How can you help me, help you? A geotechnical perspective.

A geotechnical perspective. Christopher Bell

I am often asked what information is needed to prepare a geotechnical investigation tender/proposal. In this post I will share what I perceive as the minimum information needed to prepare a proposal/tender for a geotechnical investigation and why.

The tender/proposal preparation process commences as a desktop assessment and involves a review of readily available public and proprietary data to identify the potential subsurface conditions, and to determine the most appropriate investigation techniques for the development – be it a multi-storey building with multiple basement levels; a bridge and approach embankment; a tunnel or a pipeline.

With this in mind, the minimum information about the development should generally include:

  1. Site location;
  2. Existing ground surface levels;
  3. Proposed development details including:
    • Development footprint size;
    • Size/height of the structure;
    • Excavation levels/volumes; and
    • Bulk fill levels/volumes.

The benefits of having the above information is discussed below:

Site Location – the site location is necessary for reviewing readily available desktop information including: published geological mapping; survey information; historical aerial photography; previous geotechnical information data; and to review the neighbouring developments and the potential impact of this on the proposed development or vice versa.

Existing Survey Data – existing ground surface levels would be used to assess if there is any potential trigger for specific regulatory requirements. Examples include:

  • sampling and testing to assess the presence or otherwise of acid sulfate soils;
  • to assess the potential for encountering groundwater; and
  • for identifying the requirements for a robust foundation to resist prescribed scour zone levels etc.

Development Details – information about the development will ultimately steer the investigation techniques (eg a multi-storey development versus a tunnel), and the depths for targeting specific items as follows:

  • Development Footprint: What proportion of the site will the development cover and what components are sited where? This will determine the areal extent and depths of the test locations to assess the subsurface conditions.
  • Size/Height of Structure: The size of the development would be used to assess the magnitude of loads transmitted to the foundation. This would assist in determining the depth of investigation (or a minimum foundation strength), to economically support the loads, and to inform the requirements for specific laboratory testing (eg consolidation in areas of soft soils; rock strengths and discontinuities for excavatability etc).
  • Excavation Levels/Volumes: The bulk excavation level/volume would assist in determining whether or not one of the aforementioned regulatory triggers would apply. The excavation depth and lateral extent would assist in guiding the investigation type to determine: excavatability characteristics; excavation retention options; and options for temporary and permanent groundwater control.
  • Bulk Earthworks Levels: Will the site be cut or filled? This will assist in determining the requirement for any regulatory triggers and associated sampling/testing, the depth of investigation, and also the need for targeted sampling depths for subsequent laboratory tests. For example, would you recover a bulk subgrade sample for CBR testing when the site will have metres of fill placed across it?

So the above indicates that the type and quality of information provided at request for tender or proposal stage, would greatly assist the preparation a geotechnical proposal/tender. Nonetheless, a skilled geotechnical professional would seek the above information to provide a cost effective, site specific investigation proposal that adds value to the development.

So I ask you now – how can you help me, help you?