5.3.3 Selection of procedures for fill placement and compaction
(1)P Compaction criteria shall be established for each zone or layer of fill, related to its purpose and performance requirements.
(2)P The procedures for fill placement and compaction shall be specified in such a way that stability of the fill is ensured during the entire construction period and the natural subsoil is not adversely affected.
(3)P The compaction procedure for fill shall be specified depending on the compaction criteria and on the following:
- the origin and nature of the material;
- the placement method;
- the placement water content and its possible variations;
- the initial and final thickness of the lift;
- the local climatic conditions;
- the uniformity of compaction;
- the nature of underlying ground.
(4) In order to develop an appropriate procedure for compaction, a trial compaction should be performed at the site using the intended material and compaction equipment. This allows the determination of the compaction procedure (method of placement, compaction equipment, layer thickness, number of passes, adequate techniques for transportation, amount of water that shall be added) to be followed. A trial compaction may also be used to establish the control criteria.
(5) Where there is a possibility of rainfall during the placement of cohesive fill material, the fill surface should at all stages be profiled so as to permit adequate run-off.
(6) At temperatures below freezing, fill may require heating before placement and frost protection of the fill surface. The need of these measures should be evaluated case-by-case, taking into account the quality of the fill material and the required degree of compaction.
(7)P Backfill placed around foundations and beneath floor slabs shall be compacted such that damaging subsidence does not occur.
(8) Fill should be placed on an undisturbed and drained ground surface. Any mixing of the fill with the ground should be prevented by using a filter textile or filter layer.
(9) Before placing fill underwater, all soft material encountered should be removed by dredging or other means.
5.3.4 Checking the fill
(1)P Fill shall be inspected or tested to ensure that the material, its placement water content and the compaction procedures comply with the specification.
(2) Testing need not be performed for some combinations of materials and compaction procedures if the compaction procedure has been proved by a field trial or by comparable experience.
(3) Compaction should be tested by one of the following methods:
- measurement of dry density and, if required by the design, measurement of the water content;
- measurement of properties such as, for example, penetration resistance or stiffness. Such measurement cannot always determine if satisfactory compaction has been achieved in cohesive soils.
(4) Minimum fill densities determined, for example, by Proctor percentages, should be specified and checked on site.
(5) For rock fill or fill containing a large amount of coarse particles, compaction should be checked by field methods. The Proctor test is not applicable to these materials.
(6) Site checking (see EN 1997-2) may be made by one of the following:
- ensuring that compaction has been performed according to the procedure deduced from a field trial or from comparable experience;
- checking that the settlement induced by an additional pass of the compaction equipment is equal to or less than a specified value;
- plate loading tests;
- seismic or dynamic methods.
(7)P In cases where over-compaction is not acceptable, an upper bound limit for the compaction shall be specified.
(8) Over-compaction can cause the following undesirable effects:
- the development of slickensides and high soil stiffnesses in slopes;
- high earth pressures on buried and earth retaining structures;
- crushing of materials such as soft rocks, slags and volcanic sands used as light weight fills.