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Jet Grouting and Deep Soil Mixing

Jet grouting involves the erosion of the soil by cement grout, jets of water, and/or compressed air, and the mixture of the grout with the soil to form grouted columns or walls. The grout pipe with jets may be self-drilled or inserted into a predrilled hole. The jetting fluids are pumped at high pressure while the pipe is withdrawn with rotation for columns or without rotation for walls.

Jet grouting’s ability to construct soilcrete in limited spaces and around subsurface obstacles such as utilities, provides  unique design flexibility. In any situation requiring control of groundwater or excavation of unstable soil (water-bearing or otherwise) jet grouting is often a preferred solution.


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نمایی از ستون های جت گروتینگ اجرا شده

Constructed Jetgrouting columns view

Jet Grouting

The procedure for jet grouting operation typically includes the following steps:

1. Position the drilling machine at a desired location for injection.
2. Drill into the ground to the desired depth. The tolerance of the drilling location should be less than 50 mm.
3. Insert the jet grouting pipe to the predrilled grout hole. Some machine installs the jet grouting pipe directly with an auger at the tip.
4. Once the pipe reaches the desired depth, start the jet grouting process by withdrawing the pipe and injecting grout until reaching a desired elevation. The operation should follow the specifications for injection pressure, injection rate, injection time, pipe withdrawal rate, and pipe rotation rate.
5. Clean up the injection pipe and other associated tools.
6. Move to the next jet grouting location.

During Jet grouting procedure,  high pressure flow of cement grout erodes soil body and makes a soil-cement combined column that can improve mechanical properties of soil.

جت آب خروجی از نازل دستگاه حفاری

Water jet and drilling equipment

Selection of Jet System

The single jet system injects neat cement grout through a small nozzle at high pressure, which is mixed with in-situ soil. This method produces the most homogeneous soil–cement columns or walls with the highest strength and the least amount of grout spoil return. It is the simplest system among the three systems and has more choices of qualified specialty contractors.

The double jet system injects neat cement grout at a lower pressure, which is aided by a cone of compressed air. The air reduces the friction loss and allows the grout to travel a farther distance to produce a greater column diameter. However, the presence of the air reduces the strength of the column and produces more spoil return than the single jet.

The triple system injects water at high pressure, which is aided by a cone of compressed air. This process produces an air lifting effect, which erodes the soil. The grout is injected at a lower pressure through a separate nozzle below the water and air nozzles to fill the void created by the air lifting process.

Different systems of jet grouting

سه مکانیزم اندرکنشی در جت گروتینگ

Different interaction mechanisms of soil and jet flow

روش های مختلف تزریق پرفشار

Different systems of jet grouting

Design principles

Jet grouting columns are used to increase bearing capacity, control settlement, mitigate liquefaction potential and reduce soil permeability. In order to reach to aforementioned goals jet grouting columns can be designed like piles. For instance toe and skin friction resistance of jet grouting columns should be considered in design procedure and their capacity will be determined based on their geotechnical  and structural capacity.

Structural capacity is determined using unconfined compression strength of soilcrete samples. Unconfined compresion strength of soilcrete can be obtained from 28-day samples taken from sacrificial columns.

According to following figure, Unconfined compression strength of soilcrete samples is dependent to consumed amount of cement.  

Soil TypeSingle fluidDouble fluidTriple fluid
Sandy soils10-307.5-1510-20
Clayey soils1.5-101.5-51.5-7.5

Unconfined compression strength of improved soil with jet grouting method MPa (Kauschinger et al., 1989)

رابطه نسبت سیمان مصرفی و مقاومت فشاری ستون تزریق پرفشار

Unconfined compression strength of jet grouting columns vs. consumed cement

Application:

Jet grouting is a widely used method of soil improvement and can be acceptable in various soil conditions. Jet grouting has been used for the following applications:

  • Densification of granular soils
  • Raising settled structures
  • Settlement control
  • Underpinning of existing foundations
  • Excavation support
  • Protection of existing structures during tunneling
  • Liquefaction mitigation
  • Water control
محدوده کاربرد روش جت گروت در مقایسه با سایر روش های بهسازی بستر

Application of  jet grouting method comparing to other methods

Quality control and assurance:

The general procedure for quality control and assurance are similar to that for deep mixing. Quality control for grouting may include the following steps:

  • Evaluate the grouting record including the grout volume, injection pressure, rate of injection, time of injection.
  • Inspect the integrity and uniformity of the grouted column or wall.
  • Verify the dimensions of the grouted column or wall.
  • Evaluate the strength of the grouted column.

Quality assurance includes the following field testing:

  • Core samples of grouted columns or walls to verify the strength and stiffness of the stabilized soil
  • SPT and CPT
  • Single column and composite foundation loading tests
  • Cross-hole geophysical testing

Advantages and Limitations

Jet grouting has the following advantages as compared with alternate technologies:

  • No need for removal and replacement
  • Effective for underpinning and protecting existing structures
  • Easy to access and operate within constrained space

The limitations associated with grouting are:

  • Quantity of grout is hard to estimate.
  • Effectiveness of some applications cannot be predicted.
  • Area of improvement is sometimes uncertain.
  • Grouting may cause ground movement and distresses to existing structures.
  • Certain chemical grouts may contain toxicity and have adverse impact to groundwater and underground environment.
  • Specialty contractors are required for the operation.
ترکیب شمع های اجرا شده جت گروتی و انکرهای کششی

Combination of jet grouting columns and ground anchors

Deep soil mixing

The deep mixing (DM) method mixes in-situ soil with a hardening agent (cement, lime, slag, or other binders) at depths by augers. Deep mixing can be accomplished by a wet or dry method. The equipment for the wet method may have one to eight rotary hollow shafts with cutting tools and mixing blades above the tip. The binder slurry is introduced into the ground through each hollow shaft and exits from the nozzle while the shaft penetrates into the soil or is withdrawn. Some equipment has mixing blades rotating in opposite directions (i.e., double mixing) to improve the uniformity of the soil–binder mixture. The equipment for the dry method may have single or dual rotary shafts with cutting tools and mixing blades above the tip. The binder powder is introduced into the ground through each hollow shaft and the nozzle by air pressure.

ستون های اجرا شده توسط روش اختلاط عمقی خاک

Deep soil mixing columns

Design principles 

The design procedure for deep mixing is similar to jet grouting columns design and also depends on the type of application. For foundation support, the following design procedure may be followed:

  1. Based on the geotechnical conditions, select the type of method (dry or wet method) and type of binder for deep mixing.
  2. Based on the geometry of the superstructure and the distribution and magnitude of loads, select the pattern of deep mixing (individual columns, blocks, walls, and grids).
  3. Based on the required ultimate bearing capacity of the foundation and the soil strength, determine the required column capacity if the area replacement ratio is assumed or the area replacement ratio if the column strength is assumed.
  4. Based on the column capacity requirement, determine the required minimum laboratory unconfined compressive strength of the stabilized soil considering the field to laboratory conversion factor and the scale factor.
  5. Determine the minimum column length.
  6. Determine the settlement of the column-reinforced foundation using the stress reduction method, the piled-raft method, or the column penetration method.
  7. Determine the degree of consolidation of the column-reinforced foundation.
  8. If columns are used to support embankments, stability analysis should be conducted. A numerical method is preferred, but a simplified limit equilibrium method may be used by limiting the column strength.
  9. Iterations may be needed if at least one of the calculations does not meet the project requirement.
فلوچارت طراحی و ساخت ستون‏ های اختلاط عمیق در پروژه‏ ها

Design and construction flowchart of Deep soil mixing method

Construction procedure

The basic DM procedure by the wet method is illustrated in Figure. This method includes:

  1. Position the auger at a desired location.
  2. Drill and drive the mixing shaft at a preset rate into the ground with rotation of the blades. There are two injection sequences of binder slurry: (a) binder slurry injected during the penetration and (b) binder slurry injected during the withdrawal. The injection outlet for the penetration injection method is located at the toe of the mixing blades while that for the withdrawal injection method is located at the top of the mixing blades.
  3. After reaching the desired depth, remain at the position and continue mixing binder slurry with soil for a certain time period to generate a uniform mix.
  4. Withdraw the mixing shaft gradually at a preset rate and continue mixing the binder slurry with soil (for some machines, the direction of the mixing blade rotation is reversed during the withdrawal).
  5. Complete the installation of the mixed column until the mixing shaft reaches the desired elevation (mostly at the ground surface but sometimes at a preset depth). The spoil soil should be excavated and removed during or after the installation of each column to minimize the interference with the next installation
مراحل اجرایی روش اختلاط عمیق خاک

Construction stages of deep soil mixing

Applications

Columns have been used for many applications in soft soils:

  1. support of superstructures, including buildings, walls, embankments
  2. waterfront and marine applications including quay walls, wharf structures, and breakwaters
  3. stabilization of slopes
  4. lateral support
  5. containment of water and pollutant
  6. liquefaction mitigation
  7. vibration reduction.

Deep mixed columns have also been used for roadway widening to support new embankments and mitigation of ground heave due to expansive soil. In these applications, DM columns are used to increase bearing capacity, reduce settlement, enhance slope stability, provide lateral support, contain water and pollutant movement, mitigate liquefaction, and reduce vibration.

محدوده کاربرد روش های مختلف بهسازی بستر

Application of deep soil mixing 

Quality Control and Assurance

The typical quality control (QC) and quality assurance (QA) procedure for deep mixing is presented in Figure. It starts with the design or targeted strength of a stabilized soil. Laboratory mix design tests should be performed to determine the binder content to reach the targeted strength. Sometimes, field trial is implemented to verify the field strengths of test columns. During the installation, all the parameters for deep mixing should be well controlled, including binder content. After the installation, the quality of columns should be confirmed by sampling, coring, and in-situ testing. Field instrumentation and monitoring may be conducted for large and/or complicated projects. Quality control and quality assurance can be achieved by evaluating the installation records of the columns and the test results of laboratory and field verification tests. Each column installation record should include a chart-log with the construction information, such as date and time of execution, length of column, penetration/withdrawal rates of the mixing shaft, blade rotation rate, pressure and flow rate of pumped slurry or injected powder, and total slurry or powder consumption per column.

نمونه ای از کنترل هندسی ستون های اختلاط عمیق

Geometry control of deep soil mixing columns

روند کنترل و تضمین کیفیت

 Quality control and assurance procedure

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