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Geotechnics > Soil mechanics Lecture 5 > Flashcards

Soil mechanics Lecture 5 Flashcards

1
Q

What is the equation for the degree of saturation?

A

Sr = Vw/Vv = wGs

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2
Q

What is the equation for bulk unit weight?

A

gamma = rhog = Mg/V = ((Gs+Sre)/(1+e))gamma_w

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3
Q

What is the equation for relative compaction?

A 
RC = rho_d/rho_dmax
rho_d = dry density
rho_dmax = maximum dry density
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4
Q

What is the equation for relative density?

A

Dr = (emax - e)/(emax-emin)

e: void ration
emax: maximum void ration
emin: minimum void ration

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5
Q

What is the definition of compaction?

A
  • Partially saturated soils
  • soil densification through application of external loads and mechanical mixing
  • reduction in total volume through removal of air
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6
Q

What is the definition of consolidation?

A
  • Saturates soils
  • soil densification through application of external loads
  • reduction in total volume through removal of water
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7
Q

What is common through both compaction and consolidation?

A

Volume of voids decrease in both cases

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8
Q

What are the benefits of compacting soil?

A
  • Increase stiffness (reduce settlement)
  • increase strength (increase bearing capacity, liquefaction resistance)
  • reduce permeability (forms an impermeable layer)
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9
Q

What are some methods of compaction?

A
  • static load/ pressure
  • kneading
  • impact
  • vibration
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10
Q

\What is the effect of static load/ pressure on fine and coarse grained soil?

A

Fine-grained - ok/ineffective

coarse-grained - ok/ineffective

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11
Q

What is the effect of kneading on fine-grained and coarse-grained soils?

A

fine-grained: ok

coarse-grained: coarse/ ineffective

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12
Q

What is the effect of impact on fine-grained and coarse-grained soils?

A

fine-grained: ok

coarse-grained: ok/ineffective

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13
Q

What is the effect of vibration on fine-grained and coarse-grained soils?

A

fine-grained: ineffective

coarse-grained: ok

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14
Q

What is the point of the proctor method?

A

To determine the optimum moisture content at which a given soil type will become the most dense and achieve its maximum dry density

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15
Q

How does a field compaction test work (sand cone method)?

A
  • 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
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16
Q

How does a field compaction test work (baloon densometer)?

A

Similair to a sand cone test, apart from a liquid filled balloon is forced into the hole to find the volume

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17
Q

What kind of pressure is present in geotechnics?

A

hydrostatic

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18
Q

What defines vertical stress in soils?

A

Total and effective stress

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19
Q

How do you calculate the vertical stress from the self weight of the soil?

A

sigma_z = rho_dgz (if dry)
sigma_z + u if wet
where u is the pore water pressure

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20
Q

How is pore pressure calculated?

A

equation of fluid statics

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21
Q

How is the total vertical stress calculated?

A

Total stress = pore water pressure + effective stress

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22
Q

What causes capillary rise?

A

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

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23
Q

What is the equation for cappillary rize?

A

hc is proportional to 1/d
hc = capilary rise
d= capillary tube diammeter

24
Q

What intrinsic parrameter can particle size distribution give you?

A
  • Coefficient of uniformity

- Cu

25
Q

What intrinsic parameters can the minimum density give you?

A
  • The maximum void ratio

- emax

26
Q

What intrinsic paramaters do finding the atterbeerg limits give you?

A
  • The plastic limit

- the liquid limit

27
Q

What intrinsic paramaters do finding the atterbeerg limits give you?

A
  • The plastic limit

- the liquid limit

28
Q

Which test results would be useful to design a road built on expansive clay subgrade?

A
  • the atterberg limits
29
Q

Which intrinsic parameters would be needed to assess the relative density of a gravelly pipeline backfill?

A
  • The minimum density (correlates to maximum void ratio)

- emax

30
Q

How are intrinsic parameters used in practice?

A
  • 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)
31
Q

What are the fundamental quantities?

A
  • Density of water
  • Density of solids
  • Specific Gravity
  • Unit weight of water
  • Water content
  • Void ratio
  • Bulk Density
32
Q

What is the mass of water equal to?

A

Mw = wGsrho_w

33
Q

What is the mass of the solids equal to?

A

Ms = Gs*rho_w

34
Q

What is 1+e equal to?

A

The total volume in a soil

35
Q

What is e equal to?

A

The volume of the voids when you take the volume of solids = 1

36
Q

What allows us to make the bulk unit weight into the saturated unit weight?

A

set Sr = 1

37
Q

What allows us to transform the bulk unit weight to the dry unit weight?

A

Set Sr = 0

38
Q

What is the buoyant unit weight?

A

gamma’ = gamma - gamma_w

It is the submerged unit weight

39
Q

What is the equation for liquidity index?

A 
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
40
Q

Why is the relative compaction formula particularly useful?

A

It helps to understand how the existing density relates to the maximum possible density of the soil

41
Q

What is the principle behind compaction?

A

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

42
Q

Does compaction or consolidation occur on saturated soils?

A

Consolidation

43
Q

What is the difference of the methods between compaction and consolidation?

A

Compaction: application of external loads and mechanical mixing
Consolidation: application of just external loads

44
Q

Does consolidation improve the density of soil?

A

Yes, it removes water, reducing the volume of the voids

45
Q

Where are compacted soils useful?

A

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

46
Q

What are geomaterials?

A

The use of alternate materials to fulfill functions in geotechnical engineering

47
Q

Where are sustainable geomaterials used?

A

They use rubber from scrap tyres and expansive soil, using the rubber to limit the expansion potential to build roads

48
Q

What are the main methods for compacting in the field?

A

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

49
Q

What compaction methods are used in the lab?

A

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

50
Q

What is graph is plotted when trying to optimise the compaction process in the lab?

A

Dry density vs water content (%)

51
Q

What do we need from a compaction curve?

A

to define the maximum value of a dry density that can exist at a certain water content.

52
Q

How do you obtain a compaction curve?

A

By compacting soils with different water contents, starting from a low water content and increasing it

53
Q

Why is there are a maximum water content for comapction?

A

When the soil becomes fully saturated, you start to replace mass with water, reducing the the compaction

54
Q

How does a proctor compaction test work?

A
  • 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
55
Q

What does the proctor method allow you to calculate?

A

the water content and the dry density

Geotechnics (12 decks)

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