Construction Control of Leca® LWA
It has been found that in order to maximise the benefits of Leca® LWA, careful consideration
in how the material is placed, prior to delivery, allows the contractor additional time saving on
site.
This document is designed to assist both the contractor and the specifier when using Leca®
LWA within both general and structural fill applications.
Delivery
As Leca® LWA lightweight aggregate is at least 5 times lighter than conventional fills,
delivery of the aggregate is based on volume rather than mass as with conventional
aggregates. As such deliveries are typically 60m³. This is equivalent to at least four 20
tonnes deliveries of conventional fills, thus having a substantial impact on site traffic.
Site Storage
Leca® LWA can be stockpiled as with typical aggregates. As Leca® LWA is free draining
the surface of the stockpile need not be sealed from inclement weather
On confined sites where stockpiling is not possible, Leca® LWA can be pneumatically blown
by a specialist truck into position. Please speak to a Leca® UK representative regarding this
unique feature of Leca® LWA.
Delivered moisture content
Deliveries of Leca® LWA are from a number of ports situated around the UK. As such these
stockpiles are subjected to UK weather conditions. Due to the lightweight nature of the
material, standard moisture content testing can show high levels of moisture content values
for the delivered material. This is due to the density of water within the material being
greater than that of Leca® LWA. Moisture content testing values of 35% can be expected
and is within the acceptable range of the material.
Optimal Moisture content
With conventional fills optimal moisture content testing is normally conducted to confirm the
optimal moisture content required for compaction. This relationship is generally conducted
using the 2.5kg rammer method in accordance with BS 1377: Part 4 : 1990 Clause 3.3. This
test has been conducted using the 10-20mm Leca® LWA for moisture contents ranging from
10% to 40%. The testing revealed that with Leca® LWA 10-20mm material, the moisture
content of the material had little or no effect on the density achieved after compaction,
therefore variable moisture contents between the range of 10% - 40% for delivered material
is still acceptable for use. This area is explored in more detail in the technical note related to
moisture content.
Updated April 2011 Work Prior to placement of Leca® LWA material
The underlying soils should be prepared as per the contract specifications.
A geotextile separator should be installed to prevent migration of underlying existing soils
into the Leca® LWA fill. Typically this geotextile will be similar to a Terram 1000, however
different geotextile separators can be used based upon the application.
Geogrids can be used between the underlying soils and the Leca® LWA fill however they
should be used in conjunction with a suitable geotextile separator. If Geogrids are to be
used beneath the Leca® LWA fill, the geotextile should be a non woven, needle punched
type and be placed beneath the Geogrid. Overlaps of the geogrids and geotextile should be
as per the manufacturers recommendations.
Placement of Leca® LWA Material
Leca® LWA should be placed the same as with conventional aggregates. This can be either
by tipping and spreading using a tracked dozer, or tracked excavator. Please note that all
spreading plant will need to be tracked and the maximum ground bearing pressure of the
tracked vehicle shall not exceed 50kN/m² for 10/20mm grade Leca® LWA and 80kN/m²
should a well graded Leca® LWA fill be used. This is to limit potential surface crushing of
the Leca® LWA compaction layer. Layers should be placed in 1000mm lifts. If placement is
within a confined site, behind a retaining structure or close to foundations then layer depths
should be decreased to 600mm and compacted with smaller equipment such as a vibrating
plate compactor.
Figure 1 – Use Vibrating plate, tracked excavator and tracked dozer for spreading and compacting
Updated April 2011 Alternatively the Leca® LWA material can be blown into position with an ‘auger fed’ trailer
which can be useful when access is restricted. When material is blown into position layer
depths should be as stated above. Pneumatic pumps should not be considered with this
material.
Figure 2 – Leca® LWA being blown into position
To create an embankment using Leca® LWA, confining bunds at the edge of the
embankment footprint need to be used. These bunds need to be constructed to at least the
maximum compaction depth of the Leca® LWA. The confining bunds need to constructed
using normal aggregates, the final angle of repose of the embankment is dependant upon
the properties of the fill used within the confining bunds, but should not exceed a 1:2
inclination. Figure 3 shows a simplified embankment constructed this way. If the structure is
in a cut, the Leca® LWA material may be adequately confined by the existing earth cut face.
Figure 3 – Use of conventional fill bunds to retain Leca® LWA fill
Updated April 2011 If the Leca® LWA fill is to be used behind a retaining structure, bunds of conventional fill
should be placed at the outside edge of the area to be filled with Leca® LWA fill to allow
adequate compaction of the lightweight aggregate.
Figure 4A-4C shows the sequence of this method of construction using Leca® LWA.
Figure 4A – Bund Creation
Figure 4B – Leca® LWA (ex maxit) installed in layers
Figure 4C – Final construction
Updated April 2011 Compaction of Leca® LWA Fill
Compaction of Leca® LWA material requires substantially less compaction effort when
compared to conventional fills. Therefore layer depths can be greater, typically up to
1000mm. Compaction should be conducted using a tracked vehicle with a maximum ground
bearing pressure of 50kN/m². Information on ground bearing pressures of site plant can be
obtained from the manufacturers. In the past Caterpillar D6 Tracked tractors have proved
suitable. Typically 3-4 passes of this vehicle are required for adequate compaction.
Compaction is expected to provide a reduction in volume of 8-12% with the average being
approximately 10%. This reduction can be measured using profile boards. Passes of this
vehicle should not exceed 5 to prevent excessive surface crushing of the Leca® LWA
Material and thus cause a further reduction in volume.
If the Leca® LWA has been placed pneumatically then compaction of 4-5% is achieved with
this operation. Therefore compaction plant should be limited to 2-3 passes to achieve
adequate compaction.
Behind retaining structures, on confined sites or near foundations layer depths of 600mm are
typically used. Compaction of this reduced layer thickness should be completed using a
vibrating plate compactor
If Leca® LWA is to be used to create an embankment then the maximum slope angle should
not exceed 1:2. This however is dependant upon the loading upon the embankment.
Compaction close to the face is not possible in this application, therefore a geotextile
wraparound facing may be used together with rock fill to prevent losses at the face of the
embankment (figure 5).
Figure 5 – Alternative method of geotextile wrap to retain Leca® LWA fill
Updated April 2011 Table 1 below summarises the type of compaction plant.
Table 1
Cover to Leca® LWA
Once the Leca® LWA has been placed, it is important that the cover to the Leca® LWA is
placed without delay. The cover is typically 600mm, however the cover depth should be as
per the contract specifications. A geotextile separator should be used between the Leca®
LWA and the cover material to prevent future migration of the capping layer into the Leca®
LWA fill. If the specification calls for a geogrid, this should be placed above the geotextile.
Figure 6 shows a typical section of Leca® LWA used within a road embankment. It should
be noted that the Leca® LWA materials have a capping layer to protect the material.
Figure 6 – Typical embankment layout
In some locations the slope face gradient may need to be in excess of the allowable gradient
when using Leca® LWA. In these situations and on embankments where it is not possible to
install a slope capping layer a Geotextile wraparound solution is available. The grade and
spacing of Geotextile will have to be specified by the scheme designer. If the Geotextile is to
be left uncovered then the designer should ensure that the specified Geotextile is
adequately U.V stabilised. Figures 5 and 7 show typical Geotextile reinforced
embankments.
Updated April 2011 Figure 7 – Typical layout with no slope cover
(no geotextile required between layers of Leca® LWA (ex maxit fill))
In Situ Testing
Currently there is no suitable test available to measure the in situ density of Leca® LWA.
The best method of site control for checking that adequate compaction of Leca® LWA is
achieved is by site monitoring of the settlement achieved during construction. Therefore
when compacting 1000mm of Leca® fill a reduction in height of 80-120mm should be
achieved for adequate compaction. For compaction layers of 600mm, height reduction of
48-72mm should be expected.
Plate bearing testing can be used to measure the modulus of a compacted layer of Leca®
LWA. The minimum plate size that should be used is 600mm. Some variations are required
to the test to ensure erroneous results do not occur. The test needs to be limited to a
maximum pressure from the plate of 100kN/m², to keep all loads on the Leca® LWA Fill
within its elastic limit. Immediately before the test commences, a sustained load of 1520kN/m² should be exerted from the plate onto the surface of the Leca® LWA for a period
not exceeding 10 minutes. This is to remove surface movement of the Leca® LWA particles
from the testing results. The plate bearing test should then commence in accordance with
BS1377:Part 9: clause 4.1.
Expected The K762 sub grade reaction test is not normally the most suitable test for use with
Leca® LWA fills. The movement tested for this test is normally less than 3mm. With a
mobile fill such as Leca® LWA this test can result in spurious results.
Although it is possible to conduct in-situ testing on the Leca® LWA material, it is
recommended that testing is conducted on a minimum 300mm granular capping layer to the
Leca® LWA material or on top of the capping layer. The result of this testing will reflect the
stiffness and bearing capacity of the entire structure, not just the Lightweight Leca® LWA
material.
Updated April 2011 Precautions when testing directly upon Leca® LWA
When testing directly upon Leca® LWA fill materials care needs to be taken in the testing
procedure to prevent unexpected results.
•
Ensure that the plate has been bedded down evenly as stated above under a load of
20kN/m² for 10 minutes. Measurements should be taken before and after the
bedding in process and recorded.
•
600mm Plates tend to be heavy and are normally wheeled into place. This may not
be possible on Leca® LWA.
•
The plate should be placed carefully upon the fill and not dropped into place
•
When taking measurements for the test, it is normal practice for the tester to lie down
beside the testing equipment. Care should be taken as movement by the tester
could cause movement of the plate due to the mobile nature of the Leca® LWA
surfacing
•
It is good practice to lightly compact the surface to be tested using a light plate
compactor.
Updated April 2011