During a foundation inspection in Hawaii, you find pahoehoe lava rock at the excavation bottom. The architect specifies a 2-foot deep foundation. What is the most appropriate action?
Correct Answer
B) Consult the engineer about bearing on rock surface
Pahoehoe lava rock provides excellent bearing capacity, but the foundation design should be reviewed by the engineer since the bearing conditions differ from the original soil assumptions.
Why This Is the Correct Answer
When the actual subsurface conditions differ from those assumed during design, the engineer of record must be consulted. Pahoehoe lava rock has high bearing capacity — often exceeding soil — but the foundation design was based on different assumed conditions. The engineer must confirm whether the existing design is appropriate for bearing on rock or requires modification (e.g., depth, reinforcement, anchor bolts into rock).
Why the Other Options Are Wrong
Option A: Excavate deeper into the rock as specified
Excavating deeper into solid rock per the original specification is unnecessary and potentially destructive. The specification assumed soil conditions; solid rock at the bottom may eliminate the need for the specified depth. Proceeding without engineering review ignores the changed conditions.
Option C: Add gravel base before pouring concrete
Adding a gravel base before pouring concrete is inappropriate when bearing on solid rock. Gravel bases are used to improve drainage and provide a working surface over poor soils — they are not needed on solid rock and would actually reduce the load path efficiency.
Option D: Pour concrete directly on the rock surface
Pouring concrete directly on rock without engineering review bypasses the engineer's authority over design changes. Even though rock is often better than soil, the contractor cannot unilaterally make this substitution — the engineer must approve the changed bearing conditions.
Memory Technique
Unexpected condition = call the engineer. Always. Even good surprises (solid rock instead of soil) require engineering sign-off. Remember: 'Surprise in the hole? Call before you pour.'
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