Road Construction Materials

Question 1

Give the 3 Rock classes and their characteristics?

Answer 1

• Igneous: formed as magma cools either deep in the earth (intrusive) or near the surface (extrusive). Typically requires drilling / blasting.
• Metamorphic: Rocks that have been subject to intense heat and pressure that change their chemical composition. Gneiss/Slate. Vary in ripability.
• Sedimentary: Derived from reconstituted sediment that forms layers.

Question 2

What are the types of OM?

Answer 2

Defined by particle size

Boulders, cobbles and topsoil: little value.

Coarse Grained soils: gravels, sands

Fine grained soils: cohesive (may be a problem when wet) clays and silts.

Well graded: mix of coarse grained soils with minor amounts of cohesive materials to act as a bonding agent

Question 3

What are gradation curves?

Answer 3

• graph showing particle size on the x-axis and % of soil on the Y-axis
• Build one by progressively passing soils through finer sieves and measuring the proportion that passes.
• Optimum mix is 70% gravel, 25% sand and 5% fines.

Question 4

Define Capilarity

Answer 4

A fine grained soils capacity to absorb water and transmit it in all directions.

damages road fills

water softens and expands subgrade

prevent it with a layer of coarse textured material between water source and capping.

Question 5

Define permeability

Answer 5

Ability of a soil to pass water (sands and gravels) do this but fines don’t.

Question 6

Compressibility and Consolidation

Answer 6

Compressibility: reduction in soil volume due to application of a force. Squeeze out the air and water so that the soil occupies less space. A little water helps the road compress and consolidate

Consolidation: increase in density of soil under actual service conditions. Caused by traffic over time

Question 7

Elasticity

Answer 7

Ability of a soil to return to its original shape after load is removed. Roads with elastic soils wear out fast due to expansion / compression.

Question 8

What is Shearing Resistance

Give some attributes of high bearing strength soils

Answer 8

Ability of soil particles to resist sliding against one another when a force is applied.

High Bearing Capacity soils have high Shear resistance as Internal friction contributes to shear strength and is created by large rough shaped particles (sands and gravels)

Cohesion or stickiness contributes to shear and is higher in clay soils.

-High Bearing Capacity soils have a mix of angular sands and gravels, plus enough clay to provide cohesion

Question 9

Shrinkage

Answer 9

Volume decrease with a drop in water content

Sands and Gravels don’t shrink and Clay shrinks a lot

Soils that shrink and swell make very poor foundations for roads and bridges

Question 10

Angle of internal friction

Answer 10

Also known as the angle of repose.
Angle of repose for a given soil is the angle at which shear failure just occurs

Friction between particles can increase shear strength where water will reduce it.

Question 11

Critical angle of repose

Answer 11

Critical Angle of Repose - the steepest angle at which a material on the slope face will be on the verge of sliding

Question 12

What things effect internal friction?

Answer 12

– the relative density (loose vs. dense) compact soil.

– the particle shape (round vs. angular) hard to control

– the gradation (uniformly graded vs. well graded) hard to control

– water content (wet sand works better for sand castles – but not too wet) control by installing adequately spaced and sized culverts.

Question 13

Friction angles of some common materials

Answer 13

Rounded gravels 41 - 44

– Sandy gravel 45 o

– Mixed talus 37 o (magic number above which most slopes will fail)

– Sandy clay (with 45% sand) 29 o

– Sandy clay (with 35% sand) 38 o

– Dry Sand 30 o

– Clays: +/- 15 o Beware of clay in your road!

Question 14

What variables effect fill slopes?

Answer 14

Forces that cause instability (gravity and water)

Forces that oppose instability (shear strength internal friction and cohesion).

Question 15

How can you increase fill stability?

Answer 15

1. Build embankments at a more gentle angle
2. Compact the fill materials (Denser increases the shearing resistance of the soil)
3. Control Soil Moisture
   – If partial or full saturation of the fill occurs fill slopes may fail
   – Provide good drainage of the fill to reduce the build-up of water pressure
4. Vegetate for surface erosion but it won’t protect against deep failures

Question 16

Desperation measures for fill stability

Answer 16

• Retaining walls are expensive but work as a last resort. Common in highway construction but not cheap for forest roads
• Use of coarser materials is theoretically possible but it would mean trucking in materials. Expensive and not practical.

Question 17

What differentiates cut from fill slopes?

Answer 17

Cut slopes are stable at steeper angles than fill slopes of made of the same materials

– denser state than similar type materials placed in a fill

– roots to help bind the soil

Question 18

Why are cut slopes steeper than fills in general?

Answer 18

-Cut slope angles are generally steeper to:

minimize site disturbance (area occupied by roads is reduced so more area to grow trees)

Expose less area to surface erosion

reduce excavation costs since Flatter angles create larger volumes of excavation

Question 19

What portion of the soil profile are we interested in for roadbuilding?

Answer 19

Rock or Unconsolidated materials (OM): Aka the Mineral Portion