Chapter 3: Natural Soil Deposits and Subsoil Exploration

What is the scope of natural soil deposits and soil exploration programs?

Study soil origin, distribution, and properties, and investigate subsurface conditions for safe foundation design

What are the broad categories of soil from weathering processes?

Soils from mechanical (physical breakdown) and chemical (mineral alteration) weathering; these affect particle size and mineral composition

How are residual soil deposits formed?

Form in place from weathering of parent rock without transport; properties reflect the underlying rock

What is gravity-transported soil?

Soil moved downslope by gravity (colluvium); typically loose and unstable

What are alluvial soil deposits?

Soils deposited by rivers; braided streams deposit coarse materials, meandering streams deposit finer soils

How are lacustrine deposits formed?

Soils deposited in lakes; usually fine-grained due to calm water conditions

How are glacial deposits formed?

Deposited by glaciers; can be unsorted (till) or sorted (outwash), often dense and variable

What are aeolian soil deposits?

Soils transported by wind (loess, dunes); typically well-sorted and loose

What are organic soil deposits?

Soils with decomposed plant material; highly compressible and weak

What are some local terms used for soils?

Examples: loess, till, marl, peat; names reflect origin or composition

What is the scope of subsoil exploration?

Determine soil profile, properties, groundwater, and suitability for construction

What are the components of a subsoil exploration program?

Field investigation, sampling, lab testing, and reporting

What are common exploratory boring methods?

Auger boring, wash boring, rotary drilling

What types of soil samples are needed?

Disturbed (classification) and undisturbed (strength/compressibility)

What is the standard penetration test (SPT)?

Split spoon sampler test; N-value indicates soil density/strength

Why is a scraper bucket used in soil exploration?

To clean boreholes and remove loose material for better sampling

Why is a thin-walled tube sampler used?

To obtain undisturbed samples of cohesive soils

How does a piston sampler work?

Uses a piston to minimize disturbance; ideal for soft clays

Why is observing the water table important after drilling?

Groundwater affects effective stress, strength, and design

What is the vane shear test?

Measures undrained shear strength of soft clays using a rotating vane

What is the cone penetration test (CPT)?

Cone pushed into soil; resistance correlates with strength and layering

What is the pressuremeter test?

Expands probe in soil to measure stiffness and strength

What is the dilatometer test?

Expands membrane in soil; gives stiffness and stress history correlations

What is the Iowa borehole shear test?

Measures in-situ shear strength in a borehole

What is the stepped-blade test used for?

Determines at-rest earth pressure coefficient (K₀)

What is rock coring?

Extracting cylindrical rock samples to evaluate strength and quality

What is a boring log?

Record of soil layers, depths, test results, and groundwater

What are geophysical exploration methods?

Seismic refraction, cross-hole seismic, resistivity surveys

What is included in a soil exploration report?

Site description, soil profile, test results, analysis, and recommendations