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Injection Solutions for Underground Construction

Find out the differences between pre-injection and post injection system when it comes to underground construction. It helps to make the right choice of materials for the right situation at the design stage.

September 18, 2020. 5:19 AM

Injection can be defined as the introduction of a material with pressure into the ground or a structure for waterproofing and for consolidating voids, cracks and porosity. Generally, the injection can be categorised as Pre Injection and Post Injection.

Pre Injection is cost effective in the following:

Water Ingress reduction
Ground zones with fractured rocks or weathered rocks
Unstable ground

Post Injection is carried out in the following situations:

Rehabilitations
Structural cracks in concrete
Unexpected water ingress
Rehabilitation in concrete structures

Pre-Injection depends on Ground Permeability and Water ingress. The objective of carrying out Pre-Injection is to decrease the cost impact of post grouting and decrease the water in tunnel so as to have a smooth excavation. For Pre injections, the packers are placed in sound solid rock to ensure that they are fixed well, and to avoid grout to leak back around the packer. To ensure a successful injection, the packers should be fixed in already injected rock. If not, you will easily get out-leaks through cracks and fissures. Normally, the packers are set 1.5 – 2 meters deep. If packers with non-return valves are used, they should not be mounted before they are ready to be injected. This makes it possible to see if there is contact between the holes.

The packer is attached to the injection pipes and pushed into the drilled hole. The innermost of the two injection pipes has a tightening lever for fixing the packer. The packer is expanded in the drilled hole using the tightening lever. Cement mixture or chemical substance is injected into the rock’s crack system through the rubber valve in the packer.

Mapei provides different products for injections for pre-grouting

Microfine Cement MICROCEM 8000
Ultrafine Cement MICROCEM 12000
Injection Resins RESFOAM 1KM /FOAMJET / MAPEJET SYSTEM

Microfine cement and Ultrafine cement blaine value varies from 6000 to 12000 compared to cement which is around 3500 cm2/g. Wherever the finer penetration is required, the blaine value plays an important role. For pre-grouting, various important features have to be considered. These include:

Bleeding
Desired fluidity
Setting time / gelling time
Strength required

Bleeding has to be less than 2% and fluidity when tested in Marsh cone should be 35 second. Fluidity plays an important role in injections. If fluidity has to be higher, then a grout has to balance between fluidity and bleeding. Lower the water cement ratio fluidity decreases, to balance it we can add viscosity fluidifier with a dosage that is is 2-6% by weight of cement. To accelerate the setting time, an accelerator can be added. The mechanical strength in 24 hours vary from 2 Mpa to 20 Mpa depending on the water cement ratio varying from 0.7 to 1.00

Filtration Ability Against Pressure
The coefficient of filtration ability against pressure shows the ability of the fluid to keep the initial characteristics even in the presence of pressure.

In case of heavy ingress of water, it’s better to go for a fast setting injection of resins. Generally, polyurethanes like RESFOAM 1KM or FOAMJET are better. Double component are faster in setting whereas the time of setting of Single component can be controlled if it has to be injected for a longer distance with an accelerator. Also, Nano Silica based injection material is used; it generally has particle sizes of less than 0.1 micron like the Mapejet System NS15.

Colloidal silica gel is a “mineral grout” and not a chemical grout It is a stable liquid containing single, sub-microscopic particles of silica oxide. To make it gel, a weak solution of salt water is mixed prior to pumping. The more salt water added, the faster the gel time. It is environmentally friendly and durable, as it is simply composed of quartz sand, water and salt. Having the consistency of water, it penetrates sands and fine fissures very easily. It is extremely user-friendly as a standard cement grout equipment can be used and cleaned with water.

In Grouting the most important is the GIN (Grout Intensity Number). GIN grouting intensity values were defined as the product of the final grouting pressure and absorption per meter for a given single borehole, within a certain period at the end of grouting the result is a constant value.

GIN = PV

where P is the final grouting pressure and V is the ultimate grout take which means the energy consumption per unit grouting period

Alluvial Soil Grout pressure: low, average 2-4 bar, maximum 7-10 bar
Cracks in Rock Grout pressure: High, average 30-40 bar, maximum 70-80 bar

GIN value varies with different geological conditions, but GIN is constant under similar conditions. It needs to be noted that the GIN in each envelope curve includes a maximum pressure and maximum absorption slurry in addition to a GIN value. When the injection rate reaches a pre-determined limit, or pressure reaches the limit, or the product of the two factors achieve the selected GIN, the grouting can be ended.

Post Injection: Basic features
Reactive mindset: post-injection is conducted when collapses have occured already or uncontrolled water inflow is occuring; or where rehabilitation of leaking tunnels is required; or drilling holes to intersect joints with high water inflow, or pushing slotted pipes into unstable ground masses. These may need specialised drilling equipment such as Odex, Self drilling anchors and packer injections, utilization of expensive, highly reactive chemicals - usually PUs in geotechnical problems and acrylates in rehabilitation problems.

post injection

Single Component Polyurethane or Double Component Polyurethanes can be used, or Acrylates. To stop water ingress immediately the Polyurethane can be injected and then acrylates can be injected into joints or cracks. Various Tunnel linings have used the leakage control method with Polyurethanes like RESFOAM / FOAMJET and Mapegel UTT.

Injection in fissures:

Extremely low viscosity
Resin should also be injectable when gelling has commenced
Gelling properties, accurately adjustable

Technical solution for rehabilitation of old tunnels

Acrylates for the injections in and immediately behind deteriorated concrete lining
Polyurethane (Resfoam 1km) for the improvement of ground, void filling and water seepage reduction
Taylored cement-based grout for improvement of ground behind concrete and masonry lining
Sprayable waterproofing membrane on old tunnel lining
Eventual sprayed concrete

Grouting is a design choice: Pre-injection vs Post Injection

Pre-injection – proactive, relies on effective probing and decision making at the face, cheap materials, 10 to 50 times then post, pre-injection often difficult in soft ground tunneling.

Post-injection – reactive, the last resort, expensive chemicals, unstable conditions and uncontrollable water inflows.

Make the right choice of materials for the right situation at the design stage. Mapei provides a complete system of Pre Injection and Post Injection for Civil Engineering Projects. Find out more information from UTT Mapei.