Section 1 Flashcards
How are soils made?
Soils are formed over time as a consequence of climate, mineral, and biological processes
Types of Rock
Igneous, sedimentary, metamorphic
Types of weathering
mechanical weathering
chemical weathering
What are some examples of mechanical weathering?
moving water, glacial movement, thermal expansion and contraction, stress relief, plant roots, wave action, wind, freezing and thawing of water in cracks
What is meant by ‘particle’?
layers of silca + layers of alumina + the interlayer (how the layers of silica and alumina are bonded) = particle
examples of chemical weathering
oxidation, hydration, reduction, carbonation, solution, hydrolysis, leaching, cation exchange
What is colluvium?
colluvium is soil transported by gravity
what is alluvium?
alluvium is soil transported by running water
what is lacustrine or marine transportation?
Lacustrine or marine transportation is soil transported via still water
what is till?
Till or glacial till is soil that has been transported via glacier(s)
What is Diagenesis?
The process in which sediments compact under pressure, expel connate fluids, and gradually become solid rock (lithification)
What is cementation?
Cementation involves ions carried by groundwater chemically precipitating to form new crystalline material within sediment pores
What is pedogenesis?
soil formation
What are soil profiles?
Soil profiles are tools that geotechnical engineers use to map out the strata of soil that will be encountered during an engineering project.
What is the boundary in grain size between fine grain (silt and clay) and course grain (sand) soils?
0.075mm which corresponds to the Sieve No. 200 is the grain size boundary between fine grain and course grain soils
What is the boundary in grain size between course grain soils and gravel?
4.75mm (which corresponds to the sieve number 4) is the grain size that determines course grains from gravel
What are silica tetrahedrons composed of?
silicon and oxygen
What are the two clay microstructures?
- silica (tetrahedral) sheet
- alumina (octahedral) sheet
What are alumina octahedrons composed of?
alumuna and either oxygen or hydroxyl
What is the microstructure of Kaolinite?
kaolinite is a 1 to 1 alumina to silica sheet held together by hydrogen bonds
Note
- kaolinite is the largest clay microstructure
What is the microstructure of Illite?
Illite is a 2 to 1 silica to alumina sheet, so two silica sheets sandwiching the alumina sheet
- this microstructure is held together by potassium ions (medium strength bond)
- medium size clay microstructure
What is the microstructure of montmorillonite?
Montmorillonite is a a 2 to one sillica to alumina sheet, so two silica sheets sandwiching the alumina sheet
- this microstructure is held together by Van der Waals forces (weak bond) so these bonds can be easily infiltrated by water
- smallest clay structure
What’s the big idea with specific surface?
The more surface area (smaller particles) the more opportunity for water to stick to the surface
What is adsorbed water? (pg. 12)
The surface charges on fine-grained soils are negative (anions). These negative surface charges attract cations and the positively charged side of water molecules from the surrounding water. This thin film/layer of water bonded to the mineral surface is known as adsorbed water.
What are some of the grain size descriptors associated with the grain size distribution graph?
median grain size (D50), effective grain size (D10), coefficient of uniformity (Cu), coefficient of curvature (Cc)
What are some values you can assume in solving for phase relationships?
V (total volume) = 1
or
Vs (Volume of solids) = 1
or
Gs (specific gravity of solids) = 2.7
What is the purpose of subsurface characterization?
- to evaluate the general suitability of a site for a proposed project
- to enable for an adequate and economical design
- to disclose and make provisions for difficulties that may arise during construction due to subsurface conditions
What are some items that should be included in the subsurface characterization?
- nature of the deposits including geologic origin and subsequent deposition, erosion, weathering etc.
- depth, thickness, lateral extent, elevations, and composition of each soil and rock stratum
- ground water elevations, their differences across the site and changes with time and environmental conditions
- engineering properties of the soil and rock strata that affect the performance of the structure
What should you reference in conducting reconnaissance for subssurface characterization?
- results of previous site investigations
- geologic maps
- topographical maps
- soils maps
- water well logs
- google earth
- aerial photographs
- remote sensing data
- utility locations
In a site visit to classify the subsurface characteristics what are you looking for?
- checking out previous grading and development
- performance of nearby structures
o the effect of new construction on existing
structures - surface drainage conditions
- exposed soil and rock types
- site access
What is involved in a detailed site investigation for subsurface characterization?
- geophysical methods
- soil borings and sampling
o number, location, depth, and type - in situ tests
- laboratory tests
- groundwater monitoring
What are some drilling methods used in geotechnical engineering?
- test pits
- augering (solid and hollow stem)
- rotary drilling
- percussion drilling
How do you know how many boreholes to make at a site?
there is a standard for the number of boreholes to make based on the building type (building v. subdivision)
What are some items to know about borehole depth?
- minimum depth of 6 meters unless rock or very dense material is encountered
- must penetrate below soft or compressible layers (e.g., fill)
- at least 3m into rock
- stress increase due to foundation load is less than 10% of geostatic stress
What are some drilling methods involved in geotechnical engineering?
- test pits
- augering (solid stem and hollow stem)
- rotary drilling
- percussion drilling
What determines the number of boreholes that will be made at a site?
Guidelines for the minimum number of boreholes are based on whether the construction is for a building or for subdivisions and are provided in an ASTM standard in a table
How deep should boreholes be?
- minimum depth of 6m unless rock or very dense material is encountered
- must penetrate below soft or compressible layers (e.g., fill)
- at least 3m into rock
- 1 to 3 times maximum foundation width
- stress increase due to foundation load is less than 10% of geostatic stress
What are augers?
Augers are truck mounted and equipped with continuous-flight augeres that bore a hole 100mm to 250mm in diameter. Augers can have a solid or hollow stem.
What are some advantages of hollow stem augers?
- fast
- inexpensive
- equipment readily available
- samples easily obtained
- drilling fluids not required
- groundwater levels easily obtained
What are some disadvantages of hollow stem augers?
- depths limited to 15 to 45m
- cannot be used in Rock
What is Rotary Drilling/Wash Borings?
Rotary drilling or wash boring is a drill bit advanced by the weight of the drill string and down-force of the drilling rig and rotated by a motor. Drilling fluids are used to remove the cuttings and maintain the stability of the borehole
What are some advantages of rotary drilling/wash borings?
- fast
- drills into Soil and Rock
- large depths possible
- casing not required
What are some disadvantages of rotary drilling/wash borings?
- drilling fluid required with associated time and cost
- difficulty in identifying groundwater level during drilling
- sampling not possible during drilling
What types of samples can we obtain in geotechnical engineering?
- disturbed samples
- and “undisturbed” samples
Which samples are disturbed samples?
- bulk samples
- split-barrel (split-spoon) samples
Which samples are “undisturbed” samples?
- Hand Trimmed samples
- thin-walled samples
- piston samples
- Denison samples
- Coring
How does taking split-barrel samples work?
- sampler is driven into the soil at the bottom of the boring using repeated blows of a 140lb hammer falling 30in.
- used in both coarse-grained and fine-grained soils
- samples obtained as part of the Standard Penetration Test (SPT)
How do thin-walled Shelby Tube Samples work?
- sampler is pushed into the soil at the bottom of the boring using the hydraulic system on a drilling rig
- most often used in fine-grained soils
- amount of disturbance related to inside clearance ratio
What is the Standard Penetration Test (SPT)?
- the standard penetration test is the most common in situ test worldwide
- Method: Drive a split-barrel sampler into the ground and measure the number of blows required to advance it 1ft.
What does ASTM D1586 (SPT Standard) specify about this test?
That you should record the number of blows required for each 6inch. of penetration or fraction thereof:
- the first 6in. is considered to be a seating drive
- the “standard penetration resistance”, “blow count”, or “N-value” is the sum of the blows for the second and third 6-in. increments and is expressed as “blows per foot (bpf)”
- if the sampler is driven less than 18in. the number of blows for each 6in. increments and fractions thereof should be recorded (e.g. 50/4in.)
What are some components/calculations involved in calculation for SPT?
- theoretical free-fall energy of the SPT hammer: E(theo) = (weight of hammer) * (height the hammer falls) = 140lb * (30in) = 4200in.-lbs
- friction and eccentric loading case losses: E(actual) = ER(r) * E(theo)
- standard rod energy ratios of 60%
NOTE: ER(r) obtained via calibration or assumed based on typical values
What are the advantages and disadvantages of SPT?
Advantages
- obtain a sample and a number
- simple and rugged
- suitable in many soil types
Disadvantages
- obtain a sample and a number (and that’s it??)
- disturbed sample suitable for index tests only
- crude number for analysis
What is the Cone Penetration Test (CPT)?
- the cone penetration test involves a steel probe with a 60 degree apex tip and internal strain gages or load cells to measure tip stress, sleeve friction, resistance, and (optionally) pore pressure.
- hydraulically pushed at a rate of 2cm/sec
- no boring, no samples, no cuttings, no spoil
- continuous readings of tip stress, sleeve friction, resistance, and (optionally) pore pressure, and other parameters
- ASTM D 5778
What are the advantages and disadvantages of Cone Penetration Tests?
Advantages
- fast and continuous profiling
- results not operator dependent
Disadvantages
- no sample is obtained
- high capital investment
- requires skilled operator and calibration
- may not work in all soils types
What do the index properties of COARSE GRAIN soils tell us?
Index properties of coarse grain soils tell us the relative density of the soil
What factors affect the engineering behavior of FINE GRAINED soils
Factors that affect the engineering behavior of fine grain soils include:
- clay mineralogy
- chemical and electrical bonds between clay particles
- interaction between clay particles and water
How can we identify the index properties of fine grained soils?
Atterberg Limits
What is the purpose of the Atterberg limits?
Atterberg limits are simple, arbitrary tests to quantify the complex interaction between clay particles and water. They are ‘low-tech’ tests that provide useful indicators of engineering behavior of fine-grained soils such as strength and compressibility.
What are the Atterberg limits?
- Liquid limit (LL)
- Plastic limit (PL)
- Plasticity Index (PI)
