4.6.5 Suitability of ground conditions
The ground to be filled over shall be suitably stable and appropriate for the proposed earthworks and end-use. Issues to be taken into account include:
- unsuitable ground conditions
- obstructions
- compressible soils
- unstable ground
- groundwater conditions.
The geotechnical design should be prepared by a suitably qualified and experienced Engineer who should assess the ground and be satisfied that it is suitable for engineered fill. Prior to earthworks commencing, the formation level should be inspected and tested in accordance with the earthworks specification.
Engineered fill may be placed on natural ground or made ground – both need to have been appropriately investigated to assess their suitability to provide long-term support to the engineered fill and proposed end-use. Where engineered fill is to be placed on made ground, the made ground will be considered as a residual geotechnical risk for the purposes of geotechnical design.
It may be acceptable to partially excavate existing ground and recompact it in place in order to provide a stiffened layer with improved bearing capacity, settlement characteristics and reduced permeability (as required). Settlement of both the engineered fill layer and the ground beneath should be assessed.
4.6.5.1 Unsuitable ground conditions#
Prior to the execution of earthworks filling, unsuitable materials should be removed from the formation level including:
- topsoil, tree roots, highly organic matters, or other degradable materials
- frozen soils
- excessively wet or soft soils such as silty soils or clays with an undrained shear strength of less than 40kPa, where not treated to adjust their soil properties
- contaminated soils, and soils containing slag or combustible materials.
Unsuitable materials may be treated, in some circumstances, to adjust and improve the soil properties. Further assessment would be required to determine their suitability after treatment and prior to any subsequent placement as engineered fill.
4.6.5.2 Obstructions#
Obstructions including oversized materials or relic foundations that may impede future works in the ground or adversely affect earthworks filling or drainage of the ground should be removed.
It may be impractical to remove some obstructions beneath engineered fill such as former piles etc. These should be adequately surveyed, and their locations accurately recorded. Any retained relic structure or obstructions should be a minimum of 2m beneath the underside of proposed building foundations.
4.6.5.3 Compressible soils#
Where compressible soils are present such as soft clays, disturbed ground or peat, the effects of earthworks should be assessed including the magnitude and duration of induced settlements.
Where significant settlements of the underlying soils are predicted, it may be necessary to undertake compaction works, removal or ground improvement works, such as the installation of vertical drainage, or surcharging prior to filling of earthworks.
The effects of ‘drying out’ of compressible soils such as peat beneath engineered fill, should be assessed where earthworks may reduce water infiltration, or the development may cause lowering of groundwater levels.
4.6.5.4 Unstable ground#
Voids that might collapse or cause excessive settlement (ie, dissolution features, mine workings and entries, sewers and drains) should be adequately investigated and suitably infilled or grouted prior to the commencement of any earthworks filling or building foundations.
Unstable natural deposits or materials susceptible to solifluction (materials that have undergone progressive, gravitation and downslope movement) should be adequately investigated and addressed by the geotechnical design.
4.6.5.5 Groundwater conditions#
The following conditions may adversely affect the long-term performance of engineered fill, if not properly considered and allowed for within the design and specification:
- lowering of the water table to facilitate filling may cause settlement of existing adjacent buildings
- rising or recovery of groundwater levels after filling may cause inundation settlement of fill or wash-out of ‘fines’ and settlement
- falling groundwater could cause settlement of soft strata, including peat
- lack of surface water management (ie, non-provision of adequate capping and drainage) following earthworks to ensure no subsequent deterioration over time.
Last updated: 2nd January 2024