Grout Line - Paolo Gazzarrini

An Innovative Application of Epoxy Resin Injection
- Peter Town

As an injection material, epoxy resin is traditionally used for filling cracks in concrete structures.

The function of the injected material is mainly twofold - fill the crack to prevent water ingress and act as a structural filler to transfer loads across the crack. The perceived benefit of “gluing” a crack together is largely a misconception as tensile properties of cured injected epoxy usually surpass that of concrete. Without understanding why a crack initially develops, and then implementing appropriate remedial procedures to prevent a recurrence, simply using epoxy as a crack bonding agent frequently results in crack propagation at another location.

The functions of epoxy injection to act as both crack filler and structural component were exploited for innovative repairs during the construction of a new research facility at a high security government establishment.

Measurement accuracy and working tolerances in the new facility were expected to far exceed normal engineering requirements and to isolate the work area from external influences, the facility was being constructed as a building within a building - the internal structure being disconnected from the external shell to cut off any interference from wind loading, vibration and other main site activities.

The main foundations featured multiple small diameter piles (300mmØ) that were integrity tested after installation.  Integrity testing attaches a transducer to a pile head which is then struck with a hammer and the reflected waves recorded; wave patterns are analysed and graphically displayed on a computer. The test signals revealed anomalies in several piles indicating cracks at depths varying between 10 and 15 metres below ground level. As test signals were detected passing beyond each anomaly, it was deduced the cracks may only extend partially across the pile or if all the way through, were hairline in nature.

The cracked piles did not meet the design specification but a congested site and tight programme precluded the piles from being replaced and an in-situ repair was sought.

In discussions with the piling contractor and employer, use of epoxy injection was proposed. The principle was verbally accepted and a formal proposal and offer requested. Subsequently, official instructions to proceed were issued.

Gaining access to the fault zones involved drilling a hole down through the pile centre; drilling then continued for approximately 1.00m past the fault – extending the drilling served two purposes 1) ensure the fault zone was fully exposed and 2) form a pocket to contain any falling detritus from the drilling. Drilling was undertaken using a diamond core rig and 65mm diameter core bit.  Inspection of recovered cores from the fault zones did not reveal any meaningful information as all samples were damaged during recovery.

It was also essential to physically confirm the fault location in each pile. An inflatable packer (see Figure #1) was lowered into each core hole in turn. The attached grout line was marked at 1.00m intervals and then sub-divided into 0.25m divisions so provided easy monitoring of packer depth.

Having lowered the packer to full depth of the cored holes, the packer was inflated to seal against the pile and an attempt made to pump water through the grout line and packer. When test water filled the void below the packer,
continued pumping induced a pressure increase until the pump stalled. The lack of further water take indicated the pile at this test level was not compromised. The packer was then deflated, lifted 0.25m reinflated, water again pumped and pressurised. This process was repeated until a level was reached where continual water take occurred indicating the packer was sited over the fracture level.

A typical section when water take occurred is shown in Figure #2.

Depths where water takes occurred were recorded. Water injection at the fault zones then continued for several minutes to flush diamond drilling detritus and/or other residue from the crack.

Ground water levels in the pile core holes had been previously taken with an electronic dip meter and a plot was now drawn to obtain the correlation between fractures and ground water level. This information was used to compute

the probable minimum quantity of resin to be injected at each location.

The injection product selected for the project was a low viscosity, two component pure epoxy. The material had a known capability to be successfully injected under water and properties of the cured resin were:

Tensile Strength 59.1 N/mm2

Compressive Strength 102.1 N/mm2

Bending Strength 82.3 N/mm2

E-modulus 2900 – 3100 N/ mm2

The client approved the injection product as being appropriate to meet their design requirements.

The mixed epoxy is formulated with a pot life of 20 minutes at 20°C but as the project was undertaken during a December cold spell it was necessary to store resin components in a heated cabin near the work area. Resin and hardener were only removed from storage as and when required for injection. Monitoring and recording air temperature was deemed unnecessary as epoxy exposure to ambient conditions was limited to mixing and injection operations. Prior to establishing likely injection quantities on site, it was proposed the epoxy be injected through a specialised two component proportioning pump but on setting up, it was apparent the output would be insufficient to achieve adequate flow rates for the anticipated injection quantities.

Injection then reverted to using an air powered single component piston pump typically as used for injecting polyurethane water-stop resins; it was therefore necessary to pre-mix the epoxy and then pour into the pump hopper. In practice this did not hinder the injection operation.

The packer was again inserted into the first pile and the injection ports located at the fracture zone, after inflating the packer the epoxy injection commenced. Specific gravity of the mixed resin is 1.1 – 1.15 kg/l and the injected resin initially sank to the bottom of the drilled core hole; with continued injection the core filled displacing water in the process and subsequently introducing resin into the pile fracture. Resin volumes appropriate to achieve crack penetration were already calculated from the previous measurements of core depths and fracture locations.

Once the calculated volume had been injected, a further assessed quantity was injected to provide optimum resin saturation of the fracture under the existing site conditions. The packer was then withdrawn a further distance and an additional quantity of resin injected into the core. This secondary injection provided a positive head of liquid resin to counter hydrostatic pressure from the ground water, again injection volumes were assessed from the relationship between crack and water table level.

The packer could then be fully withdrawn and the procedure repeated at the next pile. Throughout the injection process, samples of mixed resin were taken and set aside to ensure each batch of epoxy fully cured.

Whilst injection continued to other piles, the level of epoxy in previously injected core holes was monitored. No increases in epoxy levels were observed and this was taken to indicate that ground water was effectively excluded from re-entering the injected cracks. Further monitoring confirmed when the resin in the core holes had cured.

On completion of injection to all piles and verification of resin cure, the remaining sections of open, partially water filled core holes were groutedwith a high strength cementitious grout formulated to withstand washout. The final grouting was undertaken through a small tremmie set-up.

All injected piles were subsequently re-tested and passed the integrity proving assessment.

This project was spread over 5 consecutive shifts, the last 2 shifts being part days only. Site personnel were 3 trained operatives and an Injection Engineer.

Peter Town, FRSA, MASCE, A. Inst

SSE, email: peter.town@h2ox.net,