A modified NX-borehole jack with flexible loading platens

Kai Zhang, Rongbin Hou, Ranjith Pathegama Gamage

Research output: Contribution to journalArticleResearchpeer-review

1 Citation (Scopus)


The NX borehole jack test is the most widely used method to determine the deformation modulus of rock mass in the field. The common borehole jack, also known as the Goodman jack, has two drawbacks. The first is that the bending of the platens induces extra values in the displacement gauges (LVDTs), which causes the calculated modulus to be smaller than the actual figure. The second drawback is the demand for a perfect rock/platen contact.A new modified borehole jack has been designed in the present research, which overcomes the drawbacks of the Goodman jack. The new jack incorporates three improvements: (1) LVDTs located in the middle of the jack, (2) flexible platens for full rock/platen contact, (3) a pressure sensor installed at the end of the jack for accurate loading pressure measurement. A detailed theoretical analysis was carried out for the new jack, and experiments on blocks of concrete and sandstone were performed to verify the viability of the conception. The preliminary results suggest that the modified jack has a good theoretical basis, and may lower test requirements for borehole preparation.The process of borehole jack testing was simulated under plane strain conditions. The results indicate that the rock/platen contact condition of the Goodman jack varies with the modulus of rock and the loading pressure, while the contact state of the modified jack is insensitive to the modulus of the rock. The precisions of the jacks were also analyzed. The results show that the precision of the jack is acceptable when the loading pressure is over 30 MPa; however, when the modulus of rock is estimated to be over 65 GPa, the precisions of the Goodman and modified jack are both quite low.

Original languageEnglish
Pages (from-to)152-162
Number of pages11
JournalEngineering Geology
Publication statusPublished - 15 Jul 2016


  • Flexible loading platen
  • Goodman jack
  • In-situ test
  • Modulus of deformation
  • NX borehole jack

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