A 12-section interactive reference guide covering the core geotechnical engineering topics used daily in foundation design, site investigation, and earthwork. Includes USCS soil classification, SPT/CPT methods, bearing capacity equations (Terzaghi), deep foundation design, slope stability analysis, Rankine earth pressure, and seismic liquefaction procedures (ASCE 7).
Each section targets a core geotechnical topic: soil classification by USCS (ASTM D2487) and AASHTO M145; site investigation methods including SPT (ASTM D1586), CPT (ASTM D5778), vane shear, and rock coring with RQD; laboratory testing from Atterberg limits to triaxial shear and consolidation; shallow foundation bearing capacity using Terzaghi's equation with IBC presumptive values; deep foundations including driven piles, drilled shafts, and group effects; slope stability methods from infinite slope analysis to Bishop Simplified; retaining wall types with Rankine active/passive pressure; ground improvement methods (dynamic compaction, PVDs, deep soil mixing, geosynthetics); seismic site classification (A–F) and the Seed-Idriss liquefaction procedure; geotechnical report content; and a master formula quick-reference.
Use the Prev / Next buttons at the bottom of the reader, or press the left/right arrow keys on your keyboard. Click the ☰ menu button in the top-right corner to open the table of contents and jump directly to any of the 12 sections. The gold progress bar at the top tracks your position through the guide.
This guide references ASCE 7-22 (seismic site classification and loads), IBC 2021 (soil investigation requirements, Section 1803; presumptive bearing values, Table 1806.2), FHWA design manuals for deep foundations, MSE walls, and retaining structures, ASTM D2487 (USCS classification), ASTM D1586 (SPT), ASTM D5778 (CPT), and ASTM D2435 (consolidation). Always verify which edition of IBC and ASCE 7 has been adopted by your local authority having jurisdiction (AHJ).
Bearing capacity factors Nc, Nq, and Nγ are functions of the soil friction angle φ' — use published tables (Terzaghi, Meyerhof, or Hansen) or software for precise values. For SPT-based correlations, apply the N60 energy correction (CE × CB × CR × CS) and the overburden correction (CN) to obtain (N1)60 before comparing to liquefaction resistance curves. Consolidation time calculations require the drainage path length Hdr (half the layer thickness for double drainage) and Cv from laboratory consolidation testing.
The Standard Penetration Test (SPT, ASTM D1586) drives a split-spoon sampler 18 inches and counts blows — it recovers a disturbed soil sample and gives the familiar N-value. The Cone Penetration Test (CPT, ASTM D5778) continuously pushes an instrumented cone and records tip resistance and sleeve friction at millimeter resolution, but does not recover samples. SPT is preferred when soil samples are needed for visual identification and laboratory testing, or in gravelly soils where CPT cone refusal occurs. CPT is preferred when continuous profiling and high-resolution stratigraphic data are needed, for liquefaction analysis, and for soft to medium soils where disturbed SPT samples are inadequate.
Use Terzaghi's bearing capacity equation: qu = c·Nc·Fcs + q·Nq·Fqs + 0.5·γ·B·Nγ·Fγs, where Nc, Nq, and Nγ are bearing capacity factors based on soil friction angle φ', c is cohesion, q is the overburden pressure at footing depth, γ is the soil unit weight, and B is the footing width. Apply a factor of safety of 2.5–3.0: q_allowable = qu / FS. For preliminary design on unimproved sites where soil data is unavailable, IBC Table 1806.2 provides presumptive bearing values by soil type (sandy gravel = 3,000 psf; sand = 2,000 psf; soft clay = 1,000 psf).
A liquefaction assessment is typically required when the site seismic design category is C, D, E, or F (per ASCE 7) AND the site contains saturated cohesionless soils (sands, silts) within the upper 50 feet. The simplified Seed-Idriss procedure compares the Cyclic Stress Ratio (CSR, from ground motion and overburden) to the Cyclic Resistance Ratio (CRR, from SPT N-values or CPT qc values). If CSR/CRR > 1.0, liquefaction is triggered. Mitigation options include densification (vibratory compaction, stone columns), deep foundations that bypass the liquefiable layer, or drainage improvements.
A standard geotech report recommends: (1) allowable bearing capacity (psf or ksf) at the proposed footing depth, specifying whether it's for dead load only or combined D+L, (2) minimum footing depth (frost depth or competent bearing stratum, whichever is deeper), (3) foundation type recommendation (spread footings vs. mat vs. deep foundations) based on soil conditions and structural loads, (4) estimated settlement (total and differential), (5) earthwork specifications (subgrade preparation, compaction requirements for fill), (6) lateral earth pressure parameters for retaining walls, and (7) seismic site class. Always clarify whether stated bearing values are allowable or ultimate.