7 Considerations related to design

7.1 General

7.1.1 The design of ground treatment by deep vibration shall be in accordance with EN 1990, EN 1997-1 and prEN 1997-2.

7.1.2 The information on ground conditions shall be assessed to determine the suitability of the ground for treatment by deep vibration.

7.1.3 The following shall be defined in the design of the ground treatment:

  • technical objective of the treatment (e.g. increased bearing capacity, reduced settlement; reduced liquefaction potential, reduced potential for collapse settlement on wetting or reduced permeability);
  • required geotechnical properties of the treated ground (e.g. shear strength, stiffness, or permeability);
  • criteria on which treatment depth, spacing and extent are decided;
  • target performance and the way in which treatment is to be assessed in terms of measurable parameters;
  • where excavation subsequent to treatment takes place, proposals for recompaction if necessary.

7.1.4 Due to the nature of ground, variations are to be expected even after treatment and this should be taken into consideration.

7.1.5 Heave or settlement occurring during treatment should be anticipated and allowed for in the design. Where infill is not added during deep vibratory compaction, significant surface settlement is likely and shall be estimated.

7.1.6 Proposed changes in ground level subsequent to treatment shall be assessed and the effect this may have on the treatment shall be evaluated.

7.1.7 Some deep vibration processes require large quantities of water. The effects, if any, of pumping water into the ground on the treatment volume and the surrounding ground shall be allowed for in the design.

7.1.8 The possibility of adverse effects on existing adjacent foundation systems, buildings and services, earthworks, slopes, retaining structures and buried structures, arising from ground movements, pore pressures or vibrations induced by the ground treatment shall be considered in the design.

7.1.9 In cases where adverse effects on adjacent buildings and infrastructure are of concern, a precondition survey should be carried out as described in 11.4.

7.1.10 Where stone columns are required to perform as drains, adequate provision should be made for surface drainage. If the existing surface soil is not adequate a surface drainage blanket should be provided. Attention should be given to the drainage properties of the working platform.

7.1.11 Where deep vibratory compaction is not intended to compact the surface layer, rollers or tampers should be used to compact this layer. Alternatively, compaction can be executed from a level above final foundation level.

7.2 Selection of treatment method

7.2.1 The selected method for executing the ground treatment shall be capable of fulfilling the design requirements. Descriptions of the treatment processes are given in Annexes A and B.

7.2.2 Where previous experience of the treatment method on the type of soils prevailing at the site is limited, preliminary trials are advisable to demonstrate the feasibility of the treatment method or to optimise the design of the treatment.

7.2.3 The dry top-feed process shall only be used where the hole formed by the depth vibrator remains open. In other conditions either a dry bottom-feed process or a wet process shall be used.

7.3 Design verification

7.3.1 It is recommended that it is verified that the objectives of the treatment have been achieved.

7.3.2 Suitable means of verifying that the required treatment objectives have been achieved should be identified prior to commencement of ground treatment, in terms of the results of defined tests.

7.3.3 Appropriate methods of testing are described in 9.2.

7.3.4 The type and frequency of testing which is compatible with the requirements for quality control and performance of the treatment shall be specified. The execution and interpretation of geotechnical laboratory and field tests shall comply with the requirements of prEN 1997-2.

7.3.5 Testing, although necessary, 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.

7.4 Extent and layout of treatment

7.4.1 The design layout shall include the extent and location of treatment points, and the upper and lower levels of the treatment.

7.4.2 Each treatment point shall be identified by a reference number and its plan location shall be related to fixed reference points or lines.

7.4.3 Natural and artificial obstructions are commonly found and this possibility shall be considered in the design.

7.4.4 A horizontal deviation of up to 150 mm from the plan location can normally be accepted.

7.4.5 The treatment may need to extend beyond the area of the foundation of a structure to be built on the treated ground. This is the case where, for example, a primary objective of treatment is to reduce the vulnerability to liquefaction in an earthquake.

7.4.6 For foundation support, the depth of treatment shall be determined as follows:

  • in full-depth treatment the depth vibrator or compaction probe shall penetrate to a competent stratum identified from the site investigation;
  • in partial-depth treatment the depth of the treatment shall be adequate to improve the depth of ground loaded by the particular foundation system in order to fulfil the requirements for bearing capacity and total and differential settlement criteria. The potential for movement occurring below the depth of treatment shall be assessed.

7.5 Sequence of treatment

The sequence in which treatment should be executed at individual treatment points and the general direction of progress across large areas should be considered at design stage with regard to the effectiveness of the treatment and the presence of adjacent, buried or overhead structures and services.

8 Execution

8.1 General

Work shall be carried out to written procedures. The method of treatment and equipment employed shall be suitable to achieve the depth of treatment and the design objectives in the prevailing ground conditions. Account shall be taken of concurrent site activities and all equipment movement necessary to properly carry out the ground treatment. Suitably trained and experienced personnel shall be in charge of the execution works.

8.2 Site preparation

8.2.1 The site shall be prepared in such a way that operations can be carried out safely and efficiently. Prior to commencement of ground treatment, services in the ground and overhead shall be identified and clearly marked on site or relocated.

8.2.2 Any near-surface natural or artificial obstructions shall be identified prior to the commencement of ground treatment and broken out with the resulting voids filled with specified granular material, or the layout of the treatment shall be altered to accommodate the obstructions.

8.2.3 Working platforms shall be designed, prepared and maintained in a manner suitable for the safe movement and working of the ground treatment equipment. Material used to provide working platforms shall be suitable for the ground conditions on which it is placed and shall not prevent penetration of the vibrating tool.

8.2.4 Special considerations will apply when working over water.

8.3 Setting out

All treatment points shall be set out within the specified tolerances. The levels of the working platform relative to the specified upper and lower treatment levels shall be checked.

8.4 Treatment

8.4.1 General

8.4.1.1 Ground treatment shall be carried out by penetrating the ground with a vibrating tool. The appropriate method of installation shall be determined with due regard to ground and groundwater conditions in accordance with 7.2.

8.4.1.2 Pre-boring or pre-excavation may be necessary in certain circumstances.

8.4.1.3 Where unforeseen obstructions are encountered below ground level, it shall be determined whether the obstruction is to be removed, or whether the treatment points shall be relocated or adapted. Where it is decided to remove the obstruction, the void shall be backfilled with granular material suitable for compaction.

8.4.2 Deep vibratory compaction

Deep compaction is achieved by penetrating the ground with a depth vibrator or with a compaction probe. Penetration and removal can be assisted by water or air. The treatment method can be used with or without the addition of granular backfill from the ground surface. Deep vibratory compaction is described in Annex A.

8.4.3 Vibrated stone columns

8.4.3.1 Continuous columns shall be formed from the maximum required depth of penetration up to the upper design level as a minimum. A compacted stone column has to be built up by adding successive discrete charges of specified granular material and compacting each one to a chosen level of power consumption.

8.4.3.2 The most appropriate treatment process should be selected from the three principal methods for installing vibrated stone columns described in Annex B, namely the dry top-feed process, the wet process and the dry bottom-feed process.

8.4.3.3 Where the dry bottom-feed process is used, the depth vibrator shall not be removed from the ground during column construction.

8.4.3.4 Where the wet process is used, the water demand is substantial and the water supply shall be determined. Slurry shall be disposed of in accordance with local regulations. When slurry has been disposed in lagoons, on completion of ground treatment the lagoons shall be backfilled and the site reinstated to a defined standard.

8.4.3.5 With the wet process, the depth vibrator shall be kept in the hole during column formation.

8.4.3.6 Stone columns shall be constructed as near vertical as possible. The depth vibrator shall not deviate from the vertical by more than 1 in 20 during column formation.

EN 14731:2005 Execution of special geotechnical works - Ground treatment by deep vibration