COLLAR

COLLAR-SLEEVE COMIBINATION AS A SCOUR PROTECTION DEVICE AROUND A CIRCULAR PIER

C.P. Singh Baldev Setia D.V.S. Verma

Assistant Director Asstt. Professor Professor

Haryana Irrigation Research Civil Engg. Dept. Civil Engg. Dept.

and Management Institute Regional Engg. College Regional Engg. College

Kurukshetra (India) Kurukshetra (India) Kurukshetra (India)

Address For Correspondence :

Baldev Setia

Asstt. Professor

Civil Engineering Department

Regional Engineering College

Kurukshetra (India) – 136 119

Tel. : 01744 – 24350

Fax : 01744 – 20065

E-mail : setia_b@rediffmail.com

Abstract: The primary cause of local scour around bridge piers is horse-shoe vortex system. The growth of vortex can be arrested by retaining the vortex on rigid surface as a collar plate or by confining it within an enclosure like a sleeve skirting around the pier. A collar plate equal to twice the diameter of the pier when placed at the bed is able to reduce scour by 68% while a sleeve equal to 1.5 times the diameter of the pier reduces scour by 38% in comparison to that of an unprotected pier. The two devices in a suitable combination are able to do away with scour altogether for uniform as well as unsteady hydrographic-like flow. The two devices being axisymmetric, are efficient even for large change in the angle of attack of flow. However, in the eventuality of lowering of the general bed level its efficiency decreases.

Keywords : local scour, horse-shoe vortex, collar plate, sleeve.

1 INTRODUCTION

Foundation of a bridge pier in an erodible river bed is quite expensive as it has to be taken deep enough to provide the minimum anchorage length for safety of the foundation. For example, the recently constructed Ganga Bridge at Varanasi, has a well foundation 70m deep, with inside and outside diameters of the well as 8m and 13m, respectively (Thakkar, 1992; Ref. Setia, 1997). Hence some attempts have to be made to reduce scour by providing scour protection devices linked to cost safety benefit criterion. Garde and Ranga Raju (1993), in their article on ‘Research Needs in Fluvial Hydraulics,’ advocated the necessity of evolving the best scour protection device based on cost and effective performance.

Several scour protection devices have been tested by various researchers in laboratory. These includes piles (Tison, 1952; Setia, 1977), slot (Chiew, 1992; Setia, 1997), Rip-rap (Engel, 1893; Rao, 1998), Collar plate (Tanaka and Yano, 1967; Ettema, 1980; Vitall et al. 1994; Setia, 1997 and Kumar, 1999) caisson with lip (Shen and Schneider, 1970; Rao, 1998), a delta wing like passive device (Gupta and Gangadharaiah, 1993; Setia, 1997) (Ref. Singh C.P., 2000).

The present paper is the result of an experimental study carried out on a collar plate, a sleeve and combination of the two devices to test their efficacy as scour reduction modes. Sleeve is an axisymmetric, open ended enclosure encircling the pier. Its objective is to confine the horse-shoe vortex inside it and release it downstream of the pier. A definition sketch showing details of the protection devices along with their parameters has been shown in Figure 1.

2 EXPERIMENTAL SETUP

Experiments were conducted in hydraulic laboratory of the Civil Engineering Department of Regional Engineering College, Kurukshetra. A 12 m long, 0.6 wide and 0.75 m deep flume containing fine sediment (d₅₀= 0.285 mm; s_g = 2.51) was used for the study.

Preliminary investigations were carried out on a rigid bed with the help of wet paint technique (Setia, 1997) to visualise the flow phenomenon on the bed. Mobile bed studies were carried out for a uniform flow velocity equal to incipient velocity of the sediment. Duration of the test run was maintained as 300 minutes. The best results were validated for live bed conditions and also for unsteady flow in the form of a single peaked hydrograph.

Model piers of diameters 25 mm and 62 mm were used for studies on rigid bed and mobile bed, respectively. Commercially available PVC pipes were suitably cut to form the sleeves while collar plates were cut out of a perspex sheet. Scheme of experiments has been presented in Table 1. Performance of a scour protection device was given by performance potential (Setia, 1997), defined as (1-H_sm/H_smo) x 100

For wet pain impression technique studies for rigid bed experiments were conducted on (a) Pier Model Diameter D = 25mm (unprotected), (b) Caisson D_cn = 2D; Z_cn = D and (c) Collar plate diameter D_c = 2D; Z_c =D. Results are shown in Figure 2 from which it is observed that a thin collar plate results in a smaller size of separation zone impression as compared to Caisson.

3 RESULTS AND DISCUSSION

(A) Collar plate

Figure 3 shows the effect of various sizes of collar plates on maximum scour depth. As the size of the collar plate increases from D_c = 1.5D to D_c = 2.5D, the maximum scour depth decreases from 0.55D to nearly zero because of the larger shielding effect of the bigger plate. A comparison of the results with studies by Setia (1997) and Kumar (1999) shows a similar trend (Figure 4).

Efficacy of the collar plate is also a function of its vertical location. Figure 5 presents the effect of vertical location of a collar plate D_c = 2D on maximum scour depth. It is easy to observe from the figure that as the gap between the ambient bed level and the collar plate increases, the maximum scour depth also goes on increasing. For a plate elevation Z_c= 0.5D, the performance potential is only 25%. The best location, however is at 0.1D below the average bed level where a collar plate of twice of diameter of the pier gives a performance potential of 91%. A comparison of the results of present study with that of Setia (1997) shows the similar trends (Figure 6).

(B) Sleeve

Three sleeves of sizes D_s = 1.5D, 2.0D and 2.5D were investigated around a pier. Figure 7 presents the results of effect of various sizes of sleeves when placed with their tops flush with the bed level. It was observed that for a sleeve encircling the pier there were two distinct locations of deep scour – one inside the sleeve (H_{sm, in}) and the other outside the sleeve (H_{sm, out}). It is interesting to note from the figure that with decrease in the size of sleeve for D_s = 2.5D to D_s = 1.5D, inside scour, H_sm, _in decreases from 1.05D to 0.68 D. However reverse trend was observed for maximum outside scour (H_{sm, out}) during the test run duration of 300 minutes.

Lesser maximum inside scour for D_s = 1.5D may be attributed to its confining ability of the horse-shoe vortex inside the sleeve. The results have been compared with that of Singh et al. (1995) in Figure 8. Since, the sleeve of size D_s = 2.5D was not able to confine the vortex inside it, it resulted in larger inside scour. Also because of its larger size, it was associated with larger outside scour. Hence not tested further.

Figure 9 presents the effect of vertical location of sleeves D_s = 2.0D and D_s = 1.5D and also their comparison with each other. For D_s= 2D, it is observed that inside scour gradually decreases from 0.98D to 0.1D for the two extreme location of –1D to +1D. However the outside scour has different trend altogether. When the sleeve location is above bed the outside scour grows considerably. At the location of 1D above bed it has a very high scour value of 1.24D. Which is more than the scour of an unprotected pier. The point of intersection between the curves of inside and outside scours balances when the sleeve is 0.2D above the bed thus giving a scour depth of 0.75D. Similar trends have been observed for a sleeve of size D_s = 1.5D.

A comparison of the study conducted for sleeves of sizes D_s = 2.0D and D_s = 1.5D shows that the values of inside scour for smaller size sleeve of D_s = 1.5D are less than that of sleeve D_s = 2.0D for all vertical location. Also, for vertical locations above bed at +1.0D, outside scour is less i.e. 0.89D for sleeve of D_s = 1.5D as compared to 1.24D for sleeve D_s = 2.0D. Thirdly, balancing point of inside and outside scour of 0.70D at vertical location of –0.05D is lower for sleeve D_s = 1.5D as compared to 0.75D at vertical location of +0.2D. Hence, smaller size of sleeve of D_s = 1.5D performed better than the sleeve of size D_s = 2.0D in all respect.

(C) Collar plate and sleeve combination

To enhance the performance potential and reliability of the device, it was decided to use collar plates and sleeve in combination. The various combination patterns, that were investigated are shown in Fig. 10. It was observed that a combination of sleeve D_s = 1.5D with collar plate of D_c = 2.0D and sleeve sealed at D/4 below top of sleeve Figure 10(c) performed excellent. The device gives no inside and outside scour under uniform flow condition as well as under unsteady flow conditions in the form of a hydrograph (Figure 11).

4 CONCLUSIONS

(1) Collar Plate

① Efficacy of collar plate to prevent scour is a function of its size and its vertical location. As the size of collar plate increases the scour decreases. Collar plates of sizes D_s = 1.5D, 2D and 2.5D reduce scour by 50%, 68% and 100% respectively of unprotected pier.

② Collar plate of size D_c = 2.0D when placed at 0.1D below bed gives a maximum efficacy of 91%. However, when the plate is located at 0.5D above bed reduces scour by 25% only.

(2) Sleeve

① Efficacy of the sleeve is also a function of its size and its vertical location. As size of the sleeve decreases the inside and outside scour decreases showing reverse trend as that of a collar plate where scour decreases with increase in the size of collar plate.

② Sleeve of size D_s = 1.5D performs better than sleeve of size D_s = 2D in all respect. Sleeve of size D_s = 1.5D when placed its top flushed with bed gives less inside scour of 0.68D as compared to 0.86D for sleeve of D_s = 2.0D. It gives less outside scour of 0.89D as compared to 1.24D for size D_s= 2.0D when placed at vertical location of +1D. Also sleeve of size D_s= 1.5D performs better giving less balancing scour of 0.7D at its vertical location of –0.1D as compared to balancing scour of 0.75D for sleeve of D_s = 2.0D placed at 0.2D above bed.

(3) Collar Plate And Sleeve Combination

① Combination of a collar plate D_c = 2.0D and sleeve D_s = 1.5D sealed at D/4 from top of sleeve reduce scour altogether when tested under uniform flow and unsteady flow conditions.

② For live bed conditions, the maximum scour depth if 0.37D which is equal to half of the size of the ripple height, that formed during the test run.

From the study, it has been observed that collar plate and sleeve individually and in combination are potential devices. Being axisymmetric these are not affected by the angle of attack of flow. However, in an eventually of lowering of the bed level, the efficacy of the devices is adversely affected.

List of symbols

Symbol Description

s_g Geometric standard deviation

B Width of flume

D, D_c, D_cn, D_s Diameters of cylindrical pier, collate plate, caisson, sleeve, respectively

d₅₀ size for which 50% of the sediments is finer

H_smMaximum scour depth below average bed level

H_sm,in, H_sm-out Maximum scour depth inside and outside the sleeve, respectively

H_sm Maximum scour depth around an unprotected pier

h Depth of flow

Q Discharge (m³/s)

q Discharge per meter width of flume (m³/s/m)

Z_c, Z_s Height of collar plate and sleeve respectively w. r. t. average bed level

References

Chiew, Y.M. (1982), “Local Socur at Bridge Piers.” University of Aukland, Department of Civil Engineering, Report No. 290, 79 pp.

Ettema, R. (1980), “Scour at Bridge Piers.” University of Aukland, School of Engineering, Report No. 216.

Gangadharaiah, T., Setia, Baldev and Muzzammil, M. (2000) “Aid to Visualization,” National Conference on “Recent Trends in Experimental Mechanis,” Indian Institute of Technology, Kanpur, India.

Garde, R.J., Ranga Raju, K.G. and Kothyari, U.C. (1989), “Research Report on Effect of Unsteadiness and Stratification on Local Scour.” CBIP Sponsored Project, Civil Engg. Department, University of Roorkee, Roorkee, India.

Setia, Baldev (1997), “Scour Around Bridge Piers : Mechanism and Protection.” Ph.D. Thesis, Department of civil Engineering, Indian Institute of Technology, Kanpur, India.

Singh, C.P., Setia, B. and Verma, D.V.S. (2000), “Effect of Collar Sleeve Combination on Scour around a Circular Pier,” M.Tech. Thesis, Department of Civil Engineering, Regional Engineering College, submitted to Kurukshetra. University. Kurukshetra.