INFLUENCE OF COHESION ON LOCAL SCOUR

Abstract

Realistic estimation of maximum scour depth around bridge piers is required for safe and economic design of bridge piers and their foundations. Likewise, information on local scour due to ajet is also required in the design ofmany engineering structures like spillways, grade control structures etc. Various studies have been carried out in the past by different investigators on local scour in cohesionless sediments. However, in actual field conditions, the river bed material can be cohesive due to the presence ofclay. The present investigation was taken up to study the influence of cohesion on the phenomenon of local scour around bridge piers and due to submerged vertical circular jets. Although several investigations are available on the problem of scour around bridge piers (e.g. Raudkivi and Ettema, Melville, Kothyari et al. etc.), no study is available on the influence of cohesion on characteristics of scour around bridge piers. Likewise, many investigators, e.g. Clarke, Sarma and Sivasankar, Westrich and Kobus, Rajaratnam, Raudkivi and Breusers, Aderibigbe and Rajaratnam etc. have studied the scour due to a submerged circular jet of water in a cohesionless sediment. Limited number of studies related to cohesive sediments are available. Moore and Masch, Hanson, Hanson and Robinson etc. studied the variation in rate of scour due to submerged vertical circular jet in cohesive sediments. Kuti and Yen studied experimentally scouring of a cohesive bed downstream of a spillway. Abt and Ruff studied the culvert scour in cohesive bed materials. Stein et al. developed a relationship for equilibrium scour in cohesive and noncohesive sediments under a two - dimensional jet. The present study was taken up in the above background. An extensive set of experiments was conducted on scour around bridge piers and that under jets. Experiments were also conducted on the incipient motion condition for cohesive sediments. For (i) experiments on incipient motion conditions and on scour around bridge piers, a fixed bed masonry flume having a length of 30 m, width of 1.0m and a depth of 0.60 m was used. A working section having a length 4.0 m was prepared in which cohesive sediments were placed for experimentation. For pier scour, initial runs were taken with cohesionless sediments having uniformsize of 0.27 mm Next, clay material was mixed with sand, varying the clay proportion from 5% to 60% by weight and the same was placed in the test section under different conditions of antecedent moisture content and dry density. The clay material used herein had the following characteristics : Median size, d50 = 0.0053 mm, Kaoline =10%, Illite = 74.5%, Montmorillonite = 15.5%, Liquid limit = 53%, Plasticity index = 26%, cohesion, Cu (at Optimum Moisture Content, O.M.C.) = 65.66 KN/m2, Angle of internal friction, <j>c (at O.M.C.) = 14° and Group ofclay = CH. For experiments on pier scour in cohesive sediments, only onepierhaving diameter of 0.1125 m was used. Both maximum scour depth and temporal variation of scour depth were observed. Experiments on jet scour were carried out in a circular constant head tank having diameter of 1.25 m and depth of 1.25 m filled with the desired sediment upto a height of 0.65m. The nozzle was centrally located in the circular tank and was always submerged below the water. Experiments were first carried out on jet scour using uniform sand of size 0.27 mmwith varying nozzle diameter, jet velocity and jet height. Next, cohesive bed was prepared by mixing clay having above-stated characteristics with this sand varying the clay proportion from 10% to 60% with different conditions of antecedent moisture content and dry density. Twojet velocities of 1.7 m/s and 2.0 m/s were used with jet heights of 0.15 m and 0.20 m respectively. Nozzle diameter of 12.5 mm was used. Both maximum scour depth and temporal variation of scour depth were observed. In addition to this, the data existing in literature on the above aspects are also compiled for use in the present study. (ii) INCIPIENT MOTION OF COHESIVE SEDIMENTS Analysis of all the data indicated the following relation to hold good for critical shear stress of cohesive sediments t,cc =o.ooi (i+p/y — i0-(°m«+2-3) wit" r^ =r^/(Ay,.rf.) Here t,cc is the critical shear stress for cohesive sediments, A/, is the difference in the specific weights ofthe sediment and fluid and is equal to y,-yf,da is the arithmetic mean size of^kcohesive sediments, P/is the plasticity index, Wis the antecedent moisture content, W* is the moisture content at saturation (W* is equal to Liquid Limit for plastic sediments) and e is the void ratio. SCOUR AROUND BRIDGE PEERS The experiments reveal that the geometry, location and extent of scour hole in cohesive sediments are very much different from that in cohesionless sediments. The process of temporal variation of scour around bridge piers in cohesive sediments is modelled by suitably modifying the mathematical foimulation for cohesionless sediments developed by Kothyari et al. The horseshoe vortex is considered to be the prime agent causing scour at bridge piers. Based on experimental evidence, the shear stress under the horseshoe vortex at the nose ofthe pier before scour begins is taken as 4rH while it is taken as 10rB at the pier sides. Herer„ is the shear stress in the approach flow. Maximum scour would occur at the (iii) upstream nose of the pier for 4r„> zcc, while maximum scour occurs at the sides of the pier when 10rl(>rC(,>4ri( and no scour occurs when rw>10r1(. The processes of scour occurring at pier nose and at pier sides are separately modelled herein. As the scour hole develops the cross-sectional area of the scouring vortex is considered to increase as At ~ A0 + A ,v where A„ is the initial area of cross - section of piincipal vortex at pier nose or side and A,, is the cross - sectional area ofthe scour hole at the respective locations. The shear stress at the pier nose and sides respectively is assumed to vary with time as per the ratio between A„ and A,. For given shear stress, the time, t*c required for single particle to get scorned is estimated by using the equation C,-da POJcU*j with u*t = Jr^, Ipf or Jr~7pJ depending upon the location of the deepest scom" depth. Here, r , and r^, are the shear stresses at the pier nose and at the pier sides at time t respectively, pf is the mass density of fluid, p0,,c is the average probabihty of movement of cohesive sediments particles and C3 is a coefficient. The average probabihty of movement of cohesive sediment particles is considered to be related to the shear stress as below /V = 0.0027 ( \3'45 llL\ ;I^<5.56 VT„ J Tcc r, = 1.0 : -**- > 5.56 Here x t is the shear stress at the pier at any location at time t. (iv) The analysis of data indicated that C3 = 8.0 for scour at pier nose and C3 = 0.006 for scour at pier sides. The above scheme was used for estimation oftemporal variation of scour around bridge piers in cohesive sediments and the same is validated by using the data collected during present investigation. The non-dimensional parameters affecting the maximum scour depth are also identified based upon theoretical considerations. Based on these the following relationships for maximum scour depth were derived : When PI = 0 0.35 '& \0,J when PI > 0 6.02-10.82 W_ + 5.31 fw]2 0.2 Here, d.mic and d.m, are the maximum scour depths in cohesive and cohesionless sediments respectively, C* is equal to P .C ————; Pc being clay content in fraction and C„ is the value of cohesion of clay determined at O.M.C. and ^* is equal to P,.tan^+(1.0-Pc)tan^ tan^, •J, being the angle of internal fiiction for clay at O.M.C. and </)s the angle of repose of pure sand. (v) JET SCOUR A modified form ofthe relationship developed by Aderibigbe and Rajaratnam is found to satisfactorily predict the maximum scour depth under submerged vertical circular jets in case of cohesionless sediments. For cohesive sediments, the geometry and the depth of scour are found to change significantly due to influence ofcohesion. The following relationships are found to hold good for scour in cohesive sediments under a submerged vertical circular jet. ForP/ = 0 ^- =0.38|— and Vm» =0.2l'C 0.15 V. .(j>J For PI > 0 and V v_ 1.5 = L11 W_ Wj -0.37 W_ w_ WJ 0.11 v.- 2.0 ^fyy. <r, Here, VsmC and Vma are the volumes of scour hole in cohesive and cohesionless sediments respectively, yd is the dry unit weight of cohesive sediments and ym in the unit weight of water. (