Chapter 5 Field performance of surface layer of multi-layered cover system
5.1 General
5.3.1 Construction of MLCS
The configuration of MLCS considered in this study is as per the recommendations of USEPA (1989) and detailed in Table 5.3.
Table 5.3 Configuration of multi-layered cover system
Sl.
No. Layer Thickness Purpose
1 Surface protection
layer 0.4m To safeguard the remaining layers from direct interaction with atmospheric variants
2 Filter layer ~0.005m To protect the drainage layer from clogging due to piping of surface soil
3 Drainage layer 0.3m To divert the rain water infiltrated from surface soil layer outside cover system
4 Additional
drainage ~0.005m For extra safety, considering the long design life of MLCS
5 Additional Barrier ~0.01m Composite GCL that has multiple protection facilities compacted in thin membrane 6 Conventional
Barrier 0.4m Low permeable clay layer that restricts moisture transfer into deeper sections.
7 Fill soil ~0.1m The foundation layer, that bears the soil layers above it.
The usual recommendation of cover slope angle is 3 to 7° (USEPA 1989), however to understand the worst probable performance a slope of 10° is adopted in this study. The schematic representation of field cover system is depicted in Figure 5.2. Figure also details the position of rainfall simulator, the erosion collection chamber, and the vegetated surface.
Figure 5.2 Schematic representation of field cover system
Figure 5.3 Pictorial image of constructed field setup
The photograph of the constructed setup is shown in Figure 5.3. Facility was provided for collecting eroded soil at the toe portion and collecting seepage water from drainage layers as shown in Figure 5.3. The construction sequence and calibration of rainfall simulator are detailed in forthcoming sections. Mixing of the soil in definite proportions to desired water contents was done as shown in Figure 5.4 (a) and Figure 5.4 (b). Care was taken to avoid clod formation while mixing cohesive soil with water shown in Figure 5.4 (c). The whole
Toe section
Sloping direction
Erosion collection chambers Seepage
collection chamber
Boundary walls Crest
section
mixture was transferred into closed drums and isolated for 7 days as shown in Figure 5.4 (d) for uniform distribution of moisture.
Figure 5.4 (a) Figure 5.4 (b)
Figure 5.4 (c) Figure 5.4 (d)
Figure 5.4 Preparation of materials to be filled in multi-layer cover system
The filling and compaction of barrier soil was done as shown in Figure 5.5 (a). The water content was verified at regular instants during filling. After reaching about half the depth, the sensors for monitoring moisture variations were laid in position exactly at the center of the barrier layer along its length. The placement of sensor was done by taking care of the zone of influence from the boundary walls, bottom surface, and from the adjacent sensor as shown in Figure 5.5 (b). The soil layer immediately above the sensors needs to be gently compacted as shown in Figure 5.5 (c). After completion of barrier layer, a layer of GCl was laid above it for additional protection as shown in Figure 5.5 (d). The profile probe casings were provided at designated location as shown in Figure 5.5 (e). The alignment of profile probe casings was repeatedly verified at every stage of compaction, to avoid any leakage along casing surface and easy insertion of probe through it. Immediately
(e). The GDL layer was overlaid by a thick layer of river sand with the position of sensors in as shown in Figure 5.5 (f). The toe of the drainage layer sand is prefilled with fine gravel and covered with thick nonwoven geotextile layer to avoid any animal intrusion from drainage outlets as shown in Figure 5.5 (g). The completed drainage surface was covered by a nonwoven geotextile filter layer (GFL) as shown in Figure 5.5 (h) to avoid piping from surface protection layer. The surface protection layer was filled and compacted similar to other soil layers. The completed cover surface is as shown in Figure 5.5 (i).
Figure 5.5 (a) Figure 5.5 (b)
Figure 5.5 (c) Figure 5.5 (d)
Figure 5.5 (e) Figure 5.5 (f)
Figure 5.5 (g) Figure 5.5(h)
Figure 5.5 (i)
Figure 5.5 Construction of field cover system 5.3.2 Rainfall simulator
Figure 5.6 Experimental setup for evaluating erosion and infiltration of surface soil in multi-layered cover system
The rainfall simulator as shown in Figure 5.6 was made with four “Full Cone square spray - 1/2HH-40WSQ” nozzles procured from Spraying Systems Co. fitted to a 12mm pipe at a height of 2m from ground surface (Strauss et al. 2000; Francisco et al. 2010). The inlet pressure for rain simulator was controlled by an interferential, single jet, super dry straight reading type, hermetically sealed water meter confirming to ISO-4064. The spatial uniformity is established in-terms of coefficient of uniformity (Cu) defined by Christiansen (1941) as shown below
𝐶𝑢 = (1 −∑ |𝑋𝑁1 𝑖− 𝑋̅|
𝑁𝑋̅ ) 100 5.1
where, Xi is the rainfall at any measurement location, 𝑋̅ is the mean rainfall amount and N is the number of locations chosen for measuring rainfall.
The rainfall characteristics of simulated events are summarized in Table 5.4. From the table it can be seen that simulated rainfall intensity and rainfall drop size increased with the inlet pressure. The uniformity of simulated rainfall was satisfactorily recorded between 80 to 90%. The raindrop size was evaluated using flour pellet method defined by Kincaid et al. (1996). A plate of fly ash was placed below the simulated rainfall for fraction of seconds. The observed raindrop impact was measured and the average value is reported in this study.
Table 5.4 Characteristics of simulated rainfall events Nozzle
type
Orifice diameter
(mm)
Inlet pressure
(kPa)
Flow (L/Min)
Rain drop size (mm)
Rainfall intensity (mm/hr.)
Coefficient of uniformity
(Cu) % Full Cone
square spray - 1/2HH- 40WSQ
6.4 mm (maximum free passage
3.2 mm)
70 kPa 15.2 2.1±0.1 60±5 mm/hr
87.3 90 kPa 17.8 2.4±0.1 80±5
mm/hr
81.4 120 kPa 19.7 2.9±0.2 100±5
mm/hr
89.7 5.3.3 Weather station
A micro weather station (Meter Group inc. 2017) as shown Figure 5.7 was equipped with rain gauge (ECRN 100 high-resolution double-spoon tipping bucket type rain gauge), temperature and relative humidity sensor (VP3 relative humidity and air temperature sensor), wind speed (Davis cup anemometer) and solar radiation (PYR solar radiation) sensors. The weather station was installed in the immediate vicinity of field MLCS. The
weekly average climatic variations recorded during the experimental period are depicted in Figure 5.8.
The average daily temperatures results presented in this study include day and night recordings. From the variation in average daily temperature, it can be observed that the average temperature varied between 28±5°C during the monsoon and pre-monsoon periods, while in winter session it varied between 22±4°C. However, during some days in June-October the maximum temperatures were nearly about 35°C with night temperatures around 25°C. These days contribute to significant evaporation. The days with minimum average temperatures during this period are mostly the days that received rainfall, which result in relatively low evaporation. The temperature variation was relatively low in winter, which contributes to constant rate of evaporation, there by helps in maintaining constant rate of drying in simulated rainfall experiments. The temperatures were low in December and January, where the average temperature was close to 20°C while November and February had close to 24°C.
Figure 5.7 Micro weather station used in this study
The daily average relative humidity (RH) recorded at the test location broadly varied between 80±15% during monsoon and pre-monsoon periods while it varied between 75±10% during winter. The days with high RH corresponding to rainfall days and days
humidity was almost constant around 78%, which gradually decreased until early February and reached about 68%. These constant and gradual variations result in similar climatic conditions during rainfall simulation experiments. The pre-monsoon in March has further shown higher RH close to 85%.
Figure 5.8 (d) summarizes the variation in daily average solar radiation with time.
It was observed that except rainy days the solar radiation followed almost constant rate of decrement from June to December and then increased until March. The November- February session showed almost same amount of solar radiation providing similar climatic conditions during rainfall simulation experiments. Incoming solar radiation on days with rainfall was significantly affected due to presence of clouds.
(a) (b)
(c) (d)
Figure 5.8 Weekly average climatic variations in the study area during the monitoring period