Soil Mechanics Lecture 9 Flashcards
What defines a small mass movement?
- mass movement is within a small, contained area
- retaining structures can be designed/ built to ‘retain’ the collapsed soil
- Additional stabilisation methods are usually not required
What defines a large scale mass movement
- Movements over a large area - slop stability analysis required
- retaining structures may be too expensive to build
- alternative stabilisation/ monitoring systems may be needed
What is are examples of large scale mass failure?
Falls - detatched from a steep slope; descends mostly throught the air by free fall, leaping rolling. very to extremely rapid movements
Topples - forward rotation about a pivot point under the action of gravity
Slide -
Lateral spread - shear failure or liquefaction along nearly horizontal soil layers
Flow - flow slide in sand
What different types of slides are there?
- Shear failure causing slump
- Debris slide in shear become a flow slide
- Translational movement of a major part of the slip surface
What are the purposes of retaining structures?
- Create temporary or permanent space (contruction sites, underground parking, subway stations)
- Retain soil
- stabilise slopes or excavations
What is the design goal of retaining structures?
To keep withing the servicibility limit states and to prevent damage to existing infrastructure
What are different kinds ultimate limit states?
- Bearing capacity failure
- Sliding
- Overturning
- General instability
What are the most checked ultiate limite states?
Sliding and Overturning
What are some examples of externally stable retaining walls?
- Gravity
- Cantilever
- Tieback
- Braced
What are some examples of an internally stable retaining wall?
- Reinforced soil (soil nails (natural soil), geotextiles/grids (fill), metallic strips(fill))
Where does the resistance come from in a gravity wall?
The resistance comes from self weight - bear in mind the soil has to support the weight of the wall - also bear in mind that it is retaining soil, so it also needs to counteract the load from the pressure of the soil
The reistance comes from the weight and the friction
Where does the resistance come from in a cantilever wall?
the volume below the ground surface
Where does the resistance come from in a tieback wall?
The volume of the wall below the ground surface and the friction from the tieback - the lenghth and angle of the tieback has to be good enough to make sure it goes to other zones that are not going to collapse
Where does the resistance come from in a braced wall?
the braces
How do metallic strips work?
They pull the surface under tension and keep the soil behind stable
How do geogrids/ geotextiles work?
Geogrids -
Geotextiles - carpet goes under soil level, over the front, and then a wee bit over the top
built in sections over the entire height of the surface
What are the 3 kinds of soil structure interactions?
- at-rest
- active (soil towards wall)
- passive (wall towards soil)
What is K0?
the at-rest lateral earth pressure coefficient
K0 = 1 - sin(theta) (mohr’s circle of stress)
Where theta is the angle of internal shearing resistance
We do not need to know how to calculate this but it may be given in an exam question
What do the points on the mohr circle of stress represent?
The 2D stress state, sigma’ and tau on diffent planes passing through P
What is KA?
The active lateral earth pressure coefficient?
The mohrs circle becomes bigger as the horizontal stress increases, verticle doesn’t (expands to the left)
what is the equation for KA?
KA = (1-sin(theta))/(1+sin(theta))
What happens the mohrs scenario for passive state?
The mohrs circle shifts to the right
What is Kp
The passive lateral eath pressure coefficient
What is the equation for Kp
(1+sin(theta))/(1-sin(theta))
