Pressure Grouting Applications – Pros and Cons

Pressure grouting, also known as jet grouting or compaction grouting is a technique used to improve the strength and stability of soil or rock formations.

It  involves injecting a high-pressure jet of grout into the ground to create columns or bulbs of grouted soil.

The grout may be cementitious, resinous, or chemical, depending on the purpose and soil conditions. It is used to improve the strength, stiffness, and permeability of the soil, as well as to form water cut-offs and stabilize foundations.

What is Pressure Grouting?

The grout fills the spaces and forms a solid mass that bonds with the surrounding material. Pressure grouting can be used for various purposes, such as repairing foundations, sealing leaks, stabilising slopes, increasing bearing capacity, reducing settlement and controlling groundwater flow.

The grout is injected into the targeted areas through pre-drilled holes or cracks, filling any voids or gaps and creating a solid and stable mass.

The injection pressure used in pressure grouting is determined by the depth, soil or rock conditions, and the required grout penetration. A pressure gauge is used to monitor the injection pressure and ensure that it is within the range required for the specific application.

Pressure grouting is an important technique in civil engineering used to strengthen and waterproof concrete. It involves injecting a fluid grout mixture under pressure into cracks, voids, and honeycombs within concrete members.

The pressure forces the grout to penetrate and fill these defects, consolidating the concrete. There are several pressure grouting methods including gravity feed, single and double packers, and sleeve port grouting. The grout used is often a cementitious or chemical mixture tailored to the application.

Chemical grouts like sodium silicate and acrylamide are commonly used for sealing fine cracks and voids.

Pressure Grouting Specifications

When grouting concrete, the pressure must be carefully controlled. Excessive pressure can damage the concrete or push the grout where it is not wanted. Typical grouting pressure ranges from 5-500 psi depending on the application.

Low pressure gravity feed grouting involves simply pouring grout into existing vertical holes or openings and letting gravity move the grout. Higher pressure injection grouting is needed for horizontal and overhead applications.

This requires drill access holes and installable injection ports called packers. Double packer sleeves straddle the crack or void to target injection in just the defect zone. Port grouting uses fixed sleeve ports installed in new concrete placements so defects can be accessed later.

Pressure Grouting in Concreting

Concrete pressure grouting has many applications. It is commonly used to repair honeycombing defects in concrete structures. The network of voids in honeycombed concrete allows water intrusion and compromise strength.

Pressure grouting fills the voids, waterproofs and strengthens the concrete. Grouting can also repair cracks and defects around concrete infrastructure like tunnels, dams, and parking garages.

Chemical grouting is ideal to seal dynamic cracks subject to movement. The acrylamide gel can flex and maintain the seal. Epoxy injections permanently bond structural cracks. Mastering various pressure grouting techniques allows civil engineers to efficiently rehabilitate and strengthen aging concrete infrastructure.

Grouting Pressure Formulas

1. Cubic Law Formula – Used to calculate injection flow rate based on crack width and desired injection pressure:

Q = WL(P/12ηL)

Where Q is flow rate (cm3/min), W is crack width (cm), L is crack length (cm), P is pressure (bars), and η is grout viscosity (Poise).

2. Equivalent Pipe Flow Formula – Used to relate grout flow in a crack to flow in a pipe by equating crack width to pipe diameter:

Q = (π/128η)(PD4)

Where D is the equivalent pipe diameter (cm).

3. Mayer Equation – Used to relate injection pressure to crack width and depth for a given flow rate:

P = 12ηLQ/WD3

Where P is pressure (bars), Q is flow rate (cm3/min), η is viscosity (Poise), W is crack width (cm), D is crack depth (cm), L is crack length (cm).

4. Radial Flow Equation – Used to calculate grout spread from an injection point into surrounding concrete:

r = (Qt/πh)1/2

Where r is radius of grout flow (cm), Q is injection rate (cm3/min), t is time (min), and h is concrete thickness (cm).

5. Packer Spacing Equation – Used to determine optimal packer spacing:

S = (EIRD/Q)1/2

Where S is packer spacing (cm), E is grout expansion factor, I is number of grout lines, R is radius of grout spread (cm), D is concrete thickness (cm), Q is injection rate (cm3/min).

Grouting Operations

Grouting for dams is done by drilling of holes through fill, overburden, concrete, and rock. Drill holes are required for grout injection, water pressure tests and monitoring.

For rockmass, typically 50 to 80 mm diameter drill holes are used for grouting and 80 to 120 mm drill holes used for testing. Inclination of holes may vary from vertical in upward to vertically downward direction.

After drilling, the washing out of loose material, silt and clay from the seams and fissures are important for proper grout injection. In dry ground, water must be injected into the boreholes prior to grouting to avoid any reduction in water cement ratio of grout.

• A test section should be done before going for production grouting. Test section is made to finalize the grouting method, the layout and spacing of boreholes, pressures for grouting etc.
• Various proposed grouts are tested in test section to choose right grout mix for achieving the final objective.
• A water test is made on the test pad before and after grouting to evaluate efficacy of grouting. Resurgence and uplift is also measured in test section.
• The grout curtain may be made by performing grouting in sequence of primary, secondary, tertiary holes.
• For example first grouting may be done in primary boreholes say 8 m apart, then secondary grouting will be performed in-between primary holes reducing spacing between holes to 4m and similarly for tertiary holes spacing will be reduced to 2m.

The spacing of grout holes is decided based on grouting test pad results and also considering water pressure test results done during the production grouting.

Upstage (Backward) and Downstage (Forward) Grouting

Grouting in the deep holes are performed in stages. In general, grouting is carried out by upstage of backwards method.

In this method boring is done through the entire length, and grouting is performed from bottom of hole to top, generally in stage of 3 m, with systematic use of a single packer.

In downstage (forward) grouting method the holes drilled for 1^st stage from top then grouting of that stage is done.

This stage was redrilled and 2^nd stage is drilled further down now 2^nd stage is grouted using 2 packers (at top and bottom of 2^nd stage).

This procedure continues till bottom of the hole. In certain geological conditions downstage (forward) grouting method is essential, as it serves to first seal and stabilize the upper zone.

Monitoring

Resurgences of grout and upheaval of the ground shall be monitored during grouting operations and grouting shall be stopped if observed.

Monitoring and a complete recording of the main parameters of each grouting stage shall be done in real time.

The most important parameters to be analyzed are pressure vs time; flow rate vs time; pressure vs grout intake and penetrability vs grout take.

The real time interpretation of the plots of these functions allows to characterize the grouting process and to anticipate any hydro-jacking or hydro- fracturing and thus to adapt the grouting process to the real conditions of the rock mass.

Applications of pressure grouting

Pressure grouting is used in a variety of applications, such as:

Soil stabilization 

Pressure grouting can be used to stabilise soil by filling voids, cracks, and fissures in the ground. This can prevent soil erosion and collapse, improving the load-bearing capacity of the soil.

Foundation repair

Pressure grouting can be used to repair foundation cracks, improving the stability of buildings, bridges, and other structures.

Tunneling 

It can be used to seal leaks in tunnels and prevent water infiltration.

Seismic retrofitting 

It can be used to retrofit existing buildings and structures to make them more resistant to earthquakes.

Dam repair 

Pressure grouting can be used to seal leaks and cracks in dams and prevent water from seeping through.

Pressure grouting procedure

It is a versatile and cost-effective method for enhancing the geotechnical performance of various structures. However, it requires careful planning, execution, and supervision to ensure its safety and quality.

Source: https://en.wikipedia.org/wiki/Pressure_grouting#/

4 Steps of Pressure Grouting

1.  Site investigation: This involves collecting and analysing the soil or rock samples, conducting geotechnical tests, identifying the groutable zones, and designing the grouting program.
2. Drilling and installation: This involves drilling the boreholes for grout injection, installing the grout pipes or packers, and performing pre-grouting tests to determine the optimum grout pressure and flow rate.
3. Grout injection: This involves mixing and pumping the grout into the boreholes under controlled pressure and flow rate, monitoring the grout take and pressure, and adjusting the grouting parameters as needed.
4. Post-grouting evaluation: This involves conducting post-grouting tests to verify the effectiveness of grouting, measuring the improvement in soil or rock properties, and documenting the grouting results.

Pressure grouting in concrete

Concrete slabs are the most important structural elements in building construction. They act as a load carrying barriers that  transfer the entire load from the superstructure to the beams and columns.

Sometimes there is a chance of damage or deterioration of these slabs in the form of voids or cracks  that may be due to various reasons and may lead to damage to the entire structure . For such type of damage in concrete slabs pressure grouting is the best technique that helps to deal with it.

Procedure for Pressure Grouting in Concrete Slabs

Pressure grouting for concrete slabs is the best technique to repair and restore damaged concrete structures. It requires proper planning, execution and quality control to obtain successful results. 

The process starts with identifying the damaged areas of the concrete slab that require treatment. This is usually done by conducting a thorough inspection to locate voids, cracks, or areas with reduced structural integrity. Once the problem areas are identified, the following  is the stepwise procedure  to be followed:

• Preparation of the area to be grouted – first the damaged  area of the concrete  slab is  identified , cleaned and made ready for injection of grout. This may involve removing loose debris, dust, or any existing surface coatings.
• Drilling of holes – if required holes are drilled into the concrete to access the cracked or the voids formed in the slab. This helps for the easy injection of grout into cracks and voids and obtain a good bond between them.
• Injection of grout– the grout  is mixed according to the requirement of the project. It is now injected into the drilled holes using an injection pump or hand-held applicator under controlled pressure. 
• Monitoring – the pressure and flow of the grout material are monitored to ensure consistent and uniform grouting .The injection pressure should be adjusted to avoid overfilling or underfilling the cracks or voids. The  process continues until the grout material fills the voids and effectively binds the surrounding concrete.
• Curing– finally once the grout material is injected, it needs sufficient time to cure and harden and allow enough time for the grout to achieve its desired strength.

Advantages

Pressure grouting can provide several benefits for concrete slabs, such as:

• Restoring the structural integrity and durability of the slab by closing the cracks and voids that can weaken it or allow water to seep in and cause corrosion or freeze-thaw damage.
• Improving load-bearing capacity of the slab and preventing further deterioration.
• Raising the level of the slab by expanding the grout material and exerting an upward force on the slab, which can correct unevenness or settlement issues.
• Enhancing the resistance of the slab to external loads by increasing its stiffness and strength, which can prevent further cracking or deformation.

FAQ – Pressure Grouting

Overall, pressure grouting is an effective technique for improving the stability and strength of soil, rock, and concrete structures. Its applications are diverse and can be customized to suit different soil and rock conditions.

What is the material used for pressure grouting?

The materials used for pressure grouting may vary depending on the mode of application and the type of the structure or the soil being treated.

Some of them are a mixture of Portland cement, water, and sometimes additives such as sand, fly ash, and bentonite, synthetic resins, polyurethanes, or acrylics that are mixed with water or a catalyst, epoxy resins, Polyurethane etc.

How much pressure is required in pressure grouting?

The pressure required in pressure grouting depends on several factors, such as the depth, size, and orientation of the fractures, the type and viscosity of the grout, and the in-situ stress of the rock mass.

One way to estimate the maximum pressure that can be applied without causing irreversible damage to the rock is 

According to this formula, the grout pressure should be less than:

(1) Δ P g < 3 ρ · g · H k 2 · (1 + H I + 1 3 H I 2 – P w ρ · g · H

where ΔP g is the grout pressure, ρ is the density of rock mass, g is gravity, H is the depth of the fracture, k 2 is a groutability factor, I is the grout spread distance, and P w is water pressure.

However The equipment should be  capable of grouting at least to a pressure of 100 psi (700 MPa), and a pressure gauge having a full- scale reading of not more than 300 psi (2000 MPa).

What are the disadvantages of pressure grouting?

1.  It may not be suitable for coarse-grained or soft soils which have low self-support times and high risk of soil movement.
2.  It may be less cost-effective for small repair jobs than other methods such as concrete piling or replacement.
3. It may sometimes cause unnecessary spill of grout into areas that are not intended to be filled, such as drains, water systems, or the surface, if there are unknown cracks or fissures in the ground or structure.
4.  If too much grout is injected or if the pressure is too high, especially for old or delicate constructions, it may affect the structure or foundation.