Marine geotechnical investigation: borings for soil exploration around waterfront structures are generally extended to what depth below the low-water level to capture foundation-critical strata?

Introduction / Context:
Reliable foundation design for quay walls, dolphins, and breakwaters requires adequate subsurface data. Marine borings must extend sufficiently below low-water level (L.W.L.) to characterize bearing layers, potential weak strata, and liquefiable or compressible soils that influence stability and settlement.

Given Data / Assumptions:

• Exploration is for typical harbour or nearshore structures.
• Goal is to intercept all relevant strata affecting load transfer and stability.
• Standard practice uses a rule-of-thumb depth where detailed project-specific analyses are unavailable.

Concept / Approach:
As a practical guideline, borings are often taken to a minimum of about 30 m below L.W.L. unless refusal or engineering judgment dictates otherwise. This depth commonly captures soft marine clays, sand lenses, and denser bearing layers needed for design parameters.

Step-by-Step Solution:
Establish baseline: ensure borings extend beyond the influence zone of foundation stresses.Apply rule-of-thumb: adopt ≈30 m below L.W.L. in absence of site-specific criteria.Adjust in detailed design as per CPT/SPT results and expected foundation type (piles vs gravity).

Verification / Alternative check:
Project specifications frequently state minimum depths on the order of tens of metres below L.W.L.; 30 m serves as a common baseline.

Why Other Options Are Wrong:

• 35–45 m may be required at some sites but is not the typical minimum guideline; the question asks the general practice.

Common Pitfalls:

• Stopping borings at refusal without alternative methods, missing weaker deeper strata.

Final Answer:
30 m