皂市水利枢纽工程坝基岩体利用研究

Study on dam foundation rock mass utilization in Zaoshi hydro-complex project

皂市大坝坝基岩体为石英砂岩,岩石抗压强度高,变形模量低,具有“硬、脆、碎”的岩体特征,在未扰动条件下,岩体强度较高,一经扰动,岩体完整性大幅下降,具有开挖松弛的特性。按已有规范中的评价体系,该类岩体大部分不满足作为大坝建基岩体的质量要求;但若严格按已有规范确定建基岩体,则开挖深度很大,且结合类似工程的经验教训,会面临随开挖随松弛,进而演变为挖不胜挖的两难局面。因此,需要建立一套针对该类岩体合理的评价方法,以有效利用该类岩体作为大坝建基岩体。提出从岩组岩性、风化特征、结构类型、完整程度、物理力学性质和透水性等方面综合评价岩体基本质量,研究岩体工程特性、力学性质、岩体质量在垂直方向上的差异,进而提出岩体质量及建基岩体选择的评价标准。同时在确定建基面高程时,不以岩体完整性和声波波速为准,而且将岩体裂隙的风化变异程度和夹泥充填情况作为主要指标来确定建基面。以该标准确定的建基面相较已有规范方法确定的建基面抬高了10~15 m,减少了开挖工程量和混凝土填筑量、节约了工期。该方案得到工程实践检验,成果具有一定的工程参考价值。

The rock mass of Zaoshi dam foundation is quartz sandstone with high compressive strength and low deformation modulus, showing characteristics of hard, brittle and broken.Under undisturbed conditions, the rock mass strength is high, however once disturbed, the integrity of the rock mass decreases greatly, showing obvious excavation relaxation features.According to the evaluation system in the existing specifications, most of this type of rock mass does not meet the quality requirements of the rock mass used as the foundation of a dam.However, if the foundation rock mass is determined in strict accordance with the existing specifications, the excavation depth will be very deep.According to the experience and lessons of similar projects, we will face a dilemma of rock relaxation increasing with excavation process.Therefore, it is necessary to establish a set of reasonable evaluation methods for this kind of rock mass, so as to effectively use it as the dam foundation rock mass.It is proposed to comprehensively evaluate the basic quality of rock mass from the aspects of rock formation lithology, weathering characteristics, structural type, integrity, physical and mechanical properties, and water permeability, and to study the difference of rock mass engineering characteristics, mechanical properties and rock mass quality in the vertical direction.Based on these results, we propose an evaluation criterion for rock mass quality to determine a reasonable foundation elevation.By this criterion, the elevation of foundation surface would not be determined based on the integrity and acoustic wave velocity of rock mass, but determined by the weathering variation degree and mud filling condition of rock fissures.As a result, the foundation surface was raised by 10 m to 15 m compared with the existing standard method, and the excavation quantity and concrete filling quantity were reduced, the construction period was saved.The proposed method has been verified by engineering practice, and the achievements have certain engineering reference value.