Soil mechanics Lecture 5 Flashcards
What is the equation for the degree of saturation?
Sr = Vw/Vv = wGs
What is the equation for bulk unit weight?
gamma = rhog = Mg/V = ((Gs+Sre)/(1+e))gamma_w
What is the equation for relative compaction?
RC = rho_d/rho_dmax rho_d = dry density rho_dmax = maximum dry density
What is the equation for relative density?
Dr = (emax - e)/(emax-emin)
e: void ration
emax: maximum void ration
emin: minimum void ration
What is the definition of compaction?
- Partially saturated soils
- soil densification through application of external loads and mechanical mixing
- reduction in total volume through removal of air
What is the definition of consolidation?
- Saturates soils
- soil densification through application of external loads
- reduction in total volume through removal of water
What is common through both compaction and consolidation?
Volume of voids decrease in both cases
What are the benefits of compacting soil?
- Increase stiffness (reduce settlement)
- increase strength (increase bearing capacity, liquefaction resistance)
- reduce permeability (forms an impermeable layer)
What are some methods of compaction?
- static load/ pressure
- kneading
- impact
- vibration
\What is the effect of static load/ pressure on fine and coarse grained soil?
Fine-grained - ok/ineffective
coarse-grained - ok/ineffective
What is the effect of kneading on fine-grained and coarse-grained soils?
fine-grained: ok
coarse-grained: coarse/ ineffective
What is the effect of impact on fine-grained and coarse-grained soils?
fine-grained: ok
coarse-grained: ok/ineffective
What is the effect of vibration on fine-grained and coarse-grained soils?
fine-grained: ineffective
coarse-grained: ok
What is the point of the proctor method?
To determine the optimum moisture content at which a given soil type will become the most dense and achieve its maximum dry density
How does a field compaction test work (sand cone method)?
- dig a small hole in the compacted soil
- remove and weigh the soil
- then dry and weigh the soil to determine the moisture content
- fill the hole with a pre-calculated amount of sand from a jar and cone device
- the dry weight of the soil removed is divided by the volume of dry sand needed to fill the hole, this gives the density of the compacted soil
How does a field compaction test work (baloon densometer)?
Similair to a sand cone test, apart from a liquid filled balloon is forced into the hole to find the volume
What kind of pressure is present in geotechnics?
hydrostatic
What defines vertical stress in soils?
Total and effective stress
How do you calculate the vertical stress from the self weight of the soil?
sigma_z = rho_dgz (if dry)
sigma_z + u if wet
where u is the pore water pressure
How is pore pressure calculated?
equation of fluid statics
How is the total vertical stress calculated?
Total stress = pore water pressure + effective stress
What causes capillary rise?
Surface tension forces will pull water inside the tube until their vertical components are in equilibrium with the weigh of the water in the tube
What is the equation for cappillary rize?
hc is proportional to 1/d
hc = capilary rise
d= capillary tube diammeter
What intrinsic parrameter can particle size distribution give you?
- Coefficient of uniformity
- Cu
What intrinsic parameters can the minimum density give you?
- The maximum void ratio
- emax
What intrinsic paramaters do finding the atterbeerg limits give you?
- The plastic limit
- the liquid limit
What intrinsic paramaters do finding the atterbeerg limits give you?
- The plastic limit
- the liquid limit
Which test results would be useful to design a road built on expansive clay subgrade?
- the atterberg limits
Which intrinsic parameters would be needed to assess the relative density of a gravelly pipeline backfill?
- The minimum density (correlates to maximum void ratio)
- emax
How are intrinsic parameters used in practice?
- You can use these parameters to decide what kind of soil you are dealing with and make engineering decisions based on that conclusion
- More generally you can make decisions based on the soil type and the engineering use (there are tables to help at the preliminary phase)
What are the fundamental quantities?
- Density of water
- Density of solids
- Specific Gravity
- Unit weight of water
- Water content
- Void ratio
- Bulk Density
What is the mass of water equal to?
Mw = wGsrho_w
What is the mass of the solids equal to?
Ms = Gs*rho_w
What is 1+e equal to?
The total volume in a soil
What is e equal to?
The volume of the voids when you take the volume of solids = 1
What allows us to make the bulk unit weight into the saturated unit weight?
set Sr = 1
What allows us to transform the bulk unit weight to the dry unit weight?
Set Sr = 0
What is the buoyant unit weight?
gamma’ = gamma - gamma_w
It is the submerged unit weight
What is the equation for liquidity index?
Il = (w-w_p)/(w_l-w_p) w = water content w_p = water content at plastic limit w_l = water content at liquid limit
Why is the relative compaction formula particularly useful?
It helps to understand how the existing density relates to the maximum possible density of the soil
What is the principle behind compaction?
It increases the density of the soil
from:
rho_d = Ms/V
As we decrease the volume of a air, the mass does not change, and so the value of density increases
Does compaction or consolidation occur on saturated soils?
Consolidation
What is the difference of the methods between compaction and consolidation?
Compaction: application of external loads and mechanical mixing
Consolidation: application of just external loads
Does consolidation improve the density of soil?
Yes, it removes water, reducing the volume of the voids
Where are compacted soils useful?
Retaining walls, where soil is uses as a backfill
In dams or embankments they use layers of compacted soils
In underwater pipelines they use compacted soils to stop pipelines moving sideways
What are geomaterials?
The use of alternate materials to fulfill functions in geotechnical engineering
Where are sustainable geomaterials used?
They use rubber from scrap tyres and expansive soil, using the rubber to limit the expansion potential to build roads
What are the main methods for compacting in the field?
1) rollers are used on soils needing static load/ pressure
2) kneading achieved using rollers
3) a crane with a weight attatched is used on soils needing impact compaction
4) vibrating rollers are used on soils needing vibration compactio
What compaction methods are used in the lab?
1) static pressure system for static laod/pressure application
2) the kneading process is done by a clamp with a damper
3) the impact involves dropping a heavy weight into a mould
4) vibration techniques involves a vibrating labatory table
What is graph is plotted when trying to optimise the compaction process in the lab?
Dry density vs water content (%)
What do we need from a compaction curve?
to define the maximum value of a dry density that can exist at a certain water content.
How do you obtain a compaction curve?
By compacting soils with different water contents, starting from a low water content and increasing it
Why is there are a maximum water content for comapction?
When the soil becomes fully saturated, you start to replace mass with water, reducing the the compaction
How does a proctor compaction test work?
- Drop a hammer onto a given soil layer in many different positions, densifying and compacting the soil
- repeat the process for an additional soil layer
- do this until you reach the top of your steel mould
- remove the soil that is above the height of the mould
- calculate the volume of the mould and weigh the sample
What does the proctor method allow you to calculate?
the water content and the dry density
