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B units (linear digits)

The frequency output from VW sensors is not proportional to the applied change in length of the wire. In order to overcome the problem of a linear conversion, the frequency value can be squared, thereby rendering it linear, but quite large. To reduce its size, it is often divided by 1000 (or multiplied by 10-3). The expression Hz²/1000 (or Hz² x 10-3) is the most commonly adopted as a ‘linear’ digital output, often referred to as B Units.


A material used to replace excavated material.

Bearing pressure

The total stress transferred from the structure to the foundation, then to the soil below the foundation.


Strong rock underlying surface deposits of soil and weathered rock.


Bentonite is a naturally occurring impure clay consisting mostly of montmorillonite found in large quantities in USA and often referred to as Wyoming Bentonite. These Bentonites contain sodium montmorillonites which swell with water whereas those found elsewhere tend to be calcium montmorillonites which do not swell in water. These are therefore activated with sodium to give them swelling properties. Available as powder, granules and pellets they are used widely in Geotechnical instrumentation for sealing boreholes and response zones in standpipe piezometers. When mixed with cement and water it is known as a cement-Bentonite grout.

Borehole extensometers

See extensometers - borehole.

Borros anchor

Typically a three-pronged mechanical anchor lowered into a borehole used to secure the base of a settlement or heave monitoring instrument in soft soils.


Soil particle over 305 mm in size.

Braced excavation

The use of bracing to laterally support the side-walls of temporary trenches or cuts in the form of sheet, soldier, secant and contiguous piles often used in combination with internal support struts and/or tie-backs. The use of instrumentation is used for performance and proof testing of several elements in and around a braced excavation.

Bulk density

Soil density. The total mass of water and soil particles contained in a unit volume of soil.