🪨 Geotechnical Field Investigation Methods: Borings, Soundings, and Subsurface Profiling

Purpose and Planning

A geotechnical investigation translates subsurface uncertainty into informed engineering decisions. The scope must be matched to the structure type, load magnitude, site complexity, and available data. ASCE 7 Section 11.8.3 and IBC Section 1803 mandate investigations for specific seismic design categories. FHWA Geotechnical Engineering Circular 5 (FHWA-NHI-01-031) provides detailed guidance on investigation planning for transportation projects.

The investigation report must be sufficient to: classify soils (USCS or AASHTO), characterize strength and compressibility, identify groundwater conditions, detect subsurface anomalies (voids, weak zones, contamination), and support foundation type selection. Over-scoped investigations are wasteful; under-scoped investigations lead to claims, change orders, and failures.

Boring Types

• Hollow-stem auger (HSA): most common method for onshore borings in soils without boulders. Auger rotates to advance with soil traveling up the flutes; hollow center allows SPT sampler through without removing drill string. Efficient to 30 m depth; soil disturbance on auger limits detailed stratigraphy from auger cuttings.
• Mud rotary: pressurized drilling fluid (bentonite or polymer slurry) lifts cuttings and stabilizes borehole walls. Required in unstable soils at depth (>30 m), below groundwater in sandy soils, or for NX rock coring. Best for deep borings and rock transitions.
• Casing advancing: steel casing driven and soil removed by wash boring or bailing. Used in very unstable conditions or when SPT in gravel layers is required (casing prevents borehole collapse).
• Hand auger / Geoprobe: for shallow investigations (< 5 m), environmental screening, or locations inaccessible to a drill rig. Limited to soft soils.

In-Situ Testing Methods

SPT (ASTM D1586): covered in detail at SPT Correlations. Provides disturbed sample + blow count. Universal in North America; high variability.

CPT (ASTM D5778): a cone-tipped rod advanced at 20 mm/s; measures tip resistance qc, sleeve friction fs, and pore pressure u2 continuously at ~2 cm intervals. Classified by soil behavior type (SBTn, Robertson 2009). No sample recovered but best for continuous profiling, thin-layer detection, and liquefaction assessment. Piezocone (CPTu) adds u2 pore pressure; excellent for soft clay characterization and dissipation tests for Cv.

Flat plate dilatometer (DMT, ASTM D6635): a flat blade pushed into soil; two pressure readings (A and B) give lateral stress index KD, material index ID, and dilatometer modulus ED. Strong correlations to preconsolidation pressure OCR and Ko in clay; useful where CPT cannot distinguish stress history.

Vane shear test (VST, ASTM D2573): a four-bladed vane rotated in soft to medium clay to measure peak undrained shear strength Su and remolded strength Sr. Sensitivity = Su/Sr. Best performed below 5 m to avoid surface disturbance effects. Recommended for Su < 50 kPa where SPT is unreliable. Vane correction factor μ = 1.05 − 0.015·PI (Bjerrum, 1973) adjusts for anisotropy and strain rate effects.

Pressuremeter test (PMT, ASTM D4719): a cylindrical probe expanded in a borehole to measure pressure-volume response. Provides Ménard modulus EM, limit pressure pL, and creep pressure pf. Used for lateral pile design (p-y curves), foundation modulus, and in fractured rock where SPT is meaningless.

Rock Coring and RQD

Rock cores recovered by double or triple tube core barrels. Standard sizes: NQ (47.6 mm core), HQ (63.5 mm), PQ (85 mm). NX core (54.7 mm) is historical standard referenced in many specifications.

Rock Quality Designation (RQD) is computed from the sum of intact core pieces ≥ 100 mm long, divided by total core run length: RQD = (Σ intact pieces ≥ 100 mm) / total run × 100%.

• RQD 90–100%: Excellent
• RQD 75–90%: Good
• RQD 50–75%: Fair
• RQD 25–50%: Poor
• RQD < 25%: Very Poor

RQD is used in rock mass classification systems (RMR, Q-system) for tunnel and foundation design. Always record drilling fluid losses, recovery percentage (total core / run), and qualitative description of fracture condition and infilling.

Groundwater Monitoring

Groundwater depth at time of drilling is unreliable — it takes days to weeks for water level in a borehole to equilibrate in low-permeability soils. Install piezometers for reliable data:

• Open standpipe (Casagrande) piezometer: slotted PVC pipe at the zone of interest with bentonite seal above and below. Measures piezometric level; simple but slow response in clay.
• Vibrating wire piezometer: electronic sensor provides real-time data and rapid response. Preferred for monitoring during construction or for artesian conditions.
• Multi-level piezometer: separate sensors at different depths to detect perched water tables or artesian pressures in distinct layers.

Field Permeability Tests

Slug test (ASTM D4044): a slug of water is rapidly inserted into or removed from a piezometer; the rate of water-level recovery provides hydraulic conductivity k. Tested volume is small (near-borehole zone). Best for k = 10⁻³ to 10⁻⁷ m/s.

Packer test (Lugeon test): pressurized water is pumped into an isolated rock zone between inflatable packers. Lugeon value (L) = flow rate per unit length per 1 MPa pressure. Used for dam and tunnel grouting design in rock.

Pumping test: pump from a well, measure drawdown in observation wells. Provides aquifer transmissivity T = k·b (b = saturated thickness) over a large influence radius. Required for dewatering design for large excavations.

Boring Layout and Number — IBC/ASCE Requirements

IBC Section 1803.5 requires at minimum one boring or test pit per building column location and at least one boring per 230 m² (2,500 ft²) of building footprint. Boring depth must extend below the zone of influence of the proposed foundation, typically to a depth of 1.5–2 × the least foundation dimension below foundation level, or to bedrock, or until N > 50 in dense sand/gravel for three consecutive 150 mm intervals.

AASHTO LRFD Section 10.4.2 specifies boring spacing for bridges: at each pier and abutment for simple structures; additional borings as needed to define the subsurface profile between locations. FHWA recommends a minimum of three borings per structure unless subsurface conditions are very uniform and previously characterized.

For large sites, a phased investigation approach is most cost-effective: Phase 1 (desktop study, widely spaced borings) defines general conditions; Phase 2 (targeted borings/CPTs at foundation locations) provides design-level data.