Lecture 12 Flashcards
How does the development and construction of civil engineering infrastructure play a role in the interaction with soil?
Alters the state of the soil(s)
What must be accounted for in geotechnical analysis?
Changes in the soil state, e.g., density, stress, fabric
What increases as you go deeper into the soil?
Gravity and stress state
When is there an isotropic stress condition?
If 3 arrows have the same magnitude
When is there a bi-axial stress condition and an axes symmetric stress condition?
If only 2 arrows same magnitude
What are the characteristics of stress?
- Non scalar quantity
- Direction-dependent
- Uses a matrix
What are characteristics of pressure?
- Scalar quantity โ single parameter
- Same in all directions
What does hydrostatic mean?
- Complete saturated conditions, no flow
- That value of pressure will increase from zero at the top and the increase will be linear
How can pore water pressure be calculated?
u = ๐พ_wz = ฯ_wgz
What is the pressure head?
Pressure head = energy in a material divided by height in the water
What stresses increase the most in soil?
Vertical
How are vertical stresses defined?
Typically defined in terms of total and effective stress components
Why do vertical stresses exist?
This may be due to the self-weight of soil and/or external loads
How does total vertical stress change with depth?
Increases linearly
What is the total vertical stress in dry homogeneous soil?
ฯ_z=๐พ_dz
What is the total vertical stress in water or saturated soil?
ฯ_z=๐พ_sz
What is pore pressure?
This is the pressure in the water filling the voids of saturated soil
How does pore pressure change with depth?
Increases linearly
What is the effective stress?
- Net stress supported by the soil skeleton
- Upwards direction
- Controlled by soils fabric
How can effective stress be calculated?
ฯโ_z=ฯ_z-u
What do changes in effective stress affect?
Compression, distortion, and shearing resistance
How do effective vertical stresses change with depth in a homogeneous soil?
Increase linearly
How does water rise in capillary tubes?
Due to surface tension effect
- Surface tension forces pull water inside the tube until their vertical components are in equilibrium with the weight of water inside the tube
How does capillary rise work?
Like a suction
What will the pore pressure be at the height of the water in capillary tube?
Negative
What does a smaller pore space mean?
More suction of water
What is capillary rise height directly proportional to?
1/d
How can pore water pressure in the capillary zone in sands be estimated?
u = -(h_c๐พ_wS_r)
When does the equation for pore water pressure in sand not apply?
Doesnโt apply when underwater and in completely dry conditions
How can the height of the sand column in contact with water be calculated?
h = C/e*D_10
- e void ratio
- C - constant
- D_10 effective size