CRITICAL STUDY OF RCC BALANCING TANK

Transcription

1 CRITICAL STUDY OF RCC BALANCING TANK 1 PRIYANKA DEEPAK HARKAL, 2 M. M. MAHAJAN 1 M. Tech. Student, Visvesvaraya National Institute of Technology, Nagpur 2 Professor, Visvesvaraya National Institute of Technology, Nagpur 1 priyankaharkal@gmail.com, 2 mukundmmahajan@gmail.com Abstract Water supply system is the important part of the society. This system has various stages to supply water from source to population. One of the important stage is balancing tank which receives the water from main source under the gravity flow or pressure flow and pumped to the next level. Analysis and design of such reservoir is important as any failure to reservoir will directly affect on water requirement of community. An excel program has been made for analysis and design of cantilever type retaining wall of balancing tank for various probable critical cases and load combinations. Effect of different soil parameters such as cohesive and non cohesive soil around tank on wall stability has been studied which shows that cohesive soil around the soil gives higher section of tank wall than non cohesive soil. Also the comparison made between IS 1893:1984 and IS 1893:Part3;2014 for cohesive soil and non cohesive soil. Uplift effect due to seepage and water bodies near tank wall such as canal has been studied for static, earthquake analysis with seismic acceleration coefficient 0.15 for 5.5m and 6.5m height of tank wall. Effect of shear key on stability of tank wall studied for different soil parameters, uplift due to various critical conditions, and height of tank wall. It is observed that when shear key provided, section size ultimately concrete quantity reduces. Index Terms Balancing Tank, Cantilever Retaining Wall, Cohesive Soil, Non Cohesive Soil, Uplift. I. INTRODUCTION Balancing tank is huge widely spread tank on ground whose plan area is in square km which stores the huge amount of water during water supply scheme. There are many structural systems which stores or retain the water such structures are known as water retaining structures. The wall of tank is retaining wall which may be gravity retaining wall, cantilever retaining wall, counter fort retaining wall, and buttress retaining wall. In this paper focus of study is on cantilever type retaining wall for various critical situations and soil parameters considered outside of tank wall. The various critical load combinations are studied from start of construction to final working condition of balancing tank for stability and safety. Tank is also studied for variation of height of tank, effect of shear key on stability of wall, different types of soil such as non cohesive and cohesive soil to be filled from outside of tank after hydraulic testing. Also the effect of uplift pressure due to seepage and water bodies such as canal nearby tank wall. II. SYSTEMATIC APPROACH TO STABILITY CHECK A. Selection of type of tank wall For construction of balancing tank gravity retaining wall, counter fort wall, buttress wall, cantilever wall are chosen according to size, height and capacity of tank. B. Geometry fixation-capacity and plan Geometry of tank is dependent on the ground profile and capacity of tank. The shape of tank may be or may not be regular as it varies according to undulation of ground and availability of land. C. Stability Checks Stability check is performed for tank wall for all expected and critical loads such as dead load, hydrostatic load, soil pressure, wind load, earthquake load, uplift etc. and their combinations. D. Expected loads Expected loads are the loads which are generally acts on tank wall which includes dead load i.e. self weight of tank wall, live load i.e. hydrostatic pressure when tank is filled with water, soil pressure when soil is present outside of tank wall. E. Critical loads Critical loads include the wind load and earthquake loads according to tank situated in different region and different earthquake zone. Wind load is considered when tank is empty i.e. no water inside and no soil outside of tank. While earthquake is considered when tank is empty as well as tank is full with water and soil so dynamic earth pressure and hydrodynamic pressure will act on wall. III. PARAMETERS CONSIDERED FOR ANALYSIS Following parameters are considered for carrying out analysis. i. Safe bearing capacity 250kN/m 2, coefficient of friction between soil and concrete =0.5, unit weight of water =10 kn/m 3, angle of internal friction for soil below foundation 30 0, unit weight of soil below foundation =18 kn/m 3, intensity of wind=1.4 kn/m 2, free board for water=0.5m, height of water to be stored=5m, 6m, horizontal seismic coefficient=0.15, friction angle δ= 2/3φ ii. Soil properties outside tank wall Two types of soils are considered around the 63

2 tank wall, non cohesive soil (six types of non cohesive soil) and cohesive soil (two types of cohesive soil) listed in Table I. I. Soil properties around tank wall Fig.1 and Fig. 2 shows the levels of water and soil for 5.5m and 6.5m height of wall respectively. This is typical shape of tank wall and shape of toe, heel, and stem can vary to rectangular or trapezoidal. Fig. 3 Water levels prior to hydraulic testing (y 3=5m,6m,y 4=1m) After complete construction of tank wall, hydraulic testing is carried out and water levels are observed. In Fig. 3 water levels for hydraulic testing are shown. γ -Unit weight of soil in kn/m 3 ф-angle of internal friction in degrees c-cohesion in kn/m 2 Fig. 4 Canal flowing near tank wall (X 1=distance of canal from tank wall=100m,x 2=X 1+Base width of tank wall, y=water level in canal from datum when canal full, y 1=Water height when tank is empty =1m,y 2=Water level due to hydraulic grade line) Fig. 1 General fig. and levels for height 5.5m Fig. 2 General fig. and levels for height 6.5m Here critical case is considered that canal is running at some distance and water levels are shown in Fig. 4 The safety factors are considered for overturning 1.4, sliding 1.4, maximum pressure less than safe bearing capacity, and minimum pressure greater than zero. Design is carried out by working stress method. An excel program has been prepared for carrying out analysis. iii. Load cases and load combinations Table II shows the possible load cases which will occur from start of construction to final working stage of balancing tank. A] Construction Phase Analysis is carried out for tank wall when it is in construction phase for wind and earthquake. During construction phase tank having no water inside and no soil outside of tank but there are chances of wind and earthquake load acting on tank wall. B] Construction stage prior to testing After completion of construction of tank and before actual working condition of tank hydraulic testing is carried out by filling the tank with water only and levels of water are observed. In this case analysis is 64

3 carried out for hydrostatic, hydrodynamic and uplift pressure. C] Regular After hydraulic testing area inside the tank wall is treated with geo membrane and tank is filled with water and soil from outside of tank wall. In this tank full with water and full soil from outside analysis is carried out for static and earthquake loads. D] Tank empty When tank is empty and soil from outside of tank, analysis is carried out for static and earthquake loads. II. Load cases and combinations in static and earthquake case Fig. 6 Percentage saving in concrete quantity due to shear key when uplift is not considered b. Uplift due to seepage prior to hydraulic testing Tank wall which is already safe in no uplift condition is analyzed for uplift effect. Uplift pressure will act during the hydraulic testing of tank. Due to uplift section which was safe in no uplift condition is failing in meeting the safety criteria so modified and anchor bars are provided in the base wall as shown in Fig. 7. These anchor bars are inserted in ground through base slab for 1m depth in duracon grouting. Fig.7 Location of anchor bolts IV. ANALYSIS RESULTS Concrete quantity is worked out in cubic meter per 100m length of tank wall. i. Non cohesive soil outside of tank wall a. No uplift considered: Analysis is carried out load cases listed in Table II with and without shear key when non cohesive soil outside of tank wall and uplift is neglected. Fig. 5 shows concrete quantity in static and earthquake case with and without shear key. Fig. 6 gives the percentage saving in concrete quantity due to shear key. Fig. 8 shows concrete quantity in static and earthquake case with and without shear key. Fig. 9 gives the percentage saving in concrete quantity due to shear key. Fig.8 Effect of shear key when uplift considered and anchor bolts used Fig. 5 Effect of shear key on concrete quantity when uplift is not considered Fig. 9 Percentage saving in concrete quantity due to shear key when uplift considered 65

4 Fig. 10 shows the effect of uplift on tank wall when earthquake is considered. Fig. 11 shows the percentage increase in concrete quantity due to uplift. when only soil is outside of tank wall, IS 1893:1984 gives higher section for cohesive soil than revised code. Fig. 10 Effect of uplift on concrete quantity due to earthquake Fig.12 Concrete quantity for cohesive soil when IS 1893:1984 and IS 1893:Part 3;2014 used b. Uplift due to seepage prior to testing: Cohesive soil is considered around the tank wall and analysis is carried out for static load only listed in table II as provision for calculation of dynamic passive soil pressure is not given in IS 1893:Part 3;2014. Comparison is made in Fig 13 for concrete quantity when no uplift and with uplift. Fig.11 Percentage increase in concrete quantity due to uplift when earthquake considered c. Uplift due to canal flowing near tank wall Critical study is carried out considering canal flowing near tank wall and uplift effect is observed while canal running full and canal empty. Sections which were safe in static and earthquake cases for hydraulic testing analyzed for uplift effect due to canal. It is observed that when canal is running full, sections which were safe in static and earthquake case doesn t meet the safety criteria. Hence, provision is made for anchor bolts as shown in Fig.7 are made and shear key is provided. ii. Cohesive soil outside of tank wall a. Comparison between IS 1893:1984 and IS 1893:Part 3;2014: In IS 1893 the provision for calculation of active earth pressure for cohesive soil due to earthquake is not given while in revised code of IS 1893:Part 3;2014, provision is made. When this provision was not in code and cohesive backfill with earthquake is to be analyzed then IS 1893:1984 code is used neglecting cohesion. Also the same cohesive soil is analyzed using revised code and comparison done between these two analyses for concrete quantity. Fig. 12 shows the comparison of concrete quantity for IS 1893:1984 and is 1893:Part3;2014. Only case 4 i.e. tank empty condition is analyzed for comparison. Fig. 12 shows the concrete quantity. It is observed that Fig. 13 Effect of shear key on concrete quantity when uplift considered Percentage saving in concrete quantity is shown in Fig. 14. Fig. 14 Percentage saving in concrete quantity due to shear key when uplift considered iii. Effect of soil parameter around the tank wall: Here comparison is made between non cohesive and cohesive soil outside the tank wall in static case when uplift is considered and analysis is carried out for load 66

5 cases listed in Table II for static load cases only. Concrete quantities are worked out for height 5.5m and 6.5m when non cohesive soil is outside of tank and when cohesive soil outside of tank wall. Fig. 15 shows the concrete quantities for soil parameters around the tank wall. parameters are considered outside of tank wall gives higher section size higher than non cohesive soil is outside the tank so careful soil investigation is necessary for soil around the tank. Uplift is the most dangerous phenomenon to structure which reduces the structural stability. When uplift and earthquake effect has been considered in the study it is observed that, to make the structure safe anchor bars are required. Study shows that when water bodies near tank such as canal makes the tank wall unsafe due to uplift when canal is flowing full. If in future such water bodies are going to incorporate or already present, it is necessary to take effect of such water bodies. REFERENCES Fig.15 Effect of soil parameter around the tank wall Percentage increase in concrete quantity shown in Fig. 16. It is observed that when cohesive soil is around the tank wall it gives higher section for tank wall than non cohesive soil. Fig.16 Percentage increase in concrete quantity due to soil parameter around the tank wall CONCLUSION Form analysis results it is observed that when shear key provided in wall it reduces the section size both in static and earthquake cases. When height of wall increases the percentage saving in concrete quantity due to shear key is also increases and there is considerable more saving in concrete quantity when earthquake effect is taken into account. So it is beneficial to use shear key. When cohesive soil [1] IS Criteria for earthquake resistant Design of Structure: Bridges and retaining walls, Fourth Revision, UDC : [2] IS: 1893:Part 3;2014, Criteria for earthquake resistant design of Structures (Part 3)- Bridges and retaining walls, Doc : CED 39(7739). [3] IS Plain and reinforced cement concrete-code of practice, Fourth revision ICS [4] IS3370:Part1(2009) Concrete structures for storage of liquids-general requirements, First revision, ICS I; [5] B. C. Punmia, A.K. Jain, RCC Designs, Eight edition, Laxmi Publications (p), Ltd., Chapter 18, pp August [6] R. Chandra, Limit state design, First edition, Standard book house, chapter 16, pp , September [7] Saran, S. Analysis and design of substructure, second edition, Oxford and IBH Publications Pvt. Ltd.,Chapter 11, pp , [8] Prakash, S. Analysis of Rigid Retaining Walls During Earthquakes, first international conference on advances in geotechnical engineering and soil dynamics, April26-3May, [9] Choudhury, D., Subba Rao, K. S., and Ghosh, S. Passive earth pressure distribution under seismic condition 15th Engineering Mechanics Conference of ASCE, Columbia University, New York,2002. [10] Choudhury, D., Seismic passive resistance at soil-wall interface, 13th World Conference on Earthquake Engineering Vancouver, B.C., Canada, Paper No. 2746, August 1-6, [11] Zangar Hydrodynamic pressure on dam due to horizontal earthquake effect, Engineering monograph by United states department of Interior Bureau of Reclamation [12] Khassaf Al-Saadi,(2011) Optimum Location and Angle of Inclination of Cut-off to Control Exit Gradient and Uplift Pressure Head under Hydraulic Structures, Jordan Journal of Civil Engineering, Volume 5, No. 3,