The "Field Compressiometer Test" (FCT), also called "Screw Plate Test", consists in carrying out a vertical plate loading test at a given depth measuring all deformations (settlements) consequent to the applied loads (1).

The loads are applied to an auger shaped plate which is screwed into the soil at the wanted depth.

The test can be carried out at the bottom of a borehole (screwing the plate just to overpass the remoulded tract at the hole bottom) or screwing the plate into the soil directly from the surface. Using one "Pagani’s penetrometer" the second mode appears to be more practical.

The test is usually carried out applying further loading steps, as per standard plate load tests carried out on the surface, and measuring all relevant deformations of the ground underneath.

The test is preferably planned in pervious (granular) soils where drainage rapidly occurs and settlements under each loading step ends in a reasonably short time. In clayey soils the consolidation process generally occurs very slowly, the test can require weeks becoming uneconomical.

Both depth and frequency of the loading tests are stated by the Engineer; generally the minimum interval between tests is 1 m, in any case not smaller than 5 plate diameters (3).

In presence of relatively homogeneous soils, tests can be even several meters distant.

The Engineer must also state:

• the amount of each load step (generally 5 steps for each test are effected);
• any possible loading and unloading cycle.

Insertion of the plate must be carefully carried out in order to minimise any possible disturbance to the soil around.

Experience has proved that acting by a rotating device having the same pitch as the plate can complicate the insertion due to heterogeneity of the soil to be crossed. 

It is recommended to act during rotation applying a moderate starting vertical load, and then follow the helix screwing up by the pull-down of the penetrometer.

After each meter of helix penetration, a section of exagonal casing is added together with an inner loading rod.

Once the test depth is reached, the rotating head is removed, the loading cell is applied at the top of the rods, the dial gauge is placed and all sensors are set to zero.

Generally, the first loading step corresponds to the effective overburden. 

The weight of the inner rods must be considered

Each load is kept till the primary consolidation occurs.

Settlements are read at specific time intervals enabling the issue of a graph settlements versus log of time (or square root of time).

In relatively pervious soils each loading step requires a settling time between 5 and 20 minutes.

At the end the plate is taken out unscrewing the casing by rotation.

The ‘’modulus number’’ of vertical deformation (m), typical of each type of soil and depending on its relatively stiffness, may be evaluated by the graph having each step of load (on the abscissa) and the correspondent final settlements (on the ordinate). 

In determining the "m" modulus number, the effective stress acting at the test depth (4), (5), must be considered.

Then, the "constrained modulus " or "oedometric modulus" (M) may be evaluated provided that the other parameter affecting the value of "M", say the "stress exponent "a", is known..

The "a" exponent for sands can be assumed equal to 0.5; for other soils such an evaluation is more complex and requires the knowledge of studies and researches carried out on the specific matter.

FCT results must be checked with those obtained by other types of geotechnical tests and/or comparing with measured settlements of buildings already built on the same soils.

Several different factors can in fact affect the FCT results; for example:

• the shape of the auger plate
• frictions developing between inside rods and hexagonal casing
• effects due to deformation of the plate under stress
• the influence of the stress status developing on the upper side of the loading plate
• the soil disturbance caused by the insertion of the plate.

• F. Cestari (1991): "Prove Geotecniche in sito" –Ed. Geograph, Segrate (Milano)
• N. Janbu, K. Senneset (1973): "Field Compressiometer – Principles and Applications"- Proceedings of the Eighth Int. Conf. On Soil Mechanics and Foundation Engineering, (Mosca - Russia).
• J. M. Strout (1998): "Deformation and Strain Fields Beneath an Embedded Circular Plate" Proceedings of ISC ’98, (Atlanta –USA).
• J. M. Strout, K Senneset (1998): "International Development of the Field Compressiometer" Proceedings of ISC ’98, (Atlanta –USA).
• J. M. Strout, K Senneset (1998): "The Field Compressiometer Test in Norway", Proceedings of ISC ’98, (Atlanta –USA).