Architects are often required to commission the services and fee proposals from geotechnical engineers. As we are not experts in this field it is prudent to ask the structural and or civil engineer to detail what geotechnical input they require for the specific project, or to directly commission the services. The geotechnical engineer needs to understand the project brief, complexity and scope to enable them to use their professional experience and knowledge to determine and identify relevant geotechnical risks and opportunities and how these may be appropriately addressed in their scope.
Be sure not to outline the required number of bore holes or other detail to the geotechnical engineer and request their professional opinion on the site for the proposed development.
Complex sites may be best approached with a staged investigation, whereby the findings from the first stage help to coalesce design concepts (eg number of basement levels, retention and foundation types) and likewise better inform any subsequent investigations or engineering design as the project evolves.
Geotechnical and hydro-geological reports can be obtained in isolation or in combination with environmental and contaminated land reports. However, these are usually separate disciplines and so it can be beneficial to engage respective specialists to work as part of the overall project team.
Works requested from geotechnical consultants can include:
- Desktop review, which includes:
- aerial photos, topographical, geological and hydro-geological maps and surveys
- hazard identification such as landslides and other known risks from council records
- investigation of past land use, both anecdotal and researched
- potential for land subsidence (eg mine activities)
- earthquake zones and historical records
- flood-prone areas and overland-flow paths
- previous nearby geotechnical investigations, usually company specific but there has also been an emergence of public databases around the world
- previous nearby construction records (unfortunately these are usually only available to the specific builder or piling contractor).
- Site investigation, which includes:
- identifying the subsurface soil and rock profile, varying both horizontally and with depth
- presence and quality of existing fill
- soil characteristics including strength, compressibility, permeability, swell, shrinkage and aggressiveness to concrete and steel
- classification and characteristics of bedrock including weathering and strength
- profiles along with the presence of geological structures and defects such as boulders, dykes, faults, seams, bedding, foliations and joints
- rock core photography and downhole imagery
- presence of underground structures (eg footings, rail, pedestrian, services or tunnels)
- hydrogeology of site
- identifying groundwater depth and monitoring over time
- evidence of natural drainage both past and present.
- Interpretation and modelling, which includes:
- interpretation of subsurface profile between investigation points and deriving engineering design parameters for soil and rock materials
- geotechnical and hydro-geological risk assessment
- settlement characteristics and deflection of retaining walls etc
- slope risk assessment and stability analysis
- water seepage into site
- coupled soil-structure interaction between site and nearby buildings, roads etc (zones of influence)
- other sub-artesian or terrain characteristics
- ground-borne construction vibration estimates.
- Design and recommendations including:
- retaining methods including temporary or permanent slopes, anchors, piling, shoring or other walls
- shallow and deep foundation design including bearing capacities
- measures to address soil shrinkage or swell movements
- excavatability of rock and earthworks
- combined soil-structure performance to lateral loads such as wind and earthquakes
- effects on or other requirements for adjoining structures, roads, footpaths, etc
- construction methods and constructability advice.
Importantly information is only obtained from specific test boreholes and standpipes, and then interpolated as the basis for design and construction. These test locations may miss underground boulders or other features which can impact upon both engineering and construction methods, and thus impact on costs and works program (sometimes significantly).
Construction Support
- Verification of ground conditions
- Assessment and stability of slopes following excavation or filling.
- Assessment and verification of foundations and piles
- Vibration monitoring during construction works on adjacent structures
- Water seepage into site
The geotechnical consultant requires the following information from the client:
- accurate survey information including site identification, a datum to relate bore-hole position and depth; and specific location of existing services (size, depth, invert levels etc) both on the site and adjoining land.
And from project civil or structural consultants:
- any significant site feature(s) and area(s) which are questionable
- proposed development's siting, extent and intended design including potential loads
- any geotechnical reports available from neighbouring sites.
The extent of investigations is ideally determined by the geotechnical consultant in consultation with the structural or civil consultant having considered the proposed development works.
If a geotechnical investigation is not undertaken, a prudent architect may either obtain a letter from the client or confirm in writing that the client acknowledges design implications, significant risks and limitations arising from little or no geotechnical information. You may also find that structural engineers will refuse to continue without this data.
Where there is no geotechnical investigation, the architect can note site conditions and, if appropriate, within construction documents place responsibility on the contractor to confirm footing design and obtain engineering certification.
More complex projects may require greater geotechnical involvement to model nonlinear soil structure interaction in earthquake or cyclonic zones, slope stability and settlement stress release during excavation or vibration impact from excavation activities on adjoining underground basements and structures, and provide monitoring equipment and services.
On urban sites special access rigs may be required to drill boreholes within basements or other confined spaces, and these machines often have longer lead-in times on availability.
On rare occasions, geotechnical consultants may also carry out non-destructible radar testing to locate in-ground pipes, tendons in slabs, voids in columns, or other density discrepancies in a range of materials.
Relevant Australian Standards include, but are certainly not limited to:
- AS 1726:2017 Geotechnical Site Investigations
- AS 2159-2009 Piling – Design and Installation
- AS 2870-2011 Residential Slabs and Footings
- AS 3798-2007 Guidelines On Earthworks For Commercial And Residential Developments
- AS 4678-2002 Earth-retaining Structures
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