8.1 Method statement
8.1.1 Before the execution of deep mixing, the following method statement shall be delivered as a minimum:
- a) identification, objective and scope of the deep mixing work;
- b) soil description in compliance with EN ISO 14688-1:2002 and EN ISO 14688-2:2004;
- c) shape of the deep mixing column;
- d) deep mixing method;
- e) mixing tool: shape/dimensions/configuration of rotating unit(s), position(s) of outlet(s), shape and length of mixing shaft;
- f) working procedure (penetration and retrieval, mixing and sequence of execution (see table in 188.8.131.52);
- g) installation accuracy;
- h) deep mixing parameters: binder type and composition, binder content/binder factor, volume ratio, water binder ratio, admixture, filler (see table in 184.108.40.206);
- i) precautions against heave and settlement;
- j) site installation and working areas;
- k) plant and equipment;
- l) spoil management;
- m) quality control procedures as required by the contract;
- n) procedures regarding possible interruptions during the deep mixing operations;
- o) possible modification of the deep mixing parameters during the works;
- p) verification testing methods;
- q) working documents (layout, drawings, reports);
- r) safety and environmental risk assessment.
8.2 Preparation of the site
8.2.1 The preparation shall be carried out in accordance with the design specifications and the specific environmental site conditions. This shall include suitable access for plant and machinery, excavation, cleaning and levelling of the working platform, providing adequate ground bearing capacity for equipment, receipt, quality control and storage of material.
8.2.2 All materials and products for deep mixing delivered to the site shall be identified and checked against the design specifications (see 6.1.2).
8.2.3 The binders shall be protected from ingress of moisture or air that could otherwise detrimentally affect their use and/or performance.
8.3 Field trials
8.3.1 In cases where comparable previous experience is not available, representative field trials shall be performed in order to confirm that the design requirements can be achieved and to establish the critical control values, using the same equipment, materials, technique and procedure as specified for the main execution of the work.
8.3.2 The execution control values shall include:
- a) penetration and retrieval speed of mixing tool;
- b) rotation speed of the rotating unit(s) of mixing tool;
- c) air pressure (in case of dry mixing);
- d) feed rate of binder/slurry.
NOTE In certain circumstances other parameters should be monitored, where they have a direct influence on the quality and performance of the work, for instance overlapping width, where a wall is providing containment, or torque where columns are founded in stiff strata.
8.4 Execution tolerances
220.127.116.11 Before the installation of the columns, the position of each column shall be located and identified.
18.104.22.168 The columns shall be constructed within the geometrical tolerances set in the design.
22.214.171.124 Verticality and inclination measurements can be carried out by means of inclinometers.
8.5 Quality control and quality assurance
8.5.1 A quality plan shall be provided, detailing the methods and frequency of the checks to be made during the construction and verification process, and defining the procedure for dealing with any non-conformance (see EN ISO 9000). The quality plan shall also identify all documents (drawings, method statements, plans etc.) that are necessary for the execution of the works.
8.5.2 The tests on the treated soils set out in 9.3 shall be performed and the methods of testing, presented in Annex B, shall be used in accordance with the specifications of the design (see 7.4 and 9.3).
8.5.3 If the conditions encountered during execution do not correspond with those expected in the design, this shall be reported immediately to those responsible for the project.
8.6 Deep mixing
126.96.36.199 The execution of deep mixing can be carried out by either dry or wet mixing. These two methods are described in detail in Annex A.
NOTE 1 Deep mixing is executed by mechanical disaggregation of the soil using mainly vertical movement of rotating mixing unit(s) and introduction of a binder, which is homogenised with the soil during the penetration and/or retrieval. The execution of deep mixing can be carried out either by dry or wet mixing. These two methods are described in detail in Annex A.
NOTE 2 In the dry mixing method, the medium of transportation of the binder is normally compressed air.
NOTE 3 In the wet mixing method, the medium of transportation of the binder is normally water.
188.8.131.52 The equipment and the mixing tool shall be correctly positioned at each column location in accordance with the execution tolerances specified in the design.
184.108.40.206 The quantity of binder along the column shall be measured during installation of each column.
220.127.116.11 Equipment used for recording of the supplied quantity of binder or solids for the slurry shall be calibrated.
18.104.22.168 Any spoil generated shall be collected and disposed of in accordance with the legal or statutory requirements.
8.6.2 Dry mixing
22.214.171.124 The execution procedure of dry mixing shall follow the specifications given in the design.
NOTE 1 The installation is usually carried out according to the following procedure:
- the mixing tool is correctly positioned;
- the mixing shaft penetrates to the prescribed depth of treatment with simultaneous disaggregation of the soil by the mixing tool;
- after reaching the prescribed depth of treatment, the shaft is withdrawn and, at the same time, the binder in granular or powdered form is injected into the soil and the mixing tool, which rotates in the horizontal plane, mixes the soil and the binder.
NOTE 2 The binder may also be injected and mixed with the soil during the penetration stage.
126.96.36.199 The equipment and mixing tools shall be compatible with the execution procedure, the depth of the soil to be treated and the execution tolerances specified in the design.
NOTE When the binder is injected and mixed with the soil during the penetration stage, the injection outlet shall be positioned at, or under, the mixing tool.
188.8.131.52 The rotation speed of the rotating unit(s) and the rate of penetration and retrieval of the mixing tool shall be adjusted to produce sufficiently homogeneous treated soil.
NOTE 1 Current penetration or retrieval rates of mixing shaft are usually 10 mm/rev to 50 mm/rev and the blade rotation numbers are usually 200 to 500.
NOTE 2 The amount of mixing work involved in producing a dry-mixed column depends on the type and quantity of binder and type of soil. Cement as binder requires a higher mixing energy than lime only.
184.108.40.206 In dry mixing the air pressure shall be kept as low as possible during the mixing process to avoid problems of air entrainment and ground movements.
NOTE If the air pressure is too low, the binder may not spread into the whole cross-sectional area of the column.
220.127.116.11 The amount of binder and the air pressure shall be monitored during installation of the columns.
18.104.22.168 Mixing energy should be monitored to achieve uniform treated soil.
8.6.3 Wet mixing
22.214.171.124 The execution procedure of wet mixing shall take into account the specifications given in the design.
NOTE The installation is usually carried out according to the following procedure:
- the mixing tool is correctly positioned;
- the mixing shaft penetrates to the prescribed depth of treatment with simultaneous disaggregation of the soil by the mixing tool and/or injection of the slurry, usually cement slurry and possible fillers and additives;
- after reaching the prescribed depth of treatment, the shaft is withdrawn and in some cases the slurry at the same time is injected into the soil and mixed with it.
126.96.36.199 The equipment and the mixing tools shall be compatible with the execution procedure, the depth of soil to be treated and the execution tolerances specified in the design.
NOTE 1 For machines with the outlet below the mixing tool, the slurry shall not be added during the retrieval phase.
NOTE 2 Whereas a continuous flight auger may be sufficient for predominantly granular soils, cohesive soils require more sophisticated mixing tools. The rotary drives, turning the shaft, need to have enough power to destroy the matrix of the soil for intimate mixture with the slurry.
188.8.131.52 The rotational speed of the rotating unit(s) and the rate of penetration and retrieval of the mixing tool shall be adjusted to produce sufficiently homogeneous treated soil.
NOTE Current rotation speed of the mixing blades are usually 25 rev./min to 50 rev./min and the blade rotation numbers usually greater than 350.
184.108.40.206 During mixing the slurry shall be delivered by pumping in a continuous flow to the soil to be treated.
220.127.116.11 The wet mixing process may 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.
18.104.22.168 Restroking may be used to redistribute slurry to a certain portion of the treated column, re-fluidise a portion of the stratum on the penetration or as a means of keeping the rotating units in motion during a hold or waiting period.
22.214.171.124 The density of the slurry shall be tested by a suitable device at least twice per working shift at each batching/mixing plant. In the case of manual batching, the frequency of testing shall be increased.
8.7 Installation of structural reinforcement
8.7.1 Structural reinforcement (steel bars, steel cages or steal beams) may be installed into the fresh mixed-in-place columns or elements.
NOTE The aid of a vibrator may be required for the installation process.
8.7.2 Any structural reinforcement shall be installed in accordance with the specifications of the design (see 7.5.1 h)).