This method is used to increase the soil strength and to reduce the pore volume. Main advantage of grouting is that it can be used in small, difficult-to-access areas, under existing structures, and to treat specified zones and depths without causing significant disturbances to overlying layers. However, this technique is mostly very expensive, and generally preferred where other techniques are not feasible (Sutton and Mc Alexsander, 1987; Adalier, 1996; Drumheller et al., 1997). Compaction grouting is used to densify the soil, thus it is discussed under densification techniques. Non-densifying grouting techniques are: (i) jet grouting, (ii) permeation grouting, (iii) electro-kinetic injection, and (iv) intrusion grouting. Deep soil mixing, on the other hand, which can also considered as insitu reinforcement, modifies the mechanical properties of the soil such as cohesion.
Detail analysis can be found in Sutton and Mc Alexander (1987), and Drumheller et al.
(1997). Figure 2.8 shows schematic diagrams of soil modification process using jet grouting, permeation grouting, and intrusion grouting, respectively.
(b) (c) (a)
Fig. 2.8: Schematic Diagrams of (a) Jet, (b) Permeation, and (c) Intrusion Grouting (Source: http://www.haywardbaker.com)
A variety of materials such as bentonite, cement, fly ash, lime, and combinations can be used as admixtures to treat soils depending on the particular application, availability of admixtures, site access, relative degrees of improvement required, costs, and time of construction. The characteristics of the soils to be used in the mix, whether at the site or imported to the site, must be well understood and controlled, including grain size distribution, plasticity, density, moisture content, and strength.
2.4.1 Jet Grouting
Jet grouting, which has been in use for many years, can be defined as a method based on the introduction of hydraulic (some times combined with pneumatic) energy in order to erode soil and mix/replace the eroded material with an engineered grout (Fig. 2.8a) to
form a solidified in situ element known as soilcrete (Drumheller et al., 1997). The elements are most often interconnected to provide underpinning to structures, excavation support, ground water control, or in situ stabilization for a variety of civil and environmental applications, especially in and around sensitive structures where property and personal safety are imperative.
Table 2.10 gives two examples of case histories, where this method was utilized to mitigate liquefaction hazards.
Table 2.10: Selected Jet Grouting Projects as a Liquefaction Countermeasure
Site Site
Characteristics
Reasons for Method Selection
Construction
Program Performance Power plant
structure, Sacramento, CA.
Decaying timber pile foundation in loose sands and silty sands.
Existing
buildings. Encapsulated pile foundation to prevent foundation settlement and liquefaction damage by jet grouting to depths of 13.7 m.
No data given.
Transit station, Taipei, Taiwan.
Dense gravelly layer between depths of 2 and 6 m. Loose to medium dense silty sand between 8 and 26 m.
Site 30 m from residential buildings.
Soil-cement-sodium silicate columns 14 m in depth, spaced 2 m apart. Grout pressures ranged from 16 to 18 Mpa in loose sands, and 18 to 20 Mpa in clayey soils and medium dense sands. Withdrawal rate of 190 mm/min. in loose soil.
Cores taken from center of column met the minimum 28 days unconfined
compressive strength requirement of 1.4 Mpa.
(Source: Andrus and Chung, 1995)
2.4.2 Permeation (Chemical) Grouting
In this method, the grout is forced to permeate into the soil displacing pore fluid without significantly altering the physical structure of the soil (Adalier, 1996). Usually a pair of binding agents, a grout and a reactant (hardener), is pumped at high pressure into liquefiable zones (Fig. 2.8b). Grout and the hardener react to form a gel or other agents that fill the voids and facilitate the formation of soil particle bonds (Thevanayagam and Jia, 2000). Chemical grout effectiveness can be measured by the following factors (Sutton and Mc Alexander, 1987):
• Controllability of gel-time: It is essential for the chemical to penetrate into the soil to a desired extent before hardening.
• Permanency: Issues relating the durabilty of the gel/bonding medium should be considered carefully.
• Cost: Usually this technique is very expensive. However, circumstances such as existing structures may lead to choose this method.
• Viscosity: Low viscous fluid can permeate into small pores.
• Environmental Factors: This is a common issue for any kind of chemical grouting technique.
Chemical grouting can be used economically for sands having low fines content (usually less than 20%). Table 2.11 provides a summary of selected permeation grouting projects.
Table 2.11: Selected Permeation Grouting Projects as a Liquefaction Countermeasure
Site Site
Characteristics
Reasons for Method Selection
Construction
Program Performance Riverside
Avenue bridge, Santa Cruz, CA.
Loose to medium dense gravelly sand. River level at high tide 2.7 m above bottom of concrete slab- apron.
Treatment beneath existing concrete noise pier and slab- apron; limited working space.
Grout composed of sodium silicate N grade, MC 500 micro-fine cement, and less than 0.1% by vol. of phosphoric acid to control set time.
No settlement or detrimental ground movement reported after 1989 Loma Prieta earthquake;
amax=0.45g.
Roosevelt Junior High School, San Francisco, CA.
Loose to medium dense silty sand and sand
extending to depth of 4.6 m. N- values ranged from 3 to 15 before treatment.
Existing building and limited working space.
Sodium silicate based grout used. Stage down grouting in 0.3 m intervals.
Unconfined
compressive strength ranged from 269 kPa to 879 kPa. No settlement reported after 1989 Loma Prieta earthquake;
amax about 0.15g.
(Source: Andrus and Chung, 1995)
2.4.3 Intrusion Grouting
Although not a common method for liquefaction mitigation, this method can be used to strengthen small volumes of week, liquefiable soils that are embedded into a firm deposit. This technique involves injection of fluid grout under high pressures with the evolution of controlled fracturing (Adalier, 1996) (Fig. 2.8c).
2.4.4 Electro-Kinetic Injection
This technique is similar in principle to that of permeation grouting, but utilizes dc current to introduce the grout into the soil. This technique is well suited for low permeable silty soils especially beneath existing structures (Ahmad, 2001), and not good for the soils with high electrical conductivity (Adalier, 1996). However, this technique is at research level.
2.4.5 Deep Soil Mixing
In this method, mixing augurs are used to mechanically mix the cementitious grout with soil. Grout is injected from the auger shaft tip and blended with the soil as the augers advance into the soil, as well as they are withdrawn, leaving behind stabilized soil columns (Andrus and Chung, 1995; Adalier, 1996; Drumheller et al., 1997). Usually this method is very expensive in the United States (Andrus and Chung, 1995).
Table 2.12 provides two of the case histories, where this technique was selected for liquefaction remediation.
Table 2.12: Selected Deep Soil Mixing Projects as a Liquefaction Countermeasure
Site Site Characteristics
Reasons for Method Selection
Construction
Program Performance Jackson
Lake Dam, WY.
Loose gravel and sand extending to depths of 30 m.
Simpler quality control
program than other methods.
Not affected by artesian pressures.
Where upstream and downstream slopes of the new dam would be, soil mixed panels forming open hexagonal cells with 15 m sides extending to a depth of 33 m. A two-shaft auger was used. Shaft diameter was 0.9 m. Grout had a water-cement ratio of 1.25:1 by weight. Mixed soil contained 337 kg of cement per m3.
Unconfined
compressive strength of core specimens ranged from 1.4 to 8 Mpa.
Spill tanks at pulp and paper mill, Vancouver, BC.
Loose sand-silt fill between depth of 1.8 and 5.5 m.
The fill is underlain by 1.2 m of medium dense beach sand.
Provided optimal “cost- benefit”
solution.
Tank constructed on ring of tangent columns extending 0.9 m into dense silt. A single 3.6 m-diameter shaft auger was used. Grout had a water-cement ratio of 1.8:1 by weight. Mixed soil contained 177 kg of cement per m3.
Unconfined
compressive strength of core specimens ranged from 1 to 3 Mpa.
(Source: Andrus and Chung, 1995)
2.4.6 Compacted Soil-Cement Mix
This method involves mixing soil with small amounts of cementitious (usually Portland cement) materials prior to filling / backfilling, and compacting insitu. This method may require large area dewatering, and it is usually expensive. This technique was used to eliminate the possibility of liquefaction in saturated loose sands underlying a Nuclear Power Plant in South Africa, where the soils was removed, treated with 5%
cement and recompacted (source: Adalier, 1996).