Central Subway Tunneling

Pre-conditioning and compensation grouting before and during tunnel boring activities to stabilize sensitive structures


Grouting is an efficient, cost-effective technique that can stiffen, strengthen and reduce the permeability of various types of soils. Condon-Johnson’s team of geotechnologists utilize state-of-the-art, real-time computer monitoring equipment to maximize the effectiveness of our grouting applications.

Centinella West

Installation of compaction grouting to mitigate significant settlement on newly constructed embankment

Tacoma Micro Tunneling

Jet grout plugs to provide excavation support and ground water control for micro tunneling operations
Compaction Grouting

Compaction grouting involves the injection of a low-slump, mortar-like grout under high pressure at a controlled rate to displace and densify loose granular soils to increase bearing capacity and reduce liquefaction potential. Compaction grouting can also be used to reinforce fine-grained soils to increase bearing capacity, decrease/arrest future settlement and lift settled structures. The low slump grout is typically injected as the injection pipe is extracted in 2 feet to 3 feet stages; the low slump grout does not penetrate soil pores, but displaces the subsurface soils by forming a semi-homogenous grout bulb near the grout pipe tip. As the pipe is extracted, the series of injections will create a column of overlapping grout bulbs.

The grout is a blend of sand, 20-30 percent silt or flyash, water, and cement to create a thixotropic, viscous, low slump grout which enables injection at high pressures while maintaining the integrity of the grout bulb during installation. The spacing, grout pressure and grout quantity are controlled to densify or reinforce the in-situ soil, while minimizing adverse ground heave or fracturing of the soil.

Jet Grouting

Jet grouting involves the use of high energy horizontal fluid jets to hydraulically erode and mix the native soils to create a specific geometry (typically columns) of soil-cement elements. Portland cement is the most common base component of the grout mix, however other binders or reagents can be used in lieu of cement depending on the goals of the treatment. Jet grouting can be used in essentially any soil type; common uses include shoring/underpinning, excavation bottom seals, water control for excavations and pre-treatment for tunneling in unstable conditions.

A small diameter hole is drilled through the layer of soil requiring treatment. Once at the bottom of the proposed column, grout is pumped at high pressure through the drill rods to the monitor where the fluid exits horizontally at a very high velocity through one or more small diameter injection nozzles. While high pressure grout injection is maintained, the drilling rod is automatically withdrawn at a controlled speed and rotational rate. The rods can be rotated from 0 to 360 degrees during jetting to create any geometry, from circular columns to flat panels.

Calaveras Dam Replacement

Foundation and permeation grouting for grout curtain beneath the new dam embankment
Permeation/Dam Grouting

Permeation grouting, also known as dam/curtain grouting or high mobility grouting, is the injection of specially designed cement, asphalt, polyurethane, or sodium silicate mixes into the pore spaces of granular soils or into the voids, open joints, or shear zones within rock formations. The selection and design of the grout mix/type to be injected into the native soil or rock formations is selected to increase the strength/stiffness and/or decrease the permeability of the formation.

Within rock formations, typically foundation grouting for dams, a small diameter hole is drilled to intercept voids, fissures, or cracks and grout is pumped under pressure into these features to improve overall stability and reduce permeability. Within soils, a small diameter hole is drilled and a sleeve port packer pipe is grouted in place to isolate the various treatment depths within the soil profile to allow controlled discrete injections of grout.

Condon-Johnson has state-of-the-art, real-time computer monitoring equipment that monitors and records all grouting parameters. This allows the engineer to adjust the grout mix, pressure, flow rate, and cutoff volumes based on real-time data. All data collected can be used to produce grouting reports along with 3D drawings depicting the location, depth, orientation, and grout takes for each stage of each grout injection location.

Tabor Dam Montana

Dewatering, core drilling and void grouting from inside a limited access outlet tunnel
Compensation Grouting

Compensation grouting is a ground improvement method used to mitigate or reverse settlement. The intent of this technique is to protect surface structures from settlement during subsurface activities, typically tunneling. Sleeve port pipes are installed in the zone above the proposed tunnel bore and beneath the surface improvements that are at risk to tunneling induced settlement. The soil is repeatedly “pre-treated” with grout injections prior to the tunnel approach until Condon-Johnson can reliably raise or heave the ground above the tunnel alignment. Once the soil has been pre-treated, real time monitoring is carried out during active tunneling operations to monitor the ground for settlement beyond the allowable threshold. When monitoring indicates that action is required, grout is injected into the appropriate ports corresponding with the monitored movement to heave the ground and offset the settlement being initiated by ground loss from tunneling operations.

This method is most applicable to fine grained soils which can be repeatedly frac’d. Although this method can be used in granular soils, the choice of the pre-treatment grout has to be carefully selected to avoid creating a grouted mass so stiff that it cannot be reliably heaved upon demand.