Ground Improvement with Waste Plastic

GROUND IMPROVEMENT WITH WASTE PLASTIC

G Venkatappa Rao, Indian Institute of Technology, New Delhi, India

R.K.Dutta,National Institute of Technology, Hamirpur, India

ABSTRACT

The results of conventional drained triaxial compression tests conducted on 100 mm diameter x 200 mm high specimens of sand with two types of waste plastics are presented in this paper. This experimental data is utilised to assess the overall influence of such reinforced material on the bearing capacity improvement of granular trench. It has been concluded that inclusion of waste plastic strips in sand improves the bearing capacity of granular trench.

Keywords: Triaxial test, Waste plastic, Sand, Granular trench, Bearing capacity.

1. INTRODUCTION

The amount of wastes has increased year by year and the disposal becomes a serious problem. Particularly, recycling ratio of the plastic wastes in life and industry is low and many of them have been reclaimed for the reason of unsuitable ones for incineration. It is necessary to utilise the wastes effectively with technical development in each field.

The estimated municipal solid waste production in India upto the year 2000 was of the order of 39 million tons per year. This figure is most likely to touch 56 million tons per year by the year 2010 [4]. The typical percentage of plastic in the municipal solid waste produced in India is 1 %. The best way to handle waste plastic is to utilize it for engineering application after shredding in order to conserve the scarce natural valuable resource like sand.

The paper presents the test results of conventional consolidated drained triaxial on sand reinforced with two types of plastic wastes. The results have been further utilised to assess the influence of such reinforced material on the bearing capacity improvement of granular trench.

1. LITERATURE REVIEW

Many investigators have conducted the studies on fiber-reinforced materials. The results of direct shear tests performed on sand specimens [7] indicated increased shear strength, increased ductility, and reduced post peak strength loss due to the inclusion of discrete fibers. These results were supported by a number of researchers. Investigations were also conducted to determine the behaviour of material properties of fiber-reinforced sands. The failure envelopes for fiber-sand composites were bilinear [7]. The critical confining stress was a function of surface friction properties of the fibers and

soil. The inclusion of discrete fibers increased both the cohesionand angle of internal friction of the specimens[8].Theimprovement of the engineering properties due to the inclusion of discrete fibers was determined to be a function of a variety of parameters including fiber type, fiber length, aspect ratios,, fiber content, orientation, and soil properties. The peak strength reportedly increased with increasing fiber content and length up to a limiting amount of each beyond which no additional benefits were observed [5], [6], [7] and [8].

Cut pieces of HDPE waste milk jugs [1] when mixed with sand have shown that there is an increase in strength, CBR and secant modulus of sand and friction angle increase was as large as 18 degrees. The laboratory study on soils which are mechanically stabilized with short thin plastic strips of different lengths and contents [3] have shown an enhancement of strength and load bearing capacity.

Thus it is evident that not much work has been reported on the sand reinforced with wasteplastic for its application to granular trench problems.

3. EXPERIMENTAL PROGRAMME

3.1 Parameters Varied

To investigate the effects of test parameters on the mechanical behaviour of unreinforced sand and reinforced sand, a total of 56 triaxial compression tests were performed. The test parameters included: four confining pressures (34.5 kPa to 276 kPa), 2 types waste plastic strips with percentage varying from 0.05 % to 0.15 % for Type I and 0.25 % to 2 % for Type II, 2 types of strips and two different lengths of strip.

3.2 Test Materials

3.2.1 Sand

The investigation was carried out on locally available Badarpur sand which is medium grained uniform quarry sand having sub-angular particles of weathered quartzite. It had a specific gravity of 2.66, maximum particle size of 1.20 mm, minimum particle size of 0.07 mm, mean particle diameter (D50) of 0.42 mm, coefficient of uniformity (Cu) of 2.11 and coefficient of curvature (Cc) of 0.96. Minimum and maximum void ratios were 0.56 and 1.12 while the corresponding dry unit weights were 16.70 kN/m3 and 12.30 kN/m3 respectively. The sand was classified as SP-SW.

3.2.2 Waste plastic strips

The reinforcement consisted of two types of plastic waste. For the first one (designated as Type I) used plastic carry bags of LDPE having a mass per unit area of 30 gsm and a thickness of 0.05 mm were chosen. From these, 12 mm wide strips were cut. Further these strips were cut into pieces of 24 mm and 12 mm length. The resulting strips of size 24 mm x 12 mm are designated as Type I A and 12 mm x 12 mm strips are designated as Type I B. The second material studied was used packaging strips made of HDPE (designated as Type II) having a width of 12 mm, and a thickness of 0.45 mm and a mass of 3.8 g /m. These were cut into lengths of 24 mm (designated as Type II A and 12 mm (designated as Type II B ) lengths. Type I strips (with a width of 12 mm) had an ultimate tensile strength of 0.011 kN and the percent elongation at failure was 20. The ultimate tensile strength of Type II (with a width of 12 mm) strip was 0.32 kN and percent elongation at failure was 25. It may be noted that 1 % of Type II A inclusions resulted in 280 strips whereas 0.15 % of Type I A contained 276 strips. This is attributed to difference in their thickness.

3.3 Experimental Procedure

A standard triaxial apparatus was used for testing sand with and without plastic strips. The specimen was of 100 mm diameter and 200 mm high. A standard procedure [2] for preparing and testing samples for saturated cohesionless soil as recommended was adopted. The required percentage of the plastic strips were first uniformly mixed with the sand in dry condition. The sand was then soaked. The sand was then deposited in layers into the rubber membrane inside a split mould former. The samples were compacted in three layers through tamping with a rubber tamper consisting of a circular disk attached to a aluminium rod. Each sand layer was given the required number blows with the rubber tamper to achieve the required density. The density of sand specimen with Type I and Type II strips was maintained at 15.080.18 kN/m3 and 14.880.42 kN/m3 respectively for different samples. Conventional consolidated drained triaxial tests were then conducted at a deformation rate of 1.016 mm/min.

3.4 Results

The summary of the typical triaxial test results on sand with strip Type I and Type II are presented in Tables 1to 5.

Table 1 Values of major principal stress at failure

Type of inclusion / % strip / (1)f at different confining pressures ( kPa)
A / B
34.5 / 69 / 138 / 276 / 34.5 / 69 / 138 / 276
0 / 135.01 / 329.6 / 548.64 / 1171.03 / 135.01 / 329.6 / 548.64 / 1171.03
Type I / 0.05 / 201.52 / 387.51 / 599.92 / 1295.89 / 184.72 / 378.38 / 593.01 / 1285.94
0.10 / 205.39 / 408.04 / 611.9 / 1313.63 / 187.23 / 404.49 / 602.81 / 1300.09
0.15 / 210.46 / 427.35 / 626.27 / 1336.72 / 191.15 / 418.37 / 621.68 / 1313.35
Type II / 0.25 / 166.33 / 389.39 / 614.25 / 1294.49 / 156.22 / 374.98 / 595.55 / 1291.66
0.50 / 171.35 / 409.32 / 640.63 / 1309.68 / 159.95 / 386.15 / 609.4 / 1299.38
1 / 177.45 / 433.7 / 680.74 / 1371.25 / 164.7 / 402.01 / 628.98 / 1324.52
2 / 181.82 / 443.08 / 700.3 / 1455.69 / 170.88 / 416.96 / 647.25 / 1377.66

Table 2 Strength parameters for sand with strip Type I A

Range of 3 (kPa) / Strength parameter / Percentage inclusion
0 / 0.05 / 0.10 / 0.15
<69 / c (kPa) / 0 / 0 / 0 / 0
 (deg.) / 38 / 44.2 / 45.3 / 46.2
69 to 276 / c (kPa) / 0 / 7.7 / 11.5 / 14.8
 (deg.) / 38 / 39.6 / 39.6 / 39.8

Table 3 Strength parameters for sand with strip Type I B

Range of
3 (kPa) / Strength parameter / Percentage inclusion
0 / 0.05 / 0.10 / 0.15
<69 / c (kPa) / 0 / 0 / 0 / 0
 (deg.) / 38 / 43.7 / 45.1 / 45.8
69 to 276 / c (kPa) / 0 / 6.0 / 11.2 / 15.3
 (deg.) / 38 / 39.6 / 39.4 / 39.3

Table 4 Strength parameters for sand with strip Type II A

Range of
3 (kPa) / Strength parameter / Percentage inclusion
0 / 0.25 / 0.50 / 1 / 2
<69 / c (kPa) / 0 / 0 / 0 / 0 / 0
 (deg.) / 38 / 44.4 / 45.5 / 46.7 / 47.1
69 to 276 / c (kPa) / 0 / 10.5 / 16.8 / 19.8 / 13.5
 (deg.) / 38 / 39.4 / 39.2 / 40.1 / 41.8

Table 5 Strength parameters for sand with strip Type II B

Range of
3 (kPa) / Strength parameter / Percentage inclusion
0 / 0.25 / 0.50 / 1 / 2
<69 / c (kPa) / 0 / 0 / 0 / 0 / 0
 (deg.) / 38 / 43.7 / 44.3 / 45.2 / 45.9
69 to 276 / c (kPa) / 0 / 4.9 / 8.4 / 11.4 / 10.4
 (deg.) / 38 / 39.8 / 39.6 / 39.9 / 40.8

1. APPLICATION TO GRANULAR TRENCH

An analysis has been carried out to understand the changes brought out in ultimate bearing capacity of a footing on granular trench (Fig. 1) when the waste plastic strips are introduced into the trench materials following the procedure [9] developed.

Fig. 1 Granular trench with and without waste plastic strips

For this, the weak clay deposit has been assumed to possess cohesion (C2) of 20 kPa. The values of CR of reinforced material for granular trench (C1 is replaced by CR of reinforced material) adopted herein are based on pseudo-cohesion concept suggested [10]. In this study the values of CR have been extracted from the results of triaxial tests conducted on the corresponding material. The footing is placed at a depth (Df) of 1.0 m below ground level and rests directly on granular trench. The granular trench width (A) is so varied as to obtain A/B ratios from 0.4 to 2.0 in steps of 0.4. The unit weight of clay is 15.70 kN/m3.

The typical variations of BCR (ratio of ultimate bearing capacity of clay with sand in the granular trench to the ultimate bearing capacity of clay with sand reinforced with waste plastic strips in the granular trench) with A/B ratio for B= 1.0 m are illustrated in Figs. 2 & 3 for sand with strip Type I and Type II respectively. A study of these figures reveal the following.

1. The values of BCR increase with increase in A/B in a bilinear manner.
2. The values of BCR increase with increase in strip percentage.
3. The values of BCR also increase with the increase in the length of reinforcing strips i.e. they are higher for Type I A and II A strips than Type I B and II B strips. This is as expected.
4. In Figure 3, at 0.50 % of Type II A and 2 % of Type II B, the BCR values are comparable.

Fig. 2 Variation of BCR with A/B ratio for sand with strip Type I.

Fig. 3 Variation of BCR with A/B ratio for sand with strip Type II.

4.1 Comparison

The BCR values computedforsand in the granular trench reinforcedwithstrip Type I and Type IIinthe present study are compared and the results are shown in Fig. 4. A study of Fig. 4 indicate that:

1. With 2 % Type II A inclusions, the BCR values are the maximum followed by 1 % Type II A strip. However the BCR values are comparable when 2 % Type II B, 0.5 % Type II A and 0.15 % Type I A strips were added to the sand.
2. From Fig. 4 it can also be seen that with 0.15 % Type I B strip, the BCR values are more than those of 1.0 % of Type II B strip when added to the sand. Similarly for 0.10 % Type I A and 0.10 % Type I B, the BCR values are more than those of 0.25 % Type II A and 0.50 % Type II B strips when added to sand.
3. Further when 0.05 % Type I A and 0.05 % Type I B strips are added to the sand, the BCR values are more than when 0.25 % of Type II B strips are added to the sand.

Fig. 4 Comparison of BCR with A/B ratio for sand with strip Type I and Type II.

4.2 Effect of Footing Width

Herein, an analysis has been carried out to understand the effect of footing width on the BCR values. For this the values of footing width (B) were taken as 1.0 m, 1.5 m and 2.0 m. The typical variation of BCR with footing width for different A/B ratios for sand in the granular trench reinforced with 0.15 %Type I A and 2 % Type II A strips are presented in Fig. 5. A study of Fig. 5 reveals that there is marginal decrease in the BCR with the increase in footing width. This trend is observed at all A/B ratios. Similar study is conducted for the other cases also.

Fig. 5 Variation of BCR with footing width for different A/B ratios.

4.3 Comparison with Geogrid Micro Mesh Reinforced Sand

The BCR values computedforsand reinforcedwith0.15 % Type I A and 2 % Type II Astripsinthe present study are compared with the BCR values reported [11] on stone dust reinforced with 0.72 % geogrid micro mesh (GMM) for B = 1.0 m for different A/B ratios. The sizes of GMM used by them were 30 mm * 30 mm and 50 mm * 50 mm. The typical variation of BCR with A/B ratio is shown in Fig. 6. A study of this figure indicate that:

1. The BCR values are comparable when 2 % Type II A strips and 0.72 % GMM were added to the sand.
2. The BCR values are sufficiently low when 0.15 % Type I A strips were added to sand.

Fig. 6 Variation of BCR with A/B ratio for 0.72 % GMM, 0.15 % Type I A and 2 %Type II A strips.

5. CONCLUSIONS

On the basis of the results and analysis presented in this paper, it can be concluded that sand-waste plastic mixtures improve the bearing capacity of granular trench and consequently the bearing capacity ratios for all cases. How ever it may be noted that the analysis carried out herein is only indicative of the possible improvements as the actual improvement depends on the choice of correct reinforced soil parameters and the dimensions and depth of foundation/trench.

NOTATIONS

’ Angle of shearing resistance

c’ Cohesion

1f Major principal stress at failure

3 Minor principal stress

BCR Bearing capacity ratio

CRPseudo-cohesion of reinforced sand

C2Cohesion of clay

A Width of granular trench

B Width of the footing

Df Depth of foundation below ground level.

GMM Geogrid micro mesh

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