Journal of Engineering Geology

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Introduction
Geofoames are used as a light weight fill material in those places which soil borrows is not cost effective for engineering or economic purposes. In general, geofoames are highly capable of improving some of geotechnical properties of soils such as inflation creation, reduction of density, and etc., due to their light weight, no change of volume against water, low permeability, and relatively proper strength. Using mixture of geofoam beads and soil has been recently taken into consideration by researchers. The mixture causes tangible reduction of soil density and severe drop of active pressure of retaining walls. Also, using the mixture in seismic zones is of special importance. In the paper, effect of mixing geofoam (4 different percent) and three types of poorly graded sandy soils have been dealt with. The research innovation has been compared to previous ones is using poorly graded sandy soil, separating geofoam beads based on their diameter, and reviewing the effect of adding various percentage of geofoam on improvement of poorly graded sandy soil’s properties.
Materials and Test Method
Tests have been performed in direct shear box (10 ^cm x10 ^cm) under three stress levels of 50, 100, and 150kPa. First type of soil has been Firoozkooh sand (#161) with specific gravity of 2.65, as uniformly graded sand (SP). Second type of soil has been mixture of uniformly graded sand and 10% silt (SM-SP); and, third type of soil has been mixture of Firoozkooh sand and 20% silt (SM). The three above types of soils have been named as soil 1, soil 2, and soil 3, respectively.
Geofoam beads have been all fine grained, passing through sieve No. 10; and, their added weighted values have been 0, 0.2, 0.4, and 0.6% of weighted percentage of soil. All of tests have been performed with optimum moisture content of geofoam and soil mixture. Due to diversity of soil types and ratio of geofoam-soil mixtures, soil compaction test has been performed on each direct shear test’s sample to specify optimum moisture content of various types of mixtures; because there have been various types of soils used, and also various ratios of soil and geofoam mixtures.
Results
According to the results, using geofoam beads leads to considerable reduction of soil density. Decrease made in density will be more tangible when higher percentages of geofoam are added to the soil. Also, as far as geofoam absorbs water, optimum level of moisture will be increased through increase of geofoam percentage in soil-geofoam mixture.
Since geofoam beads are less rigid compared to grains of sand, sand and geofoam interlocking and friction level is lower than sand interlocked to sand; and shear strength has been decreased through increase of geofoam percentage in soil. The point to be remembered is that, reduction level of shear strength in soils containing various percentages of geofoam is not so tangible compared to the soil itself. In its worst case, the reduction would be about 12%.
Adding geofoam beads to all of the three types of soil has led to their increase of apparent cohesion. Moreover, through increase of mixture percentages, more increase has been made in apparent cohesion of mixture. The results are indicative of significant effect of mixing geofoam and soil 1 in increase of soil cohesion up to 9 times. The cohesion increase has been about 4 and 2 times for soils type 2 and 3 respectively. So, it could be concluded that the lower the soil cohesion, the higher would be effect on cohesion increase of soil, through increase of geofoam percentage.
In figure 1, chart of internal friction angle is shown based on mixture percentage of geofoam for those types of soils being tested. Considering decrease of internal friction angle through increase of geofoam percentage, the important point is slope drop observed when geofoam percentage added has been 0.4%. Therefore, reduction speed of internal friction angle has become slower, after this level. Considering the figure, internal friction angles of soils type 1, 2, and 3 have shown respectively 15, 16, and 18% of reduction, through highest percentage of geofoam added (0.6%).
Figure 1- Internal friction angle based on geofoam percentage mixed with different soils
Comparing the results from present and previous researches, it could be concluded that adding higher percentages of geofoam results in cohesion increase of sandy soils; however, the increase level is different for various types of soils. The lower the initial cohesion of sandy soils and the more uniform their gradation, the more the effect of adding geofoam on increase of cohesion coefficient of soil. Also, downward trend of internal friction angle for well graded and poorly grades sandy soils is almost similar.
Using the results from present research and considering acceptable level of reduction made in internal friction angle of the soil mixed with geofoam against cohesion increase and reduction of soil density; mixture of geofoam beads and soil could be used in construction of embankments, retaining walls and other earth structures, appropriately.