Several Key Technology Study For Deep-Thick Covering Layer Riverbed Closure

For river closure in deep-thickness covering layer riverbed, in the case of greater discharge, water head and gap flow velocity, due to low anti-scour ability of covering layer, unreasonable protection will cause scour damage and leakage damage, sometimes cause the instability of bottom protection system and embankment collapsing etc., which may threat the safety of construction staff and machine, prolong closure difficulty time. If closure material shortage appears or closure material can not meet the anti-scour requirement, it even causes river closure failure and endangers the project safety. In the aspects of safety and economy, river closure in deep-thickness covering layer riverbed increases the difficulty of river closure and requires higher construction technology. In this article, based on the research of previous period, by researching the key technology problems involved in river closure of deep-thickness covering layer riverbed as closure material stability, gap covering layer stability and the method to reduce river closure difficulty etc., the following research results are acquired:1) After analyzing the application conditions of the classic equations used to compute dumped stone stability, it is found out that the classic equations developed by Isbash and Xiaohuanxiong etc. can be applied only when gap flow velocity distribution is rectangle in its vertical direction during difficult period and the river bed is rough in some extent. Otherwise, the factors as water depth H, velocity distribution coefficientα, relative roughness△/D and circumferential motion coefficientξetc. should be considered comprehensively. By quoting the research results about friction coefficient f and circumferential motion coefficientξin China and abroad, simplifying the basic equation of stability computation, considering the important factors as depth H, velocity distribution coefficientα, relative roughness△/D and circumferential motion coefficientξetc., adopting a series of model experiments, a series of practical equations is developed to compute dumped stone stability, which can be applied to different period of end-dump closure or Transverse dumping closure and different dike head form. The computing results is verified to match the practical situation.2) For the stability of cubic steel reinforcement cage, the experiment research is carried out in the aspect of its weight, shape and bottom protection condition etc. The deduced computation equations consider the shape factor and riverbed roughness factor, can be used to compute stopping flow velocity and threshold velocity. The equations are well verified by model test results. For the stability of tetrahedral steel reinforcement cage, the experiment research is carried out in the aspect of its weight and the bottom protection condition etc. The research results show that the stopping stability of tetrahedral steel reinforcement cage is obviously better than that of cubic steel reinforcement cage with the same weight, there is little difference between their starting stability.3) The related factors affecting the stability of covering layer riverbed is analyzed. The reasons why there is coefficient difference existing in the present starting velocity equation are analyzed and the concerned problems are revealed during computing starting velocity of covering layer. For wide graduation covering layer, the related factors as pebble shape, pebble arrayal and coarsing acting are analyzed. The applicable starting velocity computing equation is put forward.4) The computing method is put forward to compute general scour depth and local scour depth in covering layer riverbed closure. The present research method of local scour computation and the existed problems are analyzed.the coarsing effect on scour depth is analyzed, and the corresponding computation method is put forward.5) The concept of river closure difficulty, its related factors and measuring index are explored and analyzed. It is pointed out that river closure difficulty is only a relative concept, the essential problem is the comprehensive balance of river closure safety, economy and technology. It is pointed out that covering layer increases river closure difficulty in the aspect of safety and economy and requires higher construction technology. It is pointed out that the difficulty indexes of river closure should include construction technology indexes as dumping intensity etc. and economy evaluating indexes.6) The usual construction measures to reduce river closure difficulty are listed and some of them are fit to be used in covering layer riverbed closure. The pointed methods to reduce covering layer riverbed closure difficulty are put forward, the effectiveness problem of bottom protection measures and the collapsing preventing measures in silting riverbed are analyzed with the emphasis on.7) According to the conservation of kinetic energy, by analyzing the flow energy acting on dumped stone and the acting force condition of closure block resisting flow, a equilibrium equation is set up to represent the interacting of flow and dumped stone in the condition of closure material being washed away. According to the conservation of energy, an equation is developed to represent gap flow energy during end-dumping closure. Introducing the relation of embankment advancing velocity and the lost quantity of closure material, and the equation deducing process of closure material lost quantity.