Technical report Recycled utility arisings in trench reinstatement: compaction trial Quality control during trench reinstatement works is important in ‘getting it right first time’. This research investigated the compaction behaviour of hydraulically bound mixtures (HBMs) manufactured from recycled trench arisings, and evaluated a range of in situ test devices for their suitability for compliance and/or control testing. The study found that HBMs can require less compaction than GSB1/Type1, and that a range of portable in situ test devices are suitable for quality control testing, but a proven compaction method specification is essential. Project code: MRF106 ISBN: [Add reference] Research date: August 2008 to March 2009 Date: August 2009
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Technical report
Recycled utility arisings in trench
reinstatement: compaction trial
Quality control during trench reinstatement works is important in ‘getting it right first time’. This research investigated the compaction behaviour of hydraulically bound mixtures (HBMs) manufactured from recycled trench arisings, and evaluated a range of in situ test devices for their suitability for compliance and/or control testing. The study found that HBMs can require less compaction than GSB1/Type1, and that a range of portable in situ test devices are suitable for quality control testing, but a proven compaction method specification is essential.
Project code: MRF106 ISBN: [Add reference]
Research date: August 2008 to March 2009 Date: August 2009
MRF106). Report prepared by J Edwards, P Edwards, L Robinson and A Buttress. Banbury, WRAP
Written by: J Edwards (Scott Wilson Ltd), P Edwards (Lafarge A&C UK), L Robinson (Scott Wilson Ltd) and
A Buttress (Scott Wilson Ltd)
Front cover photography: In situ testing during the compaction trial (courtesy of Scott Wilson Ltd)
WRAP (Waste and Resources Action Programme) and Scott Wilson Ltd believe the content of this report to be correct as at the date of writing. However, factors such as
prices, levels of recycled content and regulatory requirements are subject to change and users of the report should check with their suppliers to confirm the current
situation. In addition, care should be taken in using any of the cost information provided as it is based upon numerous project-specific assumptions (such as scale,
location, tender context, etc.).
The report does not claim to be exhaustive, nor does it claim to cover all relevant products and specifications available on the market. While steps have been taken to
ensure accuracy, WRAP cannot accept responsibility or be held liable to any person for any loss or damage arising out of or in connection with this information being
inaccurate, incomplete or misleading. It is the responsibility of the potential user of a material or product to consult with the supplier or manufacturer and ascertain
whether a particular product will satisfy their specific requirements. The listing or featuring of a particular product or company does not constitute an endorsement by
WRAP and WRAP cannot guarantee the performance of individual products or materials. This material is copyrighted. It may be reproduced free of charge subject to the
material being accurate and not used in a misleading context. The source of the material must be identified and the copyright status acknowledged. This material must
not be used to endorse or used to suggest WRAP’s endorsement of a commercial product or service. For more detail, please refer to WRAP’s Terms & Conditions on its
web site: www.wrap.org.uk
Recycled utility arisings in trench reinstatement: compaction trial 1
Executive summary
Trench arisings have the potential to be recycled into hydraulically bound mixtures (HBMs) for use as backfill and
in the structural layers of pavements. Using recycled trench arisings within HBMs offers the potential to:
� improve the performance of the reinstatement;
� reduce the cost and carbon associated with the works by replacing asphalt materials; and
� increase the recycling of trench arisings, diverting recoverable materials from landfill.
All non flowable materials (including GSB1/Type1, unbound backfill materials, asphalt and HBMs) require
adequate compaction; and acceptable performance within a trench reinstatement is dependent on this. For HBMs
and asphalts, inadequate compaction will affect durability (ability to withstand long term environmental
degradation), while for GSB1/Type1, inadequate compaction is likely to result in settlement.
The aim of this research is to investigate the compaction behaviour of HBMs to assess the suitability of adopting a
method specification for their installation (compaction); and evaluate a range of in situ test devices to assess
their suitability for compliance and/or control testing. Therefore, the following objectives were defined:
� objective 1: to evaluate compaction behaviour of two HBMs with regard to the compaction methodology; and
� objective 2: to evaluate available indirect test methods for compliance and/or control testing of HBM
compaction.
The research was initiated with a desk study that reviewed available compaction methodologies and in situ test
methods to facilitate the development of a test matrix for the compaction trial. Two HBMs were produced from
recycled trench arisings; the ‘HBM Fine’ was produced from screened trench arisings while the ‘HBM Coarse’
comprised screened trench arisings with additional recycled concrete aggregate. A typical primary GSB1/Type1
reinstatement material was used as the control. The influence of workmanship factors such as lift thickness,
compactive effort and the effect of delayed compaction were evaluated to assess the compaction behaviour.
Adequate compaction in accordance with Clause 870 of the manual of contract documents for highways works,
volume 1 - MCHW1 (HA, 2009a) was taken as a suitable target. This is defined as; compaction to an average wet
density of not less than 95% of the average wet density of test specimens compacted to refusal.
A calibrated nuclear density meter (NDM) was used, in direct transmission mode as specified in MCHW1 (HA,
2009a), to determine the in situ density of the materials, hence degree of compaction achieved, for comparative
analysis across the range of compaction methodologies. It also provided a baseline against which to compare the
data from selected in situ test devices. The eight different in situ test devices used in the trial are:
� nuclear density meter (NDM);
� asphalt pavement quality indicator (PQI);
� lightweight deflectometer (LWD);
� German dynamic plate (GDP);
� bearing capacity and deflectometer (BC&D);
� Clegg impact hammer;
� Panda2 variable energy dynamic penetrometer; and
� dynamic cone penetrometer (DCP).
All the test devices selected were portable, rapid to use and could, with varying degrees of ease and speed, be
operated by one or two operatives. However, the parameters measured by the majority of devices did not
correlate with relative density, and as such did not indicate when adequate compaction had been achieved.
As a result of this project, the following conclusions are made concerning the placement and assessment of
adequate compaction of HBMs for trench reinstatement:
� the two HBMs selected for the trial required less compactive effort than the GSB1/Type1 to achieve adequate
compaction, defined as complying with Clause 870 of MCHW1 (HA, 2009a);
� only direct density determination can be used to demonstrate adequate compaction and compliance with a
density specification, however, in situ test devices may be used to monitor consistency as part of a quality
control test regime, this requires development of material specific threshold values (pass/fail); and
� compliance with a suitable compaction method specification for HBMs should be sufficient to achieve
adequate compaction. Development of such a method specification would require a demonstration trial
utilising direct density determination to prove adequate compaction (Appendix A).
Recycled utility arisings in trench reinstatement: compaction trial 2
tests were used immediately after compaction and at several months age. The work indicates that, the higher the
coarse aggregate content (with all other variables being equal), within the overall context of the HBM particle size
distribution (PSD), the more mechanically stable the mixture. In summary, the previous project demonstrated
that appropriately designed HBMs perform better than GSB1/Type1, and that they should be designed for both
short and long term performance.
Recycled utility arisings in trench reinstatement: compaction trial 9
Performance of HBMs relies not only on a suitable mixture design but also on adequate compaction of the
material. Therefore, an assessment of the various methods of determining in situ density (and hence compaction)
of the reinstated material was undertaken within this project.
2.0 Compaction and in situ testing equipment The selection of a compaction methodology for trench reinstatement depends on a number of factors including:
� type and size of the compaction plant that is practical for use in particular reinstatement works;
� lift thickness and number of passes required to achieve compaction of each lift; and
� the tolerances in lift and layer thickness.
In general terms, the heavier the compaction equipment, the more compaction energy is transferred into a
material when it is operating. If insufficient energy is used when compacting the material, then it will not be
compacted to the correct density. For trench reinstatements (and other applications), this could result in:
� high air voids content;
� poor particle interlock;
� settlement issues; and
� insufficient performance and durability, resulting in the need to undertake remedial action.
However, the use of compaction equipment that has excessive compaction energy could result in:
� crushing of aggregate particles in granular soils;
� shearing of fine grained soils;
� less stable backfill;
� risk of damage to buried services; and
� unnecessary exposure of the operative to hand arm vibration.
Inadequate compaction could result in reduced durability or, in the case of unbound mixtures (GSB1/Type1)
settlement. Therefore, it is a key requirement for reinstatement works that a reliable method (including
appropriate plant, trained operators, suitable materials, effective installation procedure and necessary health and
safety considerations) is used to achieve adequate compaction.
In situ testing can be used either to ensure compliance with a specification requirement, as a quality control
check or to determine performance (for example stiffness or resistance to rutting). Compliance testing may
include control test methods but control testing will not necessarily indicate compliance unless the test is included
in the specification. For example CBR tests may be conducted to produce comparative data for control testing (to
check consistency), but will only be utilised as a compliance test if a minimum and/or mean CBR value is given in
the specification.
Given the importance of adequate compaction, the direct measurement of in situ density is highly desirable. The
nuclear density meter (NDM) is the test device specified by the Highways Agency in MCHW1 (HA, 2009a) for the
determination of in situ wet density. However, there are health and safety issues related to exposure to a
radioactive source (see Section 4.3) and many authorities involved in road construction are seeking an
alternative. In addition, the operation of this device is generally impractical for all but the largest of trench
reinstatement projects; and robust and reliable in situ test devices which measure a property that can be used for
quality control monitoring are preferred.
The following Sections review the compaction plant available (Section 4.1) and the tests for directly measuring
density (Section 4.2) or providing some indirect measure for use in quality control (Section 4.3). Section 4.4
explains the selection of compaction methodology and in situ test methods used in the compaction trial.
2.1 Compaction methodologies This Section reviews the available compaction plant for trench reinstatement works; the review was used to
select representative plant for use in the site compaction trial.
2.1.1 Roller compactors Roller compactors, also known as road rollers, include pedestrian and ride on rollers which can be a single or twin
drum set up. The type of drum is selected for the material to be compacted and the range includes smooth-
wheeled and pneumatic-tyre rollers, dead-weight sheep’s foot, tamping and grid rollers. They are generally
Recycled utility arisings in trench reinstatement: compaction trial 10
utilised on large scale works such as road construction and earthworks and unless specifically designed for the
narrow confines of a trench are not suitable for trench reinstatement. Therefore, roller compactors were not
considered further for this study.
2.1.2 Vibrating plate compactors Vibrating plates are low amplitude and high frequency compactors, designed to compact granular soils and
asphalt. They have a flat plate in contact with the soil on which either one or two eccentrically weighted shafts
are mounted. The power unit and control handles for a pedestrian operator are attached to a chassis supported
above the base plate on flexible mountings, usually in the form of springs or rubber cushions. Many machines are
equipped with some form of wheeled undercarriage which can be used to assist in transit between working areas
(Parsons, 1992). The SROH (HAUC, 2002) specifies a weight category over 1800 kg/m2 for a compacted lift
thickness up to 150 mm. However, vibrating plate compactors are not generally used for reinstating the lower
layers of a trench so were not incorporated into the compaction trial.
2.1.3 Vibrotampers The vibrotamper is also known as a power rammer, trench compactor, elephant’s foot or jumping jack. A high
impact force (high amplitude) is delivered from an engine driven reciprocating mechanism, which acts on a spring
system through which vertical oscillations are set up in a base plate (Parsons, 1992). Vibrotampers have masses
between 50 and 150 kg, and usually operate at a frequency of about 10 Hz. They are often used in confined and
small areas due to their portability and manoeuvrability. For situations where fumes from internal combustion
engines may be a hazard, electrically driven vibrotampers are available (Parsons, 1992).
Vibrotampers are commonly used in trench reinstatements. The compaction effort applied to the reinstatement
can be controlled by varying the number of passes with the vibrotamper and by changing the size of the foot; a
smaller foot will result in higher compactive effort compared to a large foot fitted to the same vibrotamper.
Compaction using a vibrotamper was selected for the trial as it is representative of current practice and
technically suitable for compacting the selected materials (Section 3.0).
2.2 Direct density determination Few devices are able to give a direct determination of density. Those tests that are used to determine in situ
density are often labour intensive and time consuming. Hence, widespread adoption of the methods described in
the following sections would not be practical in all but the largest scale utility reinstatement works.
2.2.1 Nuclear density meter The principal method for the direct determination of in situ density within MCHW1 (HA, 2009a) is the use of a
nuclear density meter (NDM), also referred to as a nuclear density gauge. The gamma radiation source may be
hazardous to the health of users, unless proper precautions are taken; therefore, the NDM requires a special
licence, with accompanying legal requirements for its storage and operation. The NDM is operated by technicians
who are specially trained in its use so is not widely adopted for routine assessment of compaction and in situ wet
density within trench reinstatement work. However, this test was used in the compaction trial to provide baseline
data of direct density measurements for comparative analysis.
Ionizing radiation is applied to the material with the amount of radiation detected decreasing in proportion to the
wet density of the material between source and receiver. Measurement is possible in two modes; backscatter and
direct transmission. Backscatter mode can be used to determine density, but is generally used to determine water
content while direct transmission is the favoured method for density. In backscatter mode, the source and
detector are placed on the surface of the material, and the test only penetrates to a depth of 70 to 80 mm (BSI,
1990a), whereas in direct transmission mode the radioactive source is positioned within the soil profile rather
than at the surface. The dry density is calculated from wet density and determined water content.
The device is portable and testing is relatively quick (less than 5 minutes) once the NDM has been calibrated for
the material to be tested. NDM calibration is specified in BS 1924 -2 (BSI, 1990a) and comprises compacting the
test material into a gauge block of known volume to calculate the wet density.
2.2.2 Sand replacement or sand cone method The sand replacement method is designed for testing soils and is specified in BS 1377-9 (BSI, 1990b), it involves
the excavation of a cylindrical hole in the material, which is then filled with dry uniformly graded sand. The
excavated material is carefully collected and weighed, and the water content is determined. The apparatus is
Recycled utility arisings in trench reinstatement: compaction trial 11
calibrated prior to testing, and includes determination of the density of the sand and the amount of sand
contained within the cone.
The test utilises a pouring cylinder incorporating a cone at the base, designed to generate a cone of sand of
known volume above the sand filled void to ensure that the excavated hole is completely filled. The sand
remaining in the pouring cylinder and cone is used to calculate the mass required to fill the hole and determine
the exact volume of the excavated hole. This in combination with the mass of the soil excavated from the hole, is
used to calculate and the wet and dry density of the soil. For accuracy, it is essential that the surface in which the
hole is excavated is flat and that the soil surrounding the excavated hole remains undisturbed.
The size of hole required for the test, and corresponding pouring cylinder, depends on the particle size
distribution of the soil. The British standard test method (BSI, 1990b) specifies a 100 mm diameter hole for fine
and medium grained soils, whereas a 200 mm diameter hole is required for coarse grained soils. This test method
was developed for the assessment of soils and has potential limitations associated with excavation into bound
materials and coarse aggregates.
2.2.3 The water replacement method The water replacement method is another test for soils described in BS 1377-9 (BSI, 1990b). It is intended for
use with coarse and very coarse soils, where other methods for determining the field density (Section 2.2.2)
would be unsuitable because the volume sampled would be too small to be representative. A circular density ring
is placed on the surface of the ground and a hole is excavated inside the ring, the excavated material is weighed
as in the sand replacement method. Flexible plastic sheets are used to line the excavated hole to retain water
which is poured into the hole. The volume of the hole is then determined from the mass of water; the wet density
(and dry density) of the soil can be calculated using the excavated material. This test method is not widely used
as it is relatively complicated and time consuming. Its application to bound materials will have similar limitations
to the sand replacement method.
2.2.4 The rubber balloon method The rubber balloon method was used during early research into the quality of compaction in trenches (Fleming
and Cooper, 1995). It is specified as ASTM D 2167-08 (ASTM, 2008); there is no British standard available for this
method. In principle, this method of determining the density of soil in situ is very similar to the sand replacement
method. A hole is excavated in the compacted material and the removed soil is weighed. Its dry weight is
measured as part of the water content determination. The volume of the excavated hole is determined from the
volume of water required to inflate a rubber balloon to completely fill the hole. The water is contained in a
calibrated vessel, enclosed at its base by the rubber balloon. A means of pressurising the liquid is normally
incorporated in the design of the apparatus. An initial reading is taken by placing the rubber balloon apparatus on
the surface of the soil at the location of the intended test hole, and the final reading is taken after excavation of
the hole. The difference between the two readings gives the volume of the excavated hole; which is then used to
calculate the density of the soil.
The test is not recommended for soils that deform easily (particularly where the apparatus is used under
pressure). It is also noted that the test may not be suitable for soils that contain sharp edges that could puncture
the rubber balloon (ASTM, 2008), which further limits its use.
2.2.5 The core cutter or drive cylinder method The core cutter method outlined in BS 1377-9 (BSI, 1990b) consists of driving a thin walled open ended steel
cylinder (100 mm diameter and 130 mm long) into the soil. The cylinder containing the soil is excavated and
removed from the ground, and the soil protruding from the cylinder ends is trimmed off. The mass of soil
contained in the known volume of the cylinder provides the wet density, and the dry density is calculated
following a water content determination. The method is only applicable to unhardened fine grained materials
(BSI, 1990a), therefore, not suitable for coarse granular material or many HBMs.
Recycled utility arisings in trench reinstatement: compaction trial 12
2.3 Indirect determination of compaction A variety of in situ test devices are available for pavement and subgrade evaluation, however, these devices are
designed to measure specific parameters and do not measure density directly. The following sections describe
methodologies which were assessed for use in the compaction trial. The devices selected for the trial have been
grouped into four main categories based on the parameter measured by each test:
� electrical impedance:
o asphalt pavement quality indicator (PQI);
� surface stiffness (modulus):
o lightweight deflectometer (LWD);
o German dynamic plate (GDP);
o bearing capacity and deflectometer (BC&D);
� resistance to impact:
o Clegg impact hammer;
� resistance to penetration:
o Panda2 variable energy dynamic penetrometer;
o dynamic cone penetrometer (DCP).
This list covers a range of test devices used within UK civil, utility and construction works. Static plate loading
devices, typically used for determination of resistance to monotonic loading, such as a crane outrigger or building
foundation, have not been included since they are not portable and can require significant kentledge.
A number of the test devices selected report values in terms of CBR (California bearing ratio). The CBR was
introduced into the UK following the Second World War (Croney and Croney, 1997) and has been in continued
use since then (although not in California where it has been replaced). The CBR is limited in its application as it is
only suitable for testing soils with a maximum particle size not exceeding 20 mm (BSI, 1990b), due to the size of
the plunger (50 mm diameter). The CBR test is relatively robust and cost effective; however, it is not a
fundamental property of the material and does not replicate the behaviour of materials under pavement type
loadings (Brown, 1997). In addition, its application to HBM is relatively limited (WRAP, 2008) so it was not
included in this study. For the test devices which provide a CBR value, the CBR should only be seen as a test
specific output and any quality control testing using target CBR values would need to be based upon an
established relationship between the test device and material type.
2.3.1 Electrical impedance The asphalt pavement quality indicator (PQI) was designed to give measurements of the density of bituminous
bound mixtures in a non destructive, non nuclear format (Sawchuk, 1998). The density of asphalt pavement is
directly proportional to the measured dielectric constant and composite resistivity of the material. The PQI uses a
low frequency constant current and a toroidal (doughnut shaped) electrical sensing field to measure changes in
electrical impedance of the material matrix. The electronics in the PQI convert the field signals into material
density readings and displays the results. Once calibrated, direct density readings can be consistently obtained for
bituminous bound mixtures. The company behind the PQI have recently developed a soil density gauge (SDG)
based on the same principles as the PQI; although it was not available at the time of this work.
2.3.2 Surface stiffness Lightweight dynamic plate tests are used to determine a surface modulus (stiffness), where the response of a
material under dynamic loading (its transient deflection) and the applied stress approximates to those
experienced in-service. A variety of designs are available such as the lightweight deflectometer (LWD), the
German dynamic plate (GDP) and the bearing capacity and deflectometer (BC&D). The test involves dropping a
weight onto a bearing plate which is placed on to the surface of the material. The area of loading and applied
stress can be readily controlled, and usually a damping mechanism is incorporated to control the loading time.
Lightweight dynamic plates may not be suitable to test thicker and/or stiffer materials as the result is dependent
on deflection during the test. It is important to note that the equipment cannot provide stiffness values for
individual lifts compacted in a trench reinstatement or discrete layers because the stress applied to the surface
travels down through the material. Therefore, the actual stiffness reading is a composite value of the material
down to the bottom of the zone of influence. The zone of influence of these tests is typically considered to be a
bulb of significant stress which is dependent on the size of the bearing plate and applied load.
Recycled utility arisings in trench reinstatement: compaction trial 13
LWD and GDP have been correlated against the larger falling weight deflectometer (FWD), which is used in the
UK to assess the condition of pavements; whereas the BC&D has been recently introduced to the UK so limited
data are available. The BC&D provides a measurement of surface modulus similar to the LWD and GDP.
The LWD testing procedure used in the compaction trial was developed for pavement foundations and is detailed
in the Highways Agency design guidance - IAN73/06 (HA, 2009c). The key requirements are:
� the equipment is capable of delivering a load pulse of peak magnitude in the range 4 to 15 kN, of total
duration 15 to 60 ms, to a rigid circular plate of 300 mm diameter;
� both the applied load and the transient deflection are measured directly on the tested surface. The deflection
measurement transducer must be capable of measuring deflections up to 2000 microns;
� the peak stress applied during each test is within the range 50 to 200 kPa. A peak stress of 100 kPa was
targeted, unless the deflection measurement fell outside the range 100 to 1000 microns, in which case the
applied stress was increased in order to achieve a realistically measurable deflection;
� at each test point, three initial seating drops, to bed the plate into the surface are undertaken. Three further
drops are then carried out. The results (measured load and deflection) from the last set of three drops are
then averaged to give the surface modulus applicable to that test point; and
� the surface modulus is computed at each point tested, using the following formula:
D
RPE
)1(2 2ν−
=
where:
E = surface modulus (MPa)
v = Poisson’s ratio (default = 0.35)
P = contact pressure (kPa)
D = deflection (microns)
R = plate radius (150 mm)
A disadvantage of the lightweight dynamic plate tests, with regard to testing trench reinstatement, is the
standard 300 mm plate diameter (although this can be reduced) due to the influence of lateral support. Testing
of the same material within two different trenches (with varying degrees of lateral support), is likely to give
different results (surface modulus being higher for higher levels of lateral support). However, the influence of
lateral support (and the trench itself) is likely to be more significant when assessing the performance of unbound
materials (GSB1/Type1) than HBMs. The stiffness value derived from the test is a composite stiffness value of the
layers and lateral support within the zone of influence. The zone of influence is (typically taken to be 10% of the
applied stress) and normally reaches a depth of around 1.5 times the plate diameter, so for a 300 mm diameter
bearing plate (standard for the LWD and GDP) it typically reaches to a depth of 450 mm below the surface.
2.3.3 Resistance to impact The Clegg impact hammer was developed in Australia during the 1970s by Dr Baden Clegg for the in situ
evaluation of granular base course (Clegg. 1976). It works by measuring the deceleration of a drop hammer,
registering the deceleration in units of ‘Impact Value’ (IV). It is specified as ASTM D 5874- 02 (ASTM, 2007) and
AS 1289.6.9.1 (AS, 2000). A range of hammer masses is available, namely a standard (4.5 kg), medium (2.25
kg), light (0.5 kg) and heavy (20kg). The drop height for the standard and medium hammers is 450 mm, and 300
mm for the light and heavy hammers. The standard hammer is most commonly used for trench reinstatement, so
was selected for the compaction trial.
The impact value is dependent upon a combination of density, soil type and water content. Snowdon (1992)
reported that the Clegg impact hammer should only be used for the purpose it was devised for, and that any
application of the Clegg to control or monitor dry density and state of compaction for earthworks should not be
considered.
The advantages of this in situ test are practicality, ease of use and reliability. The rapid nature of the test means
that large data sets can be readily gathered, which is ideal for checking material consistency. In addition, the
latest models convert IV to CBR in real time.
Recycled utility arisings in trench reinstatement: compaction trial 14
The main disadvantage of the standard test is the ratio between the contact area and maximum aggregate size.
The standard hammer diameter of 50 mm (which makes contact with the test surface) has the following
limitations:
� testing of materials which contain coarse aggregates (such as GSB1/Type1 and the majority of the recycled
trench arisings) is questionable, high variability might be expected depending on whether the test area is
wholly, partly or not at all over a large aggregate particle;
� the hammer diameter means that the zone of influence (the zone that is significantly stressed under the drop
hammer) is likely to be in the order of 50 to 150 mm; so low density zones at the base of thick lifts may be
missed.
The heavy (20 kg) Clegg hammer has a diameter of 130 mm so can be used to test coarser material and has a
larger zone of significant stress than the other Clegg hammers, however, the standard Clegg hammer is more
commonly used within the UK.
2.3.4 Resistance to penetration Cone penetrometers are used to determine resistance to penetration and give a profile with depth; and as with
any intrusive test, care should be taken when using it to assess material under which buried services are present.
Various sizes of portable dynamic and static field cone penetrometers are available. Static cone devices, such as
the mexicone, are generally designed to assess soft and medium fine grained subgrades, as the cone is pushed
into the soil, and are not considered suitable for assessing reinstatement materials.
The dynamic cone penetrometer (DCP), developed by CNS Farnell in conjunction with TRL (Clark, 2000), is
commonly used for the evaluation of subgrade strength under pavements – see IAN73/06 (HA, 2009c). The DCP
is driven into the ground under the action of an 8 kg steel drop weight falling vertically through 575 mm and
making contact with a steel anvil, attached via steel rods (less than 20 mm diameter) to a 20 mm diameter 60o
steel cone, which is thus driven vertically into the ground. It is suited to stronger and coarser materials than other
penetrometers, such as the Mackintosh probe. The Mackintosh probe is a light dynamic cone penetrometer that
can be operated manually and is generally suited to site investigation in soft deposits. The probe has a 4.5 kg
hammer which falls through a 300 mm drop height. Due to the relatively low energy, the Mackintosh probe is
limited in the depths and materials it can penetrate (Clayton et al, 1995); therefore, it is not considered practical
for testing bound material.
The accumulated blows are recorded against depth (rate of penetration), expressed as mm/blow, which for the
DCP can be converted to a CBR value using the following relationship (HA, 2009c):
Log10(CBR) = 2.48 – 1.057 × Log10(mm/blow)
Subsequent plots of accumulated blows (or CBR) against depth can be used to characterise layer depth and are
indicative of material strength. However, this strength measure will not normally be specified for materials other
than subgrade, since results are highly dependent on particle size and can, without calibration to specific
materials, be misleading (HA, 2009c).
The type of cone penetrometer specified in IAN73/06 (HA, 2009c) is the DCP developed by CNS Farnell and TRL,
if any alternative is to be used, then it should be carefully calibrated against equipment complying with the
specification for the specific types of material encountered.
The variable energy dynamic penetrometer (Panda2), unlike the DCP, has been designed specifically for use in
trench reinstatements, and can be used above utility services as it includes a user defined depth warning system.
The Panda2 was developed in France where it has become widely used and accepted (Langton, 1999), and is
supported by the French standard XP P 94-105 (AFNOR, 2000). A hammer is used to apply the force necessary to
drive the cone into the ground, rather than a fixed weight being dropped a standard height. This variable energy
input is determined using the output from two sensors, which measure the speed of impact and the depth of
cone penetration, recorded by a microprocessor. The results may be correlated to a CBR but this is likely to be a
material specific relationship. The dynamic cone resistance (qd) and its current depth are then calculated and
displayed in real time on the screen of the microprocessor. The dynamic cone resistance is calculated using the
formula given in XP P 94-105 (AFNOR, 2000) and has been modified to the following (Langton, 1999):
Recycled utility arisings in trench reinstatement: compaction trial 15
°
⋅
+
⋅=
90
2
1
1
.21
1
x
M
P
VM
Aqd
where:
x90° = penetration from one drop (90° cone)
A = area of the cone
M = weight of the striking mass
P = weight of the struck mass
V = speed of impact (of the hammer)
The software can also plot material specific reference curves (tolerance lines) which may be applied as pass/fail
criteria when assessing compaction compliance, as they are based on the relationship between the degree of
compaction and the dynamic cone resistance for a given material. The tolerance lines are established in
accordance with XP P 94-105 (AFNOR, 2000) and correspond to the maximum dry density, obtained from the
Proctor test. A choice of materials with predefined tolerance lines is available in the software (Langton, 1999).
2.4 Selection of test methodologies for site trial A number of test methodologies for the determination of in situ density both directly and indirectly have been
reviewed. For the purposes of the trial and the development of appropriate guidance, the selection of test
methodologies was based on the ease of use, cost and availability.
The NDM is the most commonly available method for directly measuring in situ density. It is the direct density
measurement procedure specified for use with HBMs within MCHW1 (HA, 2009a). Although the test is accurate, it
does have the following disadvantages; it:
� requires calibration using material compacted in gauge blocks;
� can only be used by a trained operator; and
� has special health and safety requirements for transport, storage and operation.
For these reasons, it is not a practical test for reinstatement crews to use in the field for standard reinstatement
works and industry are looking for a non nuclear alternative. However, for the detailed assessment of the
behaviour of an HBM at the design stage (for example, evaluating compaction behaviour or developing a
compaction method specification) or as part of a two year approval trial (HAUC, 2002), the use of an NDM for
density measurement may be appropriate. The other in situ direct density measurement methods, such as the
various replacement methods specified in BS 1377-9 (BSI, 1990b), are difficult and time consuming to undertake,
and are of limited use for bound and coarse grained materials. This makes them impractical and is most likely the
principal reason they have not been widely adopted in the UK. For the purposes of the compaction trial, the NDM
was selected for the direct method of determining in situ density.
The following test methodologies were selected for assessment during the compaction trial:
� asphalt pavement quality indicator (PQI);
� lightweight deflectometer (LWD);
� German dynamic plate (GDP);
� bearing capacity and deflectometer (BC&D);
� Clegg impact hammer;
� dynamic cone penetrometer (DCP); and
� variable energy dynamic cone penetrometer (Panda2).
The test methodologies selected for assessment measure a number of different material properties which may or
may not correlate to actual in situ material density. The PQI measures density through the direct correlation with
electrical impedance and can present the results in the same units as the NDM. However, the PQI is principally
used for asphalt testing; its application to HBMs in reinstatements is beyond the purpose that it was designed for.
The appropriate counterpart of the PQI, the soil density gauge (SDG) was not available in the UK during the trial
so was not included.
The LWD, GDP and BC&D are used to measure surface stiffness. Surface stiffness is a fundamental property of
the material’s ability to support the overlying pavement IAN73/06 (HA, 2009c). The LWD and GDP are commonly
Recycled utility arisings in trench reinstatement: compaction trial 16
used in pavement evaluation so were included in the trial. The relatively new BC&D works on a similar principle to
the LWD and GDP so was also incorporated into the trial.
The Clegg impact hammer and Panda2 were incorporated into the trial to assess compaction because they have
been adopted by UK practitioners for the assessment of trench reinstatements. The DCP was included in the trial
as it is a specified test for the evaluation of subgrades (HA, 2009c). Only the DCP and Panda2 have the ability to
give a profile with depth. The remainder of the tests are undertaken on the surface of the installed material, with
the zone of influence dependent on the material tested, the contact diameter and stress level applied during the
test.
3.0 Site work The compaction trial was designed to meet the research project aim and objectives (Section 1.0). The following
Section details the trial methodology, materials, test matrix and construction.
3.1 Methodology The in situ test devices selected for use on the trial comprised the nuclear density meter (NDM) and a range of
portable test devices, with potential for use as compliance and/or control tests during the installation of trench
reinstatement materials (Section 2.4). The testing requirements were rationalised to permit the works to be
undertaken within the duration of the trial.
The same compaction equipment was used for the site trial construction, for consistency (Section 5.3), the
variables included:
� the degree of compaction (controlled by the number of compaction passes);
� reinstatement material;
� lift thickness; and
� time delay between material production and use.
All testing and installation works were undertaken by appropriately trained and accredited staff.
Three reinstatement materials were selected for use in the compaction trial including:
� two generic HBMs produced from recycled trench arisings; and
� GSB1/Type1 control from a primary source, representing a ‘traditional’ reinstatement material.
A pipe bedding material was also selected to produce a consistent and representative starting layer for the
installation of the HBMs and the GSB1/Type1. Further details of the materials are given in Section 3.1.1.
A site was selected to permit experimentation and levels of compaction that would not be allowed in a highway.
The site selected for the trial was located on brownfield land owned by Derbyshire County Council, adjacent to
the offices of the Markham Vale Environment Centre near Chesterfield. The site consisted of made ground to a
depth of approximately two metres, the upper 200 mm of which had been in situ stabilised with cement to serve
as a working platform. The made ground comprised firm cohesive soils, which are representative of the lower
layers (subgrade) of many trench reinstatements. The ground conditions, in terms of confinement during the
material installation, are considered representative of full depth in typical highway or footpath reinstatements.
Details of the site trial construction are given in Section 3.2.
An additional complication with testing HBMs is their strength gain over time, which is related to a number of
variables; therefore, additional testing was undertaken after the trial to monitor potential strength gains. Post
compaction trial testing was undertaken at 7 days using the LWD and GDP; and 28 days using the LWD, GDP,
Clegg impact hammer, Panda2 and DCP. This testing provided additional information on the performance of the
trench reinstatement materials (Section 6.0).
3.1.1 Materials The following three materials were selected for use in the compaction trial:
� HBM Fine;
� HBM Coarse; and
� GSB1/Type1.
Recycled utility arisings in trench reinstatement: compaction trial 17
The HBMs were stored for 24 hours before compaction to investigate the influence of delayed compaction (DC):
� HBM Fine (DC); and
� HBM Coarse (DC).
The two HBMs were chosen as representative particle size distributions of processed trench arisings from a range
of sources across the UK (WRAP, 2008). The HBM Fine grading represents trench arisings where the oversize
material (>63 mm material comprising asphalt, concrete and bricks) has been screened out (removed). This
recycling option has the advantage that the oversize material can be crushed to produce recycled aggregate
feedstock for other products, but the lack of coarse aggregate (dependent on the feedstock) has to be
accommodated within the HBM design, typically by increasing the hydraulic binder additions.
The HBM Coarse grading represents the same trench arisings, but where the oversized material has been
screened and crushed to produce a <40 mm coarse aggregate, which has been blended back into the processed
trench arisings. Other sources of coarse aggregate can also be introduced to enable the production of a more
consistent material.
The previous study indicated that the HBM Coarse grading would be advantageous during installation and in the
short term, when compared to the HBM Fine grading. This is because; in the short term HBM performance is
related to the unbound aggregate mixture performance, which is dependent on the grading and quality of fines
(WRAP, 2008). In short, the higher the coarse aggregate content, within the overall context of the HBM particle
size distribution (PSD), the more mechanically stable the mixture is in the short term (WRAP, 2008).
The GSB1/Type1 aggregate (Figure 1) control material was taken from a local primary source to represent
‘traditional’ reinstatement practice. In addition, Envirosand (Figure 2), produced from recycled glass, was selected
for use as bedding sand in the trial, and recycled aggregate (Figure 3) required to optimise the grading of the
HBM Coarse, were sourced from the Lafarge A&C UK recycling hub located at Chaddesden, Derbyshire. All the
recycled aggregates were produced in accordance with the WRAP Quality Protocol for production of aggregates
from inert waste (WRAP, 2005b). Pulverised-fuel ash (PFA) and lime were selected for the binder additions to
produce the HBMs.
Figure 1: Primary GSB1/Type1
Figure 2: Envirosand
Figure 3: Recycled aggregate
The ‘as received’ trench arisings (Figure 4) were processed (by screening out the oversize materials) at the
Lafarge A&C UK recycling hub in accordance with the WRAP Quality Protocol (WRAP, 2005b) to produce the
feedstock material (Figure 5) for both HBMs. The coarse grading was achieved by combining the processed
trench arisings with additional recycled aggregate (Figure 3) to optimise the grading, while the fine grading
utilised the screened arisings without further modification.
Recycled utility arisings in trench reinstatement: compaction trial 18
Figure 4: ‘As received’ trench arisings
Figure 5: Processed trench arisings
The particle size distribution (PSD) of both HBM Fine and HBM Coarse were determined in accordance with BS EN
933-1 (BSI, 1997) and are shown in Figure 6. The grading curves are shown plotted against the grading
envelope taken from BS EN 14227-3 (BSI, 2004c), for a fly ash bound mixture (FABM 1) produced using 0/31.5
mm aggregate and siliceous fly ash. The HBM Coarse falls within the envelope and is compliant as an FABM 1,
while the HBM Fine falls outside the envelope; however, it is compliant as a soil treated with fly ash (SFA)
covered in BS EN 14227-14 (BSI, 2006e). Therefore, both HBMs are compliant with the appropriate BS EN,
MCHW1 (HA, 2009a) and the SROH (HAUC, 2002).
Figure 6: Particle size distribution of the HBM Fine and HBM Coarse
31.5
20.0
16.0
14.0
10.0
8.0
6.3
4.0
2.0
1.0
0.5
0
0.2
5
0.1
25
0.0
63
0
10
20
30
40
50
60
70
80
90
100
0.0 0.1 1 .0 1 0.0 1 00.0
Sieve size (mm)
Mass %
passing
HBM Fine
HBM Coarse
FABM 1 envelope min
FABM 1 envelope max
3.1.2 Test matrix The test matrix for the trial is shown in Table 2. The typical trench depth was 1000 mm, and the reinstatement
comprised 100 mm of sand to provide a consistent layer on which to place the test materials. The test materials
were placed in either 150 mm or 300 mm lifts, and testing was generally undertaken after 3, 5 and 8 passes with
the vibrotamper. These two construction profiles and the associated testing regimes were used for each of the
material types. Consultation with industry and the desk based review (Section 2.1) indicated that 8 passes would
be a pragmatic upper limit, as this is the method specification for GSB1/Type1 (HAUC, 2002).
Recycled utility arisings in trench reinstatement: compaction trial 19
Table 2: Test matrix for HBMs and GSB1/Type1 control compacted in 150 mm and 300 mm lifts
In Situ Testing
Material
Lift
thickness
(mm)
Lift
Number LWD GDP NDM PQI Clegg Panda2 or
BC&D DCP*
Sand 100 Test No test
1 Test after 3, 5 and 8 passes No test
2 No test
3 Test after 8 passes No test
4 No test
5 Test after 3, 5 and 8 passes No test
HBM Fine
HBM Coarse
GSB1/Type1
HBM Fine (DC)
HBM Coarse (DC)
150
6 Test after 8 passes
1 Test after 3, 5 and 8 passes No test
2 Test after 8 passes No test
HBM Fine
HBM Coarse
GSB1/Type1
HBM Coarse (DC)
300
3 Test after 3, 5 and 8 passes Test after
8 passes
*Note – DCP only undertaken on completed trench due to sensitivity of the test.
The test matrix (Table 2) allowed comparison of compaction behaviour and also the evaluation of the potential
of a range of in situ testing devices to be used for compliance and/or quality control purposes. A calibrated
nuclear density meter (NDM) was used, in direct transmission mode, to determine the in situ density and hence
degree of compaction achieved for the various materials across the range of installations.
Post construction testing using the LWD and GDP was undertaken on the reinstated trenches after 7 and 28 days.
In addition, Clegg, DCP and Panda2 testing was conducted on all the trenches after 28 days (Section 6.0).
3.1.3 Material reinstatement and test positions The trenches were divided into five test sections (Bays) to which an in situ test was allocated. A schematic
representation of the reinstatement profiles and test positions is given in Figure 7. The assigned bay remained
the same throughout the reinstatement for both 150 mm lifts and 300 mm lifts (Figure 8). The test bay
allocation was devised to ensure minimum modification/disruption to the surface prior to in situ testing and
facilitate on site management of the testing operations.
Figure 7: Schematic trench profile for 150 and 300 mm lifts and the in situ testing equipment
Recycled utility arisings in trench reinstatement: compaction trial 20
Figure 8: Testing bays for allocated test devices
3.2 Site trial construction The site trial was conducted over three days (12 to 14 November 2008 inclusive). The HBMs were mixed at the
site by the Independent Stabilising Company Ltd. The layout of the site trial is shown in Figure 9. The weather
varied between sunny and overcast on Day 1, to intermittent showers during Days 2 and 3.
Figure 9: Schematic plan view of site trial construction
HBM Fine
Lift thickness 150 mm
Trench number 1
Construction day 1
HBM Fine
Lift thickness 300 mm
Trench number 2
Construction day 1
GSB1/Type1
Lift thickness 150 mm
Trench number 6
Construction day 2
HBM Coarse
Lift thickness 300 mm
Trench number 4
Construction day 2
HBM Fine - delayed compaction
Lift thickness 150 mm
Trench number 7
Construction day 2
HBM Coarse - delayed compaction
Lift thickness 150 mm
Trench number 8
Construction day 3
HBM Coarse
Lift thickness 150 mmTrench number 5
Construction day 2
GSB1/Type1
Lift thickness 300 mm
Trench number 3
Construction day 1
HBM Coarse - delayed compaction
Lift thickness 300 mm
Trench number 9
Construction day 3
The trenches were excavated using a tracked excavator, supplied by NT Killingley Ltd (Figure 10), to the
nominal dimensions of 1 m deep and 0.8 m wide. The length of each trench was 2 m to allow each test device to
be assigned its own bay (Section 3.1.3) and to have an additional section that allowed safe access/egress to the
trench to conduct in situ testing.
Recycled utility arisings in trench reinstatement: compaction trial 21
Figure 10: Excavation of the trial trenches
Figure 11: Vibrotamper used in the trial
Compaction was achieved using a Wacker BS60-2 vibrotamper, shown in Figure 11. The specifications of the
particular model used in the trial are:
� Dimensions (length x width x height) 675 mm x 345 mm x 965 mm.
� Shoe size (width x length) 280 mm x 330 mm.
� Operating weight 66 kg.
� Compaction depth (depending on soil) 580 mm.
� Impact energy 85 J.
� Force/blow per CIMA - LEMB 13.4 kN.
� Percussion rate up to 700 blows/min.
� Travel speed up to 17.4 m/min.
The water content of the HBMs was assessed during the trial by moisture condition value (MCV) testing. The
excavator bucket was used to place the reinstatement material into the trench, which was then spread evenly by
an operator before an un-compacted measurement of the lift thickness was taken. Thickness was adjusted, prior
to compaction, to achieve a lift of reinstatement material of 150 mm or 300 mm. The reinstatement materials
were compacted using the designated number of passes with the vibrotamper. Testing was conducted with each
apparatus (Section 2.4) in the designated bay before the next allocation of passes was undertaken or the next lift
installed. The width of the vibrotamper foot in relation to the width of the trench meant that one pass comprised
three traverses.
The thickness of the lift was measured after 8 passes of the vibrotamper to check that the target lift thickness
had been achieved (Figure 12). Six measurements were taken along each trench and averaged. Control of the
lift thickness was readily achieved with the selected compaction methodology for the sand, GSB1/Type1, and
HBM Coarse. The HBM Fine proved more problematic and was noted to be more mobile (the material was easily
pushed around during compaction, Figure 13); although producing an acceptable mean layer thickness, there
was variability in the surface level of the individual lifts.
Figure 12: Measuring the compacted HBM lift
Figure 13: Compaction of material (HBM Fine)
Recycled utility arisings in trench reinstatement: compaction trial 22
3.3 In situ testing of reinstatement layers Eight in situ test devices were selected for use in the trial (Section 2.4) these were provided by members of the
project team and operated by trained technicians; the test devices and layout for testing within each trench are
shown in Figure 7 (Section 3.1.3). The NDM and PQI were calibrated for each material type using a gauge block
(Figure 14) with each material compacted to refusal in accordance with BS 1924-2 (BSI, 1990a).
Measurement of in situ density was determined using an NDM (Figure 14) in direct transmission and
backscatter, by trained operatives from Mid Sussex Testing Ltd. The results from the test in direct transmission
provided the baseline measurement of in situ density to which the other methodologies were compared. The
NDM was used on every lift that the indirect methodologies were used. The PQI (Figure 15) was provided by
Lafarge A&C UK and was trialled on Day 1 and part of Day 2. Its use was abandoned on Day 2 following initial
feedback from the interpretation of its data sets. In short, the usefulness of this device for testing asphalt did not
translate to other materials. Lafarge A&C UK also provided and operated an LWD (Figure 16). The surface
stiffness of the reinstated materials was measured using an LWD and a GDP (Figure 17), which were supplied by
Scott Wilson.
Figure 16: Lightweight deflectometer (LWD)
Figure 17: German dynamic plate (GDP)
The bearing capacity & deflectometer (BC&D) was provided by Loughborough University and operated by
research staff who visited on Day 2 (Figure 18). Tests were undertaken on the HBM Coarse compacted in 150
mm and 300 mm lifts and GSB1/Type1 compacted in 150 mm lifts. The Clegg impact hammer (Figure 19) was
used on Days 1 and 2. Measurements were taken at three points within its assigned bay. Unfortunately the
device malfunctioned on Day 3, so it was not possible to complete all the scheduled testing with it. However, it
was repaired and used to conduct the 28 day testing.
Figure 14: NDM gauge block for calibration and NDM