Geotechnical Engineering Flashcards
PE Licence
What is a D10 value and how is it obtained?
D10, AKA effective grain size, is the particle diameter (in mm) for which only 10% of the particles are finer.
- 10% of the soil mass passes through this sieve size
It is obtained through a sieve analysis
Why is D10 important?
- Permeability: It is used in Hazens equation to estimate the hydraulic conductivity (permeability) of sandy soils.
- Filter Design: When designing filters (drainage layers), D helps determine the appropriate gradation of materials to prevent soil migration.
- Soil Uniformity: Uniformity Coefficient, which gives insight into the range of particle sizes. Cu = D60/D10.
What soils are defined as coarse grained soils?
Gravel and Sand.
It is classified by its particle size which can be obtained by determining the D10 in a sieve analysis.
What soils are defined as fine-grained soils?
Inorganic silts and clays.
It is classified by its particle size which can be obtained by a hydrometer test. The hydrometer test is based on Stoke’s Law, whihc relates the speed of a particle falling out of suspension to its diameter and soil density.
Hazens Uniformity Coefficient is Cu = D60/D10. What does the Cu value tell you
A low Cu indicates a uniform soil. Uniform soils has most of its particles around the same size.
A high Cu indicates a Well-Graded soil. Well graded soils have a wide range of particle sizes, which generally improves compaction and stability.
Sands:
- Cu < 4 -> Uniform sand
- Cu >= 6 -> Well-Graded sand
Gravels
- Cu < 4 Uniform Gravel
- Cu> 4 Well-graded Gravekl
What makes clays different than silts and coarse grained soils.
Clays display the characteristic of plasticity. Clays exhibit a high plasticity index. Clays also have a smaller particle size.
Plasticity is how a soil interacts with water and how it can be molded or deformed without cracking or changing volume significantly. Silts have low to no plasticity and sands are non-plastic. Sands do not change consistency with moisture.
There are two soil classification systems. Name them and the differences between the two.
AASHTO (American Association of State Highway Transportation Officials) is based on sieve analysis, plasticity index, and liquid limit. It rates the materials based off their suitability for roadway subgrades. The best soils are classified as A-1 and the worst (highly organics) A-8.
USCS (Unified Soil Classification System) is also based on grain size distribution, liquid limit, and plasticity index of the soil. This is where you get the symbols: GW, SP, SM, ML, CL, etc.
- The symbols each contain two letters. The first represents the particle size, and the second is a decriptive modifier. Gravels (G), Sands (S), Silts (M), Clays (C). Descriptors: Well-Graded (W), CLay (C), Poorly Graded (P), Silt (M). For USCS use the chart in the attached images to determine if it is “above the A line) which requires PI and LL
What is a loam
What is an alluvium
A loam is a type of soil that is considered ideal for plant growth befcause is has a balanced mixture of the three main soil types: sand, silt, clay. A typical loam soil contains roughly 40% sand, 40% silt, and 20% clay. It has good drainage, good nutrient holding capacity, and is workable.
An alluvium is a term used in geology to describe loose, unconsolidated sediments that have been deposited by running water, typically in riverbeds, floodplains or deltas. Alluviums have a low bearing capacity, are subject to liquefaction, and are highly erodible.
What is a non-invasive geophysical method of analyzing a subsurface. It uses high frequency electromagnetic waves to detect and map subsurface structures and features.
Ground Penetrating Radar (GPR)
It can locate utilities, voids, structural features like rebar and post tension cables in concrete.
Name 3 In-Situ Tests that are direct/invasive.
Cone Penetration Test (CPT) - Effective for soft clays, sands, and layered soils. Can also be used to estimate undrained shear strength, friction angle, and modulus.
Standard Penetration Test (SPT) - Test best suited for sands. Estimates clay shear strength are crude and should not be used in design. Determines relative density, consistency, and bearing capacity of soils.
Vane Shear Test - Measures the undrained shear strength of saturated fine grained soils, particularly soft clays. Not suitable for sandy or gravelly soils.
Borehole Shear Test
Describe the procedure for SPT
Split-Spoon sampler is driven into the soil at the bottom of a borehole using a 140 lb hammer dropped from 30”.
The number of hammer blows required to drive the sampler 18 inches is record as the N value or blow count. The first 6 inches it is driven down is not counted towards the blow count as there is often loose or disturbed material at the bottom of the borehole.
N < 4 -> Very loose/soft soil
N = 4-10 -> Loose/Medium Soft Soil
N = 10-30 -> Medium Dense/Firm Soil
N > 30 -> Dense/Stiff Soil
- Make sure to convert to N60 to account for efficienct of the hammer
- Disturbs the soil during sampling
- Less reliable in soft clays or gravel
- Most commonly used in-situ soil test
- CPT provides reliable and continuous data.
- ASTM D1586
What is the density of water in lb/ft^3 and kg/m^3
pw = 62.4 lb/ft^3 = 1000 kg/m^3
What is the water content at which a soil changes from a liquid state to a plastic state.
Liquid Limit
“Plastic State” is when a soil becomes moldable.
It is determined using the Casagrande Cup Method.
What is the water content at which a soil changes from a plastic state to a semi-solid state.
Plastic Limit
“Semi-solid State” is when a soil becomes crumbly.
It is determined by rolling a small soil sample into a 3 mm diameter thread.The plastic limit is the moisture content at which a soil begins to crumble when rolled.
Aeolian soils are deposited by ___
Alluvial soils are deposited by ___
Aeoliab -> Wind
Alluvial -> Flowing water
Give the different unit weights of water
Does 1 cm^3 of water have a mass of 1 gram?
yw = 9.81 kN/m^3 = 1 g/cm^3 = 62.4 lb/ft^3
Yes, 1 cm^3 of water = mass of 1 gram of water
- Mass = Density x Volume = 1 g/cm^3 x 1 cm^3 = 1g
What describes the flow of water through a permeable aquifer or soil
Darcy’s Law
q = kia
q = flow rate
k = hydraulic conductivity
i = hydraulic gradient
A = cross sectional area of aquifer
Describe how soils are typically compacted in the field
Soils are compacted to increase stability, strength, enhance resistance to erosion, decreases permeability, and decrease compressibility.
The soil is placed in lifts of a few inches to a few feet thick and then mechanically compacting the lifts. The thickness of the lifts depends on the soil type and compaction equipment.
Equipment: Smooth-wheel roller, rubber-tired roller, grid roller, sheepsfoot roller.
What is the basic laboratory test used to determine the maximum dry density of compacted soils
Proctor Test
What is the difference between allowable stress design (ASD) and load and resistance factor design (LRFD)
ASD: Uses FS to reduce capacity
LRFD:
