A borehole stability study by newly designed laboratory tests on thick-walled hollow cylinders
- Authors: Hashemi, Sam , Melkoumian, Nouné , Taheri, Abbas
- Date: 2015
- Type: Text , Journal article
- Relation: Journal of Rock Mechanics and Geotechnical Engineering Vol. 7, no. 5 (2015), p. 519-531
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- Description: At several mineral exploration drilling sites in Australia, weakly consolidated formations mainly consist of sand particles that are poorly bonded by cementing agents such as clay, iron oxide cement or calcite. These formations are being encountered when drilling boreholes to the depth of up to 200 m. To study the behaviour of these materials, thick-walled hollow cylinder (TWHC) and solid cylindrical synthetic specimens were designed and prepared by adding Portland cement and water to sand grains. The effects of different parameters such as water and cement contents, grain size distribution and mixture curing time on the characteristics of the samples were studied to identify the mixture closely resembling the formation at the drilling site. The Hoek triaxial cell was modified to allow the visual monitoring of grain debonding and borehole breakout processes during the laboratory tests. The results showed the significance of real-time visual monitoring in determining the initiation of the borehole breakout. The size-scale effect study on TWHC specimens revealed that with the increasing borehole size, the ductility of the specimen decreases, however, the axial and lateral stiffnesses of the TWHC specimen remain unchanged. Under different confining pressures the lateral strain at the initiation point of borehole breakout is considerably lower in a larger size borehole (20 mm) compared to that in a smaller one (10 mm). Also, it was observed that the level of peak strength increment in TWHC specimens decreases with the increasing confining pressure. © 2015 Institute of Rock and Soil Mechanics, Chinese Academy of Sciences.
A Failure criterion for unsupported boreholes in poorly cemented sands
- Authors: Hashemi, Sam
- Date: 2016
- Type: Text , Journal article
- Relation: International Journal of Civil, Environmental, Structural, Construction and Architectural Engineering. Vol. 3, no. 11 (2016), p. 1707-1712
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- Description: The breakage of bonding between sand particles and their dislodgement from the borehole wall are the main factors resulting in a borehole failure in poorly cemented granular formations. The grain debonding usually precedes the borehole failure and it can be considered as sign that the onset of the borehole collapse ia imminent. Detecting the bonding breakage point and introducing an appropriate failure criterion will play an important role in borehole stability analysis. To study the influence of different factors on the initiation of sand bonding breakage at the borehole wall, a series of laboratory tests was designed and conducted on poorly cemented sand samples.