Abstract

Frictional shear strengths of cement sealing in rock salt have been experimentally determined by series of borehole push-out testing and direct shear testing. The results are used to assist in design of the cement seals in the rock salt to minimize brine circulation and potential leakage along a main access of salt mine. The salt specimens are prepared from 100 mm diameter cores drilled from Middle member of the MahaSarakham formation. The cement seal is prepared from commercial grade Portland-pozzolan cement, saturated brine, anti-form agent and liquid additive. The cement slurry is casted in the 25 mm diameter borehole with a length of 30 mm for the push-out testing and on the 100 mm diameter fracture saw cut surface for the direct shear testing. For all tests the cement is cured for 7 days prior to testing. The results indicate that dynamic viscosity of grout slurry is 4.53 Pa.s. When the curing time increases the intrinsic permeability of cement grout decreases. The uniaxial compressive and Brazilian tensile strengths after 28 day curing times are 20.06±3.82 MPa and 2.89±0.19 MPa, respectively. The direct shear tests results indicate the frictional resistance at cement-salt interface with a friction angle of 44 degrees and a cohesion of 2.12 MPa. The normal stiffness is 7.67 GPa/m. The shear stiffness is 6.60 GPa/m. The push-out test results show significantly the higher frictional resistance at the interface than does the direct shear testing. The axial shear strength of the borehole cement seal is 5.05 MPa. The findings are useful for determination of initial installation parameters of the cement seals in salt mine openings.

1. Introduction

Design of seals may also be required in sealing mine openings in order to control mine effluents. Generally, cementing materials are used to seal fractures and dissolved channels to prevent brine leakage along the openings. The primary function of borehole and shaft seals is to reduce hydraulic conductivity of the openings to an acceptable level. Axial load on plugs or seals in an underground salt mine leads to shear stresses along a contact between the plug and a host rock. These shear stresses may cause cracking and increased permeability along the cement plug and rock interfaces. Under extreme condition they can cause slipping of the cement plugs. Therefore, a bond between plug and rock is a critical element of the design and performance of plug in the excavated openings (Fuenkajorn and Daemen, 1996; Akgun and Daemen, 1997; Tepnarong, 2012).

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