[L15] Retaining wall design

Large scale mass movements

Movements over a large area slope → stability analysis required

Retaining structures may be too expensive to build

Alternative stabilisation methods/monitoring systems may be needed

Types of mass movements

1. Falls

• detached from a steep slope, descends mostly through air by free fall, leaping rolling

• rapid to extremely rapid movements

2. Topples

• forward rotation about some pivot point under the action of gravity & forces exerted by adjacent units or fluids in cracks

• example: clayey gravel sand topples above layer of clean sand

3. Slide → rotational landslide

• shear failure causing slump to more stable configuration

• example: clay

4. Slide → translational landslide

• debris can slide in shear or become flow slide

• example: soil on top of bedrock

5. Slide → block slide

• translational movement of major part of slip surface (common on larger slides, quick clay)

6. Lateral spread

• shear failure or liquefaction along nearly horizontal soil layers

• some parts sink, others left standing higher

7. Flow

• flow side in sand

Tailing

• By-product or waste from mineral extraction

• Geomaterials = Geotechnical or Mine engineers responsible for their safe disposal

Tailings dam construction

• some constructed w/ tailings sand or aggregate material from site

• thickened, pasted or filtered deposits depending on w/c

• Tailing pond → Impoundment → beach → freeboard → embankment → starter dam → drain → crest foundation

Tailings dam failure (Brumadinho dam)

1. start with tailings & started dike

2. new dikes are built on top of solidified mud tailings

3. dikes built upstream to hold

• issues with high water pressure → could potentially breach the dam & produce liquefaction

Small scale mass movements

Mass movement within a small, contained area

Retaining structures can be designed/built to ‘retain’ the collapsed soil

Additional stabilisation methods usually not required

Purposes of structures for small scale mass movements

1) Create temporary space Construction sites, etc.

2) Create permanent space (Underground parking Subway stations, etc.)

3) Retain soil

4) Stabilise slopes or excavations

SLS

Serviceability limit states

1. Excessive deflection towards open space leads to ground loss

2. Damage to existing infrastructure

ULS

Ultimate limit states

1. bearing capacity failure

2. sliding → most critical

3. overturning → most critical

4. general instability

Design goal

avoid limit states

Externally stable retaining structures

Gravity

Cantilever

Tieback

Braced

• resistance is developed by external mechanism

Gravity wall

resistance derived almost exclusively from self weight

examples: gabion, pre-fabricated concrete elements

Cantilever wall

examples: secant concrete piles, underground parking garage, steel sheet piles

Tieback

Resistance derived mainly from the tieback interaction with the retained soil

Bracing

Resistance derived from inclusion of additional, stiff bracing elements

Typical excavation sequence in cross-lot excavations

1. initial V-cut cantilever excavation

2. strut installation and pre-loading in small trenches in soil berms

3. V-cut excavation to next level and strut installation

4. final grade

Summary of forces on different walls

Internally stable retaining structures

Reinforced soil

Soil nails (natural soil)

Geotextiles/geogrids (fill)

Metallic strips (fill)

• resistance is developed by internal mechanism

Geotextiles

→ friction & interlocking with soil to develop tensile forces

prevents shear failure

At rest state soil-structure interaction

K₀ = at-rest lateral earth pressure coefficient = σ’_h/σ’_v

σ’_h = horizontal effective stress

K₀=1-sinΦ

Φ = angle of internal shearing resistance of soil

Activate state soil-structure interaction

• wall moves away from soil

• horizontal stress decreases

Ka=(1-sinΦ)/(1+sinΦ)

Mohr-Coulomb failure envelope → soil collapse when the soil’s stress state reaches it

Passive state

• wall moves towards the soil

• horizontal stress increases

• Mohr circle expands to the right

K_p=(1+sinΦ)/(1-sinΦ)

Front of wall

use Kp

Back of wall

use Ka