7 Considerations related to design
7.1.1 The in-situ strength of columns is influenced by several factors, such as properties of the soil to be treated, mixing condition, mixing tool and mixing process, curing condition, type and amount of binder, ground conditions, etc. Therefore it can be difficult to estimate the field strength accurately at the design stage. It is important to estimate and verify the field strength in several stages by laboratory mixing tests, accumulated experience, field trials and verification tests. The design should be modified if the requirements cannot be fulfilled.
7.1.2 The execution of deep mixing projects involves geotechnical design in several phases and can be an iterative process. The objective of the design is to produce technical documents, which enable works to be constructed with regard to safety, serviceability, economy and durability, taking into account the expected service life. It is recommended that the parties responsible for the design should be involved also during the construction.
7.1.3 The geotechnical design of deep mixing projects shall be based on ENV 1991, EN 1997-1 and prEN 1997-2. The informative Annex B summarises important parameters, which affect overall stability and settlement of the treated ground.
7.1.4 Reference to relevant experience is permitted if appropriate verification has been undertaken (e.g. by penetration tests, pressuremeter tests or other tests).
7.1.5 A method statement shall be prepared, which details the deep mixing works. As a minimum requirement, the method statement shall detail the location and the purpose of the works, the required design life, possible restrictions during the construction phase and any hazards associated with the execution of the works.
7.1.6 When there is some latitude in the selection of materials, the method statement shall emphasise the particular requirements, which can influence the final selection.
7.1.7 Preliminary design can be based on tests of laboratory mixed samples and comparable experience, taking into account the difference in characteristics between laboratory mixed samples and treated soil in-situ.
NOTE For guidance, reference is made to Annex B.
7.1.8 Testing may not be sufficient to verify the adequacy of the treatment. Appropriate supervision, monitoring and records are required. An observational approach is often appropriate and design is not normally complete until site experience is gained.
7.2 Additional design considerations
7.2.1 Loading conditions, climatic effects, hydraulic conditions, and acceptable limits of settlement, heave, distortion of structures and services, which may be affected by the deep mixing works, shall be taken into account.
7.2.2 The design should identify and take into account environmental restrictions on construction, such as noise, vibration, pollution of air and water and impact on adjacent structures.
7.2.3 Where end-bearing capacity of the column is required, the use of a suitable mixing tool and mixing process shall be prescribed to avoid forming a remoulded zone at the base of the column.
7.2.4 The specified column or wall layout and tolerances should take into account the limitations of the mixing equipment.
7.2.5 For grid type or block type or overlap columns, the specified spacing between the columns shall take into account the angular deviation and the positional tolerances.
7.2.6 Amendments necessitated by unforeseen circumstances, such as essential changes in ground or hydraulic conditions, shall be reported immediately.
7.2.7 Suitable protection and testing should be specified when treated soils are likely to be exposed to freeze/thaw cycles.
7.2.8 Consequences due to exposure of the columns to chemical and physical effects shall be considered in the design. Particular consideration regarding long-term durability is needed in marine environments or contaminated ground conditions.
7.3 Selection of the binder and the additives
7.3.1 The site and ground conditions and the nature and properties of the soil to be treated shall be considered in the selection of binder.
7.3.2 The efficiency of the binder and the admixture shall be studied by laboratory and/or in-situ tests of the treated soil, taking into account the prescriptions given in 7.4.
7.4 Laboratory and in-situ mixing and treatment tests
7.4.1 As the properties of the treated soil are influenced by numerous factors, including the execution process, preliminary trials and tests of the treated soil shall be carried out to confirm that the design requirements can be achieved.
7.4.2 When studying treated soil one should consider that laboratory test results might overestimate the achievable field results (owing to e.g. the use of more thorough mixing in the laboratory and to varying cure conditions).
7.4.3 When studying the process and the behaviour of treated soil, the improvement of soil with time should be considered. The rate of improvement depends on the type and quantity of binder utilised and the curing condition. When studying the effect of time on trial mix specimens, considerations should be given to the effect of curing conditions (temperature, curing under water, preloading etc.).
7.4.4 Sequence and rate of execution, setting and hardening time, and diameter of the columns shall be considered in order to avoid local soil failure or unacceptable settlement or heave.
7.4.5 Where deep mixing is used to immobilise contaminants or to stabilise waste deposits, or for similar purposes where there may be unpredictable interaction between binder and in-situ material, site-specific test programmes shall be executed.
7.5 Design statement
7.5.1 The design output should state the performance objectives and geometry of the treatment, the specification of materials or products assumed in the design, any further details such as phasing of the works and, where relevant, provide the following information:
- a) specifications for the deep mixing work;
- b) column requirements (strength and deformation characteristics and permeability);
- c) the width of the overlapping portion between adjacent columns;
- d) tolerances for columns in respect of length, diameter, inclination and plan position;
- e) boundaries and geometry of installation, setting out drawings;
- f) construction programme, including time schedule for loading and possible preloading, and notice of any restrictions such as construction phasing required in the design;
- g) a schedule of any testing and acceptance procedures for materials incorporated in the works and of any required testing and monitoring procedures during execution;
- h) requirements concerning possible structural reinforcement (class of material and installation procedure) and time schedule for its installation;
- i) toe penetration into bearing or impermeable stratum;
- j) reporting procedure for unforeseen circumstances, or conditions revealed that appear to be different from those assumed in the design, or if an observational procedure is adopted.
7.5.2 Whenever acceptance is defined on the basis of tests of core samples, the design shall specify location, age at test, coring equipment and procedure.
7.5.3 For mechanical tests on treated soil, the conditions of testing the samples and the criteria for acceptance shall be specified. Tolerances in respect of specified performance parameters shall consider the adequacy of the proposed test method, especially when these methods are indirect, as described in Annex B.
7.5.4 Limiting values of critical geotechnical design parameters shall be stated, as well as steps to be taken if values are likely to be exceeded.
7.5.5 Any additional or deviating requirements, falling within the permission clauses given in the standard, shall be established and agreed upon before the commencement of the work.