Water content (%) Dry density (kN/m3)
Chapter 7 Bio-degradation and treatment of WH fiber
7.2 Bio-degradation assessment of natural fibers in compacted soil
This section explores the effect of bio-degradation on the compressive strength and microbial activity of soil-fiber composite. Fiber bio-chemical composition change along with embedded time was measured and discussed in tandem with the UCS and microbial activity. This assessment gives an initial understanding of the mechanical performance of natural fibers with
degradation. The experiment was conducted in field wherein compacted soil-fiber composite was buried within the soil and exposed to natural environmental conditions.
Fig.7.1 Flowchart depicting experimental approach for measuring effects of fiber bio-degradation on UCS, microbial activity and fiber bio-chemical composition
7.2.1 Test plan and setup
The soil used and the fiber dimensions for WH, jute and coir were kept same as discussed in section 4.2. Fig.7.1 provides the flowchart depicting experimental steps for measuring effects of fiber bio-degradation on UCS, microbial activity and fiber bio-chemical composition.
Natural fibers (Coir, Jute and WH)
Compacted soil-natural fiber composite at (OMC, MDD)
UCS, bio-chemical composition, and microbial activity
t
0at t
1, t
2, t
3, t
4Buried fibers were used to make UCS samples
(OMC, MDD)
Bio-chemical composition of buried fibers were tested
Microbial activity of soils adjacent to buried fibers within the compacted samples Compacted samples were buried underground
at 5 cm below soil surface for 1 year
t0 correspond to time just after compacted samples were made.
t1, t2, t3 and t4 corresponds to 3, 6,9 and 12 months, respectively.
Cylindrical samples (50 mm X 100 mm) of soil-fiber composite were prepared at compaction condition of OMC and MDD. Thereafter, the UCS of the composite was measured along with the sample bio-activity by Fluorescence Di-acetate (FDA) analysis. The sample preparation procedure for making UCS samples as discussed in section 4.2 was adopted and only the sample compacter mould of bigger dimensions was used. The experimental matrix undertaken is tabulated in Table 7.1. In total, 36 (12 X 3) cylindrical samples were prepared and buried in soil. After the samples were buried and at different time intervals (as shown in Fig.7.1), the fiber biochemical properties were measured by taking 3 fibers from top, bottom and middle of each soil-fiber composite (coir, jute and WH). Thereafter, the soil was tested for bio-activity by FDA analysis (Green et al. 2006, Adam and Duncan 2001) while the rest of the degraded fibers was used to make samples of requisite dimension for UCS testing. The details of FDA analysis are given in Chapter 3. The new UCS samples were compacted at OMC and MDD with the degraded fibers for proper comparison of UCS regardless of field moisture status. The environmental parameters were measured by installing a micro-climate monitoring system near the burial site and the data is shown in Fig. 7.2.
Parameters such as daily average solar radiation, rainfall depth, temperature and relative humidity were measured for the embedded time.
Table 7.1 Experimental matrix for the field bio-degradation test Time
interval
Soil-degraded fiber composite Fiber properties Samples tested UCS FDA analysis Cellulose Hemi-
cellulose
Lignin
1 3 ✓ ✓ ✓ ✓ ✓
90 3 ✓ ✓ ✓ ✓ ✓
180 3 ✓ ✓ ✓ ✓ ✓
270 3 ✓ ✓ ✓ ✓ ✓
360 3 ✓ ✓ ✓ ✓ ✓
Fig. 7.2 Weather data from micro-climate monitoring system
7.2.2 Bio-chemical composition change with bio-degradation
0 10 20 30 40 50 60 70 80
1 11 21 31 41 51 61 71 81 91 101 111 121 131 141 151 161 171 181 191 201 211 221 231 241 251 261 271 281 291 301 311 321 331 341 351
Daily Rainfall Depth (mm)
Precipitation
0 5 10 15 20 25 30 35 40
1 11 21 31 41 51 61 71 81 91 101 111 121 131 141 151 161 171 181 191 201 211 221 231 241 251 261 271 281 291 301 311 321 331 341 351
Daily average Temperature ( C)
Temperature
0 20 40 60 80 100
1 11 21 31 41 51 61 71 81 91 101 111 121 131 141 151 161 171 181 191 201 211 221 231 241 251 261 271 281 291 301 311 321 331 341 351
Relative Humidity (%)
Relative Humidity
0 5 10 15 20 25
1 11 21 31 41 51 61 71 81 91 101 111 121 131 141 151 161 171 181 191 201 211 221 231 241 251 261 271 281 291 301 311 321 331 341 351
Daily average Radiation (MJ/m2)
Solar radiation
Fig.7.3 Cellulose, hemicellulose and lignin variation with bio-degradation time
Fig.7.3 showcases the change in cellulose, hemi-cellulose and lignin content of fiber with buried time. It was seen that hemicellulose decreased the most followed by cellulose and lignin.
The decrease in hemicellulose is due to its highest affinity for moisture absorption which is conducive for micro-organisms to act on it (Rowell and Stout 2007). The insolubility and
0 10 20 30 40 50 60 70
0 100 200 300 400
Cellulose (%)
Time (Days)
Coir WH Jute
0 5 10 15 20 25 30
0 100 200 300 400
Hemicellulose (%)
Time (Days)
Coir WH Jute
0 10 20 30 40 50 60 70
0 100 200 300 400
Lignin (%)
Time (Days)
Coir WH Jute
complexity of the lignin polymer allow it to resist degradation by microorganisms as reported in the literature (Campbell and Sederoff 1996, Methacanon et al. 2010). Therefore, lignin is considered to be important for geotechnical purpose as soil reinforcement, as it majorly indicates resistance to microbial degradation in soils.
Fig. 7.4 UCS variation with time for soil-degraded fiber composite
7.2.3 Effect of bio-degradation on UCS of soil-fiber composite
Fig. 7.4 presents the UCS for soil-natural fiber composites with the degraded fibers. All soil-fiber composite showed a decrease in UCS with respect to initial state (non- degraded). It can be clearly seen that for the one-year period of embedded time, all soil-natural fiber composite exhibit UCS greater than at least 1.68 times for bare soil. Thus, its feasibility for short term reinforcement is not substantially affected by bio-degradation. Among all the soil-fiber composites, jute reinforced soil showcased the highest drop from its initial state (22.72%). This can be explained with respect to moisture absorption capacity of the fiber due to its high hemi-
Bare soil
300 600 900 1200
0 50 100 150 200 250 300 350 400
Unconfined compressive strength (kPa)
Time (Days)
Soil+Coir Soil+WH Soil+Jute
cellulose content facilitating higher microbial growth. The FDA analysis result (Fig. 7.5) of all composites and bare soil with time give further evidence that microbial activity was highest in jute reinforced soil. WH and coir showcased similar drop in UCS from its initial state by 14.75% and 16.08% respectively. It was also seen that inclusion of natural fibers regardless of fiber type greatly increased the microbial activity in comparison to bare soil. This is due to the fact that the fibers provide additional food source to various micro-organisms in the soil.
Fig. 7.5 Bio-activity variation with time in terms of fluorescein for the monitored time