Stabilizing soft clay using geo-foam beads and cement bypass dust

clay

Introduction

Expanded polystyrene (EPS) geo-foam blocks are used as a lightweight fill in a wide range of geotechnical applications. The primary function of geo-foam is to provide a lightweight fill below highways, bridge approaches, embankments or parking lots (Baghdadi, Fartani, & Sabban, 1995). EPS geo-foam minimizes settlement in underground utilities. Geo-foam is also used in much broader applications, mainly as lightweight fill, green roof fill, compressible inclusions, thermal insulation, and (when appropriately formed) drainage. EPS geo-foam has been used as a geotechnical material since the 1960s. EPS geo-foam is approximately 1% of the weight of soil and less than 10% of the weight of other lightweight fill alternatives. As a lightweight fill, EPS geo-foam reduces the loads imposed on adjacent and underlying soils and structures (ACI Committee, 1999).

The intended use of EPS geo-foam is not as a general soil fill replacement material but to solve some engineering challenges. The use of EPS typically translates into benefits for construction schedules and lowers overall construction costs because EPS is easy to handle during construction, often without the need for special equipment and is unaffected by weather conditions (Howard & Hitch, 1997). EPS geo-foam can be used to replace compressible soils or in place of heavy fill materials to prevent unacceptable loading on underlying soils and adjacent structures. EPS geo-foam has a high compressive resistance and can thus adequately support traffic loadings associated with secondary and interstate highways (Shaheen, 1993). Additionally, using EPS geo-foam eliminates the need for compaction and fill testing, reduces construction time and minimizes the impact on existing roadway and adjacent structures and/or buried utilities (El-Didamony, Abo El-Enein, Ali, & Sokkary, 1997).

Industrial solid waste pollution is a serious environmental problem. The rate of waste generation is increasing daily with industrial growth. Environmental studies have shown that rising population growth is increasing the demand for industries, which is one of the main reasons for the high rate of solid waste generation, especially in countries with dry climates. This industrial waste is disposed of in company landfills in the desert, causing the wastes to become airborne and thereby endangering the surrounding urban areas. Cement dust powder is a very fine industrial waste at approximately 90 μm in size with a 2500 g/cm2 surface area and is thus becoming an increasing load on the environment. The negative effects of cement dust powder on the progress of cement manufacture, as well as the chemical and morphological modification of the clinker phases and the corresponding effect on the physical and mechanical properties of the systems constructed from these cements, have propelled efforts to determine the most suitable means for reusing and recycling ordinary Portland cement bypass dust. Previously, cement bypass dust production was not regulated, and filters were not used, such that cement bypass dust was suspended in air from chimneys at a rate of 9000 Mg/m3, resulting in air pollution. The Ministry of the Environment created an environmental protection program requiring the use of filters to cap the rate of suspension of cement bypass dust in air to 500 Mg/m3.

In this research, soft clay soil was studied because it is found in many places in Egypt, especially in canal towns. This soil is characterized by low strength and high compressibility. The effect of different environments on the sedimentary process results in significant variations in both the physical and engineering properties of clays. Furthermore, these soils exhibit high compressibility, reduced strength, low permeability and compactness and are consequently low quality soils for construction purposes (Nordin, 2010). Several studies have been conducted to investigate the use of cement dust as a stabilizer for weak soft clay soils. Bypass cement dust can be used in civil engineering as a structural/base/subbase fill, a flowable fill and in the production of the hot mix asphalt as an alternative to traditional fillers. Thus, cement bypass dust accumulates below cement flues. The disposal of these wastes has become a severe problem because all of the available landfills that have been dedicated to this purpose are almost full. In addition, most of the people that have been working with these waste materials are suffering from silicosis, a well-known industrial disease, because of long periods of exposure to calcium carbonate and silica particles (El-Gammal et al., 2011). The quantities of toxic materials in these wastes can also increase the alkalinity of water and affect soil fertility (Hamza et al., 2011).

Research objectives

(1)The main goal of this study was to improve soft clay behavior by mixing the clay with other materials, such as CBPD and foam beads, to produce a self-compacting (up to 95%), self-leveling and flowable fill.

(2)The weight of weak clay soil was reduced and used in light installations instead of heavier replacement soils.

(3)The geotechnical components of the mixture were determined and the mixture was used as replacement soil. The soil can be mixed in the required proportions and poured down foundations or behind retaining walls to reduce lateral pressure.

Our research concept can be implemented by deep mixing of the replacement soil 2.0–3.0 m below the foundation of the building under consideration or by incorporating the replacement soil in the form of columns covering part of the total area below the building.

Experimental program

The experimental study was carried out on two groups of mixtures (A&B). Different ratios of geo-foam beads to CBPD were added to these mixtures to study the effects of the additives on the geotechnical properties of the soil.

Material characteristics

The samples were slurried for 28 days. The samples were dried in the oven to determine their water content. The soft clay characteristics are listed in Table 1 (see Fig. 1).

Table 1. Properties of tested soft clay soil.
Fig. 1. Size of geo-foam beads.

The unit weight of the geo-foam beads was 15.0 kg/m3, and the geo-foam beads were approximately 5.0 mm in diameter.

The particle size distribution was assessed by conventional grain size analysis, and the results are shown in Fig. 2.

Fig. 2. Particle size distribution of clay particles.

Mixture proportions

Two groups of mixtures were studied experimentally, where each mixture had a volume of 600 cm3. Group A consisted of five subsamples without geo-foam that contained increasing percentages of CBPD (50 g each) and different water percentages. Group B consisted of five subsamples that were mixed with increasing (i.e., 5-g increments) percentages of geo-foam and different water percentages with the same weight of CBPD (see Table 2, Table 3, Table 4, Table 5).

Table 2. Tested sample mixtures of group A.
Table 3. Tested sample mixtures of group A (percentages).
Table 4. Tested sample mixtures of group B.
Table 5. Tested sample mixtures of group B (percentages).

Experimental study and results

Flow consistency (ASTM D6103)

Samples were prepared for groups A and B containing different percentages of water, as shown in Fig. 3. The flow consistency of each sample was measured. The flow consistency for group B was slightly higher than that of group A (see Table 6, Table 7). Figure 4 shows the percentage of water that was used in the B samples and the clear effect of the presence of the geo-foam beads on the flow consistency compared to that of using only bypass cement dust in soil.

Fig. 3. Sample mixtures.
Table 6. Measured flow consistency of group A.
Table 7. Measured flow consistency of group B.
Fig. 4. Flow consistency for group (B).

Unconfined compressive strength (ASTM D4832)

The studied mixtures for each group were molded and hardened. The unconfined compressive strength was measured by the unconfined compression strength (UCS) test for the studied mixtures, as shown in Fig. 5.

Fig. 5. Typical shear failure of mixtures.

Figure 6 shows that increasing the amount of cement bypass dust in a sample resulted in a significant increase of the unconfined compressive strength, especially for samples A4 and A5, for only a slight increase in the strain values. The unconfined compressive strength was equal to half of the peak stress that was obtained during the UCS test. Thus, the compressive strength values increased with the mixing rates in cement bypass dust by approximately 50%, as shown in Fig. 7. The inclusion of cement bypass dust in the mixtures had a significant effect on the compressive strength of the studied samples.

Fig. 6. Typical stress–strain curve for group A samples.
Fig. 7. Effect of cement bypass dust on compressive strength.

Shear strength (ASTM D3080)

Shear box tests were carried out on the studied samples. The samples were loaded with increasing stresses (from 50 kPa to 100 kPa to 150 kPa). The confining vertical stress of the samples was kept constant at its preshearing value, and the shear stresses were calculated from the horizontal displacement. The shear strength parameters that were obtained from the direct shear test showed that the CBPD affected the cohesion of the group A samples (see Fig. 8). In contrast, the angle of internal friction increased significantly with the percentage of geo-foam beads for the group B samples, as shown in Fig. 9.

Fig. 8. Shear strength parameters for sample A4.
Fig. 9. Shear strength parameters for sample B4.

Conclusions

In this paper, we have presented an experimental study of various samples of soft clay that were mixed with different percentages of geo-foam beads and cement bypass dust. The following conclusions may be drawn:
(1)The test results on the studied materials showed that cement bypass dust can be successfully used to produce self-compacting, self-leveling, excavatable and flowable fill material.

(2)The dry unit weight of the studied mixtures for the group without geo-foam ranged from 1.40 g/cm3 to 1.6 g/cm3 for CBPD percentages between 3.88% and 18.63%.

(3)The dry unit weight of the studied mixtures for the group with geo-foam ranged from 0.65 g/cm3 to 1.20 g/cm3 for geo-foam percentages between 0.32% and 1.35%.

(4)The unconfined compressive strength of the studied mixtures without geo-foam ranged from 271.8 kPa to 1405.14 kPa for CBPD percentages between 3.88% and 18.63%.

(5)The unconfined compressive strength of the studied mixtures containing geo-foam ranged from 230 kPa to 120 kPa for geo-foam percentages between 0.32% and 1.35%.

(6)The cohesion values for the group without geo-foam ranged from 62 kPa to 105 kPa for CBPD percentages between 3.88% and 18.63%.

(7)The cohesion values for the group with geo-foam with ranged from 50 kPa to 20 kPa for geo-foam percentages between 0.32% and 1.35%.

(8)The friction angle of the group without geo-foam ranged from 30° to 11° for CBPD percentages between 3.88% and 18.63%.

(9)The friction angle of the group with geo-foam ranged from 10° to 22° at CBPD percentages between 0.32% and 1.35%.

Source: Stabilizing soft clay using geo-foam beads and cement bypass dust

Authors: Essam Farouk Badrawi, Mahmoud Samir El-kady

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