Stiff clay or silt

Stiff clay or silt can be dredged by backhoe dredgers, trailer hopper suction dredgers or cutter suction dredgers. The dredging technique has a significant influence on the resulting properties of the soil in the reclamation area. 

When dredging stiff clay/silt with a cutter suction dredger, the clay/silt will initially be cut into pieces by the cutter head. The material will then be loaded into barges or hydraulically transported via pipelines to the discharge area. During transport in the discharge pipeline, the clay/silt lumps will erode into clay balls. The eroded particles will mix with the process water and form a low density slurry. 

The resulting geotechnical properties will strongly depend on the volume of clay balls in relation to the volume of slurry. This relationship is very difficult to predict. If the stiff clay is dredged in combination with more sandy layers, the clay balls can be encapsulated in a sand matrix. 

The research of hydraulically transported clay balls dates back to 1949 when Casa-grande studied the hydraulic reclamation of 30 million m of clay balls for the construction of the Boston airport (Casagrande, 1949). 

The results were re-evaluated by Carrier Bromwell in 1984. Their conclusion was that when the initial voids between the clay balls are large, most of the settlement is probably caused by a rearrangement of the slippery clay balls into a denser matrix (Carrier Bromwell, 1984). 

---

Stiff clay or silt (pumped) Very low plasticity 7,<20 Significant disintegration, occasionally clay ball 9.1.4... 

---