Design of Strong Light Walls

Question 1

Types

Answer 1

-Secant: Piles biting into each other with the alternate ones being reinforced
-Contiguous pile walls
-Sheet pile:
*Set up then alternate sheets hammered in
*Goes deep into the ground unlike gravity walls
*Required ties and anchors
-Diaphragm: Reinforced conc blocks, can bite into each other

Secant and diaphragm installed before excavation

Question 2

Free earth method

Answer 2

-Applies to sheet, diaphragms and piled walls
-Gives the depth of the wall, max bending moment and F in anchor/strut
-Assumes rotation about point of anchorage implying one row of anchors
-Also assumes low K0 and doesnt work for K0»Ka

Question 3

Free earth method steps

Answer 3

1. moments about O taken to get pile length, gives a cubic in d
2. Horizontal equilibrium for anchor F
3. Check vertical equilibrium when wall roughness is included
   - W is usually small and fiction often ignored
4. Plot SFD of the wall to find the point of max moment and calculate it

Question 4

Free earth method limits

Answer 4

FoS applied to the entire passive F is erroneous for high c’ and low phi’ with considerable PP so the distributions of the active and passive pressures are erroneous and pile BM is over predicted
- Fos can also be applied to strength

Solutions:
-Undrained has a couple methodologies with the equation for sigma_h’v changed
-Rowes Correction reduces costs by reducing the size of the passive distribution, its close to true passive for a loose sand

Question 5

Anchor examples

Answer 5

-Shallow plate anchors apply earth pressure theory
*alternative is injected grot anchors
-dt = Distance from the bottom of the tendon to the base of the anchor with H/2 > dt > H/3

Question 6

Anchor design

Answer 6

-Assume delta’ = 0 since block would lift otherwise
-Active/passive pressure zones assumed not to interact, ensured by anchor placement, and surcharges shouldnt be placed on the active zone
-T is found through equilibrium with active and passive forces
* Usually large for deep anchors but can be over predicted since locally contained failures are not accounted for
–> requires bearing capacity theory

For a plate with a finite width:
-Include tau cating against the anchor, alont the sides of the box (Q0’ = double integral of sigma_z’ with z then y)
* uses y=z_max.Tan(45 + phi’/2) so dy=dz.Tan(45 + phi’/2)
-Ignores side friction in the active zone
-Assumes an unlikely failure mode