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SPECIFICATIONS
DEEP SOIL MIXING
(CUTOFF WALL, RETAINING WALL, IN-SITU TREATMENT, IN-SITU FIXATION) |
Note: (This technical specification is to be used to guide the writer in the contract requirements for Deep Soil Mixing construction for a specific site. Included are _______________ to be filled in with project specific data. Also included are [ ], which denote options to be considered for specific design requirements. Optional subsections are denoted by *. Parenthetic remarks ( ) are included when appropriate to provide the writer with additional, nonessential information. Most [ ] and * are used for designs that go beyond the standard design and may include the addition of proprietary additives or the injection of air to extract vapor phase organics. All [ ],( ), and * should be filled in or omitted from the writer's specification.)
TABLE OF CONTENTS
PART 1 GENERAL
1.1 DEFINITIONS
1.2 SCOPE OF WORK
1.3 QUALIFICATIONS OF CONTRACTOR
1.4 PROPRIETARY CHEMICALS
PART 2 PRODUCTS
2.1 DSM (CUTOFF WALL, RETAINING WALL, IN-SITU TREATMENT, IN-SITU FIXATION)
2.2 MATERIALS
2.2.1 (Grout, Slurry)
2.2.2 Bentonite
2.2.3 Additives
2.2.4 Cement
2.2.5 Reinforcing Steel
2.3 EQUPMENT
2.3.1 Batching Equipment
2.3.2 Deep Soil Mixing Equipment
PART 3 EXECUTION
3.1 HORIZONTAL ALIGNMENT
3.2 VERTICAL ALIGNMENT
3.3 MIXING SHAFT SPEED
3.4 PENETRATION RATE
3.5 (GROUT, SLURRY) INTAKE
3.6 KEY
3.7 OBSTRUCTIONS/MIXING SHAFT REFUSAL
3.8 QUALITY CONTROL AND QUALITY ASSURANCE
3.8.1 Sampling
3.8.2 Testing
3.8.3 (Grout. Slurry)
3.8.4 Documentation
PART 1 GENERAL
1.1 DEFINITIONS
DEEP SOIL MIXING (DSM) is a soil improvement technology used to construct cutoff or retaining walls and to treat soils, in-situ. This is accomplished with a series of overlapping stabilized soil columns (typically 36-inch diameter). The stabilized soil columns are formed by a series of mixing shafts (2 to 4), guided by a crane-supported set of leads. As the mixing shafts are advanced into the soil, (grout, slurry) is pumped through the hollow stem of the shaft and injected into the soil at the tip. The auger flights and mixing blades on the shafts blend the soil with (grout, slurry) in pugmill fashion. The mixing shafts are positioned to overlap one another and form a continuously mixed overlapping column. When the design depth is reached, the augers are withdrawn and the mixing process is repeated on the way to the surface. Left behind are stabilized DSM columns having the following (property/properties): (low permeability, improved bearing capacity, or shear strength, immobilized contaminants that when reinforced, are able to withstand differential soil and hydrostatic loading).
SLURRY is a stable colloidal mixture of water and bentonite and/or attapulgite clay. Other additives may be used as approved by the Engineer to enhance the behavior of the slurry.
GROUT is a stable colloidal mixture of water and Portland cement. Additional materials such as bentonite clay, attapulgite clay, or fly ash may be added.
1.2 SCOPE OF WORK
This section of the specifications includes requirements for the DSM (cutoff wall, retaining wall, in-situ treatment, in-situ fixation) and related work as indicated on the Drawings and as hereinafter specified. The work consists of furnishing all plant, labor, equipment and materials to perform all operations as required to construct the DSM (cutoff wall, retaining wall, in-situ treatment, in-situ fixation).
1.3 QUALIFICATIONS OF CONTRACTOR
The Contractor shall submit evidence that he is experienced and competent to construct a DSM (cutoff wall, retaining wall, in-situ treatment, in-situ fixation). This evidence will include the successful completion of at least three similar projects. This evidence will ensure that the Contractor will have sufficient competent experienced personnel to carry out the operations specified.
1.4 PROPRIETARY CHEMICALS
Proprietary chemicals shall be approved by the Engineer based on initial bench scale testing.
PART 2 PRODUCTS
2.1 DSM (CUTOFF WALL, RETAINING WALL, IN-SITU TREATMENT, IN-SITU FIXATION)
A DSM (cutoff wall, retaining wait, in-situ treatrnent, in-situ fixation) shall be constructed to the lines, grades and cross sections as indicated on the Drawings. The DSM shall have essentially vertical columns, a minimum width of 24 inches, and shall extend through the overburden and to a minimum specified depth (of ___ feet into the aquiclude).The completed DSM (cutoff wall, retaining wall, in-situ treatment, in-situ fixation) shall be a homogeneous mixture of (grout, slurry) and the in-situ soils. Mixing is to be controlled by mixing shaft speed, penetration rate, and (grout, slurry) intake. A generalized description of the overburden through which the DSM (cutoff wall, retaining wall, in-situ treatment, in-situ fixation) is to be excavated is indicated by boring logs included in the Drawings.
2.2 MATERIALS
2.2.1 (Grout, Slurry)
The material added to the soil will be water-based (grout, slurry). The purpose of the (grout, slurry) is to assist in loosening the soil for penetration and mixing (and to lower permeability, aid in structural support, treat in-situ soils, fixate in-situ contaminants). The (grout, slurry) will be premixed in batch plants which automatically combines materials in predetermined proportions.
2.2.2 Bentonite
Bentonite used in preparing slurry shall be pulverized (powder or granular) premium-grade sodium-cation montmorillonite and shall meet the most current API Standard 13A "API Specifications for Oil-Well Drilling Fluid Materials". Fresh water, free of excessive amounts of deleterious substances that could adversely affect the properties of the (grout, slurry) shall be used to manufacturer (grout, slurry). It is the responsibility of the Contractor that the (grout, slurry) resulting from the water shall always meet the standard of this Specification.
2.2.3 Additives
Admixtures of softening agents, dispersions, retarders, or plugging and bridging agents maybe added to the water or the (grout, slurry) to permit efficient use of materials and proper workability of the (grout, slurry). However, the addition of additives must be approved by the Engineer.
2.2.4 Cement
Cement used in preparing a (grout, slurry) shall conform to ASTM Designation C-150 "Requirements for Portland Type 1-11 Cement." The cement shall be adequately protected from moisture and contamination while in transit to, and in storage at, the job site. Reclaimed cement or cement containing lumps or deleterious matter shall not be used.
2.2.5 Reinforcing Steel
Reinforcing steel shall meet ASTMA36 and free torn any defects affecting its usefulness.
2.3 EQUPMENT
2.3.1 Batching Equipment
The batch plant shall consist of colloidal mixers, volumetric screw feeders, and flow controllers. Dry materials shall be stored in silos and fed by screw feeders to the colloidal mixers for agitation and circulation. The resulting (grout, slurry) will be transferred to a surge tank for additional mixing and to supply the DSM rig.
The precise arrangement and sequence of mixing shall be based on the results of the design mix program. Commercial grade clays (i.e., bentonite and attapulgite) may be premixed with water (hydrated) in a separate mixer and the slurry stored in a holding tank. The clay-water slurry shall be transferred to a second mixer (only if premixing is necessary) where cement and/or other materials will be added and mixed into the grout.
(Grout, Slurry) shall be monitored and controlled to ensure proper proportioning. Density measurements shall be taken to verify the (grout, slurry) proportions. Dry ingredients shall be batch mixed by weight to a predetermined final density. A minimum mixing time of five minutes and a maximum holding time of three hours will be enforced.
Calibration of mixing components shall be done at the beginning of the project, and monthly thereafter. The screw feeders shall be calibrated against time to deliver the predetermined weight. Water shall be controlled by flow meter and/or by volume level indicators in the colloidal mixer.
2.3.2 Deep Soil Mixing Equipment
The DSM rig shall contain of a series of overlapping mixing shafts capable of creating a wall with a minimum thickness of 24-inches. The shafts shall have a bottom discharge capability for (grout, slurry). The mixing shafts and mixing blades shall be capable of blending the in-situ soils and (grout, slurry) into a homogeneous mixture.
The power source for driving the mixing shafts shall be sufficient to maintain required RPM and penetration rate from a stopped position at the depth specified.
PART 3 EXECUTION
3.1 HORIZONTAL ALIGNMENT
The DSM (wall, treatment area) shall be accurately staked prior to construction. A shallow pre-trench shall be constructed to define the installation location. Overlapping the auger flights and mixing blades between the shafts of the DSM rig shall accomplish continuity between the stabilized soil columns. Overlapping primary and secondary strokes, advances the (wall, treatment area). The first and last column of a secondary stroke shall overlap the last column and the first column of the primary strokes. The soil treatment shall be a sequence of overlapping and alternating primary and secondary strokes, so that (the wall is continuous, no areas are left untreated.) The placement of strokes shall be controlled by the use of a template or other approved means to gauge the distance between strokes. The template will be set according to surveyed reference points.
3.2 VERTICAL ALIGNMENT
The DSM equipment operator will control the vertical alignment of the auger stroke. Two measurements of verticality will be monitored. These are the fore-aft and left-right vertical mast positions.
These measurements will be monitored by two seno-accelerometers mounted at right angles to each other with both their sensitive axes in the horizontal plane. When mounted to the lead structure, one unit will be sensitive only to the fore-aft (pitch) of the lead, and the other to the left-right right (roll) axis of the lead. The output of these units will be routed back to the DSM console for scaling and display. Resolution and accuracy of this method will be within 0.1 degree. The outputs will be compared to preset levels of ± 0.1degree and when exceeded will trigger display lights in a panel visible by the operator. The operator will adjust the position of the equipment as necessary.
3.3 MIXING SHAFT SPEED
The mixing shaft speed (RPMs) shall be adjusted to accommodate a constant rate of mixing-shaft penetration, based on the degree of drilling difficulty. This speed can be adjusted to aid mixing of the soil column when needed such as hard drilling.
3.4 PENETRATION RATE
In order to maintain adequate mixing, the penetration rate of the mixing shafts shall be constant to ensure the proper amount of slurry is added and the proper amount of mixing time is allowed. The bottom of the columns shall be double-mixed by raising the mixing shafts 10 feet from the bottom and then reinserting them for remixing. The penetration rate and maximum depth of each stroke shall be recorded on the Daily Quality Control form.
3.5 (GROUT, SLURRY) INTAKE
The (grout, slurry) (intake, injection rate) per vertical foot of column will be adjusted to the requirements of the mix design. Positive displacement pumps will be used to transfer the (grout, slurry) from the mix plant to the DSM rig. The (grout, slurry) will be delivered separately to each auger. Flow monitoring devices will be installed in each grout line to detect any line blockage.
The rate of application may be controlled and monitored by any of the following three methods. First, the positive displacement pumps will be calibrated according to site conditions to produce a time verses flow correlation. By adjusting the pump output to the penetration rate, a preset grout intake can be achieved. Second, the transfer pumps will deliver (grout, slurry) to the DSM rig so that a return line can be adjusted to bleed-off any overflow back to the surge tank. The surge tank will be a cylindrical vessel with a constant depth-volume relationship, which can be monitored while the return line is adjusted to ensure a constant injection rate. Third, a programmable electronic controller, flow-metering, pressure-regulating robot may be employed.
Generally, the injection rate will be approximately 80 percent while the augers are moving downward and 20 percent while moving upward. These rates maybe adjusted for variable soil conditions. The overall application rate to each stroke can be monitored, calculated, and controlled. Additional mixing will be used as necessary to evenly distribute the (grout, slurry) through the entire column. The injection of (grout, slurry) to each stroke will be monitored, checked by calculation, and recorded.
3.6 KEY
Unless otherwise directed, the bottom of the DSM (wall, treatment area) will be keyed the minimum specified penetration depth of ____________ into the underlying aquiclude beneath the site, as indicated by soil borings. If drilling resistance at the key slows penetration to a rate of less than 1-foot per minute for five minutes, the final depth of the shaft shall be accepted by refusal. The final depth and penetration of the (wall, treatment area) shall be measured from shaft penetration, checked by the Contractor and approved by the Engineer immediately following penetration.
3.7 OBSTRUCTIONS/MIXING SHAFT REFUSAL
If obstructions including, but not limited to, boulders or timbers are encountered that reduce the rate of penetration to 1-foot per minute for five minutes, the stroke shall be completed in accordance with the specifications and remedial measures/investigation taken.
3.8 QUALITY CONTROL AND QUALITY ASSURANCE
Prior to construction, a mix design program shall be conducted to determine the ingredients, sequence of mixing, (grout, slurry) and soil mix properties. The basic guiding principal for the mix design program is to strive to accurately model expected field conditions.
3.8.1 Sampling
Samples will be retrieved from the (wall, treatement area) for testing (every 100 lineal feet of the DSM wall, from each day's work). A special sampling tool shall be used to take these samples at mid-depth in the DSM column immediately following installation. The sample mixture shall be placed in suitable molds, rodded to remove trapped air pockets, and then sealed. The samples shall be stored in a damp environment for curing until initial set-up has been achieved. After initial set-up, a dead weight load may be imposed on the sample to model the stress from earth pressures.
3.8.2 Testing
The samples shall be transported to an independent geotechnical laboratory for testing once they have sufficient strength gain and the transporting will not adversely affect the properties.
(Cured samples will be tested with a triaxial permeability apparatus similar to that used by Daniel, Trautwein, Boynton and Foreman (ASTM Geotecnnical Journal, September 1984, Page 113.) (Compressive strength tests will be performed in accordance with ASTM C39.))
3.8.3 (Grout, Slurry)
Based on the mix design program, field testing of the (grout, slurry) properties shall be submitted for approval. (Grout, Slurry) control may be performed by one, or a combination, of the following tests:
- Unit weight, by Mud Balance method
- Viscosity, by Marsh Funnel method
- pH, by pH meter or paper
- Filtrate, by filter press
The mix design program will determine the frequency and standards for field tests. Calculations of mix proportions will be determined by the absolute volume method. Proportion calculations will be based on the weight of the water to that of the (grout, slurry).
3.8.4 Documentation
All quality control and measurement for payment data shall be recorded on specially prepared Quality Control forms. The forms will contain the following information:
- Summary of daily activities
- Quality control test results
- Location of test samples
- Measurement of pay quantity
- Pay quantity
- Other comments as necessary
- Signatures
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