International Journal of Informative & Futuristic Research

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1 Volume 3 Issue 1 September 2015 ISSN: International Journal of Informative & Futuristic Research Design And Detailing Of Counter-Fort Retaining Paper ID IJIFR/ V3/ E1/ 011 Page No Subject Area Key Words Civil Engineering Retaining Wall, Counter-Fort, Base Pressure, Active Earth Pressure, Passive Earth Pressure Rupa B. Patil 1 G. R. Patil 2 M.E. (Structure) Student, Department Of Civil Engineering Rajarshi Shahu College of Engineering Tathawade, Pune-Maharashtra, India Assistant Professor Department Of Civil Engineering Rajarshi Shahu College of Engineering Tathawade, Pune-Maharashtra, India Abstract A retaining wall is a structure designed and constructed to resist the lateral pressure of soil when there is a desired change in ground elevation that exceeds the angle of repose of the soil. The most important consideration in proper design and installation of retaining walls is to recognize and counteract the tendency of the retained material to move down slope due to gravity. This creates lateral earth pressure behind the wall which depends on the angle of internal friction (ø) and the cohesive strength (c) of the retained material, as well as the direction and magnitude of movement the retaining structure undergoes. In many cases we have to come across the retaining wall of 7m, 8m, 9m height. So we will consider these heights for noncohesive soil conditions for different spacing of counter-forts. We studied, by changing the spacing of counter-forts what will be the change in thickness of stem as well as heel slab, what is the optimum spacing of the counter-forts, what is the effect of changing spacing of counter-forts on bending moments, and plotted a graph of optimum spacing of counter-forts vs height of wall. The data presented here in following sections clearly indicates that changing spacing of counter-forts for retaining wall results in, reduction of spacing of counter forts will result in reduction in bending moments in heel slab and stem wall, reduction of spacing of counter forts will result in reduction in thickness of heel slab and stem wall. It is also observed that for 1m, 1.5m, 2m, 3m, 3.5m, 4m spacing of counter-forts the concrete and steel quantities per meter length of retaining wall is more than at 2.5m spacing. So optimum spacing of counter-forts for 7m, 8m, 9m height retaining wall is observed to be 2.5m. Published Online On: 11/09/

2 1. Introduction 1.1 General A soil mass is stable when the slope of the surface of the soil mass is flatter than the safe slope. At some locations where the space is limited, it is not possible to provide flat slope and the soil is to be retained at a slope steeper than the surface one. In such cases, a retaining structure is required to provide lateral support to the soil mass. Retaining walls are relatively rigid walls used for supporting the soil mass laterally so that the soil can be retained at different levels on the two sides. Generally, the soil masses are vertical or nearly vertical behind the retaining structure. Thus, a retaining wall maintains the soil at different elevations on its either side. In the absence of a retaining wall, the soil on the higher side would have a tendency to slide and may not remain stable. As the supply of level building sites diminishes, the need to create level building plat forms for industrial construction sites will increase. Also, on many developed sites there is often a need to level the front and/or back yards to fully utilize the space for carports, gardens, and other industrial requirements. Thus, our paper is been cleared with the design of a counter fort retaining wall to such industrial boundaries. In many cases we have to come across the retaining wall of height 7m, 8m, 9m height. So we will consider these heights for different soil conditions for different spacing of counter forts. 1.2 Counter-Fort Retaining Wall: Counter fort walls are cantilever walls strengthened with counter forts monolithic with the back of the wall slab and base slab. The counter-forts act as tension stiffeners and connect the wall slab and the base to reduce the bending and shearing stresses. To reduce the bending moments in vertical walls of great height, counter forts are used, spaced at distances from each other equal to or slightly larger than one-half of the height Counter forts are used for high walls with heights greater than 8 to 12 m. 1.3 Objectives of the study: After having literature review and having on field experience it found necessary to study by changing the spacing of counter-forts what will be the change in thickness of stem as well as heel slab, what is the optimum spacing of the counter-forts, what is the effect of changing spacing of counter-forts on bending moments, and plotted a graph of optimum spacing of counter-forts vs height of wall for 7m, 8m,9m height retaining walls. In the proposed investigation we are going to design counter fort retaining walls of height 7m, 8m,9m for the spacing of counter forts from 1m to 4m for non-cohesive soil condition and soil bearing capacity 25T/Sq.m. And we are going to study the variation of bending moment in stem wall and heel slab for the different spacing of counter forts. This study will provide the ready reference to consultants as well as students for basic size and bending moments for the specified heights and soil conditions. 2. Modelling And Analysis Of Retaining Wall 2.1 Problem Description: In many cases we have to come across the retaining wall of 7m, 8m, 9m height. So we will consider these heights for non-cohesive soil conditions for different spacing of counter forts. By changing the spacing of counter-forts what will be the change in thickness of stem as well as heel slab. To study the optimum spacing of the counter-forts. To prepare a graph of spacing of counter-forts vs bending moments To prepare a graph of optimum spacing of counter-forts vs height of wall 62

3 2.2 Analysis of counter-fort retaining wall by using excel programme: Counter fort retaining wall 7 m height Height of retaining wall, H = 7m Grade of concrete, = 25 N/ Grade of steel, = 500 N/ R = 3.32 From design constant sheet K= 0.46 From design constant sheet SBC of soil, P = 25 T/ SBC of soil, P = 250 KN/ SBC of soil, P = 0.25 N/ Density of soil, ω = N/ Density of soil, ω = 18 KN/ Angle of internal friction, ø = = 0.50, 1- = 0.50, 1+ = 1.50 = = 0.334, = = Minimum depth of foundation required, = = m Depth of foundation provided = 1.5 m So, over all height of wall = 8.5 m Unit weight of concrete = N/ A) Preliminary dimensions of wall component i) Base width B=0.6*H = 4.2 m (a) B=0.7*H = 4.9 m (b) So, B = 4.9 m (maximum of a and b) Assume, B = 5.5 m ii) Toe projection Toe projection = C*B C = 1 -( ) C = So, toe projection = C*B = 0.5 m Assume, toe projection = 0.75 m W1 COUNTER FORT W3 H iii) Clear spacing of counter-forts (l), 1.5m l=3.5( ) = 0.389, so ( ) = 0.79 D W2 A l = m So, assume clear spacing of counter-forts (l) = 2.5 m iv) Thickness of base slab Assumed thickness of base slab = 0.3 m 63

4 B) Design of stem wall h = Height of stem wall = 8.2m Pressure intensity at base = ωh( ) = 49.23KN/ Maximum working moment, M = KN-m Factored moment, = KN-m, x = N-mm Consider 1000 mm (b) width of stem slab. Effective depth required for balanced section is (d) = d = = mm Clear cover assumed 35 mm Diameter of bar used is 16 mm Assuming an under reinforced section and to provide a suitable thickness to resist shear at base of stem, adopt an overall thickness of stem wall required = mm ~ 150 mm constant up to the top. Over all depth of section is (D) = 0.15 m So, effective depth of section is (d) = 99 mm ~ 100 mm IS: , clause G-1.1, The reinforcements in the stem are computed using the relation, = 0.87 d * + By solving we get the quadratic equation, ( ) b d + = = 0 = a = 500, 2a = 1000 b = , so -(b) = c = , 4ac = E+12-4ac = E+12 = First root = Second root = Or second way to calculate area of steel is, = ( ( ))1000d = ( >.hence safe. ). = b D In % Grade Of Steel 0.12 FE 415 & FE FE 250 = 180 Using 16 mm dia. Bars, 64

5 Spacing ( S ) of bars = mm So, provide 16 mm bars at 175 mm C/C = ( >. hence safe. ) ( >. hence safe. ) Distribution bars Using 10mm dia. Bars, Spacing ( S ) of bars = mm So, provide 10 mm bars at 300 mm C/C = ( >. Hence safe. ) C) Check for stability of wall 1) Check against over turning Consider 1m length of retaining wall Height of stem wall = 8.2 m Base width (B) = 5.5 m Heel Projection (b') = 4.6 m Depth of soil on toe projection (h) = 1.2 m Toe projection = C*B = 0.75m Sr. No. Description Of Load Magnitude Of Load In (KN) Distance Of C.G. From `A' In (M) 1 Weight of stem wall (W1) Weight of base slab (W2) Weight of earth fill over heel slab (W3) A In (KN-M) Total Horizontal earth pressure on the full height of the retaining wall tending to overturn the wall. = w = = = N acting at H/3 from A where, = m Over turning moment = Stabilizing moment = = N-m = N-m Factor of safety against overturning = = Which is greater than 2, hence ok. 2) Check for sliding Horizontal earth pressure on the full height of the retaining wall tending to slide the wall. = w = N Total force opposing sliding = µ x W= 0.58 W = Factor of safety against sliding= = 2.71 Which is greater than 1.5, hence ok? D) Maximum and Minimum pressure at the base The distance of point of application of the resultant from point A is, N 65

6 z =, z = m Eccentricity (e) = z- b/2 = m But, b/6 = m So, e < b/6 Maximum and minimum pressure at the base are given by, = H*e =1.708, = = ( ) = KN/ Maximum intensity of soil pressure at base is less than SBC hence,safe. = ( ) = KN/ Pressure diagram at the base D c b A KN/ KN/m 2 j i E f g KN/ H' Pressure at line cf at toe projection is = KN/ E) Design of toe slab Moment in toe slab Sr. No. Description Of Load Magnitude Of Load In (KN) Distance Of C.G. From `C' In (M) 1 Upward pressure 'cdif ' Upward pressure 'efi ' Total Deduct self weight of toe slab 4 Deduct weight of soil above toe slab Total 8.10 Maximum working moment in toe slab = M = KN-m Factored moment = = KN-m, x = N-m Consider 1000 mm (b) width of toe slab. Effective depth required for balanced section is (d) = C In (Knm) d = = mm Clear cover assumed 35 mm Diameter of bar used is 12 mm 66

7 Over all depth required = mm ~ 200 mm Over all depth of the section assumed is (D) = 0.3 m Assumed thickness is greater than required hence,safe Effective depth of the section is (d) = 253 mm ~ 255 mm IS: , clause G-1.1 The reinforcements in the stem are computed using the relation, = 0.87( ) d [ ] By solving we get the quadratic equation, ( ) b d + = = 0 = a = 500, 2a = 1000 b = , so -(b) = c = , 4ac = E+12-4ac = E+13 = First root = Second root = Or second way to calculate area of steel is, = ( ( ))1000d = ( >.hence safe. ). = b D in % Grade of steel 0.12 FE 415 & FE FE 250 = 360 Using 12 mm dia. Bars, Spacing ( S ) of bars = mm So, provide 12 mm bars at 130 mm C/C = ( >. hence safe. ) ( >. hence safe. ) Distribution bars Using 10mm dia. Bars, Spacing ( S ) of bars = mm So, provide 10 mm bars at 200 mm C/C = ( >. hence safe. ) E) Design of heel slab Consider 1000 mm wide strip of heel slab. Near end 'A' upward soil pressure = KN/ 67

8 Weight of soil on strip = KN/ Self weight of strip = 7.2 KN/ Total downward load on heel slab = KN/ Net downward pressure = KN/ Spacing of counter fort (l) = 2.5 m Maximum working moment in heel slab = M = KN-m Factored moment, = KN-m, x = N-mm Consider 1000 mm (b) width of stem slab. Effective depth required for balanced section is (d) = d = = mm Clear cover assumed 35 mm Diameter of bar used is 10 mm Over all depth required = mm ~120 Over all depth of the section assumed is (D) = 0.3 m Assumed thickness is greater than required, hence,safe Effective depth of the section is (d) = 253 mm ~ 255 mm IS: , clause G-1.1 The reinforcements in the stem are computed using the relation, = 0.87( ) d [ ] By solving we get the quadratic equation, ( ) b d + = = 0 = a = 500, 2a = 1000 b = , so -(b) = c = , 4ac = E+12-4ac = E+13-4ac = First root, = Second root, = Or second way to calculate area of steel is, = ( ( ))1000d = = b D in % Grade of steel 0.12 FE 415 & FE FE 250 = 360 Using 12 mm dia. Bars, Spacing (S) of bars = mm 68

9 So, provide 10 mm bars at 200 mm C/C = ( >. hence safe. ) ( >. hence safe. ) Distribution bars Using 10mm dia. Bars, Spacing ( S ) of bars = mm So, provide 10 mm bars at 200 mm C/C = ( >. hence safe. ) E) Design of counter fort Thickness provided at the top = 300 mm Thickness of counter-fort= 300 mm Centre to centre spacing of counter-fort (l) = 2.5 m Maximum working moment in counter-fort is, M = = = 0.334, Maximum working moment in counter-fort is, M = KN-m Factored moment = = KN-m, x = N-mm Consider 300 mm (b) width of counter-fort. Effective depth required for balanced section is (d) = d = = mm Clear cover assumed 35 mm Diameter of bar used is 12 mm Over all depth required = mm ~1455 mm Over all depth of the section assumed is (D) = 1.46 m Assumed thickness is greater than required, hence,safe Effective depth of the section is (d) = 1413 mm ~ 1410 mm IS: , clause G-1.1 The reinforcements in the stem are computed using the relation, = 0.87( ) d * + By solving we get the quadratic equation, ( ) b d + = = 0 = a = 500, 2a = 1000 b = , so -(b) = c = , 4ac = E+13-4ac = E+13-4ac = First root = Second root = As per IS: , clause

10 . = = = ( >.hence safe. ) Dia. of bar in mm No. of bars Total area = ( >. Hence safe. ). 3 Extract Sheets Table 3.1 Thickness of stem wall in (m) Clear spacing of the counter-fort (l) in (m) Thickness of stem wall in (m) 7m 8m 9m Table 3.2 Thickness of base slab in (m) Clear spacing of the counter-fort (l) in (m) Thickness of base slab in (m) 7m 8m 9m Table 3.3 Thickness of counter-fort in (m) Clear spacing of the counter forts (l) in (m) Thickness of the counter forts in (m) 7M 8M 9M

11 Thickness of stem wall in (m) ISSN: Table 3.4: Concrete quantity in Cu.m per meter length Spacing of counter forts in meter Concrete qty. in Cu.m per meter length 7m 8m 9m Results And Discussion Table 3.5 Steel quantity in Cu.m per meter length Spacing of Counter forts in meter Steel qty. In Kg per meter length 7 m 8 m 9 m Thickness of stem wall in m vs clear spacing of counter-fort in m M 8M 9M Clear spacing of the counter-fort(l) in (m) Figure 1- Thickness of stem wall in m vs clear spacing of counter-fort in m 71

12 Thickness of the counter forts in (m) Thickness of base slab in (m) ISSN: Thickness of base slab in m vs clear spacing of counter-fort in m THICKNESS OF BASE SLAB IN (m) 7M THICKNESS OF BASE SLAB IN (m) 8M THICKNESS OF BASE SLAB IN (m) 9M Spacing of counter-forts in (m) Figure 2- Thickness of base slab in m vs clear spacing of counter-fort in m 4.3 Thickness of counter-fort in (m) vs clear spacing of counter-fort in m Thickness of the counter forts in (m) for 7M Thickness of the counter forts in (m) for 8M Clear spacing of the counter forts (l) in (m) Thickness of the counter forts in (m) for 9M ` Figure 3- Thickness of counter-fort in m vs clear spacing of counter-fort in m 72

13 Steel qty. in Kg per meter length Concrete quantity in Cu.m per meter length ISSN: Concrete quantity in Cu.m per meter length vs clear spacing of counter-fort in m Spacing of Counter forts in meter CONCRETE QTY. IN CU.m PER METER LENGTH 9M CONCRETE QTY. IN CU.m PER METER LENGTH 8M CONCRETE QTY. IN CU.m PER METER LENGTH 7M Figure 4: Concrete quantity in Cu.m per meter length vs clear spacing of counter-fort in m 4.5 Steel quantity in Cu.m per meter length vs clear spacing of counter-fort in m m 8 m 9 m Spacing of counter forts in meter Figure 5- Steel quantity in Cu.m per meter length vs clear spacing of counter-fort in m 5 Conclusion I. Thus the data presented here in above sections clearly indicates that changing spacing of counter-forts for retaining wall results in, II. From extract sheet it is observed that for 7m height retaining wall bending moment in stem wall increases from 4.1 KN-m for 1m c/c spacing of counter-forts to KN-m for 4m 73

14 c/c spacing of counter-forts. Also bending moment in heel slab increases from 2.3 KN-m for 1m c/c spacing of counter-forts to 37 KN-m for 4m c/c spacing of counter-forts. III. For 8m height retaining wall bending moment in stem wall increases from 4.60 KN-m for 1m c/c spacing of counter-forts to KN-m for 4m c/c spacing of counter-forts. Also bending moment in heel slab increases from 3.50 KN-m for 1m c/c spacing of counter-forts to KN-m for 4m c/c spacing of counter-forts. IV. For 9m height retaining wall bending moment in stem wall increases from 5.00 KN-m for 1m c/c spacing of counter-forts to KN-m for 4m c/c spacing of counter-forts. Also bending moment in heel slab increases from 3.60 KN-m for 1m c/c spacing of counter-forts to KN-m for 4m c/c spacing of counter-forts. V. From graph 4.1 it is observed that for 7m, 8m, and 9m height of retaining walls for 1m, 1.5m and 2.0m spacing of counter-forts the thickness of stem wall required is minimum thickness 150mm. As the spacing of counter-forts increases the thickness of stem wall increases to 175mm to 300 mm for 3.0m 3.5m and 4.0m spacing of counter-forts. VI. From graph 4.2 it is observed that for 7m, 8m, and 9m height of retaining walls for 1m, 1.5m and 2.0m spacing of counter-forts the thickness of stem wall required is minimum thickness 150mm. As the spacing of counter-forts increases the thickness of stem wall increases to 175mm for 3.0m spacing 200mm for 3.5m spacing and 300mm for 4.0m spacing of counter-forts. VII. It is also observed that for 1m,1.5m, 2m, spacing of counter-forts the no. of counter-forts are more for the same length of retaining wall due to which concrete and steel quantities are more than it is for 2.5m spacing of counter-forts. For 3m, 3.5m, 4m spacing of counter-forts the basic dimensions and steel requirements are more due to which concrete and steel quantities per meter length of retaining wall is more than at 2.5m spacing. So 2.5m. clear spacing is the optimum spacing of counter-fort for 7m, 8m, and 9m height of retaining wall VIII. (5) From graph 4.3 it is observed that for 7m, 8m, and 9m height of retaining walls for 1m, 1.5m and 2.0m spacing of counter-forts the thickness of counter-fort required is 300mm. As the spacing of counter-forts increases the thickness of counter-fort increases to 500mm. IX. From table 4.6 it is observed that for 7m height of retaining wall if tapered section of stem wall is provided then we can save concrete quantity up to 1.5 to 2.5 Cu.m per meter length of retaining wall. Similarly this is true for 8m and 9m height of retaining walls. 6 References [1] Kenny Luu1, Ken O Neil2, Weimin Deng3, Design of the Counterfort Retaining Wall on the Barangaroo Headland Park Project, Sydney. [2] M. Ghazavi and V. Salvati, Sensitivity analysis and design of reinforced concrete cantilever retaining walls using bacterial foraging optimization algorithm. [3] Patil S. M. Wagh and Prof. K. S. Wagh, Reduction in construction material:effect of the provision of the loft behind the cantilever retaining wall [4] J. Khana and M. Sikderb, Design basis and economic aspects of different types of retaining walls. [5] S.N. Moghaddas Tafreshi and T. Nouri, Seismic stability of reinforced retaining wall. [6].Lohith Reddy, Y.Naga Harish and K.N.S Pavan Kumar Varma Analysis and design of retaining wall of Ganges valley school [7] Siripuram Anusha, R.Sabitharaj and M. Balakoteshwari Design of retaining wall for a minor bridge. [8] Dr. B.C. Punmia, Ashok Kumar Jain and Arun Kumar Jain (2007.) Limit State design of reinforced concrete Laxmi Publications (P) LTD, New Delhi. [9] Gopal Ranjan, and Rao, A.S.R. (2000). Basic and Applied Soil Mechanics, 2 nd Edition, New Age International (P) Ltd. Publishers, New Delhi 74

15 [10] IS 456:2000 COP Plain And Reinforced Concrete. [11] K.R. Aror (2003). Soil Mechanics and Foundation Engineering, 6th Edition, Standard Publishers Distributors, Delhi. [12] Basudhar, P.K., and Madhav, M.R. (1980), Simplified Passive Earth Pressure Analysis Journal of Geotechnical Engineering Division, ASCE, April, GT4, [13] Choudhury, D. and Singh, S. (2006)a; "New approach for estimation of static and seismic active earth pressure", Geotechnical and Geological Engineering, Springer, Netherlands, 24(1), [14] Das, B.M. (2002). Principles of Geotechnical Engineering, 5th Edition, Thomson Brooks/Cole. [15] Murthy, V.N.S. (2002). Principles of Soil Mechanics and Foundation Engineering, 5th Edition, UBS Publishers Distributors Ltd., New Delhi 75