It should be noted that the term soft implies that the soil is very wet or saturated; the consolidation failures, in fact, occur in deposits with a high water content, such as those found near lakes, oceans or in beds of pre-existing lakes and old courses where the water has changed direction and towards but the aquifer water table remained high.
Limi with high values d Ip and wL are called plastic silts incolved in Aircore Drilling. These silts show approximately the same characteristics as the soft clays. The plastic characteristics derive from contamination of the soil mass with clay minerals or organic material. Inorganic and silt slime with low clay content can be present in a loose form but the behavior is more similar to that of the sands while the design and thickening procedures are similar.
In nature there are few deposits of silt puro. Most deposits contain particles of clay (with resulting plasticity/cohesion) or a certain amount of fine to medium sand. In this regard, it should be noted that even a small amount of clay (~ 5%) can provide cohesion to the silt; a rate of 10 to 25% of clay particles can cause the deposit to be considered as clay that can be handled by Aircore Drilling.
For all these types of terrain it is necessary to carry out the most balanced estimate of the permissible load-bearing capacity to control the breakage with a suitable safety coefficient and estimate the probable consolidation failures.
The bearing capacity is most often determined by adopting the undrained shear strength obtained from quality cylindrical samples or from samples obtained from an ordinary standard penetrometric test. If the soil has high sensitivity (resistance to shear of the remodeled soil ~ 0.2 of that of undisturbed soil), tests of in situ resistance must be taken into consideration, such as the scissometric test or the static penetrometric test before Aircore Drilling.
Preconsolidated clays have fractures due to shrinkage and joints (fractured in a quantity of small blocks). The presence of structural defects sometimes makes it difficult to determine the resistance to unconfined compression. In many cases, especially over-warm (S << 100), the resistance determined through occasionally intact samples obtained by SPT or with the use of a pocket penetrometer provide adequate indications on resistance.
For best estimates it is possible to resort to tests with load plate as it is very difficult to obtain cylindrical samples of sufficient quality for carrying out triaxial tests; the cell pressure within these tests, in fact, tends to close the slits in such a way as to provide only an approximation of the shear strength in situ.