Soil mechanics Lecture 5

Question 1

What is the equation for the degree of saturation?

Answer 1

Sr = Vw/Vv = wGs

Question 2

What is the equation for bulk unit weight?

Answer 2

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

Question 3

What is the equation for relative compaction?

Answer 3

RC = rho_d/rho_dmax
rho_d = dry density
rho_dmax = maximum dry density

Question 4

What is the equation for relative density?

Answer 4

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

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

Question 5

What is the definition of compaction?

Answer 5

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

Question 6

What is the definition of consolidation?

Answer 6

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

Question 7

What is common through both compaction and consolidation?

Answer 7

Volume of voids decrease in both cases

Question 8

What are the benefits of compacting soil?

Answer 8

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

Question 9

What are some methods of compaction?

Answer 9

• static load/ pressure
• kneading
• impact
• vibration

Question 10

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

Answer 10

Fine-grained - ok/ineffective

coarse-grained - ok/ineffective

Question 11

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

Answer 11

fine-grained: ok

coarse-grained: coarse/ ineffective

Question 12

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

Answer 12

fine-grained: ok

coarse-grained: ok/ineffective

Question 13

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

Answer 13

fine-grained: ineffective

coarse-grained: ok

Question 14

What is the point of the proctor method?

Answer 14

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

Question 15

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

Answer 15

• 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

Question 16

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

Answer 16

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

Question 17

What kind of pressure is present in geotechnics?

Answer 17

hydrostatic

Question 18

What defines vertical stress in soils?

Answer 18

Total and effective stress

Question 19

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

Answer 19

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

Question 20

How is pore pressure calculated?

Answer 20

equation of fluid statics

Question 21

How is the total vertical stress calculated?

Answer 21

Total stress = pore water pressure + effective stress

Question 22

What causes capillary rise?

Answer 22

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

Question 23

What is the equation for cappillary rize?

Answer 23

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

Question 24

What intrinsic parrameter can particle size distribution give you?

Answer 24

• Coefficient of uniformity

- Cu

Question 25

What intrinsic parameters can the minimum density give you?

Answer 25

- The maximum void ratio | - emax

Question 26

What intrinsic paramaters do finding the atterbeerg limits give you?

Answer 26

- The plastic limit | - the liquid limit

Question 27

What intrinsic paramaters do finding the atterbeerg limits give you?

Answer 27

- The plastic limit | - the liquid limit

Question 28

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

Answer 28

- the atterberg limits

Question 29

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

Answer 29

- The minimum density (correlates to maximum void ratio) | - emax

Question 30

How are intrinsic parameters used in practice?

Answer 30

- 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)

Question 31

What are the fundamental quantities?

Answer 31

- Density of water - Density of solids - Specific Gravity - Unit weight of water - Water content - Void ratio - Bulk Density

Question 32

What is the mass of water equal to?

Answer 32

Mw = w*Gs*rho_w

Question 33

What is the mass of the solids equal to?

Answer 33

Ms = Gs*rho_w

Question 34

What is 1+e equal to?

Answer 34

The total volume in a soil

Question 35

What is e equal to?

Answer 35

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

Question 36

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

Answer 36

set Sr = 1

Question 37

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

Answer 37

Set Sr = 0

Question 38

What is the buoyant unit weight?

Answer 38

gamma' = gamma - gamma_w It is the submerged unit weight

Question 39

What is the equation for liquidity index?

Answer 39

``` 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 ```

Question 40

Why is the relative compaction formula particularly useful?

Answer 40

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

Question 41

What is the principle behind compaction?

Answer 41

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

Question 42

Does compaction or consolidation occur on saturated soils?

Answer 42

Consolidation

Question 43

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

Answer 43

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

Question 44

Does consolidation improve the density of soil?

Answer 44

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

Question 45

Where are compacted soils useful?

Answer 45

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

Question 46

What are geomaterials?

Answer 46

The use of alternate materials to fulfill functions in geotechnical engineering

Question 47

Where are sustainable geomaterials used?

Answer 47

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

Question 48

What are the main methods for compacting in the field?

Answer 48

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

Question 49

What compaction methods are used in the lab?

Answer 49

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

Question 50

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

Answer 50

Dry density vs water content (%)

Question 51

What do we need from a compaction curve?

Answer 51

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

Question 52

How do you obtain a compaction curve?

Answer 52

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

Question 53

Why is there are a maximum water content for comapction?

Answer 53

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

Question 54

How does a proctor compaction test work?

Answer 54

- 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

Question 55

What does the proctor method allow you to calculate?

Answer 55

the water content and the dry density