Constructon Technology Research Of Relaxation Deformation Prevention For Columnar Jointed Basalt Dam Foundation Of Left Bank Baihetan Hydropower Station

Baihetan Hydropower Plant is the second cascade developed-power plant in the main stream of Jinsha River's downstream,its barrage is of concrete dome dam,with dam length of 289 m,which is the third highest one in the world.Below elevation 660 m of the left bank dam foundation of Baihetan dome dam,it is columnar jointing basalt which is a unique rock,its structural surface is properly developed and has small spacing,so,its integrity is poor,the rock itself is relaxed and easily deformed,its relaxation depth and degree of relaxation may further be worsen as time goes go and blasting impact.So,measure will be taken in construction to prevent the relaxation of columnar jointing basalt dam foundation.This Paper summarizes a series of construction techniques based on the real construction of Baihetan Hydropower Plant;the following contents are thereby studied:As for the non-columnar jointing basalt dam foundation,in accordance with the result of basting test and in combination with the geological prediction and rock power obtained during excavation and drilling,the excavation blasting parameter is finally determined;the controlled blasting can obtain an expected result.Based on the large quantity of field tests and surveys done early indicate that columnar jointing basalt is obviously relaxed,so,blasting may seriously impact its relaxation,meanwhile,ground stress may also affect thickness of the relaxed formation.Left bank dam foundation of 660 m – 650 elevation is pre-anchoring experimental zone of excavation,the reserved rock protection layer is 2m,three zones are divided for anchoring test,including non-anchored,mortar bolt anchored and prestressed bolt anchored;based on the feedback analysis of the monitored data,ordinary mortar bolt is determined for pre-anchoring;experimental zone is selected in the similar rock area of the front edge of left bank dam foundation for smooth blasting and contrast test of presplitting blasting,presplitting blasting is selected as the blasting method of columnar jointing basalt excavation,then undertake adjustment test of blasting parameter of the presplitting blasting,the blasting parameter of 5m rock protection layer is determined accordingly,the relaxation of rock blasting can be controlled about 1.0m by strict control of blasting;furthermore,contrast test is done between the mechanical excavation of milling driver and impact hammer,plant efficiency and impact depth of excavation are also studied;consolidation grouting for the reserved rock protection layer,consolidation grouting method and inspection & acceptance standard for the geological condition of columnar jointing basalt,hole sealing of consolidation grouting and the embedded ordinary mortar bolt for pre-anchoring.By means of above said pre-anchoring test,excavation test,mechanical excavation test and consolidation grouting test,the reserved 5m rock protection layer is finally obtained,the controlled blasting is used outside the rock protection layer,pre-anchoring bolt is constructed by consolidation grout hole,excavation is done as per the controlled blasting excavation,the blasting damage of the prestrseed anchor cable(the foundation face of dome dam)will be commenced quickly after completion of excavation.Application of this construction technique has achieved the expected result based on the actual construction of the left bank dam foundation of Baihetan Dome Dam.