Wk 3 Flashcards
Blow or Heave-failure eqn
Ko eqn
eqn for OCR + what is stands for
- relationship with Ko?
Definition of shear strength of a soil
What does it mean when the shear strength of a soil has been reached?
max stress has been reached
why do we want to know shear strength
how could soil fail?
What contributes to shear strength?
Different contributions to Cohesion
what happens when soil shears
change of volume
change of pore water pressure
Define dilaton
in dense soil:
- increase in volume during shear
- pore water pressure initially drops in shear zone
Dense: packed effectively, less pores
top layer of particle moves right hand side, height increases
define contraction
in loose soil:
- reduction in volume during shear
- pore water pressure initially rises in shear zone
Loose: loads of pores
Small particles fall into the pore holes, becomes more effectively packed, volume decreases
Definitions of these conditions:
- Undrained
- partially drained
- drained
Consider this diagram when:
- surcharge of 25kN/m^2 has been applied
- applied immediately afterwards
- a long time afterwards
Consider this diagram when:
- surcharge of 25kN/m^2 has been applied
- applied immediately afterwards
- a long time afterwards
- sand is same
Consider this diagram when:
- surcharge of 25kN/m^2 has been applied
- applied immediately afterwards
- a long time afterwards
Tut 3, Q1
b) no immediate change in clay layer effective stress doesn’t change (copy it down)
- water level still at top of clay layer (has 0 in pore water pressure diagram)
- same layer has 50 due to surcharge
- 6m down, has 30 due to pore water pressure in clay layer if no surcharge (applied for long time)
Tut 3, Q2
a) clay - long time afterwards becomes like sand
b)
1) work out vertical stress normally (no surcharge)
2) clay layer- no immediate change in effective stress (copy down)
3) work out pore water pressure backwards (0-25, 126-91)
4) calculate porewater pressure normally in bottom of clay layer (as if no surcharge, 60- applied for long time)
5) work out pore water pressure normally in sand as well (60+30)
Tut 3, Q4
ai) Change is RAPID
1)pore water pressure = 0 at water level
pwp = 30 at bottom of sand, (310) to the ground water lvl
pwp = 230 at bottom of clay, (15+5+3)10 to water lvl in standpipe
aii) ‘When excavation has reached top of clay’- just when it happened
remove sand layer, only draw clay
3m reduced to 1m in standpipe above new ground level
- effective stress in clay is same (copy down)
- work backwards to get pore water pressure
- work out pore water pressure at top of sandstone layer (15+5+1)*10 = 210
b)
level of water in standpipe must be lowered to 9.8m above the base of the clay.
Tut 3, Q3
a)
1) work out vertical stress normally,
2) Artesian condition, so (4+2)*10 = 60 for pore water pressure in clay
b) 2m reduced to 1m in standpipe above ground lvl
1) vertical stress no effect
2) no immediate change in effective stress of clay (copy down)
Tut 3, Q5
Draw this diagram
- immediately after 50 kN/m^2 surcharge applied
- long time afterwards
- immediately after 50 kN/m^2 removed
____________________________________________________________________ - Clay, Ko = 0.75
- sand, Ko = 1.1
Draw diagram for total horizontal stress
(for immediately after 50 kN/m^2 removed diagram)
Worked example 1
Draw this diagram
- immediately after 50 kN/m^2 surcharge applied
Worked example 2
