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  The Idiots' Guide to Highways Maintenance

Copyright 2000/16, C.J.Summers




The moisture condition test is a method of quickly assessing the suitability of earthwork materials for use as fill, i.e. the soil / fill is at a moisture content such that it can be fully compacted by the appropriate plant.

The moisture condition test is a physical test indicating how much compactive effort is needed to compact a sample of soil at a particular moisture content.

Compactive effort is provided by dropping a captive  weight vertically a known distance on to a sample of soil in a cylindrical mould, and measuring compaction.
By counting blows to give the required compaction it will be possible to establish a Moisture Condition Value,(MCV), for a soil at a particular moisture content.

From laboratory compaction/density tests it is possible to determine an optimum moisture content at which site compaction of that soil should take place.

By performing a MCT on a soil sample at this optimum  moisture content at which it is able to be fully compacted, you are able to establish a MCV reference number. then a MCT performed on site should give a MCV  that complies, (with a small tolerance), to the MCV reference number to ensure maximum compaction of the soil is able to take place assuming correct layer thickness and compaction plant, and no change in soil type.

The MCT therefore excludes the need for a moisture content to be taken on site and allows a quick assessment of the suitability of fill material being placed.

Information on the moisture condition test, (MCT), and moisture condition value, (MCV), can be found in :-
TRRL Supplementary Report 522,
TRRL Research Report 90

The test was/is fully described in,

BS 1377-4 : Soils for civil engineering purposes : Part 4:Compaction related tests.


This device is basically a 50mm. diameter cylindrical 4.5kg.weight confined in a tube, and the length of the tube allows it to fall 450mm. before striking the surface to be tested.
The weight is attached to a "T" shaped handle for lifting and dropping, inside the handle is an accelerometer for measuring how quickly the weight stops when it hits the surface.
The theory being that a well compacted surface will cause the weight to stop more quickly than a poorly compacted surface.
Some people argue this is a strength testing apparatus and does not necessarily indicate compaction or density.

It is however relatively easy to use, but MUST be used on each compacted layer of material.

Only using it on the last layer of material to be backfilled into the trench will not give an accurate reflection of the compaction of the full depth of the trench.

TRL Report 361 - Compaction monitoring devices for earthworks,  for further information on this apparatus.

I try to refer to as few commercial sites as possible in compiling my site,
but when a site offers particularly useful information about a subject I make an exception.

For further information regarding the Clegg Impact Soil Tester, press --------------------->


The Dynamic Cone Penetrometer can be used to indicate the compaction/density of soil/fill materials.
It will not give actual figures for density .
The theory being that the resistance to penetration is an indication of the degree of compaction of the fill, and in simple applications it seems to work well.
If you need a large number of blows to cause the cone to penetrate a short distance the soil/material is well compacted, and if the cone penetrates easily with few blows the soil/material is poorly compacted or "unsuitable''.
This apparatus can be very useful in monitoring the compaction of trench reinstatements depending on the nature of the reinstatement material, it is particularly useful because the cone is on the end of a long rod, approximately a metre long.
This means a considerable depth of soil/fill can be tested, not just the surface layer.
This in turn can show where soil/fill has been placed in layers which are too thick, with only the surface being compacted.

It is also possible, in broad terms, to convert results from this apparatus to CBR figures.

The mode of operation is a standard cone, on the end of a long steel rod, is subject to a blow of an 8kgs. mass falling a distance of 575mm. onto an anvil attached to the penetrometer rod.
The results can be expressed as the number of blows for a measured length of penetration, or alternatively, the length of penetration for a single blow, and it is normal to plot results on a graph, and this way it is possible to distinguish between different layers of the pavement.

Use of the apparatus was/is described in,

BS 1377-9 : Soils for civil engineering purposes : Part 9 : In-situ tests,

Further information on the Dynamic Cone Penetrometer can be found in,
TRL Report 361 - Compaction monitoring devices for earthworks


Core cutters are used for testing the compaction of cohesive/clay soils placed as fill.
The cylindrical cores of standard volume, 13cms long and 10cms diameter., they have a sharpened edge at one end to improve penetration of the soil surface.
These cores are driven fully into the surface to be tested, they are removed from the ground without disturbing the core contents.
In the laboratory they are cut flush top and bottom and weighed.
Bulk density can be quickly calculated, and by determining the moisture content of the soil the dry density of the fill can be calculated and hence the voids percentage.
A high percentage of voids indicating poor compaction.

This test was/is fully described in,

BS 1377-9 : Soils for civil engineering purposes : Part 9 : In-situ tests,


This is the test where you try and make a "worm" about the thickness of a pencil out a sample from a clay soil.
The Plastic Limit being the moisture content at which the "worm" starts breaking up, i.e. it is at the limit of its plasticity.
Generally speaking a cohesive/clay soil at this moisture content is not far from the optimum for compaction, so do not laugh at this test, it works.
It is a good "rule of thumb" that the optimum moisture content of clay / cohesive soils for their compaction is plus or minus two percent of their plastic limit.
this is a guide, and really you should be performing compaction trials on the fill you are using to verify the correct moisture content values for optimum compaction.
The Proctor Test is a frequently used laboratory test to determine the optimum moisture content for compaction.

A simple form of the Plastic Limit Test is included as a "field" test in the,
Specification for the Reinstatement of Openings in Highways.


The conditions of use and storage of a nuclear density gauge/meter are very stringent and must be adhered to.
Technicians who use the apparatus must undergo strict medical checks on a regular basis.
Failure to comply with the appropriate regulations renders the laboratory and technician open to prosecution.
The nuclear density meter is an easily portable device employing a radioactive isotope to determine the density of road pavement layers, including earthworks, granular materials, bituminous materials and road pavement concrete.
Simply put it is the ability of the nuclear density meter to pass very, very, very small amounts of radioactivity through the various materials that you wish to test, and to measure the responding amounts of radioactivity returning to the measuring head of the meter.

Generally speaking the less dense a material the easier radioactivity will penetrate/pass through it.
A nuclear density meter is not on its own able to determine the density of a material.
The nuclear density meter MUST be calibrated against actual core densities obtained from the same material it will be used to measure.
It may be used to test similar material with the approval of the engineer.
If you want to test the density of a different material it MUST be recalibrated.

The specification for the calibration and use of a nuclear density meter was/is :-
BS 1377-9 : Soils for civil engineering purposes : Part 9 : In-situ tests,

Advice and guidance on the use of the apparatus can be found in :-
TRL Report 361 - Compaction monitoring devices for earthworks.
TRL Supplementary Report SR 754 - Nuclear gauges for measuring density of dense roadbase macadam : report of a working party


This along with the Core Cutter is one of the older, more labour intensive methods of determining compaction.
It is used on boulder clays, some granular fills and it can even  be used on cement stabilised materials, e.g. "lean concrete" used as a base (roadbase).

A cylindrical, flat bottomed, hole is dug out from the surface by a technician, approximately 200mm. deep by 200mm. diameter, using a chisel, lump hammer and small scoop.
All material excavated is very carefully saved in a moisture retaining container, and weighed in the laboratory.
Sand of a known density is poured into the hole, from the weight of the sand taken to fill the hole its volume can be determined.
Bulk density of the layer tested can be quickly calculated, and after determining the moisture content, the dry density and air voids.

This is not a sophisticated test, but it is accurate when performed with care.

The test was/is fully described in:- 

BS 1377-9 : Soils for civil engineering purposes : Part 9 : In-situ tests,


The percentage refusal density is a measure of the  density and hence the state of compaction of laid dense bitumen macadam, and the test which determines the PRD is intended as a quality control test.
The PRD test will determine whether the binder course (basecourse) and base (roadbase) has received full compaction.
This is because fully compacted material, i.e. minimum air voids, will be a stronger pavement layer than a material containing air voids.
This test is NOT usually performed on hot rolled asphalt binder course (basecourse) and base (roadbase), or any surface course (wearing course) materials.
The test is performed on dense bitumen macadam binder course (basecourse) and base (roadbase) layers.

Information on the PRD test and how to perform it was included in :-
DOT Specification for Highway Works, Road-Pavements : Bituminous Bound Materials, Clause 927,
but it is not included in, 
Interim Advice Note 101/07,
which is a complete, revised 900 Series of the Specification (MCHW1) for Road Pavements - Bituminous Bound Materials   
However, guidance on this type of compaction testing can be found in
Clause 930:EME2 Base and Binder Course Asphalt Concrete
, of IAN 101/07 in relation to the "Compaction Control for the Permanent Works" 
Details of the "PRD Test" was/is to be found in,  
BS 598-104 : 2005 : Sampling and examination of bituminous mixtures for roads and other paved areas. 
Part 104 : Methods of test for the determination of density and compaction.
(This standard may be superseded, but I have not found confirmation of this, at this time.) 

The principle of the test is that a core is taken, on site, of the material that has been laid and compacted in the road, and the density of this core is determined in the laboratory.
This core is then placed in a standard mould at a standard temperature and compacted until it refuses to compact any further, i.e. refusal density.
The density of the core is now measured again.
The original density of the core as it was in the road is now expressed as a percentage of the density of the core compacted to refusal.

The figure of 93% for PRD is considered a reasonable minimum for most dense bitumen macadam binder courses (basecourses) and base (roadbase).
BUT the PRD figure of the material on site MUST be determined and used for control purposes.
It is quite common practice for a nuclear density meter to be used to test for PRD on site once actual figures have been determined by laboratory testing, and most importantly the nuclear density meter has been calibrated against these figures.

NOTE : With effect from the 1st. of January 2008, and the introduction of the use of the new,
BS EN 13108 : 2006 : Bituminous mixtures. Material specifications "family" of standards, 
guidance for the use of the nuclear density gauge/meter with regard to compaction control, when testing compacted dense base and binder course asphalt concrete (bitumen macadam) bituminous mixtures, can now be found in, 
BS 594987:2007:Asphalts for roads and other paved areas -
Specifications for transport, laying and compaction and type testing protocols

These procedures particularly apply to "designed" bituminous mixtures.

Consult, Section 9:Compaction, and Annex C (informative)
Guidance on protocols for calibrating and operating indirect (nuclear) density gauge/meters can be found in,
I (normative)

Information and guidance can also be found in,

TRL Supplementary Report SR 754 - Nuclear gauges for measuring density of dense roadbase macadam : report of a working party

I try to refer to as few commercial sites as possible in compiling my site,
but when a site offers particularly useful information about a subject I make an exception.

For further information regarding various hand operated pieces of soil testing equipment,
press ---------------->

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