Improvement by Addition of Material

Dynamic replacement method uses the similar method and equipment like dynamic compaction method. However, the difference in this method is material (crushed stone) is added to the ground that is to be treated to deliver the dynamic compaction energy to deeper layers and back filling the pits which are typically 2.0 m in diameter with the stones during dynamic compaction.

Among the soil improvement methods, dynamic replacement method is applied for reducing the total and different settlements and for improving the mechanic and engineering properties of the soils or uncontrolled fills or to increase the bearing capacity in greater depths.

It is possible to support uniformly distributed loads, by means of a compacted transition layer and the arching effect within such a layer, that is constructed on stone column pillars.

Carrefoursa Bursa dynamic replacement and dynamic compaction application

Stone columns have the following effects on the treated soil:

  • introduction of porous elements with good engineering properties on a regular grid,
  • increase in the modulus of deformation of the whole treated mass,
  • increase in the average angle of internal friction and in overall shear strength,
  • increase in the coefficient of earth pressure at rest (K0),
  • significant increase in the rate of consolidation, with most of the settlement occurring within a short time after construction.

There are many design methods for stone column reinforcements. Most of them are only valid for loads spread over a large area.

Stone columns applications may be executed by several different methods, mainly depending on the ground conditions. These methods are;

  • The stone columns to be formed by wet top feed method with the similar equipment used in vibro-flotation or vibro-compaction method in clayed and silty soils,
  • The stone columns to be formed by dry bottom feed method with the similar equipment used in vibro-flotation or vibro-compaction method,
  • In appropriate soil conditions, driving a casing pipe and filling the hole in the ground by stones and and compacting stones after placing by vibration of casing during the withdrawal of the casing,

Stone columns can be implemented both off-shore or on land typically with 60 cm to 100 m diameters.

"WET-TOP FEED" METHOD

In Wet-top feed method vibrator is lowered down to the desired depth with the help of the water jet, and in the meantime vibrator creates a space around itself.

This space is filled by stones with appropriate gradation fed from the surface of the soil.

Wet-top feed stone column method

Wet-top feed stone column application

"DRY-BOTTOM FEED" METHOD

In dry bottom feed method vibrator is lowered down to the desired depth by the help of the air jet.

Crushed stone with appropriate gradation is placed at the bottom of the drilling area with the help of the pipe that is placed near the vibrator for forming the stone column.

In this method the stone column diameter may vary depending on the density of the surrounding soil. While the vibrator is pulled upwards by applying vibration, the stone feeding is continued from the pipe next to the vibrator and thus stone column is formed up to the surface.

Dry-bottom feed stone column method

Dry-bottom feed stone column application

In this method, a special drilling tool is used with the injection nozzles and cutting blades. The external / cutting diameter is as the diameter of the target column. The drilling tool is rotated with proper torque for drilling, through the rotary of the drilling rig, simultaneously cement / water mixture (grout) is fed through the nozzles and blades and the soil is mixed in situ. During the process of drilling and cutting the soil is grouted through nozzles. The grouting may be continued during the withdrawal to upwards or only mixing can be processed. In this method, the mechanical mixing is done by cutting blades, therefore providing an uninterrupted flow of grout is sufficent.

In this way, grout and a mixture of uniform dosage and a controlled diameter and volume mixture of the ground is obtained to achieve the improved soil-cement mixture (soilcrete) DSM columns. This method gives better results is in many ways of diameter control and in terms of uniformity and can be applied at every soil condition where mechanical drilling / cutting is possible. Obtained DSM column's strength can be adjusted to the requirements of the design by regulating the amount of mixing and the cement dosage.

DSM Method

DSM Application

In this method, the soil initially is drilled to the desired depth with conventional drilling machines with special attachment or top hammers in a diameter of typically Ø90 mm. Fluids used during the drilling are water, air, bentonite or cement grout, if necessary. Second stage is jetting with 400 bar ~ 600 bar pressure through 1 to 4 nozzles of 1.5 mm to 4.0 mm in diameter which are located at the bottom of the drilling rods which is vertical to the monitor. The grout is injected at a very high speed of 250 m / sec through nozzles and thus tears and mixes the surrounding soil with its great kinetic energy, meanwhile drilling rod is rotated and pulled upwards with a fixed speed. As a result, this operation provides a completely mixed and improved soil crete jet grout columns.

Improved jetgrout column diameters can be obtained by varying the parameters like grout speed and angle, rotation and the pullout speed, the number and diameter of nozzles. Jet grouting method gives possibility to improve various types of soils from cohesive soils like clay to granular soils like sand, gravel.

Some of the grout material comes up during jetting from the space between the rods and the ground. This indicates that there is no pressure inside the soil that is improved and thus, the desired diameter jetgrout columns are obtained. Jet grouting applications are done by three different systems. 1. mono jet is jetting with cement grout where ground cutting and filling is done by grout mixture , 2. double jet is jetting where mono jet is supported by pressurized air, 3. triple jet is jetting where water is used additionally to all those.

In jet 2 method, drill and high-pressure grouting system is composed of concentric two separate drill rods and cement mixture is given through the inner pipe with 300 ~ 600 bar pressure, while 8 ~ 12 bar pressurized air is pumped to the soil between inner and outer pipes. Pressurized air reduces the losses of air friction and allows an increase of 60% ~ 80 in the diameters of the columns. Jet 3 method has a system of 3 similar concentric drill rods where drilling uses air and water, and grouting uses cement grout. While applying 400 ~ 600 bar pressure water from the innermost tube, 8 ~ 12 bar compressed air is given through the middle of the pipe and 30 ~ 80 bar pressure grout injection is given through the outer pipe.

Enlarged diameters of Jet Grout column can be achieved by the help of air and water pressure which can penetrate deeper into the soil, compared to Jet 1 method. For this reason, jetgrout columns up to 3.00 m diameter can be obtained by Jet 2 and Jet 3 methods, while jetgrout columns up to 1.00 m diameter is obtained by Jet 1 method. The diameter in clayey soils can be 60-70 cm by Jet 1method. For the columns obtained by Jet 2 and Jet 3 systems, a slight decrease in jetgrout column strength might be mentioned, due to the excess water and air entering in the grouted material of soil and cement mixture.

Jet grouting is a solution to various foundation engineering problems.

Jet grouting equipment consists of a jetgrouting compatible drilling rig, pump unit, mixer unit and cement silo. Jet grouting drilling rig is a hydraulic and crawler rig capable of drilling by one of the methods mentioned above, up to the estimated depth within the design requirements in single pass, and obtaining jet grout column without the need for adding rods until the desired depth.

Rods are high-pressure-resistant with special alloys and pads in the connection parts are able to withstand a pressure of 600 bar. Cement taken from the cement silo is mixed with water in mixer tank and anticipated dosage of injection grout is prepared and this is transferred to pump unit. The grout transferred from high pressure pump unit to drilling rig is transmitted to the soil under high pressure, through the drilling machine and the nozzles at the bottom of the rods that are connected to the rig.

Jetgrout Method

 
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Zetas Zemin Teknolojisi A.S.

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