Chapter 7 - Soft Ground Tunneling
7.7 Soil Stabilization and Improvement .1 Purpose
Until fairly recently essentially all the design effort for tunnels in soft ground was to provide a support system or systems that would stabilize the existing ground during construction and then, perhaps with some modification, would permanently support the ground and provide an opening suitable for the long term mission of the tunnel. In the last two or three decades, however, the situation has changed such that in some applications a dual approach is taken. First, the characteristics of the ground are modified by stabilization and/or improvement to make that ground contribute more to its own stability. Then, secondly a supplementary but less costly support/lining system is installed to make the tunnel perform for its full lifetime. In this section the
various methods of soil stabilization and improvement are summarized. References with more details on these methods are also given.
7.7.2 Typical Applications
The decision to use soil stabilization or improvement must be made on each individual case. This decision may sometimes be easy with there being no other way to construct the tunnel. More often, the decision comes down to a trade off among treating the ground, using high-tech
machines, and/or a combination of the two. With all of the possibilities it can be said that there are now no unacceptable construction sites. Table 7-10 summarizes the challenging ground sites and corresponding treatment methods.
Table 7-10 Ground Treatment Methods
Challenging Ground Conditions Treatment Method(s)
Weak Soils • Vibro Compaction
• Dynamic Compaction
• Compaction Grouting
• Permeation Grouting
• Jet Grouting
Ground Water • Dewatering
• Freezing
• Grouting
Unstable Face • Soil Nails
• Spiling
• Soil Doweling
• Micro Piles
Soil Movement • Compensation Grouting
• Compaction Grouting
It is to be noted that the boundaries between both ground conditions and treatment methods are not fixed. Also, the use of vibrocompaction techniques or dynamic compaction is typically
applicable at or near the tunnel portals as these techniques are applied to the ground surface and are not effective beyond about 100 ft depth for vibro compaction and 35 ft depth for dynamic compaction. Both are generally effective only in granular soils.
Readers are referred to the Ground Improvement Methods Reference Manual (FHWA, 2004) for more detailed discussion for the soil stabilization and improvement techniques presented below.
7.7.3 Reinforcement Methods
Soil Nails Soil nails may be used to stabilize a tunnel face in soil during construction. Steel or fiberglass rods or nails are installed in the face and the resulting reinforced block(s) are analyzed for stability much as for usual slope stability analyses. Several methods (e.g., Davis, Modified Davis, German, French, Kinematrical, Golder, and Caltrans) are used for these analyses.
Walkinshaw (1992) has studied these methods and concluded that all had some level of inconsistencies, such as:
• Improper cancellation of interslice forces (Davis method)
• Lateral earth pressures inconsistent with nail force and facing pressure distribution (all)
• No redistribution of nail forces according to construction sequence and observed measurements (all except Golder)
• Complex treatment and impractical emphasis on nail stiffness (Kinematical) (after Walkinshaw, 1992: Xanthakos, 1994)
For more discussion readers are referred to GEC No.7 Soil Nail Walls (FHWA, 2003), which also recommends that the Caltrans SNAIL program be used because it will handle both nails and tiebacks. However, it must be recognized that application of that or any other program must be tempered with appropriate judgment, measurements and case history experience.
Soil Doweling Soil doweling entails the installation of larger reinforcement members than does nailing. These dowels act in tension like soil nails but are large enough in cross section that they also develop some shearing resistance where they pass through the sliding surfaces.
7.7.4 Micropiles
As they are applied to tunneling, micropiles are essentially the same as soil dowels. These are typically drilled piles two to six inches in diameter that contain a large reinforcing bar centered in the hole and the hole backfilled with concrete. As opposed to pin piles that are typically installed at the surface (and that act in compression), the pin piles placed in tunnels typically act in tension and shear across the sliding surfaces.
Soil nails, soil dowels, and pin piles are typically installed at the face of the tunnel to stabilize that face for construction. Thus, they are continually being installed and mined out of the face. For ease in this mining operation, fiberglass bars (rods) are typically used in these applications because they are much easier to mine out and cut. In contrast, spiling tends to look out around the perimeter of the tunnel, thus steel is more likely to be used for spiling bars or plates.
Readers are also referred to "Micropile Design and Construction Reference Manual" (FHWA, 2005f) for more details.
7.7.5 Grouting Methods
All grouting involves the drilling of holes into the ground, the insertion of grout pipes in the holes, and the injection of pressurized grout into the ground from those pipes. The details of the operations, however, are distinctly different. Readers are referred to the Ground Improvement Methods Reference Manual (FHWA, 2004) for more detailed discussion for the grouting techniques discussed hereafter.
Permeation Grouting Permeation grouting involves the filling of pore spaces between soil grains (perhaps displacing water). The grout may be one of a number of chemicals (but is usually sodium silicate or polyurethane) or neat cement using regular, micro- or ultra- fine cement, along with chemicals and other additives. Once injected into the pore spaces, the grout sets and converts the soil into a stable, weak sandstone material. Permeation grouting usually involves grout holes at three to four feet centers with enough secondary holes at split spacing to verify that all the ground is grouted. If necessary to get full coverage all of the split spacing holes may have to be grouted and verification performed by the tertiary holes.
Compaction Grouting Compaction grouting uses a stiffer grout than does permeation grouting. In compaction grouting the goal is to form a series of grout bulbs or zones four to six feet above and around the tunnel crown. By pumping the stiff grout in under pressure these bulbs compress (densify) the ground above the tunnel and between the tunnel and overlying facilities.
The pipes for compaction grouting are pre-positioned and drilled into place and all the grouting pumps, hoses, header pipes, instrumentation and the like are in place before the tunnel drive begins. Instrumentation is read as the tunnel approaches and passes a facility and the grouting
operation is adjusted real time in response to the movement readings. Actually, in most
applications it is possible to either pre-heave the ground or to jack it back up (at least partially) by pumping more grout at higher pressures.
Compensation Grouting Compensation grouting is, in some ways, similar to compaction grouting.
The goal is to monitor ground movements, primarily between the tunnel and any overlying facility.
When it is apparent that ground is being lost in the tunneling operation, a grout, typically slightly more liquid than the compaction grout mix, is injected to replace (compensate for) the lost ground. As indicated the differences between these two schemes are relatively minor - compaction grouting seeks to recompact the ground by forming grout bulbs, compensation grouting seeks to refill voids created by the tunneling operations.
Jet Grouting Jet grouting is the newest of the grouting methods and is rapidly becoming the most widely used. Jet grouting uses high pressure jets to break up the soils and replace them with a mixture of excavated soils and cement, typically referred to as "soilcrete". There are a number of variations of jet grouting depending on the details of the application and on the experience and expertise of both the designer and the contractor.
The design of a jet-grouted column is influenced by a number of interdependent variables related to in situ soil conditions, materials used, and operating parameters. Table 7-11 presents a summary of the principal variables of the jet grouting system and their potential impact on the three basic design aspects of the jet-grouted wall: column diameter, strength and permeability.
Table 7-11 gives typical ranges of operating parameters and results achieved by the three basic injection systems of jet grouting. It should be noted however, that the grout pressures indicated in this table are based on certain equipment and can vary. This table can be used in feasibility studies and preliminary design of jet-grouted wall systems. The actual operating parameters used in production are usually determined from initial field trials performed at the beginning of
construction.
Jet grouting is frequently used as a ground control measure in conjunction with tunneling in soft ground using Sequential Excavation Method (Chapter 9).
Table 7-11 Summary of Jet Grouting System Variables and their Impact on Basic Design Elements
Principal
Variables General Effect of the Variable on Basic Design Elements (Strength, Permeability and Column Diameter)
(a) Jet-Grouted Soil Strength Degree of mixing
of soil and grout Strength is higher and less variable for higher degree of mixing Soil type and
gradation
Sands and gravels tend to produce stronger material while clays and silts tend to produce weaker material.
Cement Factor Strength increases with an increase in cement factor (weight of cement per volume of jet-grouted mass).
Water/cement ratio of grouted mass
Strength of the jet-grouted soil mass decreases with increase in in situ water/cement ratio.
Jet grouting
system The strength of the double fluid system may be reduced due to air entrapment in the soil-grout mix.
Age of grouted
mass As the jet-grouted soil mass cures, the strength increases but usually at a slower rate than that of concrete.
(b) Wall Permeability
Wall continuity Overall permeability of a jet grout wall is almost entirely contingent on the
Table 7-11 Summary of Jet Grouting System Variables and their Impact on Basic Design Elements
Principal Variables
General Effect of the Variable on Basic Design Elements (Strength, Permeability and Column Diameter)
continuity of the wall between adjacent columns or panels. Plumb, overlapping multiple rows of columns would produce lower overall permeability. In case of obstructions (boulders, utilities, etc.) if complete encapsulations is not achieved then overall permeability may be increased due to possible leakage along the obstruction-grout interfaces.
Grout
composition Assuming complete wall continuity and complete replacement of in situ soil, the lowest permeability which can be obtained is that of the grout (typically 10-6 to 10-7 cm/sec). Lower permeabilities may be possible if bentonite or similar waterproofing additive is used.
Soil composition If complete replacement is obtained (as may be possible with a triple fluid system) then soil composition does not matter. Otherwise, if uniform mixing is achieved then finer grained soils would produce lower permeabilities as compared to granular soils.
(c) Column Diameter Jet grouting
system The diameter of the completed column increases in size as the number of fluids is increased from the single to the triple fluid systems.
Soil density and
gradation As density increases, column diameter reduces. For granular soils, the diameter increases with reducing uniformity coefficient (D60/D10).
Degree of mixing of soil and grout
Larger and more uniform diameters are possible with higher degree of mixing.
7.7.6 Ground Freezing
As with much of tunneling technology, ground freezing was developed first in the mining industry and was probably first used in sinking mine shafts. For a mine the shaft (and the mine) is located where the ore is. Thus, means of obtaining access in unfavorable ground conditions, of providing emergency support in unstable ground below the water table, and of maintaining stability of working faces below the water table, such as freezing, often had their roots in the mining industry.
In its simplest form, ground freezing involves the extraction of heat from the ground until the groundwater is frozen. Thus converting the groundwater into a cementing agent and the ground into a "frozen sandstone". The heat is extracted by circulating a cooling liquid, usually brine, in an array of pipes. Each pipe is actually two nested pipes, with the liquid flowing down the center pipe and back out through the annulus between the pipes. When the pipes are close enough and the time long enough, the cylinders of frozen soil formed at each pipe eventually coalesce into one solid frozen mass. This mass may be a ring or donut as needed to support a shaft or a solid block of whatever shape necessary to stabilize the working face or heading.
Because of the dearth of engineering data on the properties of frozen ground (especially clays) it is recommended that two steps be taken early in any design of ground freezing:
1. A qualified consultant be engaged to advise on the design and construction of the project.
Advice from such a professional is essential for the work and will pay for itself many times over.
2. Laboratory tests be designed and carried out using soil samples from the actual site.
Only in this manner can meaningful properties of frozen soil be obtained for the site involved for purposes of conceptual engineering ("scoping the problem").
However, a few general guidelines can be stated as follows (after Xanthakos, 1994).
1. Pipes are normally spaced 3 to 4 feet apart.
2. Select a spacing-to-diameter ratio <13 (for pipes 120 mm or less in diameter).
3. Use a brine temperature <25˚ C.
4. Provide 0.013 to 0.025 tons of refrigeration per foot of freeze pipe.
5. Determine typical frozen ground properties by laboratory testing.
Groundwater flow across the site requires special considerations closer pipe spacing, multiple rows of pipes and the like. Groundwater flow velocities approximately >2 m/day may impede or prevent freezing. A number of special challenges associated with ground freezing should be considered in both the design and construction stage. Those are creep of frozen ground, sensitivity of frozen ground properties to loading condition, ground heave or settlement, and others.
Readers are referred to the discussions and details of ground freezing application in Chapter 12.