Geotechnical Testing Journal, Vol. 34, No. 1
Paper ID GTJ102981
Available online at: www.astm.org
David Castelbaum,1 Mitchell R. Olson,1 Thomas C. Sale,2 and Charles D. Shackelford3
Laboratory Apparatus and Procedures for
Preparing Test Specimens of Slurry Mixed Soils
ABSTRACT: A laboratory mixing apparatus used to prepare test specimens of columns of soil mixed with slurries consisting of clay, typically
bentonite, and granular zero-valent iron (ZVI) is described. The slurries are injected and simultaneously mixed into the soil via a hollow-stem auger
to mimic the process that occurs in the field associated with the remediation of source zones contaminated with chlorinated solvents (e.g., carbon
tetrachloride and trichloroethylene) using large-diameter (e.g., 2.4 m or 8 ft) hollow-stem augers. The presentation includes descriptions of the
testing apparatus and procedures for preparing the test columns, mixing the slurries into the base (host) soil, and determining the resulting vertical
distributions in the physical properties (e.g., unit weight, water content, porosity, void ratio, clay content, ZVI content, and degree of saturation) of
the post-mixed column specimens. Example results of the vertical distributions in physical properties as well as of treatability studies for base soils
that were contaminated with chlorinated solvents are provided to illustrate the use of the laboratory apparatus and testing procedures.
KEYWORDS: bentonite, chlorinated solvents, kaolin, sand, slurries, soil mixing, zero-valent iron (ZVI)
Introduction chloride (CT), the source zones have been treated via ISSM by
using a mixture of granular zero-valent iron (ZVI) and a processed
In situ soil mixing (ISSM) is a technique that involves the introduc- clay (i.e., bentonite or kaolin) in the form of a ZVI-clay-water
tion and blending of one or more engineered additives with subsur- slurry. This approach to treating NAPL contaminant source zones
face soils. Traditionally, ISSM has been used for geotechnical en- has become referred to as the ZVI-clay technology.
gineering applications involving hydraulic cut-off walls, The ZVI in the ZVI-clay technology reacts with chlorinated sol-
excavation support walls, ground treatment, liquefaction mitiga- vents resulting in reductive dechlorination, a chemical process
tion, and in situ improvement, piles, and gravity structures (Por- whereby the chlorine (Cl) associated with the chlorinated solvents
baha 1998a, 1998b; FHWA-RD-99-138 2000; Topolnicki 2004). is replaced progressively by protons 共H+兲 and subsequently re-
ISSM also has been used extensively in environmental remediation leased to the surrounding environment as chloride 共Cl−兲 (Gillham
applications, including applications involving containment, stabili- and O’Hannesin 1994; Wadley 2002; Wadley et al. 2005). The clay
zation, and treatment of subsurface contamination (Ryan 1987;
serves to enhance sorption of the contaminants within the mixed
Gazaway and Jasperse 1992; Day and Ryan 1995; Shackelford and
zone thereby decreasing the mobility of the contaminants and in-
Jefferies 2000). In terms of immobilizing subsurface contamina-
creasing the time for reaction between the ZVI and contaminants,
tion, in situ stabilization involves mixing contaminated soils with
and decrease the hydraulic conductivity of the mixed zone thereby
reagents, typically pozzolanic materials such as cement, fly ash,
inhibiting the contaminant mass flux emanating from the mixed
lime, and slag (Jasperse and Ryan 1992; Kosinski et al. 1992;
Walker 1992; Day and Ryan 1995; Wheeler 1995; Al-Tabbaa et al. zone (Castelbaum and Shackelford 2009). Also, the clay slurry
1997; Al-Tabbaa and Evans 1999; Shackelford and Jefferies 2000; serves as a drilling fluid that reduces the mechanical energy re-
Al-Tabbaa 2003; Hernandez-Martinez and Al-Tabbaa 2004; Evans quired to mix the soils, as well as suspending the ZVI and facilitat-
2006; Osman and Al-Tabbaa 2007). ing means of improving the uniformity of the post-mixed body.
Within the past decade, ISSM also has been used increasingly to The first two field projects involving the ZVI-clay technology
deliver chemical reagents that degrade contaminants in the subsur- used kaolin as the processed clay (Shoemaker and Landis 2002;
face. Typically, this use has been applied in areas where non- Shackelford et al. 2005). However, all subsequent field projects
aqueous phase liquids (NAPLs) have infiltrated into the subsurface have used bentonite. The current preference for bentonite is due, in
soils creating contaminated bodies of soils commonly referred to as part, to the requirement for less bentonite in terms of both achiev-
“source zones” (National Research Council (NRC) 2005). For sites ing a suitable slurry viscosity and reducing the hydraulic conduc-
where the NAPLs have included chlorinated solvents, such as per- tivity of the mixed zone.
chloroethylene (PCE), trichloroethylene (TCE), and carbon tetra- Concurrent with increased field applications of the ZVI-clay
technology has been a need to conduct basic research and site spe-
Manuscript received January 14, 2010; accepted for publication August 15, cific treatability studies to resolve appropriate mixtures of reactive
2010; published online September 2010. media and clays (e.g., Olson 2005). Of 15 site specific treatability
1
Ph.D. Student, Dept. of Civil and Environmental Engineering, Colorado studies with which the authors have been involved, five have led to
State Univ., 1372 Campus Delivery, Fort Collins, CO 80523-1372.
2 full-scale field applications, six have had favorable results with
Associate Professor, Dept. of Civil and Environmental Engineering, Colo-
rado State Univ., 1372 Campus Delivery, Fort Collins, CO 80523-1372.
field projects pending, and four have resulted in decisions to pursue
3
Professor, Dept. of Civil and Environmental Engineering, Colorado State other alternatives technologies. Primary variables among the stud-
Univ., 1372 Campus Delivery, Fort Collins, CO 80523-1372 (Corresponding ies have included different types of soil (i.e., clays, silts, and sands),
author), e-mail: shackel@engr.colostate.edu different sources and amounts of additive materials (i.e., ZVI, clay,
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and/or other additives), and different types of contaminants and
contaminant concentrations.
Research and treatability studies have involved the use of a spe-
cially developed laboratory mixing apparatus and complementary
procedures for preparing representative test specimens. The labora-
tory mixing apparatus mimics the mixing that occurs during field-
scale ISSM and allows for reproducible mixing while varying a
single parameter, such as the amount of clay or ZVI. In addition to
preparing mixed specimens for treatability studies, the mixing ap-
paratus has been used to evaluate the vertical distributions of physi-
cal properties within the mixed zone (Castelbaum 2007), the de-
crease in hydraulic conductivity resulting from the addition of
bentonite to the mixed zone (Castelbaum 2007; Castelbaum and
Shackelford 2009) and the consolidation properties of the mixed
zone (Sample 2007).
Accordingly, the objective of this paper is to present and de-
scribe the mixing apparatus and related procedures for preparing
test specimens. The presentation includes a detailed description of
the mixing apparatus, specimen preparation and testing proce-
dures, and some examples of results. In addition to studies involv-
ing the ZVI-clay technology, the mixing apparatus and procedures
may be beneficial in the study of other ISSM applications, such as
contaminant containment and stabilization as well as more tradi-
tional geotechnical applications.
Mixing Apparatus and Procedures
Mixing Apparatus
FIG. 1—Schematic cross-section of mixing apparatus (not to scale).
The mixing apparatus was designed to mimic field mixing in the
laboratory. A detailed schematic and photographs of the mixing ap-
paratus are shown in Figs. 1 and 2, respectively. All system compo- The slurry displaced from the cylinder is routed through a hose
nents are mounted on a four-legged platform with wheels for mo- to a swivel connection (Figs. 1 and 2(a)–2(c)), down the HSA
bility. The apparatus has three key operations similar to field-scale (19.0-mm (0.75-in.) o.d., and 14.4-mm (0.567-in.) i.d.), and finally
soil mixing equipment: (1) vertical movement of a hollow-stem to three 6.8-mm (0.27-in.)-diameter injection ports located below
auger (HSA), (2) rotation of a HSA, and (3) injection of the slurry. the three auger mixing blades welded to the shaft at an angle of 30°
Each of these operations is independently powered and controlled. from the horizontal and spaced at 120° about the axis of the shaft
Vertical movement is driven by a 1/3 hp AC electric motor (Day- (Fig. 2(d)). The widths of the blades are 15.5 mm (0.610 in.) adja-
ton 5K537 right angle gear motor, Dayton Motor Co., Dayton, TX) cent to the shaft and 30.9 mm (1.22 in.) at the furthest extent from
that is mounted at the top of the apparatus (Fig. 1). This motor is the shaft. The slurry injection rate is determined based on the
geared to rotate two threaded screws that raise or lower the under- amount of slurry required to be delivered in a desired number of
lying platform (to which the auger and auger rotation system are downward mixing passes.
attached) at a fixed rate of 10.2 cm/min (4.0 in./min). Electrical
micro-switches (not shown) can be adjusted to control the upper Test Columns
and lower limits of travel.
The auger rotation system consists of a 1/12 hp variable speed Two different column assembly systems have been used. The first
DC electrical motor (Dayton 47726A permanent magnet gear system has been used to evaluate the vertical distribution of physi-
motor, Dayton Motor Co., Dayton, TX) and rheostat control (Figs. cal properties of mixed materials and to prepare specimens for
1, 2(a), and 2(b)). The rate of auger rotation (reversible) is adjust- measurement of hydraulic conductivity or consolidation behavior
able up to ⬃21 rpm. via connection to a separate hydraulic control system or consolida-
The slurry injection system also includes a 1/12 hp motor and tion testing apparatus, respectively. The second system has been
rheostat control (as above) attached to the apparatus as shown in used to measure rates of contaminant degradation. The systems
Figs. 1, 2(a), and 2(c). The slurry injection system also includes a consisted of either clear acrylic or clear PVC tubing with an inner
reservoir comprised of a stainless steel cylinder 50 cm (20 in.) in diameter of 10.2 cm (4.0 in.).
length and 10 cm (4 in.) in diameter (Figs. 1, 2(a), and 2(c)), and a Photographs of the first system are presented in Fig. 3. This sys-
piston. The slurry is delivered by piston displacement through the tem consists of interlocking sections of clear acrylic tubing with
cylinder via a screw driven actuator connected to the piston and lengths of 2 cm (0.8 in.), 13.4 cm (5.3 in.), 30.5 cm (12.0 in.), 45.7
motor. Piston displacement (also reversible) can be adjusted up to cm (18.0 in.), or 61.0 cm (24 in.). In all cases, the assembled col-
⬃8.6 cm/ min (3.4 in./min), resulting in a maximum slurry dis- umns include a bottom or base section that is 8.0 cm (3.1 in.) long.
placement rate of 670 cm3 / min 共43 in.3 / min兲. The purpose of the base section of the assembled column is to allow
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