Subsurface Exploration – Minimum depth below heavy embankments or dams For heavy embankments and dams of height h, what is the recommended minimum depth of soil exploration below ground level to ensure foundation adequacy and identify weak strata?

Introduction / Context:
Foundation safety for embankments and dams depends on the behavior of underlying soils. Subsurface exploration must extend deep enough to identify compressible layers, potential slip surfaces, and seepage paths influenced by the load of the structure.

Given Data / Assumptions:

• Embankment/dam height = h.
• Significant stress bulb extends below the base.
• Objective is to capture all strata that materially influence settlement and stability.

Concept / Approach:

Stress distribution beneath loaded areas extends to depths proportional to the loaded width and magnitude. For high earth structures, an exploration depth of at least 2 h is a widely used rule of thumb to intersect the stress bulb and detect weak layers that could cause excessive settlement or instability. Additional depth is required if weak strata persist or if rockhead is deeper.

Step-by-Step Solution:

Verification / Alternative check:

Design manuals recommend extending investigation to the depth where additional stresses become small relative to in-situ stresses and where no critical strata remain unchecked; for high embankments this often coincides with about 2 h or more.

Why Other Options Are Wrong:

Shallower depths (h/4, 1/2 h, h) risk missing deep weak layers; 3 h may be conservative but not the commonly cited minimum.

Common Pitfalls:

Stopping boreholes as soon as SPT values rise; ignoring valley geometry and seepage conditions; under-sampling lateral variability.

Final Answer:

2 h