Assess Feasibility: Determine if the proposed project can actually be built at the chosen location safely and economically.

Determine Design Parameters: Provide the structural engineer with accurate values for soil bearing capacity, settlement estimates, active earth pressures, and groundwater elevations.

Hazard Identification: Detect hidden geohazards such as active faults, subsurface cavities (karst), ancient landslides, or loose liquefiable soils.

Construction Planning: Allow contractors to select appropriate excavation equipment (e.g., excavators vs. blasting), design temporary excavation support systems (shoring), and plan dewatering methods.

Reviewing existing geological and topographic maps.

Studying historical aerial photographs to identify old stream beds, abandoned mines, or previous land uses.

Reviewing nearby well logs or previous geotechnical reports from adjacent properties.

Observing rock outcrops to measure strike and dip.

Noting vegetation types (e.g., willow trees indicate shallow groundwater).

Identifying signs of instability like curved tree trunks, tension cracks in the soil, or seepage zones on slopes.

Disturbed Samples (Split-Spoon): The soil structure is completely altered or destroyed during retrieval (e.g., auger cuttings, SPT split-spoon samples). The split-spoon sampler is a thick-walled steel tube that splits in half to reveal the recovered soil. These are perfectly fine for classification tests (grain size, Atterberg limits) but useless for determining the soil's actual in-situ strength or compressibility.

Undisturbed Samples (Shelby Tube): Specialized tools are used to retrieve a sample while minimizing structural disturbance. The most common is the Shelby Tube, a thin-walled, seamless steel tube with a sharpened cutting edge. It is pushed smoothly and steadily into soft to stiff clays (never hammered) using the hydraulic pressure of the drill rig. These expensive, high-quality samples are mandatory for accurate consolidation and triaxial shear strength laboratory testing.

Test Pits: Digging shallow trenches (usually $\lt 5\text{m}$) with a backhoe. This is relatively cheap, allows the engineer to visually inspect the soil profile in 3D, and take large block undisturbed samples. However, it is limited by the reach of the machine and usually cannot go below the water table without trench collapse.

Soil Borings (Auger Drilling): Using a hollow-stem auger to drill deep into the soil profile. This is the standard method for soil exploration, allowing samplers (split-spoon or Shelby tubes) to be lowered through the hollow center to sample the undisturbed soil ahead of the drill bit.

Rock Coring (Rotary Drilling): When bedrock is encountered, a diamond-studded drill bit is used to cut and retrieve intact, continuous cylindrical samples of the rock (cores). Wireline coring is the modern standard, where the inner core barrel is retrieved rapidly via a wire cable, eliminating the need to pull the entire drill string out of the hole for every sample.

$E_m$ = Hammer efficiency (can vary from 45% for a donut hammer to 90% for an automatic hammer)

$C_B$ = Borehole diameter correction

$C_S$ = Sampler correction (whether a liner is used)

$C_R$ = Rod length correction

Cone Penetration Test (CPT): Pushing an instrumented steel cone continuously into the ground at a constant rate. It measures tip resistance, sleeve friction, and pore water pressure in real-time, providing a continuous, high-resolution soil profile without retrieving physical samples. CPT is vastly superior to SPT for identifying thin, weak liquefiable sand layers.

Vane Shear Test: Used exclusively in very soft, saturated clays. A four-bladed steel vane is pushed into the clay at the bottom of a borehole and slowly rotated. The torque required to shear the clay cylinder provides a highly accurate, direct measurement of the soil's undrained shear strength ($s_u$).

Groundwater Monitoring (Piezometers): Knowing the precise location of the water table is absolutely critical for calculating effective stress and slope stability. Engineers install Piezometers (slotted PVC pipes or vibrating wire sensors) into finished boreholes to measure and monitor static groundwater levels and pore pressures over months or years.

Total Core Recovery (TCR): The total length of the core recovered (including solid rock, highly crushed rock, and soil-like clay seams) divided by the total length drilled (the core run length), expressed as a percentage. Low TCR indicates highly weathered, weak, or cavernous rock that washed away during drilling.

Solid Core Recovery (SCR): The total length of only the solid, full-diameter, cylindrical pieces of rock recovered divided by the total length drilled. This completely ignores any crushed rock or soil gouge.

0% - 25%: Very Poor (Heavily crushed, highly faulted rock)

25% - 50%: Poor

50% - 75%: Fair

75% - 90%: Good

90% - 100%: Excellent (Massive rock, extremely widely spaced joints)