Aspects of design
This annex covers some specific aspects of the design of vertical drainage systems, including the evaluation of soil characteristics and influence of drain characteristics, drain pattern and depth of drain installation. It does not cover the detailed principles or methods of geotechnical design, for which reference should be made to EN 1997-1 and EN 1997-2.
The scope of the application of vertical drainage is to handle and solve problems associated with the following aspects:
- 1) consolidation settlement in low-permeability soils (resulting from surface loading or groundwater lowering);
- 2) stability (of structures and embankments).
As a result of soil improvement by vertical drainage, the effects of dynamic and cyclic loading (e.g. in seismic regions) can be reduced as well as the effects of vibrations on structures and human beings. Vertical drainage can also be used for remediation of contaminated ground and for mitigation of liquefaction potential.
Vertical drainage design encompasses two phases, functional design and process design:
- 1) in the first phase, the need for vertical drainage needs to be quantified. This phase of functional design defines the loading and drain spacing which will produce the desired effects on rate of consolidation and settlements, and eventually on the un-drained shear strength of the soil. The objectives are linked up with improving the ground by preloading and enabling staged construction of an embankment, and also with creating satisfactory drainage paths for pore water in the case of liquefaction;
- 2) In the second phase the method of drain installation and their functioning in practice has to be designed. This phase of process design accounts for effects of drain installation on the ground, for the geometry, the nature and the dimensions of the drains, for possible buckling in case of excessive strains in some soil layers etc.
B.2 Design process
Vertical drainage may be used for different purposes. However, the process of designing vertical drainage always includes the operations listed in Figure B.1: the objective (design basis) and the ground properties (first row of boxes) interact with the settlement and stability analyses to satisfy the requirements put on the effect of the drains, i.e. to reach a given degree of global and/or local consolidation within a specific period of time.
Ground treatment by vertical drainage and the associated loading shall be designed and executed in such a manner that the structure, supported by the treated ground, during its intended life and with an appropriate degree of reliability and cost-effectiveness, will remain fit for the intended use and sustain all actions and influences that are likely to occur. This requires that the serviceability and ultimate limit states are satisfied.
The requirements for the serviceability and ultimate limit states shall be specified by the client. The design shall be in accordance with the requirements put forward in EN 1997-1. The observational method, which involves adapting the design in a planned manner, can be an important part of the design.
The design shall take into account the combinations of loads that could occur during construction and service. It shall account for the known effect of the drain installation on the properties of the ground.
The installation of vertical drains may induce excess pore water pressure and cause a short-term reduction of the un-drained shear strength.
B.3 Investigations for vertical drainage
The subsoil characteristics are usually determined by means of field investigation methods (e.g. cone penetration tests, field vane tests and pore pressure observations at various depths) in combination with sampling for laboratory analysis. The pore water pressure distribution with depth forms the basis for evaluation of the effective overburden pressure distribution with depth. This information is required to determine whether or not the soil is overconsolidated or normally consolidated. However, one needs to realise that the pore water pressure may vary considerably with time of the year and amount of precipitation. Occasional high pore water pressure, which reduces the magnitude of effective overburden pressure, can give a false impression of overconsolidated soil.
The testing should be carried out in compliance with EN 1997-2. The soil identification and classification, which is based on the results of the soil investigation, shall comply with EN ISO 14688.
The penetration resistance of the soil should be investigated to provide information for selecting the capacity of installation rigs.
B.3.2 Laboratory investigations
The consolidation and settlement parameters are conventionally determined by oedometer tests on undisturbed soil samples, taken by means of high-quality piston samplers. The results of conventional oedometer tests yield values of the compression modulus, the preconsolidation pressure and the coefficient of consolidation in vertical pore water flow. For determination of the coefficient of consolidation in horizontal pore water flow by oedometer tests, allowance for radial drainage needs to be made.
Laboratory testing also includes determination of the un-drained shear strength and sensitivity of the soil as well as unit weight, water content and index testing.
B.3.3 Field investigations
Field investigations normally comprise determination of the un-drained shear strength by field vane tests and/or cone penetration tests. The coefficient of consolidation and the permeability in horizontal pore water flow can be evaluated from cone penetration tests with a pore pressure device (CPTU). This is done by intermittent sounding accompanied by a study of the excess pore pressure dissipation caused by the sounding operation    .
Possible contamination of the pore water can be investigated by sampling of pore water at various depths .