Question 4 of 4

Question 4 (25 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

(a) Main objectives of the site investigation

For a foundation project the investigation must: (i) establish the subsurface stratigraphy and groundwater conditions across the footprint; (ii) determine geotechnical design parameters (bearing capacity, settlement, and if applicable slope-stability or lateral-earth-pressure parameters) for the encountered materials; (iii) identify geohazards that could affect the foundation (liquefaction potential, expansive soil, fill, contamination, seismicity); (iv) recommend a foundation type and allowable bearing pressure/pile capacity consistent with the structure's tolerable settlement; and (v) provide the information needed for construction planning (excavatability, dewatering, temporary shoring, disposal of excavated material).

(b) Site characterization considerations once scope is set

Once the scope of work is fixed, characterization must address: the spatial coverage of exploration points relative to the structure's footprint and any zone of influence beyond it; exploration depth sufficient to reach a competent bearing stratum or to cover the full zone of stress influence beneath the foundation (a common rule of thumb is to extend below the depth at which added stress falls to about 10% of the applied bearing pressure); the full range of geotechnical parameters needed for the specific foundation type under consideration (strength and compressibility for shallow foundations; skin friction and end-bearing for piles); groundwater conditions as covered in Question 3; and site-specific hazards (seismicity, slope stability, contamination) that may not be evident from the desk study alone and need to be confirmed or ruled out by targeted field work.

(c) Codes, guidelines, and legislative framework

In the Canadian context the applicable framework includes: the National Building Code of Canada (NBCC) Part 4 (foundation design and geotechnical requirements, including seismic site classification); the Canadian Foundation Engineering Manual (CFEM) as the accepted industry-standard reference for geotechnical practice; relevant CSA standards (e.g., CSA S6 for bridge foundations where applicable); provincial Engineers and Geoscientists legislation (in BC, the Professional Governance Act and EGBC's professional practice guidelines, including the requirement for a qualified professional to take responsibility for geotechnical work and, where triggered, for Letters of Assurance under the local building bylaw); provincial/municipal building bylaws and zoning that may impose site-specific geotechnical report requirements; and, where groundwater or contamination is present, provincial environmental legislation (e.g., BC's Environmental Management Act and contaminated-sites regulation) governing handling and disposal of contaminated material and dewatering discharge. Federal legislation (e.g., the Fisheries Act) may also apply where the site interacts with a watercourse.

(d) Primary objectives of the field exploration component

Field exploration exists to convert the desk study's inferred ground model into confirmed, site-specific data: (i) directly confirm the stratigraphic sequence and its lateral/vertical variability at the actual structure location; (ii) obtain samples of adequate quality for the laboratory testing needed to derive design parameters; (iii) measure in-situ properties that cannot be reliably obtained from samples (SPT N-values, CPT resistance, in-situ permeability, standard penetration in gravel where sampling recovery is poor); (iv) establish groundwater conditions directly (levels, and where needed, piezometric pressures); and (v) identify any physical obstructions or anomalies (boulders, buried structures, voids) that would affect construction means and methods, feeding directly into constructability and cost planning for the contractor.

(e) Major headings of the geotechnical report

A geotechnical site investigation report is conventionally organized under the following major headings: Introduction (project description, purpose and scope of the investigation, terms of reference); Site Description (location, topography, current land use, and site history from the desk study); Investigation Procedure (field methods, exploration point locations and depths, sampling and in-situ testing methods, laboratory testing program); Subsurface Conditions (stratigraphy, soil/rock descriptions, groundwater observations, presented with borehole logs and a site plan); Discussion/Interpretation (engineering assessment of the ground conditions relative to the proposed structure, including any identified hazards); Design Recommendations (foundation type and allowable bearing/pile capacities, settlement estimates, lateral earth pressures, seismic site class, groundwater/dewatering and excavation/shoring guidance as applicable); Construction Considerations (excavatability, temporary works, quality-assurance/inspection recommendations during construction); and Limitations (a standard closing statement on the report's applicability, the point-sample nature of the data, and the requirement to notify the engineer of any conditions encountered that differ from those described). Appendices typically carry the borehole/test-pit logs, laboratory test results, and site plan.