7 Considerations related to design
7.1 Field trials
7.1.1 Test areas with various drain spacing and/or various drain types may be required as a basis for the final design of the drain installation work.
7.1.2 The process of consolidation in the test areas should be monitored by settlement measurement in combination with pore pressure measurement, preferably by means of settlement gauges and piezometers placed at various depths. The strength increase due to consolidation may be assessed by laboratory tests and/or in-situ tests.
7.1.3 When relevant, the horizontal displacements along the periphery of the test area may be measured by means of inclinometer.
7.1.4 In case of test areas with partially penetrating drains the influence on the consolidation process of underlying untreated soil layers should be taken into account.
8.1 Method statement
8.1.1 A method statement shall be prepared, which details the vertical drainage works. The method statement shall detail the location, drain grid/pattern, equipment and method of installation, possible restrictions during the construction phase and any hazards associated with the execution of the work.
8.1.2 The equipment and installation method chosen by the contractor shall be assessed and approved.
8.1.3 The following method statement shall be delivered as a minimum (see Clause 4):
- a) objective and scope of drain installation;
- b) site installation and working areas;
- c) plant and equipment;
- d) control procedures;
- e) procedures regarding possible interruptions during drain installation and/or preloading;
- f) verification testing methods;
- g) working documents (layout, drawings, reports);
- h) measures to avoid puncturing of artesian ground water;
- i) safety and environmental risk assessment.
8.1.4 If judged necessary, trial installations should be carried out to confirm the installation machine suitability for the site conditions and the drain material utilised.
8.2 Preparation of the site
8.2.1 The preparation shall be carried out in accordance with the design specifications and the specific site conditions. This shall include suitable access for plant and machinery, levelling of the working platform, providing adequate ground bearing capacity for equipment and installation of a drainage blanket. The drainage blanket may be installed after the installation of the vertical drains in on land constructions.
8.2.2 All materials and products for vertical drainage delivered to the site shall be identified and checked against the materials and products specifications.
8.2.3 When the drainage blanket serves as a working platform and is installed directly on the ground surface, it should have a minimum thickness of 0,5 m with an initially horizontal upper surface. When the drainage layer is installed on top of the working platform, made of ordinary fill material, the thickness of the working platform is usually at least 0,5 m for on land construction. The drainage blanket is then at least 0,3 m thick. It should consist of gravelly sand or sandy gravel, containing less than 5 % of material with grain size < 0,06 mm. It should be protected from ingress of fine-grained material and frost that can detrimentally affect its permeability. It can also consist of an appropriate draining system of geotextile or geotextile-related products.
8.2.4 When the drainage blanket consists of a layer of granular material, instructions should be given regarding the methods and frequency of checking the grain size distribution and permeability of the drainage blanket material.
8.2.5 The drainage blanket should be protected against freezing to ensure that its permeability is not seriously reduced.
8.2.6 In the case of marine drain installation, the drainage blanket should consist of granular material, preferably sandy gravel, with a minimum thickness of 0,5 m. The drainage blanket should be placed before the drain installation.
8.3 Drain installation
8.3.1 The surface location of installation of each drain shall deviate less than 0,15 m from the specified location. The verticality of the leader shall not be less than 50 (vertical):1 (horizontal), unless obstructions make it impossible.
8.3.2 Where it is impossible to install a drain as a result of obstructions, another drain shall be installed as close as possible.
8.3.3 If the presence of fill or dense soil at the surface makes installation of drains by conventional methods difficult, hard layers should be penetrated by pre-drilling or other appropriate methods prior to drain installation.
8.3.4 Drains shall be installed to the depth specified in the design (within a tolerance of 0,15 m). In soft soil deposits with varying thickness, this depth may be defined as that of the underlying more resistant layer.
8.3.5 For each installed drain, information shall be provided regarding installation date and installation depth.
8.3.6 During installation of sand drains the amount of sand filled into the drains shall be monitored to check whether necking or failure of the drains takes place or not.
8.3.7 For band drain installation, the mandrel should leave a free inside space for the drain and should be constructed in a way to limit soil disturbance. Further information is given in Annex A.
8.3.8 The installation rig for band drains should preferably be provided with a fully automatic recorder. The following parameters should be recorded:
- a) drain identification number;
- b) date and time;
- c) depth of installation;
- d) accumulated amount of installed drain length;
- e) verticality and location.
8.4 Special aspects
8.4.1 The method of drain installation should not threaten the site stability. In particular, attention should be paid to excess pore water pressure being built up during the installation process by dynamic methods and in strain sensitive clays (quick clays) by displacement methods.
8.4.2 The execution of sand drain installation shall be performed by personnel who have previous experience of drain installation methods (closed-end casing installation, open-end mandrel installation, jet installation etc.).
8.4.3 Before band drains are installed into the soil they should be provided with an anchor, which keeps the drain in place when the mandrel is withdrawn from the soil. The soil should be prevented from intruding into the mandrel during installation (closed-end mandrel) (see Annex A).
8.4.4 After the mandrel is withdrawn the band drains shall be cut so that the drains are in adequate hydraulic connection with the drainage blanket, preferably 0,2 m to 0,25 m above the surface of the working platform.
8.4.5 For fully penetrating drains connected to a lower high-permeability layer, the penetration into this layer shall be sufficient to ensure that the drains are in adequate hydraulic connection with it.