Water content (%) Dry density (kN/m3)
Chapter 5 Hydraulic characteristics of soil-WH composite
5.2 Soil-water retention and desiccation characteristics of soil-WH fiber composite
5.2.4 CIF variation with time and drying cycles
Progressive crack propagation of the four different soil types with time (at 2, 5, 7 and 35 days) is shown in Fig. 5.5. Careful visual inspection of the images suggests that the unreinforced or bare soil (denoted as B) exhibited the greatest intensity of desiccation, whereas coir-reinforced soil (C) was subjected to the least for the same monitoring period. Jute (J) and WH-reinforced soil have intermediate intensity of desiccation. Apparently, the natural fibers, regardless of the types, were in tensile action (i.e., “bridge effect”; Tang et al. 2010, 2016) to reduce crack widening (Fig.
5.5). In general, the CIF increased with time for all soil types for a particular drying cycle.
Fig. 5.5. Progressive crack propagation with time (at 2,5,7 and 35 days) for the selected soils and bridge effect of fibers
Fig. 5.6 depicts the variation of mean CIF with standard error (taking into consideration of all three replicates) of all soil types during the entire 105 days of monitoring. For simplicity the first 5 DCs (35 days) are discussed in detail at first. For the bare soil, the mean CIF increased
almost exponentially during all five cycles, meaning that cracks were always formed as the soil continued drying. This was associated with the development of soil matric suction upon evaporation (Fig. 5.7), which induced tensile stress (disequilibrium) within the soil matrix and initiated crack formation from the weakest point of the soil. During wetting, the mean CIF dropped abruptly implying closure of cracks. Crack closure occurs in low or moderate plastic soils due to soil swelling or possibly because of clogging of cracks by particles eroded from the crack fracture surfaces during permeation (Li et al. 2011). Interestingly, although similar drying and crack formation patterns are found in the subsequent cycles, apparently there is a progressive increase in the mean peak CIF. Based on the weather data obtained during the monitoring period, soil evaporation rate Ep can be estimated by the Penman’s equation (Penman 1948).
Ep
=
∆Rn+ρ(∆+γ)λacp(es−ea)/ra (5.1) where Ep (mm/day) is evaporation rate, ∆ (kPa K−1) is slope of the saturation vapor pressure curve, Rn (MJ m−2 day−1) is net irradiance, γ (≈ 66 Pa K−1) is psychometric constant, 𝜌𝑎 = Air density (kg m-3), 𝑐𝑝 = Specific heat capacity of air (≈ 0.00101 MJ kg-1 K-1), U2 (m/s) is wind speed, es-ea = δe=Vapour pressure deficit (kPa), λ (≈ 2.26 MJ kg−1) is the latent heat of vaporization, Ta is temperature in Kelvin, es (kPa) is saturated vapor pressure of air, P is air pressure (Pa) (≈ 101.3 kPa). These parameters were obtained from the measured meteorological data by the following equations (Allen et al. (1998) and Shuttleworth (2007))
ρa = P
R𝑇𝑎 ; (5.2) Δ = 0.133 ∗ [5336
𝑇𝑎2 ∗ 𝑒(21.07−
5336 𝑇𝑎 )
]; (5.3)
𝛿𝑒 = (1 − RH) ∗ 𝑒𝑠; (5.4) 𝑒𝑠 = 0.133 ∗ [𝑒(21.07−
5336 𝑇𝑎 )
] (5.5)
Fig. 5.6. CIF variation for the selected soils for the monitoring period of 105 days
Fig. 5.7. Suction variation for the selected soils for the initial 35 days of monitoring
The calculated evaporation rate during the 35-day monitoring period is in Fig. 5.6. It is clear that the change of CIF during the five drying cycles did not correspond to that of evaporation rate, especially in the 1st, 4th and 5th cycles. Song et al. (2016) studied the evolution of desiccation cracks of highly expansive clay (CH) soil under controlled environmental conditions in the laboratory. They found that as cracks propagated, the evaporation rate changes in three stages where evaporation rate gradually decrease for a particular drying cycle. The current evaluation in Fig 5.6 implies that CIF change is not only sensitive to the evaporation rate due to the uncontrolled environment adopted in the current work. The observed trend of CIF might be explained from the soil microscopic point of view. Soils subjected to the first drying cycle after compaction should shrink and this process might cause irreversible fabric changes (Yong and Warkentin 1975).
Delage et al. (1995) who tested desiccation of compacted silt also observed this kind of soil microstructural change due to soil moisture changes. Apparently, the increase in peak CIF ceases beyond the third drying cycle for the rest of the monitoring period. The mean maximum CIF and their standard deviation are reported in Table 5.1 for the entire monitoring period. Al Wahab and El-Kedrah (1995) who tested the desiccation behavior of compacted clays of medium plasticity also showed that their dimensionless crack index became constant after three drying cycles.
Fig 5.6 also shows that the CIF of soil-lignocellulose fiber composite is always less (at least by half, except the initial two drying cycles) than that of bare soil. Coir-fiber reinforced soil has the lowest CIF, followed by WH- and Jute-reinforced soils which have similar CIF. This is consistent with the measurements of suction shown in Fig 5.7, where the magnitude of suction induced in all lignocellulose fiber-reinforced soil was always smaller than that in the bare soil.
This is because the presence of hemicellulose biopolymer in the natural fibers enhance water holding capacity of the soil. Apparently, there is no difference of induced suction among the three
types. The fiber inclusion reduces cracking potential of soil via two mechanisms – by mobilizing the tensile strength of the interlocked fiber during drying and by increasing soil-fiber interface friction; which in turn resists the soil tensile stresses induced during drying (Tang et al. 2012, Chaduvula et al. 2017). Coir has the greatest ability to reduce crack formation because it is a multifilament fiber, meaning that more number of fibers could mobilize their tensile strength simultaneously across a crack plane. Although the coir fiber has smaller tensile strength than jute and WH, the coir monofilament fiber has higher lignin content and hence a coarser surface, providing higher soil-fiber interface friction. A study reported by Hathaway and Penny (1975) show that the effects of lignin content on tensile strength of natural fiber become more significant when the fiber moisture content is higher. This might be another mechanism that the coir fiber (which has the highest lignin content among the other two), provides further tensile strength in resisting soil cracking after wetting.