Different Types of Earth Pressure Cells, Their Applications and Construction of Isobar Diagram Using Them-16

2.11 Installation of Jack-Out Pressure Cell in Slurry Trenches

The Jack-Out Pressure first needs to be assembled into the jack-out frame. The assembly is shown in Figure 2.11. The support plate has a circular hole cut in it and bolt holes to fit the Jack-Out Pressure Cell (JOPC), and is connected to one end of a double-acting hydraulic jack by means of steel struts. The support plate and reaction plate are cambered top and bottom to prevent them from snagging on the sides of the slurry trench. The reaction plate is attached to the other side of the double-acting hydraulic jack. The jack is attached firmly to the rebar cable and arranged so that the plates are free to move outwards. The hydraulic line and signal cable are tied off to one of the rebars at intervals of 1 meter (3 feet).When the rebar cage has been lowered to its proper depth, the jack is activated, forcing the two plates out against the trench walls.

Observation of the pressure indicated by the JOPC will indicate when the cell has made contact with the wall. The pressure in the jack should be increased to a level of approximately 100 KPa (≈10 psi) greater than the projected pressure of the grout at the depth of the jack. This ensures that the grout pressure will not close the jack and allow the reaction plates

Figure 2.11 - Jack-Out Pressure Cell Installation

to move away from the trench walls. The jack pressure should be checked from time to time because the pressure might bleed away if the walls of the trench are soft and yielding. Repressurise can be done as needed and the jack is left. Pressure is provided until the grout has set up.

2.12 Installation of Cells to Measure Earth Pressure at the Base of Footings, Floor Slabs, Pavements, Etc.

Experience has shown that attempts to measure contact earth pressures on the base of footings, floor slabs, pavements, etc., frequently meets with failure. The problem is twofold: First, the contact stress distribution can be inherently variable due to varying properties of the ground and varying degrees of compaction of the ground. Thus the contact stress at one location may not be typical of the surrounding locations. Secondly, a cell installed could result in the creation of an anomalous zone immediately around the cell where there may be a different, more fine grained material, under a lesser degree of compaction. (The material around the cell may be poorly compacted because of the need to avoid damage to the cell.)