PROTECTION AGAINST THE SCOUR DOWNSTREAM OF A SOLID APRON USING CONCRETE MATTRESSES

Abstract

This study explores the suitability and design approach of concrete mattress (CM) of a cabled and anchored system under subcritical turbulent flow conditions (STFC) against the scour downstream of a solid apron (SDSA). The concept of CM and its development is discussed with hydraulic considerations. The results are based on the Experimentation Works (EW) carried out on various models at Hydraulic Laboratory (HL), Indian Institute of Technology Roorkee, (India). Models made of a rich-concrete and acrylic sheet were employed confirming to the Froude Criterion for an undistorted model. The entire EW was organized: i. To study the scour profiles downstream of a solid apron for different flow conditions without any protection, ii. To compare those scour profiles with the scour profiles downstream of the apron for flow conditions with concrete mattresses of different sizes and weights under the same flow conditions, and iii. To test the stability of concrete mattresses under those flow environments. The CMs of 0.010 m thickness were found stable and working satisfactorily in STFC by shifting the Point of Maximum Scour (PMS) in flow-direction and reducing the Depth of Maximum Scour (DMS) considerably downstream of the apron. CM-2-390 was deformed following Disordered-Sinusoidal-Curvature (DSC) at lower discharges; that signifies neither too short nor too long launching length is beneficial for the specific discharge characteristics against SDSA. The optimum Gap Opening (GO) in the plan-form of a CM has ranged from 12.49 % to 12.93 % for the models to articulate the system and release the pore-water pressure. It was observed that once the blocks at the corner of the frontal row of the mattress were lifted, enough water would flow under the mattress that curled back the concrete mattress to roll onto it and slipped down the test section, which further aggravates the situation resulting into Enhanced-Curling-Effect (ECE) and ultimately the failure of the CM system. Therefore, the stability analysis of corner blocks and middle blocks of the IX frontal row is quite essential, principally, from lift considerations. The suitability of the mattresses is suggested as per Table 1. Table 1.0: Suitability of Mattresses Models Range of Suitability for 􀝍􀯠(􀝉􀬷⁄􀝏⁄􀝉) Adopt suitable factor of safety for prototype uses. CM-2-390 0.06565 < q􀭫 ≤ 0.08333 CM-2-335 0.04633 < q􀭫 ≤ 0.06565 CM-2-280 q􀭫 < 0.04633 Higher safety factor (SF = 3.248) was obtained against the target factor of safety (SFT = 3.12) as per the National Concrete Masonry Association (NCMA) method considering the most critical case of the stability of a block on the sloped bank. SF at bed is much near to SF minimum as per the Cox method (Colorado State University Factor of Safety Method) for an anchored system under very rough flow conditions. The SF about bed is satisfactory, though; there were limitations on the part of laboratory tests. EW has established the stability of the block numerically; which is consistent with the stability observed in the video clippings taken during the EW. The safety factor corresponds to the thickness of 0.21 m by Pilarcyzk’s equation for an anchored system under very rough flow conditions; equivalent to a model thickness of 0.0084 m which is much near to the adopted model thickness of 0.010 m. The CMs protected the channel beds downstream of the apron where Froude Number stands below 0.88. Additional research is essential to study the directionality effect of its shape in elevations and plan-form, its stability under the hydraulic jump, and drainage filter layer suitable for mobile bed characteristics