Surrounding Rock Excavation Stability Analysis & Study For High Pressure Headrace Tunnel Involving Of Nam Phay Hydropower Station

In order to satisfy multitask of water conservancy and hydropower engineering,it generally selects to excavate kinds of underground hydraulic tunnels to exert functions of flood discharge,power generation&irrigation upon water diversion and reservoir emptying etc.Since the high pressure headrace tunnel,as one of the modes,has sound functional performance,therefore more and more such tunnels have been applied to large water conservancy&hydropower works.But since it involves of higher requirement at aspect of tunnels alignment,performance of rocks,anisotropic load&stress and construction mode,also involves of insufficient assessment about surrounding rocks,therefore it is very necessary to carry out stability study for high pressure headrace tunnel and relevant work.Study about surrounding rocks and support is absolutely critical content.Such study is orienting with high pressure headrace tunnel from Nam Phay Hydropower Station,it carries out analog computation for tunnel sections with different surrounding rocks upon applying self-weight stress field to simulate initial stress field,and carries out analysis to obtain isotropic displacement of surrounding rocks in different construction proposals.Upon finite element method,it analyzes analog computation with simulation for underground structural excavation issue.At the same time it will carry out sensitive analysis about the obtained side pressure coefficient.The Project proposes such technical route to carry out study that it will select computation sections at upper,intermediate and lower horizontal sections respectively,then carries out structural analysis about the initial stress field,subsequently develop surrounding rocks stability&support structure safety analysis involving construction excavation period,check surrounding rocks stability'and support structure safety.Upon comprehensive analysis&computation,it can obtain the following conclusion that.Upon the final analysis and study,as for 400m-embedment depth involving ?2 surrounding rocks,the depth for maximum plastic zone of the surrounding rocks is higher,which possibly causes shotcreting layer damaged by press and causes anchorage rod under yield condition upon tensi:le.Therefore it hereby recommends applying joint shotcreing&anchorage support measures for such tunnel section to ensure stability of the surrounding rocks,but the actual support time is better not too early.Under IV surrounding rock condition,when embedment depth is over 200m,depth for plastic zone will exceed two-time of tunnel excavation diameter,therefore it needs to apply shotcreting&anchorage support measures,furthermore since stress value for shotcreting layer and anchorage rod is higher,therefore it recommends to apply shotcreting with net or steel fiber reinforced concrete so as to ensure the entirety of shotcreting;at the same time since surrounding rocks deformation and depth of plastic zone is generally larger,therefore it needs to strengthen support measures,increase anchorage rod length(adding to 3-6m with recommendation)and support strength(anchorage rod diameter 25mm or over 28mm).