The cutter soil mixing (CSM) method excavates rectangular panels while simultaneously adding water to the soil to fluidise it in place to a prescribed depth. Upon retraction, the cement grout is added and mixed with the fluidised soil to form a soil-cement mixture. To construct the shaft, the panels are interlocked to form a contiguous ring of panels.
CSM was first introduced in Europe by BAUER Maschinen GmbH and has been used in Europe, Asia, and Canada. It was recently used in the US to construct trench walls in Seattle, Washington, and 11 and 16 metre deep shafts near Sacramento, California.
The CSM is being used on a Contra Costa Water District (CCWD) project in northern California to construct 20 and 32 metre deep watertight shafts in difficult ground conditions. The shafts will be used for a 275 metre long microtunnel crossing of Old River. This is the second known application of this technology in the US for the construction of microtunnel shafts and the first known application using shotcrete-reinforced walls. The shafts penetrate soft saturated silts and clays, loose-to-dense sands, and a confined aquifer presenting over two bars of pressure opposite the jacking shaft tunnel eye. Existing construction techniques, such as secant piles and slurry walls, were not considered feasible for the deeper shaft because of concerns over pile drift during installation, which can result in “÷windows’ between the panels.
CSM in the delta
The CCWD is building a new Alternative Intake Project (AIP) near its existing pumping facilities in the Delta region, east of the San Francisco Bay. The project will provide an alternative raw water intake facility at Victoria Canal, where the water quality is much higher than that at the current intake location on Old River. The new AIP project will pump raw water directly into the CCWD’s existing Old River pipeline facilities, which convey raw water to the District’s Los Vaqueros Reservoir.
This new AIP project will divert up to 7,000 litres of raw water per second from the new intake pumping plant through a new 1,800 mm diameter pipeline. An approximately 275 metre section of this new pipeline crosses under Old River at the existing Old River intake facility. Construction of the crossing required the installation of a 2,400 mm diameter, 25 mm thick steel casing using microtunnelling. The crossing required a 28 metre deep jacking shaft and a 15 metre deep receiving shaft. CSM panels approximately 32 and 20 metres deep were constructed to embed the shaft walls below the shaft floor.
Existing conditions
The near-surface geology of the region is characterised by relatively thin deposits of peat, organic soils, and fills (typically the levee structures), which overlie deep alluvial soils. The Delta region is at the confluence of several major rivers that drain the Central Valley from the north, east, and south. As the rivers converged, the fine-grained materials consisting of sands, silts and clays, settled out, forming the thick alluvial deposits. Sandwiched in the alluvial deposits are two confined aquifers presenting over two bars of pressure, which influenced the bottom part of the jacking shaft.
The natural groundwater level is about one metre below the ground surface. The river water, which is two to five feet higher than the adjacent land elevations, is in direct communication with the natural groundwater level and identified aquifers.
The CSM technology
The CCWD opted for a performance-based, design-build specification for construction of the shafts. The specified performance for the shafts consisted of the following:
- Shafts had to be watertight, defined as groundwater infiltration into the shafts of no more than 40 litres per minute.
- Shaft tops had to be established at 0.5 metres above the 100-year flood level.
- Shaft construction could not lower the natural groundwater table by more than 0.6 metres below the lower-bound preconstruction levels.
- Construction vibrations could not exceed 13 mm per second peak particle velocity.
- Pile foundations for existing facilities could not come under the influence of jacking loads at the jacking shaft.
- A grout prism that is a minimum thickness of 1.5 metres and two tunnel diameters centred about the vertical and horizontal axes had to be developed at the receiving shaft.
The successful contractor submitted a proposal to use the CSM construction method for the shafts. This method was not known to the owner or engineers, but the contractor had successfully used it to construct wall diaphragms on a project in Seattle, Washington.
The construction methodology is similar to that used for construction of secant piles configured in a ring and interlocked to develop watertightness. Unlike the secant pile method, which completely replaces the soil with concrete, the CSM method mixes grout with the fluidised soil to develop the cementitious soil-cement panels. An appreciable amount of the fluidised soil is displaced and must be contained and disposed of as part of the construction process.
Method differentiation
Tooling and guidance control is the primary difference between the CSM method and other traditional commonly recognised soil-mixing techniques. Unlike the tools used with traditional soil mixing techniques that utilise augers mounted in a vertical axis that turn along a horizontal axis, the cutters for the CSM method are mounted on a horizontal axis and turn on a vertical axis. Additionally, an inclinometer mounted in the head assembly provides real-time data for the X and Y locations of the cutter head as it excavates through the ground.
The cutter wheels typically counter-rotate when the excavation is in progress so as to bring the cuttings up toward the centre, where shearing blades help to further break down the soil cuttings for mixing with water injected at the wheel confluence. Wheel rotation can be changed to counteract deviations of the head in the plane of the cutter wheel rotation.
Panel layout
Layout is critical to ensure the panels are correctly located and sufficiently overlapped for panel interlock. For this project, the contractor had the panel corners loaded into a total station survey instrument. This proved especially useful when locating the panels. The layout of new panels would have been very difficult for the contractor if it had relied upon string lines and offset measurements, given the mucky condition of the ground surface following each panel construction. New panel locations were established in a matter of minutes using a total station to set survey brushes that protruded through the surface muck for panel demarcation. The underground contractor was very conscientious about making sure the panels were accurately located to ensure panel interlock to satisfy the watertight performance criteria.
Shaft construction
Primary alternating panels are typically constructed first and allowed to cure. Secondary overlapping or face-to-face panels are then cut into and between the primary panels to form continuously interlocking panels.
For the jacking and receiving shaft constructed for this project, a grout strength of 3 MPa was used for the panel design. The shallower receiving shaft was designed as a single panel shaft, whereas the deeper jacking shaft was designed as a double. For the loading conditions and design grout strength, a safety factor ranging from about 3.8 to over 5 was calculated for the different stress conditions evaluated. The shafts were designed on the basis that alternating panels drifted out of alignment at a rate of 0.5 per cent of the panel depth, reducing the effective width of the panel. The 0.5 per cent level of accuracy is derived from the equipment manufacturer’s literature. As an added safety factor, the shaft interiors were designed with 150 mm of mesh-reinforced shotcrete, which increased to 300 mm with depth.
Structural reinforcement was required to transfer loads around the tunnel eyes and to distribute jacking loads into the shaft walls. The structural reinforcement was encapsulated in shotcrete.
Following construction of the receiving shaft panels, the grout strength achieved was considerably higher than 3 MPa. The 14-day grout strength ranged between
8.3 and 18 MPa. The receiving shaft design was subsequently revised to eliminate the shotcrete in the upper nine metres of shaft. Because of the higher than anticipated grout strengths achieved at the receiving shaft, the jacking shaft was revised from a double panel wall down to a single panel shaft, with double panels opposite the microtunnel reaction wall. Shotcrete in the upper nine metres of the jacking shaft was also eliminated. However, the compressive strength of the jacking shaft panels proved to be much lower than that of the receiving shaft. The exact cause for the lower strength results at the jacking shaft has not been determined.
When the microtunnel boring machine (MTBM) excavated into the shaft walls, but before the shaft walls were breached, a considerable amount of water leaked through the cold joint between the soil-cement walls and the shotcrete. This occurred only when the slurry pressures were increased to counterbalance the ambient pressure in the aquifer opposite the tunnel eye. This necessitated the injection of a hydrophobic (water reactive) material in the cold joint to prevent leakage.
Lessons learned
One aspect of the CSM method to consider for future projects is slurry control and management. This aspect needs to be addressed during the project design phase as it will impact temporary construction easements for handling, storage, and disposal, which in turn will influence production rates.
In a congested urban environment, where open areas are not as available as they were on this project, there must be an aggressive and comprehensive program to manage and dispose of the fluidised material. Specifications for use of the CSM method would need to address slurry containment within the immediate area surrounding the panel excavation.
Production rates could potentially be improved by matching cutter wheel tooling to the soil conditions. Adapting clay spades to the cutter wheels could be more efficient in cutting the clay soils into smaller particle sizes to minimise plugging of hoses or hose screens.
Conclusions
The shafts constructed by the CSM method were successful in satisfying the project specifications.
Shaft construction using the CSM method must include the following steps:
- Construction of in-the-field test panels must be done to validate the assumptions used in the design and to determine the grout injection rates.
- Panel locations (corners) must be set by survey methods using a total station survey method.
- Wet samples of constructed panels must be taken and tested – preferably 150 x 300 mm samples to minimise the influence of soil pockets on the overall sample size.
- Comprehensive slurry management plan must be set up to contain, collect, and dispose of the fluidised materials.
- Consideration must be given to disturbance of the shaft area during relocation of the CSM track-mounted rig.
- If shotcrete is used as structural reinforcing around the tunnel eye, and the exit slurry or external ambient pressures are in excess of about 10-15 psi (0.7-1.0 bar), consideration must be given to incorporating a positive seal between the shotcrete and CSM walls.
Equipment refinements in co-operation with the manufacturer could include assessments using different cutters on the wheels where soil conditions are predominantly silts, clays and sands.
This article is an edited version of a paper presented at Trenchless Australasia 2009 by Norman Joyal of Jacobs Associates, entitled “÷Microtunnel jacking and receiving shafts constructed using cutter soil mixing (CSM) technology’. Please refer to the original for more detailed information, references and acknowledgments.