Properties of soil–bentonite–cement bypass mixture for cutoff walls
Introduction
Cement bypass dust (CBPD), also known as cement kiln dust (CKD), is a fine cementitious powder that is produced as a byproduct during cement manufacturing. CBPD is mainly composed of oxidized, anhydrous, micron-sized particles, and is considered a major health hazard where the disposal of this fine dust poses an environmental threat. Many researches are being carried out, in various parts of the world, to find economical and efficient ways for recycling of CBPD in various applications; like soil stabilization, pavements, landfills and concrete mixes. Rahman et al. [12] presented a research review of CBPD usage in soil stabilization and concluded that CBPD is potentially useful in stabilizing sandy and clayey soils. Ata et al. [3] presented a pilot study of utilizing two industrial solid wastes; excess foundry sand and CBPD to produce a low strength excavatable flowable fill suitable for geotechnical applications. The compressive strength of the flowable fill mixtures ranged between 0.60 and 1.33 MPa, and the ratio of the compressive to tensile strength was 7.5 [3]. Wayne and Donald [15] discussed the basic characteristics of CBPD and concluded that CBPD can be solely used in wide variety of applications, but is often more effective when used in combination with other cementitious materials including ordinary Portland cement, fly ash and slag. Al-Aghbari et al. [1] presented experimental results for using Portland cement and CBPD to stabilize desert dune sands for possible use as a foundation bearing soil, and showed substantial improvements in the maximum dry density, unconfined compression and shear strength parameters. Mahrous and Yang [14] utilized the CBPD in manufacturing cement bricks for industrial construction by replacing ordinary Portland cement with different percentages ranging from 5% to 40%. The dimensions of the bricks were 250 mm × 120 mm × 60 mm and satisfied the density and compressive strength requirements of the Egyptian Code [10], and the cost was far less than that standard cement bricks of the same size.
In this study, different mix proportions for slurry wall backfill containing CBPD are studied to provide an economical water cutoff wall with properties and parameters suitable for most geotechnical applications. Cutoff walls or vertical barriers are widely implemented to restrict the underground movement of water, liquid wastes and polluted groundwater. The vertical barriers may be only one part of the remedial process and may be combined with other containment blockage systems such as liners; Ryan [21]. The EPA [19] has defined slurry cutoff walls as a vertical trench excavated along the perimeter of a site, using bentonite slurry for support, and is then backfilled with a mixture of low-permeability (1 × 10−8 m/s or less) material. The EPA [19] grouped vertical cut-off walls into five categories namely; slurry trench barriers, grouted barriers, deep-soil-mixed barriers, sheet-pile walls, and treatment walls. In general, vertical barriers can reach depths of 60 m and may vary in thickness between 0.6 and 1.20 m, Koch [23]. Ryan [22] pointed out that the primary design parameters of slurry wall are permeability, strength, compressibility, and durability.
This paper presents a study of recycling the industrial and hazardous byproduct of CBPD, to produce mixtures of low-permeability material to be used in backfilling slurry cutoff walls. Numerous trials are tested and assessed by mixing native soil with variable percentages of bentonite and CBPD. The study encompasses investigating the effect of the following aspects on strength and permeability of the soil–bentonite–cement bypass mix (SBCB):
- 1.
Effect of fines content in the native soil (material passing sieve No. 200 or finer than 75 μm).
- 2.
Effect of bentonite content.
- 3.
Effect of CBPD content.
- 4.
Effect of curing time.
Section snippets
Soil
Soil mixtures are prepared in the laboratory by mixing native clean siliceous sand with variable amounts of fines (silt and clay particles finer than 75 μm). The physical and mechanical properties of the soil mixtures are determined according to the Egyptian Code specification [11] and are presented in Table 1. The sand is classified as poorly graded (SP) according to the unified soil classification system (USCS), while the fines are classified as silt of high plasticity (MH). Direct shear tests
Laboratory testing program
In barrier walls, the permeability is normally specified to be a maximum of 1 × 10−8 m/s, Ryan and Day [13]. The addition of cement to the backfill blend of soil–cement–bentonite (SCB) walls allows the backfill to set and form a more rigid system that may support greater overlying surcharge loads, if exists. In order to provide a more sustainable and low cost cutoff wall, ordinary Portland cement is substituted in this laboratory investigation by CBPD, to produce SBCB. At first, preliminary
Effect of silt content
Fig. 7, Fig. 8 present the results of the UCS of various groups and mixtures for curing times of 14 and 28 days, respectively. The results indicate that for Group A, the UCS decreases with the increase of the silt content (passing #200). Initially, increasing the silt content results in a noticeable reduction in the UCS until the silt content reaches 15%, and then the reduction in the UCS diminishes. At 28 days, the maximum UCS is 450 kN/m2, corresponding to zero silt content, while the minimum
Conclusions
An experimental testing program is performed to use cement bypass dust, along with soil and bentonite to produce soil–bentonite–cement-bypass (SBCB) low permeability mixtures suitable for seepage cutoff backfilled walls. The study confirmed that cement bypass dust can effectively replace ordinary Portland cement to achieve optimal performance regarding low permeability and sufficient strength. The following are specific conclusions:
- 1.
The compressive strength of the SBCB mixtures ranged from 0.28
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