A.3.3 Wet mixing

A.3.3.1 General

Wet mixing is carried out in accordance with some general principles, which can be summarised as shown in Figure A.6.

In wet mixing the binder is usually cement slurry. Filler (sand and additives) may be added to the slurry when necessary. The specific quantity of slurry added can vary with depth. For machines with the outlet below the mixing tool the slurry need not be added during the retrieval phase.

¹⁾ Blade rotation number means the total number of mixing blades passing during 1 m of shaft movement and is defined by the equation T = ΣM(N_d/V_d + N_u/V_u), where T = blade rotation number (n/m), ΣM = total number of mixing blades, N_d = rotational speed of the blades during penetration (rev./min), V_d = mixing blade penetration velocity (m/min), N_u = rotational speed of the blades during retrieval, and V_u = mixing blade velocity during retrieval. If injection is made only during retrieval, then N_d is set equal to zero.

Whereas flight augers may be sufficient for predominantly granular soils, increasing fineness and stiffness requires more complicated mixing tools provided with mixing and cutting blades of different shapes and arrangements The rotary drives, turning the shaft, need to have enough power to destroy the matrix of the soil for intimate mixture with the slurry.

Depending on the type of soil and slurry, a mortar-like mixture is created which hardens during the hydration process. Strength and permeability depend strongly on the composition and characteristics of the soil (fines content, organic content, type of clay, shape of the grains, grain size distribution, grain hardness), the amount and type of binder and the mixing procedure.

The wet mixing process can be interrupted on condition that the slurry has not begun to harden and the mixing tool starts again at least 0,5 m in the soil already treated.

Pumps for transport of the slurry to the outlet need to have sufficient capacity (delivery rate and pressure) to safely deliver the design quantity of slurry.

Wet mixing is common in Central and Southern Europe, North America and Japan.

A.3.3.2 European technique

In Europe the installation of wet-mixed columns is either carried out by means of flight auger(s) (continuous or sectional, single or multiple) or by means of blades, depending on ground conditions and applications.

In reinforced soil wall structures, steel bars, steel cages or steel beams can be installed into the fresh mixed-in-place columns or elements. The aid of a vibrator may be required for the installation process.

A.3.3.3 Japanese technique

In Japan, the wet mixing technique has been used frequently for both on-land constructions and marine constructions [5]. In on-land construction, machines with one, two or four mixing shafts have been used. The mixing tool is composed of several stacks of mixing blades to achieve uniformity of the treated column. A steel bar fixes the distance between the two mixing shafts. The bar and sometimes additional freely rotating (undriven or counteracting) mixing blades function to prevent rotation of soil adhering to the driven mixing blades and shaft.

Blade rotation number and amount of binder are automatically controlled to achieve uniformity of the treated column. The machine has several mixing blades with a diameter of 1,0 m to 1,6 m and a capacity of installing columns to a maximum depth of 48 m. The shaft has several mixing blades at different levels.

In marine constructions, large execution vessels are usually used for rapid treatment of considerable soil volumes, Figure A.7. On the vessel, a mixing machine, a batching plant, storage tanks and a control room are installed. The machines for marine works usually have more than two mixing shafts. The deep-mixing machines currently available in Japan are capable of constructing columns with a cross-sectional area of 1,5 m² to 6,9 m² to a maximum depth from the sea water level of up to 70 m.

Key

Typical mixing conditions are shown in Tables A.3 and A.4.