Aspects of design
The aspects of design covered in this annex are related to the process design development, the choice of binder, laboratory and field-testing and the influence on the design of the column layout and performance. This annex does not cover the detailed principles or methods of geotechnical design, for which reference should be made to EN 1997-1.
As deep mixing is a ground improvement process, design encompasses two distinct aspects:
- functional design describes the way in which the treated soil and the untreated soil interact to produce the required overall behaviour;
- process design describes the means by which the required performance characteristics are obtained from the treated soil by selecting and modifying the process control parameters.
The scope of the application of deep mixing is to handle and solve problems associated with the following aspects:
- settlement reduction (e.g. of embankments and structures);
- improvement of stability (structures and embankments);
- support of slopes and excavations;
- improvement of bearing capacity and reduction of settlement and lateral spreading due to dynamic and cyclic loading (e.g. in seismic regions);
- immobilisation and/or confinement of waste deposits or polluted soils;
- construction of containment structures;
- reduction of vibrations and their effects on structures and human beings.
B.2 Design principles
The ground treated with deep mixing need to be designed and executed in such a manner that the supported structure, during its intended life and with appropriate degree of reliability and cost-effectiveness, will remain fit for the use for which it is intended and sustain all actions and influences that are likely to occur during execution and use. This requires that the serviceability and ultimate limit states are satisfied.
The requirements for the serviceability and ultimate limit states are to be specified by the client. The design shall be in accordance with the requirements put forward in EN 1997-1.
So-called iterative design, based on a follow-up of the results obtained by various testing methods, is an important part of the design. Here, the main focus is placed upon those factors that are important for the execution and the purpose of deep mixing.
The design is made for the most unfavourable combinations of loads, which could occur during construction and service.
The deep mixing process may involve a short-term decreasing resistance to failure in consequence of induced excess pore water pressure and soil displacements. The mixed-in-place columns should be arranged in a way to avoid that possible planes of weakness in some columns installed could have a negative influence on the stability. In the stability analysis it is important to take into account the differences in stress vs. strain relationship between treated and untreated soil. For excavation support, the most important parameters are the compressive strength of the treated soil and arching. Figure B.1 indicates the iterative process combining functional design and process design.
B.3 Execution process of deep mixing
The purpose of standardised laboratory tests (laboratory mixing tests) is to provide information on binder type and dosage appropriate for the actual construction. The tests should include each representative soil layer. In most of the cases there is a difference between laboratory strength and field strength. The preliminary process design is based on the laboratory test results, database and information about similar experience as shown in Figure B.1. Before the actual construction, deep mixed test columns are constructed on which field trials are carried out to confirm that the dosage, type of binder and mixing energy yield the required strength and uniformity. In case field trials fail to satisfy the requirements given in the design, the functional and process design have to be reconsidered.
B.4 Choice of binder
The binders used in dry mixing usually consist of cement or a mixture of lime and cement, in wet mixing of cement. The choice of binder is a critical aspect of deep mixing, which largely depends on the soil conditions and the purpose of deep mixing. Testing of binders with the soil to be treated is normally an essential requirement on any deep mixing project. A summary of the binders that are commonly used is given in Table B.1.
|Soil type||Suitable binder|
|Clay||Lime or lime/cement|
|Quick clay||Lime or lime/cement|
|Organic clay and gyttja||Lime/cement or cement/granulated blast furnace slag or lime/gypsum|
|Peat||Cement or cement/granulated blast furnace slag or lime/gypsum/cement|
|Sulphate soil||Cement or cement/granulated blast furnace slag|
|Silt||Lime/cement or cement|
The binder used in wet mixing is in most cases cement. Specially prepared binders may be used for highly organic soils or for extremely soft soils with high water content. Mixtures of fly ash, gypsum and cement may be used in cases where low strength of the treated soil is preferred. Bentonite is frequently used to improve rheology and stabilise the slurry mixes.