Flat Jack Test

Published: May 24, 2019 | Last updated: July 5, 2023

What Does Flat Jack Test Mean?

A flat jack test is used to measure the acting stresses or mechanical parameters of structures. The test is based on the stress release in a small area of a structure by cutting a plane perpendicular to the surface. This method is the oldest practiced method preceding overcoring and hydraulic fracturing methods. During excavation, in-situ stresses can be released causing rock burst, buckling and other ground control problems. This test along with other geotechnical investigation methods helps in the design of safe structures.


Trenchlesspedia Explains Flat Jack Test

Flat jack test method consists of a test apparatus with two steel plates, circular or square in shape that are welded around the periphery. A feeder tube inserted in the middle of the plates allows pressurization of the apparatus with oil or water.

Two pins are fixed into the walls of the excavation and the distance between them is measured. A slot is cut into the excavation between the pins. In case of compressive stress, the movement of the pins will take place together. The prepared flat jack is then placed into this slot.

The flat jack is pressurized until the pin or strain gauge readings return to their original position. At least six flat jack tests need to be conducted in six different directions to get a complete picture of the stress state in a particular area.


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