It's been many years since I used them,but the type was a round disc about 12 inches in diameter,two thin steel plates secured together at the rim. The liquid inside could bleed to a tube extending up out of the fill. Up there was a device to apply pressure to the fluid in the plates. However a valve of sorts in the tube would bleed bleed the pressure from above when the pressure in the discs was reached. That pressure was then read at the surface. It was very sensitive and small changes in soil pressure readily were measured. It measured pressure on a retaining wall some 38 feet high at several locations.. It could detect compactors working 17 feet above it, but next to the wall. From this we then found it necessary to have compactors staying more than 2 feet from the wall to prevent excessive wall pressures.

Edit: Cost was incidental since we wanted something rugged, simple and easy to use.

I installed a couple hundred of these several years ago. Made by Geokon, www.geokon.com/Pressure-Cells.

You would need to contact Geokon to get current pricing. Note that pricing likely varies based on cable length and you will need a readout unit. The type of readout will depend on the type of transducer you use.

Best of luck.

Mike Lambert