The technology of jet-grouting appeared almost simultaneously in three countries - Japan, Italy, England. The engineering idea has proved so fruitful that over the past decade, jet-grouting has instantly spread around the world, allowing not only to more effectively solve traditional problems, but also to find new solutions to many other complex problems in the field of underground construction.
The essence of the technology is to use the energy of a high-pressure jet of cement mortar to destroy and simultaneously mix the soil with the cement mortar in the "mix-in-place" mode (mixing in place).
After hardening of the solution, a new material is formed - ground concrete, which has high strength and deformation characteristics.
In comparison with traditional technologies of injection soil fixing jet-grouting allows to strengthen almost the entire range of soils - from gravel to fine clay and silt.
Another important advantage of the technology is the extremely high predictability of soil strengthening results. This makes it possible already at the stage of design and conclusion of contracts to accurately calculate the geometric and strength characteristics of the created underground structure.
From the extremely extensive list of practical applications of the technology:
Strengthening of weak soils (for example, in the construction of tunnels and sewers);
Protection of pits in watered soils;
Device of anti-filtration curtains;
Strengthening of foundations in the reconstruction and the superstructure of buildings;
Soil strengthening at the base of slab foundations;
To improve the stability of slopes and slopes;
Filling of karst cavities in fractured rocky soils.
The advantages of jet grouting technology
High speed of construction of soil-cement piles;
The ability to work in cramped conditions – in a wet basement, or near existing buildings, on slopes, etc. In this case, the object is established only small-sized drilling rig, and the whole of the injection complex is located at a more convenient off-site;
The absence of a negative impact on the foundations of closely spaced buildings, in contrast to the clogging of concrete piles, the device of soil-cement piles is performed by rotary drilling;
The small diameter of the leader well allows, for example, when strengthening foundations to perform piles with a diameter of 500-1500 mm through a hole with a diameter of 112 mm.
THE ESSENCE OF TECHNOLOGY
The device of ground-cement piles (Jet piles) is performed in two stages – in the process of forward and reverse motion of the drill string.
During the direct stroke, drilling of the leader well to the design level is performed. During the return stroke, cement mortar is fed into the nozzles of the monitor located at the lower end of the drill string under high pressure and the column is lifted with its simultaneous rotation.
EQUIPMENT FOR JET-GROUTING:
High-pressure pump >400 MPa
Mixing station with capacity >20 m3 / h
Silos for storage of cement with the screw
For the technological version of Jet2, a compressor is additionally required, and for the technological version of Jet3 – a compressor and a second pump for pumping cement.
There are three main types of technology:
Single-component technology (Jet1)
In this case, the destruction of the soil produced by a jet of cement mortar. The discharge pressure of the solution is 400-500 ATM. Jet1 technology is the easiest to implement, requires a minimum set of equipment, but the diameter of the resulting piles is also the smallest compared to other technology options. For example, in clay soils the diameter of soil cement piles does not exceed 600 mm, in sandy soils the diameter of piles is 700-800 mm.
Two-component technology (Jet2)
In this variant, compressed air energy is used to increase the length of the water-cement jet. For separate supply of cement mortar and compressed air to the monitor, double hollow rods are used. Cement mortar is fed through the inner bars, and compressed air is supplied through the outer bars. The diameter of the piles obtained by this technology in clays reaches 1200 mm, and in Sands-1500 mm.
Three-component technology (Jet3)
This option differs from the previous ones in that the water-air jet is used exclusively for the erosion of the soil and the formation of cavities in it, which are subsequently filled with cement mortar. The advantage of this option is to obtain columns of pure cement mortar. The disadvantages include the complexity of the technological scheme, which requires the use of triple rods, as well as additional technological equipment. With the right selection of technological parameters can be obtained piles with a diameter of 2500 mm.
The compressive strength of the soil cement in sandy soils is on average 5-10 MPa, in clay 2-4 MPa. In some cases, it is possible to achieve higher strengths, for this purpose, an increased consumption of cement is used and jet cementation is performed until the complete replacement of the soil.
The set of technological equipment necessary for the production of jet cementation of soils using the technological variant Jet1, includes:
Cementing high-pressure pump;
Slot for storage of cement.
For the preparation of cement mortar in large quantities, a mixing station with a capacity of 10-20 m3/hour is required.
The drilling rig used in jet cementation technology must be equipped with an additional device for automated lifting of the drill string at a given speed.