COMPACTION AND CONSOLIDATION

CONTENTS

COMPACTION, GENERAL COMMENTS
COMPACTION INFORMATION FROM THE INDUSTRY
SOIL COMPACTION
BITUMINOUS MATERIAL COMPACTION
DENSITY / COMPACTION
METHOD SPECIFICATION
PERFORMANCE SPECIFICATION
COMPACTION SPECIFICATION, WORKING PRACTICE
SOIL CONSOLIDATION
SOME SIMPLE RULES ON COMPACTION AND CONSOLIDATION

COMPACTION, GENERAL COMMENTS

Compaction in the broad civil engineering sense is usually meant to mean the increasing of the density of a material by applying compactive effort and removing the air present, i.e. decreasing air voids content.

Good compaction is not difficult.
You first need a material that is in a suitable condition to be compacted, you lay it in the correct layer thickness, and you use compaction plant capable of compacting that thickness.
A variety of rollers both in size, weight and type are available to compact soil, granular and bituminous materials, and a number of hand operated devices are available for compaction in confined spaces such as trenches and haunch widenings.

By far the most common cause of poor compaction is attempting to compact too thick a layer of material at a time.
It is necessary to have the correct combination of roller compactive effort related to the thickness of material laid for successful compaction to occur.

The importance of the correct compaction of materials cannot be stressed too highly, and you will find reference to appropriate compaction procedures in many British Standards, DOT Specifications and DOT Design Manuals, Specification for the Reinstatement of Openings in the Highway, and other publications.

SOIL COMPACTION

Soil compaction will depend upon the nature of the soil and its moisture content.
The soil can be a cohesive/clay, sand and gravel, or more likely some combination of clay and granular material, which is what makes soil engineering so interesting.

But if you have a soil, or granular material, that has cohesive properties, (i.e. exhibit plasticity), to whatever degree, moisture content becomes very important.
Too wet you will have consolidation problems at a later date, too dry and full compaction will be hard to achieve, and further compaction/consolidation will happen at a later date as equilibrium moisture content is established.

BITUMINOUS MATERIAL COMPACTION

Bituminous compaction will depend upon the grade of bitumen present in the mix ,the temperature of the material at rolling, and the weight of the roller, I.e. the compactive effort it can exert.
This will not entirely be weight related if it is a vibratory roller.
On site the grade of bitumen and the type of roller will probably have already been decided, so look to your temperatures, and ensure rolling takes place above the minimum rolling temperatures specified.


DENSITY / COMPACTION

Testing the density of materials related to highway construction and maintenance means you are monitoring the degree of compaction materials have received, and whether that amount of compactive effort has been sufficient to achieve the density required, i.e. the density determined in the laboratory as the optimum achievable with the particular fill material being used.

Higher degrees of density / compaction usually means stronger materials layers, and hence stronger road pavements and increased pavement life.
This is apart from the obvious fact that materials, whether they be soil, granular or bituminous, that are poorly compacted when laid, will compact under traffic with time and cause settlement of the road surface.
E.g. The  possibility of  wheel tracking in surface layers due to the poor compaction of the underlying binder course (basecourse).

METHOD SPECIFICATION

This means that knowing the type/class of material you are required to compact you specify the method employed to compact it, having previously established in trials that the method is sufficient to ensure complete compaction of the particular material.

Possible items included in a method specification :-

Moisture content of soils/granular material
Minimum rolling temperatures for bituminous materials
Maximum thickness of layer to be compacted
Type of roller, (smooth / sheep's-foot / vibratory / etc.)
Weight of roller
Minimum number of passes of roller   
Minimum ambient/surface temperatures for bituminous materials 

Method Specifications for soil compaction are comprehensively covered in the DOT Specification for Highway Works, Vol.1, Series 600, Earthworks.

There is also detailed information of a Method Specification for the compaction of backfill/granular material in trenches in,

The Specification for the Reinstatement of Opening in Highways.

Method Specifications for compacting bituminous materials can be found in,
BS 594 : Part 2, and BS 4987 : Part 2, for Hot Rolled Asphalt and Coated Macadam respectively.

NOTE : From 1st. of January 2008 the above standards are superseded by BS 594987, a combined standard for hot rolled asphalts and asphalt concrete (coated macadam).

The methods included in these specifications are known from previous work, or trials carried out by such bodies as,  The Transport Research Laboratory, to be capable of ensuring the amount of compactive effort required to achieve the necessary densities of the laid materials.

For a Method Specification to be effective it needs constant monitoring by an Inspector or Clerk of Works to ensure the method is adhered to.

PERFORMANCE SPECIFICATION

The principle behind Performance Testing is that you do not specify the method by which the material is to be compacted. 
But that you specify a minimum density, or more commonly a maximum air voids content, that the compacted material MUST achieve after compaction.

Example No.1 :-
It is not uncommon for cohesive soil used as fill  in an embankment to have a maximum air voids content of 10% up to 600mm. below the actual road pavement construction, and for the final 600mm. to have a maximum air voids requirement of below 5%, i.e. you are specifying the performance of the compaction not the way in which the compaction was performed.

Example No.2 :- 
It is now a requirement on DOT work to test Dense Bitumen Macadam Roadbase, and Basecourse for air void content by the Percentage Refusal Density Test.
The degree of compaction is stated a little oddly, in that the on site core density shall have a minimum density of, for instance, 93% of the maximum refusal density achieved on the same core in the laboratory.

N.B. This is not quite the same as saying it shall have a maximum air voids content of 7%, because although compacted to refusal in the laboratory the compacted material could still have a considerable air voids content.

COMPACTION SPECIFICATION, WORKING PRACTICE

Although we talk of Method Specification and Performance Specification as separate practices it is most common to employ a combination of both systems.
We may have a specification that is more biased to one or other of the systems but rarely is it solely a Method Specification or a Performance Specification that is used on site, but each method supports the other to produce a comprehensive and workable system.

SOIL CONSOLIDATION

Soil consolidation is usually meant to mean the increasing of soil density by the removal of water from the soil.
It is necessary to be aware of this problem and know how it differs from compaction.
The problem occurs in soils that are of a predominantly cohesive/clay nature, and has a moisture content above its natural level.
Soil/fill does not necessarily have to look "wet" for this problem to occur with time.
Soil consolidation happens hardly at all during initial compaction of soil in embankments, but occurs over time under the weight of the "made" ground or embankment, and the weight of the road pavement.

An embankment that has been fully compacted is still able to consolidate and alter its level, i.e. settle.

It is most important to choose fill material correctly and of the appropriate moisture content.
The consolidation of soil can also occur in the backfilling of trenches if unsuitable, i.e. "wet" fill is used.
This is particularly a problem in larger trenches, e.g. sewer trenches, especially if you use excavated cohesive material as backfill that has become wet while stockpiled.
This is why more often than not the whole of the backfill in deep trenches is specified as granular material.
In the course of time, and this can be a long continual process, the weight of the soil in the made ground / embankment will "squeeze" out the excess water present around the soil particles.

The water that can be removed is that which is in excess of the natural moisture content of the fill.
This water occupies volume in the construction, when the water is removed that space will become occupied by surrounding solids and settlement takes place.
If this settlement had not been anticipated and allowed for you will have a prolonged and expensive problem.

One way of allowing for consolidation is by constructing embankments well in advance of the main part of the contract and allowing the consolidation to take place prior to constructing the road pavement.
You cannot consolidate a material by the normal compaction process.

BS 1377 : Part 5, gives details of a relatively simple test to indicate the amount and rate of consolidation of cohesive soils

SOME SIMPLE RULES ON COMPACTION AND CONSOLIDATION

If your fill is a cohesive / clay soil, it must NOT be too wet, and it must NOT be too dry, a reasonable "rule of thumb" for cohesive (clay) soils is the moisture content range +/- 2% of the Plastic Limit. 

Lay soil fills in thin layers, maximum 200mm, compact each fully before placing next layer.

Granular fills must be well graded, particle size well distributed through the range of the material, not single sized particles.

Consult appropriate specification to choose compaction plant and numbers of passes / blows.

Ensure correct compaction plant is on site.

When compacting bituminous materials ensure they are above minimum rolling temperatures.

Maximum thickness of 40mm nominal size bituminous roadbase is 150mm, (with a BIG roller).

Do not trust any testing apparatus that requires calibration, satisfy yourself it is working correctly by using other testing methods alongside the instruments that need calibration.

Keep the testing simple and use a lot of it ! .


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