Geology 229Engineering Geology

Lecture 12

Elementary Soil Mechanics (cont. 2)(West, Ch. 7)

Outline of this Lecture

1. Compaction/Consolidation

Soil densification include compaction and consolidation.

Densification compaction + consolidation

Compaction is one kind of densification that is realized by rearrangement of soil particles without outflow of water. It is realized by application of mechanic energy. It does not involve fluid flow, but with moisture changing.

Consolidation is another kind of densification with fluid flow away. Consolidation is primarily for clayey soils. Water is squeezed out from its pores under load.

The engineering purpose of compaction is to increase the material’s density.

1. Compaction effort, 2. soil type and gradation, 3. moisture content, and 4. dry density

are the 4 control factors to the extent of compaction.

Increase permeability and decrease compressibility

West, Figure 7.11 Plasticity Chart

Lower compressibility Higher compressibility

Standard Proctor Test

Compaction Effort is calculated with the following parameters

•Mold volume = 1/30 cubic foot

•Compact in 3 layers

•25 blows/layer

•5.5 lb hammer

•12" drop

hammer

cylindricalcontainer

Dry of optimumwet of optimum

Total or wet density ρ :

t

ws

t

t

VMM

VM +

==ρ

Solid density ρs

dry density ρd

s

ss V

M=ρ

sdstt

sd VV

VM ρρρ <∴>= Q,

Also we have

wandwthatso

wVMMM

VM

VM

VMM

VM

ddd

dts

sw

t

w

t

t

t

wt

t

sd

+==+

−=−=−=−

==

1ρρρρρ

ρρρρ

The purpose of conducting the Proctor test is for finding out the optimal water content and guide the compaction project in the field.

Practically, you know the water content w, you measure the wet density ρ, and then calculate the dry density. Sure you can also bake the sample and directly measure the dry density ρd.

Zero-air-void density:

At certain water content, what is the density called if there is no air in the voids

s

ws

wGGvaz+

=1

)..( ρρ

w

ρ(zav)

swwswwwwww

swsswsw

wssssssss

s

wGwGMVorVMVbywGMwMthenMMw

GVVMthenVMV

========

====

ρρρρρ

ρρρ

//,1,,/

,/1

s

sw

s

sw

t

tdry wG

GwGwGw

wVM

w +=

+++

=+

=+

=1)1)(1(

)1()1(1

ρρρρ

Zero-air-void density:

At certain water content, what is the density called if there is no air in the voids

s

ws

wGGvaz+

=1

)..( ρρ

It is clear that in the above equation, specific gravity of the solid and the water density are constant, the zero-air-void density is inversely proportional to water content w. For a given soil and water content the best possible compaction is represented by the zero-air-voids curve. The actual compaction curve will always be below. For dry soils the unit weight increases as water is added to the soil because the waterlubricates the particles making compaction easier. As more water is added and the water content is larger than the optimum value, the void spaces become filled with water so further compaction is not possible because water is a kind like incompressible fluid. This is illustrated by the shape of the zero-air-voids curve which decreases as water content increases.

w

ρ(zav)

Now we can make another description of compaction in the engineering sense:

Compaction is for making the optimum density as close as the zero-air-void as possible

Also we can conclude that more compaction effort make the optimum density closed to ρ(zav) by the fact of the 2 linear lines have different slopes.

Compaction curves for different soils with the same compact effort (West, Figure 7.19); fine grain soil needs more water to reach optimum and coarse grain soil needs less water to reach optimum.

In contrast to compaction, consolidation is the reduction in volume of clayey soil under external loading as water drained out from pores.

∆ewater

solid

Vv=Vw=e0

Vs=1

watersolid

00

00 1,

1 eeHHthen

ee

HH

+∆

=∆+∆

=∆

Apparently, consolidation is the reason for surface subsidence in areas of groundwater or other crustal fluid over-draw.

Compaction Equipment

A. Types of Action

* Ramming

* Vibration

* Static Rollers

B. Ramming

Rammers have a low frequency (800 blows/minute), and a higher stroke (1.5" to 3.5“ height). As the machine operates the ramming action breaks down the soil, pushes the particles closer together, and forces air out of the voids.

C. Vibration

Vibratory machines are distinguished by their high frequency (2,000 to 6,000 blows/minute) and a low amplitude. The vibratory action sets the soil particles in motion, which rearranges them into a denser packing.

Types of Rollers1. Smooth Wheel Rollers or Smooth Drum Rollers are suitable for thin layers,

and for finishing operations. 2. Pneumatic Rubber-Tired Rollers have a series of rubber tires side-by-side

instead of a smooth drum. Compaction is achieved by a combination of weight and kneading action.

3. Sheepsfoot Rollers are drums with a large number of "sheepsfoot" projections. They are most effective in compacting clayey soils. The initial pass in the field compacts the lower portion of the lift, with subsequent passes compacting the middle and upper portions. The roller is said to "walk out" as the soil is compacted and the feet do not penetrate as deeply as the initial pass.

4. Vibratory Rollers work well in compacting granular soils. Vibratory action can be used with smooth drum, rubber tired, or sheepsfoot rollers.

Field CompactionA. Relative CompactionRelative compaction is a way of comparing compaction on the construction site with the laboratory compaction results. It is defined as:

B. Water Content In the field it has been found that soils compact better at water contents on the dry side of the optimum water content.

C. Determination of Dry Unit Weight in the Field Two methods in common use today to determine units weights and water contents in the field are:

the Sand Cone Method, ASTM D-1556 the nuclear and non-nuclear density meters.

Field Compaction (cont.)