Measuring soil strength Flashcards
How do soils generally fail?
shear when the shear
forces acting on the soil reach the shear strength
What does soil shear strength =?
Soil shear resistance
What is the strength of a material?
The greatest stress (shear stress) it can maintain
What is soil shear strength derived from?
Cohension and friction
What are the characteristics of soil cohesion?
Stress independent
Cementation between sand grains
Electrostatic attraction between clay particles
Stickiness
How is factor of safety calculated?
Shear resistance / shear force
How does factor of safety calculation define slope stability?
If less then or close to one slope is unstable
Is resistance stress dependant?
Yes
What is the equation for stress?
Stress (sigma) = force (N)/ area (m2)
What are 2 methods for emasuring direct shear?
Shear box - force on top in box with known area allows stress to be known when a load cell moves at a set rate (but need to reverse to continue - know remolded strength)
Ring shear - small sample rotated round - find resdiual strength - stress is the rate of turn
What will happen to normally consolidate material during shear?
WHen they moe the soil compacts
WHat happens to heavily compacted soil during shear?
Rise up and dilate - as particles move apart
What happens to loosely compacted soils during shear?
Densification
WHat will happen to stress when a soil dilates?
Will be a high peak before “collapse” to residual
What is the strength of densely packed sands?
stronger (then loosely) but weaken due to dilatancy as they shear
What is the strength of loosely compacted sands?
weaker (than dense), but strain leads to increased
density (tighter packing)
What will happen to the strength of sands as strain increases?
Strength converges to a critical value
Why might multiple tests be done for failure criteria?
Get peak stress and allow a stress envelope to be developed which can be comapred to mohr circle to see if failure will occur
What is the angle of internal friction for sand?
34* depending on grain size (relatively steep angle)
What is the angle of internal friction like for clays? (Smectite)
5* as particles finer and flatter
What happens to mohr circles as loading increases?
Mohr circle increases in diameter until failure occurs and the failure envelope is met
Will soil above the water table be drained or undrained?
Drained as water can flow
Will soil be drained or undrained when below the water table?
Undrained as nowhere to flow
What is effective stress?
The stress that controls changes in the volume and strength of a soil
Terzaghi (1925-36)
How do you calculate effective stress?
sigma - pore pressure
What happens to the mohr circle and failure envelope as water pressure increases in soil?
Effective stress lowered and failure more likely as less frictinal strength is mobilised
What is drained soil?
pore fluid is free to move in or out of the soil without affecting the soil properties. High permeability (e.g. sandy) soils and/or slow loading (give
the water time to move
What is undrained soil?
no movement of pore fluid within the soil, interaction between solid particles and pore fluid. Low permeability soils and/or rapid loading
What are the parameters of drained triaxial experiment?
Drainage allowed during cell pressure application
Increase deviatoric stress keeps cell pressure constant, sample fluid pressure constant
Water can drain so test ran slower to prevent the development of pore presssure
What sediment does drained soil strength apply to?
Sediments which can contain pore fluid but that pore fluid can move and change volume
Volume can change during strain: grains incompressible but pore fluid can escape
When will drainage occur in the undrained triaxial water experiment?
Drainge in consolidation
When will drainge occur in the draine dtriaxial water experiment?
Drainage in consolidation and shear
What will happen because water cant drain in the undrained triaxial experiment?
Pore water pressure increases and can be measured
When does sediment liquefy?
when grains no longer in contact and strength drops to zero
What happens in undrained densely and loosley compacted soils?
Dense - prevention of dilation (drop pore pressure and increase strength)
Loose - Contraction prevented (increse pore pressure and loss of strength)
What will ultimately lead to liquefaction?
as no volume chnage during shearing of loose sands
What are the stress paths of drained, undrained and liquefied?
Drained - No pore water pressure (diagonal line)
Undrained - Development of pore water pressure (efefctive stress decrease to envelope)
Liquefaction - increase to failure enevelope and decrease to 0 for normal and shear stress
when calculating soil properties what should you always ensure with the units?
convert cm3 to m3 by dividing by 1,000,000 move the number back 6
