Ground Improvement C: Stabilisation Flashcards
What are equations for:
- bulk density
- bulk unit weight
- moisture content
- dry density
What test is done for compaction?
Nuclear density meter (in-situ measurement)
For this typical compaction curve, what is the acceptable area for maximum dry density?
NB. equation for air voids is V_g / V
What is the typical equipment used for ballast compaction?
- Vibrating plate
- Dynamic track stabiliser
What is the typical equipment used for compaction of each layer of new rail embankment?
- Towed rollers
- Self-propelled rollers
What is the typical equipment used for compaction of loose in-situ granular soils?
- Vibro compaction (vibroflot)
- Dynamic compaction
Name the different types of reinforcement techniques for in-situ ground (often for soft clays)
Stone columns
Vibrated cement columns
Deep soil mixing (DSM)
Jet grouting
Give an example of how to improve embankment stability
Resistance on shear surface increased by adding stone columns
Base geosynthetic reinforcement to further increase stability
Main thing is to stiffen the subgrade
What are the two types of stone column reinforcement techniques?
Displacement
- most popular out of the two
Replacement
- water jets used to create the column (very messy)
What is a drawback of the stone column reinforcement technique?
Gives a stiffness that is too small to prevent Rayleigh wave development
Unlikely to generate a high enough stiffness for high speed trains traversing low embankments over soft in-situ cohesive soil foundations
- due to critical velocity effects
- columns not stiff enough within the wavelength zone
NB. wouldn’t use technique underneath the track (only underneath the embankment)
Discuss the advantages of the vibro stone column technique for granular soils
Advantages:
- vibro poker densifies the sand
- the stone column allows better drainage of excess pore pressures
Discuss the advantages and disadvantages of the vibro stone column technique for cohesive soils
Advantages:
- stone column acts as reinforcing element within the soft ground
- columns allow excess pore pressure dissipation
Disadvantages:
- if clay is too soft, the ground won’t hold the column in place
- may get some clay into the column (more than using a sand drain)
- if using the replacement method (instead of displacement), it can get quite messy
Give an overview of the vibrated cement columns (displacement) reinforcement technique
NB. also referred to as vibro concrete columns
Similar to stone columns, except cement is used (instead of granular material)
The head of the column is formed into a ‘mushroom’ shape as the vibroflot is removed
- mushroom effect is in denser areas (top/bottom); increases area so more stress comes onto it
A load distribution (transfer) platform is then used to distribute loads onto the columns
What is a drawback of the vibrated cement columns reinforcement technique?
Could end up creating differential settlement in between the columns
- due to variations in stiffness
Give an overview of the Deep Soil Mixing (DSM) reinforcement technique?
Mechanical technique that improves the ground by mixing the soils in-situ with various materials to form a column
What are the two types of deep soil mixing techniques?
Lime columns
- hydrated, powder form
Cement columns
- powder and slurry form
Give an overview of the two jet grouting techniques
Permeation Jet Grouting
- grout pumped into pore space or natural fissures in the soil
Hydrofracture Jet Grouting
- natural fissures can be opened up or new ones formed by applying high grout pressures
For the Ledsgard case study, what technique was used to mitigate for critical velocity through subgrade stiffening?
Deep soil mixing (DSM) cement/lime columns were used to stiffen the soil
The columns (mostly around 6/7m) were taken down to a max. depth of 13m below the railhead, to stop Rayleigh wave formation
What are the advantages of the compaction grouting technique?
- No big stress wave generated
- Get a very sturdy foundation solution
What are the disadvantages of the dynamic compaction technique?
- May generate stress waves (because a weight is being dropped)
- May generate liquefaction if there is water in the ground
Why couldn’t you use rollers for a weak cohesive soil (describe technique)
- Load applied
- Excess pore pressure generated; needs time to dissipate
- Roller would only do top 300/400mm, even if you could compact it
