Find out the difference between pre-injection and post-injection systems in underground construction. It helps to choose the right material for the right situation in the design phase.
Injection can be defined as the introduction of pressured materials into the ground or the structure for waterproofing and reinforcing voids, cracks and pores. Generally, injection can be divided into pre-injection and post-injection.
Pre-injection is cost-effective in the following aspects:
Reduce water ingress
Cracked rock or weathered rock on the ground area
Inject after execution in the following cases:
Concrete structure cracks
Accidental water ingress
Concrete structure repair
The pre-injection depends on the ground permeability and water inflow. The purpose of pre-injection is to reduce the cost impact after grouting and reduce the accumulation of water in the tunnel, so that the excavation can proceed smoothly. For pre-injection, the packers are placed in solid rock to ensure they are firmly fixed and to prevent grout from leaking back around the packer. To ensure successful injection, the packer should be fixed in the injected rock. If not, you will easily leak through cracks and cracks. Usually, the depth of the packer is 1.5 – 2 meters. If packers with check valves are used, they should not be installed before they are ready to be injected. This makes it possible to see if there is contact between the holes.
The injection packer is installed on the injection pipe and pushed into the borehole. The innermost part of the two injection pipes has a tightening rod for fixing the packer. Use the tightening rod to extend the packer in the borehole. The cement mixture or chemical substance is injected into the fracture system of the rock through the rubber valve in the packer.
Whenever finer penetration is needed, the value of simplicity plays an important role. For pre-grouting, various important characteristics must be considered. These include:
Setting time/gelling time
When tested in a cone, the bleeding must be less than 2% and the fluidity should be 35 seconds. Fluidity plays an important role in injection. If mobility must be improved, a balance must be struck between mobility and bleeding. Reduce the water-cement ratio and fluidity. In order to achieve a balance, we can add a viscosity fluidizer, the amount of which is 2% to 6% of the cement weight. To speed up the setup time, an accelerator can be added. The 24-hour mechanical strength varies from 2 Mpa to 20 Mpa, depending on the water-cement ratio, varies from 0.7 to 1.00
Pressure filtration capacity
The coefficient of filtration capacity relative to pressure shows that the fluid can maintain its original characteristics even under pressure. Filter capacity
If a large amount of water enters, it is best to inject the resin quickly. The two-component setting speed is faster, and the single-component setting time can be controlled, if you must use an accelerator to inject it into a longer distance. In addition, injection materials based on nano-silica are used.
Colloidal silica is a “mineral slurry”, not a chemical slurry. It is a stable liquid containing single submicroscopic silica particles. To make it gel, the dilute solutions are mixed together before pumping. The more saline added, the faster the gel time. It is environmentally friendly and durable because it only consists of quartz sand, water and salt. It has the consistency of water and easily penetrates sand and fine cracks. It is very user-friendly because standard cement grouting equipment can be used and cleaned with water.
In grouting, the most important thing is GIN (grouting intensity number). The GIN grouting strength value is defined as the product of the final grouting pressure and the absorption per meter of a given single borehole, and the result is a constant value within a certain period after the grouting ends.
Where P is the final grouting pressure and V is the final grouting volume, which means energy consumption per grouting cycle
Alluvial mud pressure: low, average 2-4 bar, maximum 7-10 bar
Rock grouting pressure cracks: high, average 30-40 bar, maximum 70-80 bar
The GIN value varies with different geological conditions, but GIN is constant under similar conditions. It should be noted that the GIN in each envelope includes the maximum pressure and the maximum absorption slurry in addition to the GIN value. When the injection rate reaches the predetermined limit, or the pressure reaches the limit, or the product of the two factors reaches the selected GIN, the grouting can be ended.
After injection: basic functions
Reactive thinking: post-injection treatment when collapse has occurred or uncontrollable water influx occurs; or where the leaky tunnel needs to be repaired; or drill holes to intersect the joints with high water inflows, or push slotted pipes in Unstable ground. These may require specialized drilling equipment. One-component polyurethane or two-component polyurethane or acrylate can be used. In order to immediately prevent water from entering, you can inject polyurethane first, and then inject acrylate into the crevices or cracks.
Very low viscosity
Resin should also be injected after gelation starts
Gelling performance, precise and adjustable
Technical solutions for repairing old tunnels
Used for aging concrete lining and the injected acrylate immediately after it
Polyurethane (Resfoam 1km), used to improve the ground, fill gaps and reduce water seepage
Taylor cement-based grouting, used to improve the ground after concrete and masonry lining
Sprayable waterproof membrane on old tunnel lining
Final spray concrete
Grouting is a design choice: pre-injection and post-injection
Pre-injection-active, relying on effective detection and decision-making to determine, cheap materials need 10 to 50 times, and then proceed, pre-injection is usually difficult in the construction of soft soil foundation tunnels.
After injection-the final reaction method, expensive chemicals, unstable conditions and uncontrollable water influx.
Post time: Apr-20-2021