Geotechnical Engineering 2

What are the key features of granular soils?

• Coarse grained

• Granular particles

• Can see individual grains

• Cohesionless

• Non-plastic

What affects the engineering behaviour of a granular soil?

It’s grain size and shape.

What are they features of a cohesive soil?

• Fine-grained

• Plate-like particles

• Cannot see individual grains

• Cohesive

• Plastic

• Anisotropic (orientation of the particles is significant in their mechanical response).

What is plasticity?

A soils ability to be moulded.

What is cohesiveness?

A soils ability to stick together

Describe what makes a soil a multiphase material.

It is made up of three phases: grains/particles/solids, water/moisture and air.

What makes a soil saturated?

Its voids being completely filled with water.

What makes a soil dry?

Its voids being completely filled with air.

What makes a soil partially saturated?

Some of its voids are filled with air, while others are filled with water.

What are the different types of grading characteristics?

Uniformly graded, poorly graded, medium graded, well-graded, gap graded.

What does the PSD curve of a well-graded soil look like?

The curve is more or less constant. No deficiency or excess of any particular particle size.

What does the PSD curve of a uniformly graded soil look like?

A major part of the curve is steep. The soil has a PSD extending over a limited range with most particles tending to be about the same size.

What does the PSD curve of a gap-graded soil look like?

A large percentage of its bigger and smaller particles and only a small percentage of intermediate sizes. A curve with a significantly flat section.

Which parameters determine the grading characteristic of a soil?

• Coefficient of uniformity (Cu=D60/D10)

• Coefficient of curvature (Cc=D30²/(D10*D60))

What can particle size and shape be an indicator of?

Compressibility, shear strength and hydraulic conductivity.

Which force dominates within a clay? Why?

Interparticle forces. Because they have a large SSA (specific surface area).

What is the relationship used to find the SSA?

SSA = surface area / volume.

What is absorbed water in a clay?

The layers of water molecules surrounding each clay particle.

What are the Atterberg Limits?

The threshold water contents a which certain types of engineering behaviour can be expected. The liquid limit, the shrinkage limit and the plastic limit.

What is the liquid limit?

The water content at which the soil stops acting as a liquid and starts acting as a plastic.

What is the plastic limit?

The limit at which plastic deformation changes to brittle behaviour.

What is the shrinkage limit?

The limit at which further drying of soil does not result in volume change.

Going from “dry” to “wet”, in what order are the Atterberg Limits in?

Shrinkage limit, plastic limit, liquid limit.

What is the relationship used to calculate the plasticity index?

PI (plasticity index) = LL (liquid limit) - PL (plastic limit)

What is the relationship used to calculate the liquidity index?

LI (liquidity index) =( w - PL) / PI.

What does it mean about a soil if its LI = 1?

The soil is at its liquid limit. What does it mean about a soil if its LI

What does it mean about a soil if its LI = 0?

The soil is at its plastic limit.

What does it mean about a soil if its LI < 0?

The natural water content is less than the plastic limit and we would expect the soil to behave in a brittle way. What does it mean about a soil if its LI

What does it mean about a soil if its LI > 0?

The natural water content is more than the liquid limit. The soil will act as a viscous liquid.

What does it mean about a soil if 0 < LI < 1?

The natural water content of the soil is less than the liquid limit but more than the plastic limit. It will behave plastically, or in a mouldable way.

What are the three volumetric ratios?

e (the void ratio), n (porosity) and S (degree of saturation)

Which relationship is used to find the void ratio?

e = volume of voids / volume of solids

Which relationship is used to find the porosity?

n = volume of voids / total volume

Which relationship is used to find the degree of saturation?

Sr = volume of water / volume of voids

What is the specific gravity of a material?

The ratio of the mass of a volume of the material to the mass of an equal volume of water

Which method can be used to calculate the volume of solid particles within a soil?

The fluid pycnometer method

Describe the fluid pycnometer method.

The fluid pycnometer method is based on the volume difference to of the liquid necessary to fill the pycnometer with and without the soil sample.

What is density? And what are the four density that are of interest in geotechnical engineering?

Mass per unit volume. Bulk density, particle density, dry density and saturated density.

What is bulk density?

The natural, in-situ density of the soil.

Which method can measure the volume of a granular soil (usually taken to measure bulk density)?

The sand replacement test.

Which method can measure the volume of a cohesive soil (usually taken to measure bulk density)?

The core-cutter method.

Describe the core-cutter method.

A core cutter is driven into the soil and a known volume of soil is extracted from the ground.

Which is the key relationship between 2 of the volumetric ratios?

S_r x e = w x G_s

What is the difference between the particle density and the dry density?

The particle density is density of the solid parts of the soil (excluding any voids). The dry density is the density of the entire soil (including the voids- but they are completely dry).

What are the normal units for density?

Mg/m³

What is the relationship between unit weight and density?

Unit weight = density x g

What is the buoyancy effect and why does it occur?

The weight of a soil submerged below the water table is partly balanced by the buoyancy effect of the water. (This is why we often calculate effective stress = total stress - pore water pressure).

What is the relationship used to determine the buoyant unit weight?

Buoyant unit weight = saturated unit weight - unit weight of water.

What is the density index/relative density and why is it necessary?

The density index is used to relate the field void ratio to the maximum and minimum possible values. This is necessary because granular soils can have a large range of possible void ratio values.

What is the relationship used to determine the density index/relative density?

I_D = (e_max - e) / (e_max - e_min)

How is the minimum void ratio (e_min) determined?

By vibrating a sample in a steel mould under a surcharge.

How is the maximum void ratio (e_max) determined?

By pluviation at a given rate of deposition.

How can a soil at the site be stabilized?

By compaction.

What is compaction?

A process of increasing density of a soil by packing the particles closer together with a reduction in the volume of air and no significant change in the volume of water.

Why does compaction make a soil more stable?

It increases the shear strength, which makes it stronger against loads.

What are the advantages of compaction?

• Increased soil strength

• Increased load-bearing capacity

• Increased soil stability

• Reduction in settlement (lower compressibility)

• Reduction in flow of water (hydraulic conductivity)

• Reduction in soil swelling (expansion) and collapse (contraction

What are the risks of a poorly compacted soil?

Low stability and high settlement

What is compaction a function of?

Dry density, water content, compactive effort and soil type.

What is compactive effort?

A measure of mechanical energy applied to a soil mass (energy per unit volume).

How is compactive effort measured in the field vs in the lab?

In the field, compactive effort is the number of passes and weight of the roller on a given volume of soil. In the lab, compactive effort is calculated by the mass of the rammer, height of drop, number of drops per layer, number of soil layers and volume of the mould.

What is the optimum water content of a soil?

The amount of water in a soil that allows it to be compacted to its maximum dry density (most tightly packed state).

What can be determined from lab compaction tests?

The relationship between water content and dry density, and therefore the optimum water content and maximum dry density.

Describe a soil on the dry side of the OWC.

The soil is usually stiff and difficult to compact, thus it gives a dry density lower than the maximum.

Describe a soil on the wet side of the OWC.

Dry density decreases because an increasing proportion of the soil is replaced by water.

What is the compaction curve?

Dry density plotted against water content.

What is the key method for compaction?

The drop hammer test (Proctor test)

Describe the drop hammer test.

A compactive effort is applied to a number of samples from the same soil but with different water contents. Dry density is calculated for each sample.

When is the drop hammer test meaningless and which method can be used instead?

The drop hammer test is meaningless for some highly permeable soils such as clean gravels and coarse clean sands. For those, vibrating hammer compaction is used.

What does the degree of field compaction achieved depend on?

Compactive effort, water content and type of soil.

What are the two categories of field compaction?

Static compaction and dynamic compation.

Which pieces of equipment fall under static field compaction?

Smooth roller, grid roller, sheepsfoot roller and pneumatic roller.

Which pieces of equipment fall under dynamic field compaction?

Vibratory roller, power rammer.

What is the smooth roller suitable for?

Well-graded sands and gravels; silts and clays of low plasticity.

What is the grid roller suitable for?

Well-graded sands.

What is the sheepsfoot roller suitable for?

Cohesive soils; sands and gravels with >20% fines.

What is the pneumatic-tyred roller suitable for?

Most coarse and fine soils and clays above their plastic limit.

What is the vibratory roller unsuitable for?

Wet cohesive soils.

What is the power rammer suitable for?

All soil types in confined areas.

Describe the smooth roller.

Consists of hollow steel drums.

The compaction is mostly by pressure.

Results in a smooth surface (good for finishing).

Has poor bonding between layers.

Describe the grid roller.

Consists of a towed, heavy unit made up of a grid.

The material is mainly vibrated and crushed.

They provide high contact pressure but very little kneading.

Describe the sheepsfoot roller.

Consists of hollow steel drums and numerous club-shaped feet projecting from the surface.

Because the feet have a small area, they provide high pressure.

It provides excellent bonding between layers due to kneading.

Describe the pneumatic-tyred roller.

Consists of wheels mounted close together on two axes. The rear set of wheels overlap the lines of the front set for best coverage.

There is both pressure and kneading action.

The resulting finish is smooth, resulting in poor bonding between layers.

Describe the vibratory roller.

Smooth-wheeled roller fitted with a power-driven vibration mechanism.

What is the vibratory roller suitable for?

Sands and gravels (most soils with a low to moderate fines content).

Describe the power rammer.

Consists of manually controlled rammers.

Used for small areas where access is difficult or use of larger equipment cannot be justified.

How can compaction be specified?

By either method compaction or end-product compaction.

Describe what total stress in a soil is.

The stress generated by the mass (acted upon my gravity).

What is the relationship used to calculate total stres on a soil?

σ=ρ g z or σ = y z.

σ is the total stress, ρ is the sum of densities of all materials, g is the gravitational constant (9.81), z is the thickness of the soil, and y is the unit weight.

What is the relationship used to calculate the pore water pressure?

u = ρ_w g h_w or u = y_w h_w.

ρ_w is the density of water, g is the gravitational constant, h_w is the depth below the groundwater table.

What assumptions are made in the pore water pressure calculation?

• The pore water is interconnected, and the soil is fully saturated.

• Horizontal water table.

• No added head.

• Static conditions.

What is the relationship used to calculate the effective stress?

σ’ = σ + u

Describe what effective stress is actually telling us.

The average of the contact forces over their total area.

What is the relationship used to calculate the effective horizontal stress?

σ’_h = K₀ σ’_v. K₀ is the coefficient of lateral earth pressure at rest.

What is the relationship used to calculate the total horizontal stress?

σ_h = σ’_h + u

What is capillarity?

Arising from surface tension, water rises in ‘capillaries’ above the water table.

What is the relationship between water pressure and capillary rise?

u_c = -h_c ρ_w g.

u_c is the water pressure in the capillary and h_c is the height of capillary rise.

What is the effect of the negative capillary water pressure?

In capillary zones, the effective stress can be larger than the total stress. This is because the tension in the water pulls the grains of soils together. This increases the intergranular contact stress.

What is the sign convention for GTE2 Mohr circles?

• Compressive forces are positive

• Positive shear stress produces counter-clockwise movement.

• Clockwise angles are positive.

How can you find the pole on a Mohr circle?

The pole is found by drawing a line parallel to the plane that the major principal stress is acting on, starting from the major principal axis.

How can you find the state of stress at a specific point on the Mohr circle?

σ = C + Rcos(2α)

τ = Rsin(2α)