EGL Ref: 9049 23 June 2022 Page 4 File: WAI-985-000-REP-LC-0050-Rev0.docx This report shall only be read in its entirety. While this assessment is for ground settlement effects that are caused by ground dewatering, this report is in no way to be taken as an assessment of effects on groundwater. 2.0 GROUND SETTLEMENT OVERVIEW AT WAIHI 2.1. Ground settlement mechanism Ground settlement effects at Waihi are predominately caused by dewatering within the ground, which arises because of lowering or local depressurisation of the groundwater around the mine open pits and underground workings and interconnected geological vein or fault structures. Lowering or depressurisation of the groundwater occurs within the defects and pores of the soil and the rock, resulting in a transfer of stress from the groundwater to the solid soil and rock material causing it to compress and settle. The amount of settlement due to dewatering within the ground depends on the following: 1. The change in the compressive stresses in the ground from the pore water to the soil and rock material, which depends on the difference between the initial and final groundwater pressures; 2. The compressibility of the ground which is a natural property, and which varies depending on the type of soil and rock, and whether the ground has felt the same stress before; and 3. The thicknesses of the different types of soil and rock that underlie a site which undergo a change in pressure. The rate of settlement depends on the permeability of the ground. Permeability is a measure of how easily water flows through a material. The presence of shafts and drives and other discontinuities in the ground increases the draining rate of water from the ground and hence increases the rate of settlement. Settlement of the ground without notable tilt (i.e. settlement as a uniform surface) does not cause damage to structures founded on the surface. Tilt, which is differential settlement of the ground, can cause damage to structures if it occurs over short distances. This can affect the amenity of structures and in areas of very severe tilt potentially impact the structural integrity of brittle structures. 2.2. Geology and hydrogeology interpretation The understanding of geology and hydrogeology is fundamental to understanding the observed settlement effects, and to allow estimates of future effects from extended and new projects. Different geological units beneath the ground surface present different settlement potential. The younger geological units made up of volcanic airfall, alluvial and lacustrine (lake) type deposits closer to the surface are more prone to ground settlement effects (both settlement and tilt) upon dewatering due to their shallower depth, higher permeability, and higher compressibility. The older andesite geological
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