Using its geopolymer method to lift and stabilise a section of the M54 meant ground engineering contractor Geobear could lengthen the lifecycle of this part of the motorway by an estimated nine years.
Ground engineering specialist Geobear was contracted to re-level and stabilise concrete slabs that had been found to be defective on a section of the M54. By using its geopolymer injection method to treat the slabs, Geobear was able to extend the lifecycle of this part of the motorway by several years.
The M54 is a 23-mile (37km) east-to-west dual carriageway passing through the counties of Shropshire and Staffordshire. The original dual carriageway was constructed in sections in 1975-83. It is a two-lane dual carriageway for most of its length, with some shorter three-lane sections.
The M54 forms part of the strategic route connecting England to North Wales. It is the only motorway in Shropshire and forms an important part of the county’s economic corridor. It is managed by Highways England and included in the West Midlands region known as Area 9. The highway maintenance contract for Area 9 is operated by Kier Highways under an extension to its asset support contract arrangements.
Works were carried out on the eastbound section from Junction 3 Tong Interchange to Junction 2 Coven Heath Interchange, an eight-mile (17.8km) stretch of two-lane dual carriageway that carries an average annual daily traffic flow of 50,300 vehicles. It is constructed of unreinforced concrete (URC) slabs laid over sub-base materials.
Pavement defects and solutions
An investigation of the carriageway had been carried out by others and a programme of pavement maintenance requirements had been prepared. The URC slabs in lane 1 had undergone settlement and an average uplift of 10mm was required to bring the traffic running surface back to level with adjacent slabs. The URC slabs were constructed in bays and were typically 4,000mm wide x 6,000mm long x 280mm thick. Those in lane 1 had been identified for geopolymer injection treatment, for slab stabilisation and re-levelling.
Geopolymer injection process
The specialist site injection equipment used by Geobear Infrastructure utilises a self-contained vehicle that acts as a confining bund for the material components and contains the following equipment:
- A pumping unit capable of injecting geopolymer material is used to pump material through a drilled hole, beneath the slab. The pump can control the flow rate of the material to provide the required amount of geopolymer to stabilise and lift the slab
- Control devices are used to maintain the temperature and proportionate mixing of the geopolymer material according to Geobear’s patented specification
- Electric drills are used to cut 16mm diameter holes through the concrete pavement
- Laser levelling devices detect compaction and slab movement of 0.5mm increments with an instantaneous readout system
Injection method
A series of 16mm diameter holes were drilled at predetermined intervals throughout the concrete slab. They were drilled to sufficient depth to penetrate the concrete pavement and the geotechnical layers. Drill holes were kept as perpendicular as possible to the pavement surface.
Geobear’s geopolymers are formed when a number of chemical components are combined during the injection process. A small injection pressure causes a controlled chemical reaction known as polymerisation. During the polymerisation, the newly formed geopolymer changes from a liquid into a solid and undergoes expansion of up to 10 times its volume depending on the specific geopolymer properties and the level of confinement provided by the surrounding ground.
The geopolymer material was injected under the slab to fill any air voids. The amount of slab movement was controlled by the pumping unit and injection gun, by measuring the rate of injection. The actual slab lifting process was measured in 0.5mm increments until the proposed profile elevations were reached.
The geopolymer was also injected into the sub-base layer to stabilise and enhance the load-carrying characteristics of the granular material.
Once completed, the injection tubes were removed and the holes filled with a non-shrinking sand cement grout. The drill arisings and any excess geopolymer material were cleaned up at the end of each shift before the motorway was opened to traffic.
Test methods and procedures
Onsite testing was subcontracted to materials testing specialist Nicholls Colton Group, which is UKAS-accredited for onsite testing activities including sampling through coring and dynamic cone penetrometer (DCP) testing.
Sampling and coring
In total, 22 cores – 12 of nominal 150mm diameter, C1 to C12, and 10 of nominal 100mm diameter, C13 to C22 – were taken through the road pavement using a trailer-mounted coring rig at locations identified in the scope of works. Each core was examined and logged onsite prior to wrapping and was transferred to the laboratory for storage.
DCP testing
The dynamic cone penetrometer is an instrument designed for the rapid in-situ measurement of the structural properties of existing road pavements constructed of unbound materials.
The robust, simple and portable design means the DCP is quick and easy to use. A typical test takes only a few minutes so the DCP provided an efficient method of obtaining pavement information.
An office-based study had already identified that there were no areas of potential risk of buried services in the vicinity of the treatment areas. However, the test locations were also checked for these using a cable avoidance tool (CAT) scanner so that a permit-to-work certificate could be issued and any areas of potential uncharted cable strikes could be avoided.
The DCP test was carried out through a 150/100mm diameter core hole extracted from the 280mm thick concrete pavement at the base of the core hole. This approach removed the requirement for digging test pits and consequently DCP was judged suitable for use.
The core holes were located in the centre of each lane. This approach prevented core hole reinstatements being carried out in the wheel path of traffic, reducing the risk of future potholing. The overall geopolymer injection process was carried out under a full weekend-closure traffic management system.
The frequency of the test positions was predetermined so that their number and location could be easily managed.
To measure the improvement in load bearing capacity, it was decided to carry out the DCP testing before and after the geopolymer injection process. The testing frequency was divided into two types:
- A treatment section consisting of continuous sections of multiple 6m-long bays, with each bay tested in sequence – bay 1 pre-treatment, bay 2 post-treatment, bay 3 pre-treatment – with ongoing alternation to the end of the section. This meant that the pre- and post-treatment tests were carried out in adjacent bays at approximately 6m intervals.
- An isolated treatment section consisting of a single, 6m-long bay
The penetrometer, an 8kg free-fall hammer, was lifted and dropped through a height of 575mm in accordance with the manufacturer’s requirements. The distance of penetration of the 60o cone tip was recorded and the cycle repeated. The standard DCP equipment allows continuous measurements to be made to a depth of about 850mm. This meant that a CBR (California Bearing Ratio) strength profile could be established for each of the sub-base and lower foundation layers. A typical CBR profile with depth is shown in the diagrams below.
The foundation surface modulus position within the overall carriageway construction is described in the following diagram, extracted from the current version of CD 225 design for new pavement foundations.
Interpreting the results
Correlations have been established between measurements with the DCP and conventional in-situ CBR. This means that the site-measured test results can be interpreted and compared with CBR specifications used for pavement design.
In the case of the M54 project, DCP measurements were carried out before and after the geopolymer treatment process. This approach provided test data that could demonstrate the improvement in the bearing capacity of the unbound granular sub-base and foundation layers.
TRL Road Note 8 was used to convert DCP measurements to in-situ CBR percentages.
Equation 1: Log10CBR = 2.48 – 1.057 Log10 [mm/blow]
A summary of the CBR calculations are shown in Table 1 below:
The pre-treatment mean test result of 40% for the in-situ CBR measurement was typical for a well-compacted granular type 1 sub-base material.
The post-treatment mean result of 85% for the in-situ CBR measurement for granular sub-base had been increased by a factor of 2 when treated with Geobear geopolymer.
The improved CBR results can be further interpreted through conversion into standard axles, using the TRL 1132 and TRL RR87 relationship shown in equation 2 below. This relationship was used at the time of the original pavement design process, when unreinforced jointed concrete pavements were constructed on the M54 in the 1970s.
Equation 2: Ln(H) = {Ln(T) − 3.466Ln(Rc) − 0.484Ln(E) + 40.483 } / 5.094
which can be rearranged to give the design traffic directly.
Equation 3: Ln(T) = {5.094Ln(H) + 3.466Ln(Rc) + 0.484Ln(E) – 40.483 }
where:
H is the thickness (mm) of the concrete slab without a tied lane or 1m edge strip
Ln is the natural logarithm
T is the design traffic (msa)
Rc is the mean compressive cube strength (N/mm2 or MPa) at 28 days
E is the confined foundation stiffness (MPa) and relates to foundation classes shown in Table 2 below:
Note that only Classes 3 and 4 hydraulically bound materials would be compliant for use in current designs for rigid pavements.
To show the effect that the sub-base improvement had on the design traffic loading and lifecycle enhancement of the M54 concrete pavement, the following assumptions were made:
Rc = 40N/mm2
H = 280mm
E = 400MPa before and 840MPa after treatment
Using equation 3, the design traffic could be calculated as 49.4msa pre-treatment compared with 69.9msa post-treatment.
This demonstrated that the geopolymer treatment had enhanced the design life traffic loading of the M54 eastbound pavement by an additional 20.5msa.
Using traffic flow counters at Junction 4, the average annual daily flow of commercial HGV vehicles travelling in lane 1 is approximately 2,100 (2017 data). This flow of traffic equates to 2.3msa per year travelling on the eastbound carriageway.
The theoretical lifecycle of the geopolymer-treated sections of the M54 was therefore extended by nine years.
