Effect of Joint Opening and Block Protrusion on the Hydraulic Pressure Around Rock Blocks in Unlined Dam Spillways

Abstract

ABSTRACT Rock mass erosion has been a serious safety concern in recent years, especially in the case of dam spillways. Rock block removal by uplift depends on the weight of the block, shearing resistance developed along the sliding faces and hydraulic forces around the block. The effects of several geomechanical and hydraulic parameters on the hydraulic pressures around a block are being studied using a pilot plant scale laboratory spillway model. This study focuses on the effect of joint opening (JO) and block protrusion (BP) on the hydraulic pressures inside the joint and on the top of the block. The blocks are placed in 5 different configurations for testing with three different opening values between the blocks (3, 10 and 20mm) and different block protrusion heights. The test results show that the pressure on the top of the block has a dominating effect in the uplift of the block as the pressure on the bottom of the block remained almost constant with different JO and BP conditions. Test results revealed that block protrusion plays a very important role in the uplift of a block. INTRODUCTION During the flood season, the excessive water stored in the reservoirs shall be released downstream through spillways considering the safety of the dams. These spillways can be broadly classified into two types: parallel flow spillways and plunging jet spillways. In both cases, the flowing water with huge hydraulic energy could cause significant erosion of the rock mass, which may incur unnecessary expenses in repairs and public safety risks (Annandale, 2012; Pells, 2016). In recent years, the damage from the hydraulic erosion has become an important concern in frequently operating spillways. Rock mass erosion is a complex phenomenon that depends on both hydraulic and geomechanical parameters. Rock mass erosion occurs when the resistive capacity of the rock mass is exceeded by the erosive capacity of flowing water (Annandale, 2012; Boumaiza, 2019; Pells, 2016). The erosive capacity is dependent on the flow conditions and the common parameters used to represent the erosive capacity are unit stream power dissipation (Annandale, 2006; Pells, 2016), flow velocity and shear stress (Bollaert, 2010, 2016). The resistive capacity represents the ability of rock mass to withstand the hydraulic forces from the flowing water which is dependent on several geomechanical parameters like joint shear strength, joint opening, joint protrusion, joint orientation, block volume, etc. Boumaiza et al. (2019, 2021) studied the existing rock mass geomechanical indices that represents the rock mass resistive capacity against hydraulic erosion and concluded that they considered improper weightage to some parameters or completely ignore some other relevant parameters like joint opening, 3D rock block volume, while considering unimportant parameter like the unconfined compressive strength of intact rock specimens. A comprehensive description of the erosion prediction models developed in the past few decades is provided by Jalili Kashtiban et al. (2021) most of which are based on empirical relationships. The underlying mechanisms behind the erosion has not been studied to the full extent (Saiang et al., 2022).