Chapter 3 - PowerPoint Presentation

Slide1

Chapter 3

CompactionSlide2

Compaction

To improve the density and other properties of soil

Increases the solid density improves strength

Lowers its permeability

Reduces future settlementSlide3

Maximum D

ry

D

ensity

Compaction requirements are measured in terms of dry density

Example

Clay soil may not reach 2000kg/m2

Granular soil may have a dry density above clay value before compaction

Dry density will vary with its water content

Water acts as a lubricate to the grains of soil when compact

Air is forced out of the soil – leaving more space for soil solids

However when most of the air has been removed the dry density does not increase much furtherSlide4

Compaction curve

First step is to determine maximum dry density that can be expected for a soil under certain

compactive

effort and water content

Compaction curve is also called moisture density curve or proctor curve

Dry

desity

is plotted against water content

Curve is drawn through the test points

The top of the curve represents

Maximum dry density for the soil with the test

compactive

effort

The corresponding water content – know as the optimum water content (

wo

)Slide5

Standard compaction test

Soil is compacted in a 10cm diameter mold have a volume of 943.9cm3

The soil is placed in three layers

Each layer compacted 25 times with a 2.5 kg hammer falling 30cm

Usually test is conducted on only material that passes no 4 sieve

If larger grains are to be included a larger mold is used

In the test – a sample of soil is mixed with water and then compacted

Mass of the compacted sample is measured

And water content id determinedSlide6

Compaction Curve Example

Pg 99 -100

To aid in the drawing the

mositure

density curve - a maximum theoretically possible density – zero air voids (

Zav

) CURVE IS PLOTTED

Rules for

zav

curve

No point can be above the

zav

line – therefore errors are obvious

The slope of the moisture density curve on the wet side of optimum moisture content is parallel to the

zav

curve – this is helpful in sketching the curve where test results are erraticSlide7

Compaction Curve

Moisture density curve is different for each soil

Granular well graded soils generally have fairly high maximum densities at low optimum moisture contents

Clay soils have lower densities

Since compaction equipment has become better over time

Modified compaction test

4.5 kg hammer falling 45cm on each of five soil layers in the same mold

Maximum dry density obtained in the this test is higherSlide8

Compaction Curve

Compaction of soils varies – construction requirement are usually specified as a percent of maximum dry density

Example would be 95%

If the maximum dry density from the test was 2000kg/m3 at an optimum water content of 11% the required field density would be 95% of 2000 or 1900kg/m3 – moisture content should be as close as possible to 11% which reduces the required

compactive

effort

Table 3-1 on pave 103 give typical values for

aashtoSlide9

Field Density

Quality control of compaction involves measuring field density

And comparing field results to lab results

Usually made with a nuclear

densometer

– however other test are available

Sampling method

Sample of compacted material is dug out of a test hole in the soil layer being checked

Dry mass of the soil removed and the volume of the hole are measure

Field dry density equals the dry mass divided by the volume originally occupied by the sample

Two basic methods are used to measure the volume of the hole – balloon filled with a liquid and a sand cone

apparatueSlide10

Balloon apparatus

The volume of the sample hole is found by forcing a liquid filled balloon into the test hole

Rubber membrane allows the fluid to fill all the cavities in the test hole

V

olume of fluid required to do this is read on a scale on the apparatusSlide11

Sand Cone apparatus

Volume of the test hole is obtained from the mass of loose sand required to fill the hole

The sand is uniform medium sand that has an essentially constant loose density when poured into a container

The cone acts a

a

pouring funnel

The mass of the sand container with an attached cone is obtained

The container is then inverted over a test hole – value at the small end of the cone is open and sand flow into test hole

When flow stops the valve is closed – mass of the sand container with its cone is measured

Example 3-4 page 105Slide12

Quality control

Problem 3-5 page 105Slide13

Nuclear method

Main method used today

Test only takes a minute

Gamma rays are emitted into the soil – these rays are absorbed by the soil in amounts that vary according to the density of the soil

Neutron radiation is also emitted – these neutrons lose energy due to collision with hydrogen atoms in the soils

A detector measures the amount of moderated neutrons – thus moisture content can be obtainedSlide14

Compaction operations and quality control

In highway and other types of construction

The layer of si9l 15-30 cm in depth that provides the foundation for the pavement structure

Fill material used to bring the soil surface up to the

subgrade

level

Spread in layers 15-30 cm thick – each layer compacted

Backfill materials in trenches opened for utility

Layers of granular material used in the pavement

The asphalt layer Slide15

Compaction equipment

Rate of vibration of 1500 to 2500 cycles per minute

Mass of drum from 2-15

tonnes

(2-17 tons)

Very effective for gravel

snad

and silt soils

Layers .5 -1 m in depth usually require 3-5 passes

Padded wheel or sheep foot

Most effective for clay and

silty

clay soil

Pressure exerted by the pad can be 1000-5000kpa

Compact layers 15-30 cm in 3-5 passes

Pneumatic tire rollers

15cm layers

Smooth drum – non vibrating rollers

Finished or sealing passes Slide16

Deeper Compaction Methods

Last slide use to less then 1 meter deep

Vibrocompaction

methods can compact much greater depths

Large vibrator

Dynamic compaction