Question 2 of 4

Question 2 (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 deep foundation project the investigation must: (i) establish subsurface stratigraphy and groundwater conditions to the full depth over which pile or caisson capacity develops, not just the near-surface zone relevant to a shallow footing; (ii) determine geotechnical parameters for both shaft resistance (unit skin friction/adhesion by stratum) and end bearing at the anticipated founding elevation; (iii) identify geohazards that specifically affect deep foundations — obstructions or boulders that could deflect or refuse a pile, artesian pressure that could affect caisson excavation stability, and liquefiable layers that could reduce skin friction under seismic loading; (iv) recommend a foundation type (driven pile, drilled shaft/caisson, micropile) and capacity consistent with the structure's loads and tolerable settlement; and (v) supply the driveability/excavatability information the contractor needs for means-and-methods planning.

(b) Site characterization considerations once scope is set

Once scope is fixed, characterization must address: spatial coverage of exploration points across the foundation footprint, with points located at each pile/caisson group or, for a large structure, on a grid fine enough to capture lateral stratigraphic variability; exploration depth extended well below the anticipated founding elevation (a common rule of thumb is at least 3 pile-group widths, or to a depth where added stress becomes negligible, whichever governs) so that a weaker layer below the bearing stratum is not missed; the full range of parameters needed specifically for deep-foundation design (undrained/drained shear strength by stratum for skin friction correlations, SPT N or CPT tip resistance profiles for empirical capacity methods, and rock quality/strength if founding on bedrock); groundwater conditions, including any artesian pressure relevant to open-caisson excavation; and any hazard (obstructions, contamination, corrosive groundwater chemistry affecting pile durability) that the desk study flags for confirmation in the field.

(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 geotechnical practice reference, including its deep-foundation design chapters; relevant CSA standards (e.g. CSA S6 for bridge foundations, or CSA A23.1 where cast-in-place concrete piles/caissons are used); provincial Engineers and Geoscientists legislation (in BC, the Professional Governance Act and EGBC's professional practice guidelines, including sign-off and Letters of Assurance requirements where triggered by the local building bylaw); municipal building bylaws and zoning; and, where groundwater, dewatering discharge or contamination is present, provincial environmental legislation (e.g. BC's Environmental Management Act). Federal legislation (e.g. the Fisheries Act) may also apply if the works interact with a watercourse, and any dedicated pile-driving noise/vibration bylaw should be checked given the construction method.

(d) Primary objectives of the field exploration component

Field exploration converts the desk study's inferred ground model into confirmed, site-specific data for deep-foundation design: (i) directly confirm the stratigraphic sequence and its variability to the full depth of influence of the piles/caissons; (ii) obtain samples of adequate quality for the strength and index testing that skin-friction/end-bearing correlations depend on; (iii) measure in-situ parameters (SPT N, CPT resistance) at close enough depth intervals to profile capacity with depth, since deep-foundation capacity is depth-dependent in a way a shallow-footing investigation is not; (iv) establish groundwater conditions, including any artesian pressure relevant to caisson construction; and (v) identify obstructions (boulders, buried structures, very dense/hard layers) that would refuse a driven pile or require special drilling methods for a caisson, directly informing the contractor's means-and-methods planning and risk allocation.

(e) Major headings of the geotechnical report

The report follows the standard geotechnical report structure: Introduction (project description, purpose and scope, terms of reference); Site Description (location, topography, land use and history from the desk study); Investigation Procedure (field methods, exploration locations/depths, sampling and in-situ testing, laboratory program); Subsurface Conditions (stratigraphy, soil/rock descriptions and groundwater, presented with logs and a site plan); Discussion/Interpretation (engineering assessment relative to the proposed deep foundation, including any identified hazards); Design Recommendations (foundation type, allowable/factored pile or caisson capacity by depth, group effects and settlement, driveability or excavation guidance, and seismic site class); Construction Considerations (obstructions, vibration/noise, dewatering, quality-assurance and load-testing recommendations during installation); and Limitations (standard closing statement on the report's applicability). Appendices carry the borehole/CPT logs, laboratory results and site plan.