additional BC required Cum , ,21, Total 3,69,49,422 Table 6 Abstract of detailed estimation for Mechanically Stabilized Earth Wall Sl. Quantit Rate in Particular Item name Unit No. y Rupees Amount 1 Earth excavation Cum ,55, Embankment filling Cum ,27, Entire labor cost for preparation of precast panels Sq ,38, a M-35 grade concrete (thickness of the facia Sq panel = 0.18m) 18,58, b Reinforcing steel - Kg per sq. of face area (Single mesh of reinforcement steel of 10mm kg ,50, dia.; Rate 40 Rs./Kg) 4c M-15 grade PCC levelling pad of dimensions 0.35m wide X 0.15m thick Cum , , Filter media Cum , ,86, Assembling, jointing and laying of reinforcing elements Cum ,70, M25 Concrete for crash barrier and friction slab Cum , ,82, Steel Reinforcement (Fe-500) for crash Tonne barrier s , ,08, Additional WMM required Cum , ,54, Additional primer coat required Sq , Additional tack coat required above prime coat Sq , Additional DBM required Cum ,35, Additional tack coat required above DBM Sq , Additional BC required Cum , ,20,466.1 Total 1,96,10, editor@iaeme.com

9 3. CONCLUSIONS The depth of foundation will be depending on SBC of foundation soil, density of foundation soil and angle of internal friction of foundation soil. The size of cross sectional elements and reinforcement quantity will be depending on surcharge load, height of retaining wall and type of backfill material. If these parameters are varied, then construction cost will also be varied. The total cost of counterfort retaining wall is about rupees 3.69 crores. The contribution of cost of concrete and steel in RCC stepped counterfort retaining wall of different heights are about % and % respectively. Thus the key contribution in cost difference is recognized by the huge amount of concrete and steel quantity which is usually required in conventional RCC retaining wall as compared to that of mechanically stabilized earth wall. The total cost of mechanically stabilized earth wall is about rupees 1.96 crores. The total cost is mainly affected by earth filling (10.85 % of total cost) and earth reinforcement (44.22 % of total cost). In case of mechanically stabilized earth wall, about % of cost saving has been achieved in the present study. The variation in cost is depending upon the type of construction at the site. Since the construction of mechanically stabilized earth wall involves use of prefabricated concrete element, it will take less time for construction when compared with conventional RCC retaining wall. Hence it is finally concluded that the mechanically stabilized earth wall significantly more economical as compared to that of conventional RCC retaining wall for given geometrics and loading conditions considered in the present study. REFERENCES [1] Yash Chaliawala, Gunvant Solanki, Anuj.K.Chandiwala, Comparative Study of Cantilever and Counter fort Retaining wall, International Journal of Advance Engineering and Research Development, Volume 2, Issue 12, December [2] Dr. S.S Patil, Analysis and Design of Stepped Cantilever Retaining Wall, International Journal of Engineering Research & Technology (IJERT), Vol. 4, Issue 2, February-2015 [3] Ancy Joseph and Mercy Joseph, Comparative Study of Gabion Walls and Reinforced Earth Retaining Walls Vol.3, Issue 2, February 2015 [4] A. J. Khan and M. Sikder, Design Basis and Economic Aspects of different types of Retaining Walls, Journal of Civil Engineering (IEB), 2004 [5] A.M.Mhetre, M.V.Nagendra, Constructional and Techno-Economic Aspects of Reinforced Earth Construction Works, International Journal of Engineering Research, Vol.5, Issue 1, Jan [6] IRC:SP: , Guidelines for Design and Construction of Reinforced Soil Walls [7] Peter L. Anderson, Robert A. Gladstone, John E. Sankey, State of the Practice of MSE Wall Design for Highway Structures, 2010 [8] Mahi Sharma, Mr.H.S.Goliya, Design and economic analysis of reinforced earth wall, International Journal of Emerging Trends in Engineering and Development, Vol.6, Issue 4, Oct. -Nov [9] IS 6403 (1981), Code of practice for determination of bearing capacity of shallow foundations [10] IS 456:2000, Plain and Reinforced Concrete -Code of Practice, fourth revision. [11] Dr. B.C.Punmia, Er. Ashok Kumar Jain, Dr. Arun K. Jain, Reinforced Concrete Structures (R.C.C. Designs), Laxmi Publications (P) LTD, 10 th Reprint, 2006 [12] S.S.Bhavikatti, Advance R.C.C Design, New Age International (P) Limited, Publishers, 2 nd Reprint, 2013 [13] Braja M. Das, Principles of Foundation Engineering, SI, Cengage Learning India Private Limited, 1 st Reprint, editor@iaeme.com