Site Characterization and Subsurface Exploration
Planning the program
Questions in this domain ask whether you can scope an investigation and then read the results critically. Know the rules of thumb: borings at building corners and center, spacing roughly 50–100 ft for fills and 200–500 ft for linear work, and termination at the depth where the foundation-induced stress increase falls to about 10% of the effective overburden stress - often 1.5B to 2B below a footing, deeper for piles or groups.
Sampling and in-situ testing
Distinguish split-spoon (disturbed, SPT), Shelby tube (undisturbed, cohesive), and rock coring. For rock, know TCR, SCR, and RQD; RQD counts only sound pieces 100 mm (4 in) or longer measured along the core axis, and it is not recovery. SPT corrections are perennial exam material: N60 = Nm(ER/60), then N1,60 = Cn·N60 with Cn = (pa/σ'v)^0.5 capped near 2.0. Hammer type, rod length, sampler liner, and borehole diameter all matter. CPT gives continuous tip resistance, sleeve friction, and pore pressure but no sample; geophysics (refraction, MASW, resistivity) images stratigraphy and stiffness from the surface.
Traps
Reporting the water level observed the morning after drilling rather than the stabilized level - caving, perched, and artesian conditions all mislead. Also watch for sample disturbance inflating compressibility, and for confusing RQD with core recovery.
Worked rule of thumb
Field N = 18 with a 60%-efficient hammer and standard sampler: N60 = 18. At 20 ft, γ = 120 pcf, water table at 10 ft: σ'v = 2,400 − 624 = 1,776 psf. Cn = √(2,116/1,776) = 1.09, so N1,60 ≈ 20.