Hot bitumen grouting rediscovered15 October 2001
Hot bitumen grouting technology has continually evolved since its early applications almost a century ago. Alex Naudts explains why it is effective at stopping major water inflows and stabilising soils in an efficient, economic and environmentally sound way
One of the oldest references related to bitumen grouting was for the construction of the Tower of Babel recounted in The Antiquities of the Jews (IV:3), by Flavius Josephus in the first century AD. By the end of the 19th century, hot bitumen grouting was utilised for remedial repair work on dams and rock tunnels. However, deplorable selection of inappropriate grades of bitumen rendered hot bitumen grouting into extinction.
The use of hot bitumen was ‘rediscovered’ during the early eighties with the successes of the Lower Baker dam and the Stewartville dam grouting projects in Canada, and the technique enjoyed a remarkable comeback during the late 1990s.
Bitumen is a natural hydrocarbon-based product. There are numerous types of bitumen available with a wide range of characteristics. Hard, oxidised and environmentally friendly bitumen with a high solidification point is most desirable for use in grouting.
When hot bitumen is injected into a medium saturated with water, it cools quickly at the interface with water. Steam is created at that point, decreasing the viscosity of the bitumen.
The steam acts as an air lift drawing the bitumen into its pathway through small and large fissures or pore channels. The centre of the bitumen mass remains hot, and continuously breaks through the skin formed at the interface of the bitumen and water. The skin prevents the wash-out, while the sheltered hot bitumen behind the skin behaves as a Newtonian fluid, penetrating in a similar fashion as solution grouts. The behaviour of bitumen during grouting has been compared to the flow of lava.
Hot bitumen is often injected in conjunction with cement-based suspension grout for any of the following reasons:
• Reduce grout spread.
• Make bitumen less susceptible to creep.
• Cool off the bitumen faster and compensate for thermal shrinkage of the bitumen during cooling.
• Increase the mechanical strength of the end product.
• Eliminate water flows and facilitate the grouting of regular cement-based grout which otherwise could not be injected.
Hot bitumen will penetrate fractures as small as 0.1mm, and adhesion between the bitumen, cement and rock is excellent. In larger flow channels or fractures the cement-based suspension grout and hot bitumen form a swirl.
The viscosity, rheology and set characteristics of bitumen must be selected for each specific application: large fractures under conditions of high pressure flow may require the injection of a more viscous bitumen with the addition of polymers; or it must be injected in conjunction with large volumes of regular cement-based grout, or low mobility compaction grout, to fill shrinkage gaps and reduce creep.
The advantage of bitumen over other grouting systems to stop or control water flow, especially under high pressure and at high flow rates, is that blown bitumen will never wash out. In order for the grout not to wash out, it needs to possess adequate cohesion to form a conglomerate larger than the particle size that corresponds with the critical particle size under the given flow conditions. The critical particle size refers to the size of the particle that will just be moved by a flow of a given velocity. As the velocity increases, the critical particle size increases. A high cohesion of the grout – in contact with the flow – is therefore required. If a high cohesion is required to prevent wash-out, it will limit grout spread and penetrability. Bitumen combines the best of both worlds: a skin prevents wash-out, while the low viscosity bitumen penetrates fine pathways.
The overall supply and installation cost of hot bitumen is relatively low. Installed costs for hot bitumen range from US$300 to US$1000 per m3 in North America, compared to US$9000-US$20,000 for polyurethane solution grout, or even US$4000-US$7500 for the hydroblock system. Considering that using hot bitumen in conjunction with regular cement-based grout requires considerably fewer boreholes, and consumes only a fraction of the volume of material of more conventional grouting systems, the potential cost savings are substantial.
Based on the analysis of the various bitumen projects conducted during the last 20 years, it is considered safe to conclude that hot bitumen always meets objectives in the short term. To make it work in the long run requires experience, knowledge and a sound engineering design.
The equipment set-up and safety protocol are more complex for bitumen grouting than for the application of regular cement-based grouts or solution grouts. It is necessary to continuously monitor the subsurface conditions for signs of cement grout wash-out by measuring the pH of the water, evaluating temperature changes as recorded via downhole sensors to assess the spread of the bitumen, and interpreting changes in apparent Lugeon value (ie changes in flow rates and injection pressures). The latter is ideally performed using custom developed software monitoring and assessing all parameters in real-time. CAGESTM is suited to gauge the response of the formation to the grout. The apparent Lugeon value is the permeability coefficient of the formation using grout as a test fluid (see Landry et al, 2000).
The injection of hydrocarbons into soil, rock or structures immediately raises environmental concerns. However, oxidised blown bitumen has, for over 40 years, been used successfully as a lining for (potable) water reservoirs in California. In 1987 Washington and Oregon state wildlife authorities used it for lining fish hatcheries ponds.
Oxidised bitumen has also proven to be in compliance with American Water Works Association standards for leachate resistance of materials for use in potable water applications.
Lower Baker dam, Washington, US
Lower Baker dam, operated by Puget Sound Power and Light, was constructed shortly after the turn of the century. One of the oldest documented bitumen grouting operations took place more than 75 years ago to stop major leaks through the limestone foundation below the abutments. The operation was reported to be very successful in the short term but creep of the bitumen, opening up secondary flow channels, led to further erosion of clay filled seams in the limestone foundation. The procedure was repeated in the 1950s and 1960s with similar results.
In 1982, under the direction of ECO Grouting Specialists of Canada, an attempt was made to grout with polyurethane grouts. It did not work. Hot bitumen grouting was again successfully used to curtail the 2200 litres/sec inflow under a reservoir pressure of 80m water column.
At the end of two weeks of grouting, the leak was reduced to 2% of its original flow. However, the owner could not be convinced to use bitumen in conjunction with cement-based grout. The leak gradually again increased over time and has stabilised at a rate of approximately 1000 litres/sec.
Stewartville dam, Ontario, Canada
The Stewartville dam, located on the Madawaska river, is 63m high, 248m long and was constructed in 1948. The foundation is composed of predominantly massive competent limestone. Zones of weathered micaceous limestone occur on some bedding planes and joints. In these zones water eventually began to enter the foundation drainage gallery. Hot bitumen in conjunction with cement-based grout was successfully used to seal a 370 litres/sec inflow under a dam foundation under full reservoir head of 46m. This application illustrates a high degree of control to perform a surgical strike using hot bitumen, in conjunction with cement-based suspension grout, since the leaks had to be stopped while not plugging the adjacent foundation drains.
It was decided in 1983 to proceed with the injection of hot bitumen in conjunction with regular cement-based grout. ECO produced the design for the grouting operation and directed the fieldwork.
The main challenge in the grouting operation was to grout under conditions of high flow, in fractures up to 0.2m wide, and within 7.5m of the foundation drain, which had to be kept open. A durable product was also required.
After six hours of hot bitumen grouting in conjunction with cement-based grout, the inflow ceased without hampering the foundation drains.
Thermocouples installed in a number of observation holes made it possible to trace the travel of the hot bitumen with time. A mathematical model of the travel pattern based on the data obtained is described by Lukajic et al (1985). Bitumen travelled against the flow and in the direction of the flow.
A similar grouting programme conducted in 1984 beneath the northern portion of the dam reduced seepage to almost nothing from over 9000 litres/min.
Kraghammer Sattel, Germany
The Bigge reservoir in Germany, located in the Kraghammer Saddle, is situated on highly fractured permeable alternating strata of greywacke slate and sandy, partly calcareous clay slate. Bitumen grouting was used to reduce the permeability of the formation beneath the reservoir dam.
The unique feature of this project was that the success of the bitumen grouting was checked by the excavation of two inspection tunnels through the grouted formation.
Of significance is that a portion of the formation was grouted with regular cement-based grout, and the volumes injected and final permeabilities were compared to the areas of the formation grouted with hot bitumen. The results of the testing yielded three interesting findings:
• Despite lower grout takes per metre of borehole injected with hot bitumen, relative to cement-based grout, the residual permeability after hot bitumen grouting was lower than with cement grout.
• Bitumen was found to have filled seams in the rock up to 3.5m from the boreholes and penetrated seams as narrow as 0.1mm.
• Zones with initial permeability in the 1-10 Lugeon range were successfully grouted with hot bitumen proving that hot bitumen is very suitable to treat formations with low initial permeability values.
Drainage plug in abandoned open pit mine tunnel
In the mid 1990s, the plug inside an old access tunnel, connecting a very large open pit mine with a river, failed. The reservoir was filled with millions of cubic meters of liquid waste and tailings. The slurry flowed out of the impoundment reservoir into a river causing major environmental problems. The flow reached a peak of 7m3/sec. The hydrostatic pressure in the tunnel (2.5m wide, 2.5m high and 3km long), was in excess of 1MPa.
A sophisticated grouting operation began with the injection of blown bitumen and cement-based suspension grout. Within the hour, the flow of water and slurry through the tunnel was stopped. Bitumen was originally travelling upstream and downstream of the injection point. Once the flow stopped, bitumen was travelling as far as 100m against the flow, drawn into that direction by steam.
Potash mine, Canada
In 1997 inflow of fresh water into a potash mine located in Canada had gradually dissolved the salt layer between overlying shale formation and lower basalt rock, forming a large cavern. Eventually the overhanging mudstone and limestone collapsed. Inflows of fresh water in the mine ranged from 10–15,000m3/day.
The proposed method was the injection of hot bitumen, in conjunction with regular cement-based suspension grout, to fill the 19,000m3 large cavern located 700m below a large brine pond and to stop the inflow. Two 1600m long drill holes, one for injection of hot bitumen and one for injection of cement-based suspension grout, were installed from the surface using directional drilling.
ECO designed the grouting programme and directed the field operation. The client was informed before the start of the grouting programme that when the leak was stopped there was the possibility that as soon as hydrostatic pressures began to rise in the cavern, the deteriorated formation could collapse sending a tidal wave of water through the mine.
Once the bitumen grouting operation was successfully launched, the cement grouting kicked in. The real-time monitoring and assessment of the grouting programme was instrumental.
Bitumen was injected at an average flow rate of approximately 25m3/hr for more than two weeks of continuous operation. Cement-based suspension grout formulations with varying rheological properties were injected at a rate of approximately 45m3/hr.
After three days of grouting, the inflow ceased and formation pressures started to rise. After five days a major collapse of the cavern floor occurred. Immediately, the hydrostatic pressure in the formation dropped and millions of litres of water rushed into the mine over a few hours. The grouting operation continued without interruption.
The inflow was stopped again completely. On the thirteenth day of grouting the formation collapsed again. More than 23M litres of grout had been installed in what might have been one of the largest production grouting operations in the world.
It was concluded that the salt horizon had been too severely undermined and further efforts would be futile.
The fact that injection of hot bitumen and cement-based grout completely sealed an inflow of such magnitude at such a great depth – be it only temporary – is a testament to the robust nature of the technique. The failure of the plug was not a failure of the bitumen grouting technique but was a consequence of the undermining of the salt horizon surrounding the newly formed plug during the two months prior to grouting.
Jaburu dam, Brazil
Bitumen grouting applications are not only limited to large scale crisis projects. In many cases only a small volume of bitumen is required to solve a problem.
The Jarabu dam, located in Brazil’s Serra Grande region, is 47m high and 770m long. It was built in the early 1980s on jointed siltstone and shale.
In 1988 seepage of 20 litres/sec was noted under one of the dam abutments. The seepage soon increased to 47 litres/sec. A bitumen grouting programme in conjunction with cement-based suspension grout was conducted and the seepage was reduced to a steady flow of 3 litres/sec. In this instance, the most suitable bitumen was not readily available and a much less viscous, non-oxidised bitumen with lower solidification point was used without the benefit of pre-heated bitumen lines. A total of 60m3 of bitumen was used.
Sealing major inflows
Hot bitumen grouting in conjunction with cement-based grout, is the most effective and sometimes only method to stop and successfully seal major inflows under very adverse flow conditions.
Hot bitumen grouting is the solution for inflows of biblical proportions but should not be limited to just catastrophic areas, as it is capable of penetrating very fine fissures and pore channels. This makes it a suitable grout for rock grouting, coarse soil grouting, as well as sealing of water retaining structures.
When properly applied and well designed, the grouting of hot bitumen in conjunction with cement-based grouts, is a fast, safe, cost-effective, environmentally sound, predictable and durable solution.
Special thanks to Stephen Hooey, P.Eng, for his assistance in writing this paper. The author acknowledges the co-operation of numerous individuals who played a role in some of the bitumen projects described. They are: C Brawner, D Bruce, J Bruce, M Fallat, A Graetz, G Grimmig, D Haas, L Jaillard, Dr. K Kading, D Krizek, E Landry, B Lukajic, T Moody, G Moore, J Pordon, G Rorison, E Schönian, W Thrytall, M Yates.
Many thanks also to Whitman Benn, Jay Dee Contractors, and USL.