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Effect of Increased Soil Permeability with Passage of Time on the Spacing of Perforated PVC Drains
Associate Professor, Civil Engineering
Department, and Dean Civil Engineering MCE, |
Abstract
Subsurface drains were installed in a salinity control project area to lower the water table. The spacing of PVC drain pipes depends on soil permeability. It was observed that the discharge from drains increases with lapse of time or in other words soil permeability increases with passage of time. To establish this post-project permeability at number of locations in the project area was determined. It is concluded that Permeability of water logged clays increases with the passage of time
KEYWORDS:PVC drain pipes, permeability, water logged clays
INTRODUCTION
Mardan SCARP project area is located in the northwestern portion of Peshawar valley in Pakistan i.e. River Kabul lies in South, River Swat on the West and Malakand hill on the North sides. The project covers Gross Command Area of 135,000 acres with Culturable Command Area of 123,000 acres irrigated from Lower Swat Canal (LSC) System. Mardan SCARP (Salinity Control and Reclamation Project) area was brought under irrigation in 1885 by establishing Lower Swat Canal System, which includes Main Canal, ten Distributaries and several Minors. Watercourses finally convey irrigation water to the farms. A drainage system was also established. Water table in the area was around 70-80 ft below ground level (and at-least 30-35 ft below ground level in low-lying areas at the time of establishing irrigation system. By 1925 water table came up to 40 ft and in 1940 up to 25 ft below ground level. In 1960 it reached 10-12 ft and finally in 1970 it reached within 2-3 ft of the ground level and even touched the ground surface in rainy season. In late sixties it was noticed that the crop yield has dropped more than 50%. It was not before 1980 that the problem was diagnosed and remedies started. WAPDA used a number of techniques for reducing the level of water table. These techniques included construction of surface drains, installation of tube wells and construction of subsurface tile drains (Qamar,J.S.1992; Tarar, R.N.,1994). In the system of tile drains perforated PVC pipes are placed in a ditch at designed depth. The pipe is shrouded by well-designed granular filter and the trench is backfilled. The technique is fairly effective but requires expensive material and equipment. The pipe drain has to be spaced properly to optimize performance and minimize cost. The pipe spacing is controlled by soil permeability. During design stage of the SCARP Project soil permeability was given due consideration. This important soil parameter was determined at a number of locations through field tests. The spacing of the drains was subsequently determined on the basis of this data and the project was executed. A few years after the project execution field engineers of WAPDA observed that the discharge from the drains increases with time. It was deduced from this observation that the soil permeability increases with time. This increase may be attributed to leaching of salts from soil pores. The behavior of clayey soils depends on their chemical composition and the composition of effluent occupying soil pores (Mitchell, 1976). In water logged area salt concentration increases over time. These salts get deposited in soil pores and act as cementing agent. Further more the chemical interaction between clay particles increases and they come closer together. Both of these actions result in reduced permeability (Mitchell, 1976). When fresh water percolates through the same soil, these chemicals are drained out. It was hypothesized that the permeability of clay has increased over time in SCARP area due to leaching of salts. The aim of study reported here in was to test this hypothesis i.e., to establish that permeability of Mardan SCARP area soil has increased with time due to installation of drains.
METHODOLOGY OF STUDY
The field permeability data for soils of Mardan SCARP project area was collected about 20 years ago for designing the project. In Mardan SCARP the total project area was divided into grids of 3000 ft X 3000 ft and hydraulic conductivity values ‘K’ were determined through more reliable in-situ permeability tests conducted in 4” diameter augered bore holes. Field permeability tests were conducted at the same locations under the study reported here-in to determine the present soil permeability . Undisturbed samples of clay were also collected for determination of soil permeability in the laboratory for comparing field and lab values. Disturbed samples were collected for determination of index properties and soil classification. Subsequently the data was analyzed to compare past and present permeability. The spacing of drains was recomputed to assess the effect of permeability on spacing.
LITERATURE REVIEW
Reclamation of salt affected soils requires sufficient supply of water beyond crop requirement for salt leaching; invariably a well engineered economical drainage system is required to maintain water table at that depth which would create deterrent to the capillary rise of solutes to the root zone (Fairchild, W.D. 1983; Gates and Crismer, 1989). It is also suggested by them that feasibility of such projects to effectively manage saline high water table should be based on the consideration of the problem at regional scale. Problem of disposal of saline drainage effluent is also considered seriously in developed countries like USA where farmers and water developers are feeling a great need to manage increasing volume of drainage water in an economically feasible and environmentally safe way (Hall, Johnson and Miller,1988). In the tile drainage system the perforated pipes are laid at some calculated depth below ground surface. Generally the spacing between pipes is wider if they are placed at a greater depth. The depth of drainage pipes, however, depends on the general topography of the area to be drained out and the level of water in the natural stream that finally receives drained effluent. Tile drains were used for the same purpose during early days of the SCARP project but subsequently PVC pipes replaced them. The design of closed subsurface drainage system involves determination of depth, spacing and size of drains. Pipe spacing is roughly proportional to the permeability of sub-surface soil. A soil with low permeability requires close spacing and vice versa.
Figure 1. Notation
The Hooghoudt or "drain spacing" equation is a well known relationship developed by Hooghoudt in 1940 and the same was used for the Mardan SCARP design. The equation relates drain spacing to drainage rate, soil hydraulic conductivity, water table height, and the depth to an "impermeable layer" (Chandio, Arain, and Soomro, 1990). The equation is used by many states in USA for their drain spacing recommendations. The equation is given as:
L2 = 4K/ qd * (2d * Y + Y2)
where
qd = Drainage coefficient, (ft/day)
Y = Height of water table above drain center, (ft)
d = Depth from drain centers to barrier, (ft)
DWD = Design water table depth, (ft)
L = Spacing between drains calculated by Hooghoudt’s equation, (ft)
K = Hydraulic conductivity (permeability), (ft/day)
field and laboratory testing
Pre-project soil permeability was determined by WAPDA at seven locations by using the standard field permeability procedure. In this procedure bore holes are made in the field which extend up to certain depth below GWT. Initially water is bailed out of the borehole. This creates a flow of water into borehole through its perimeter and bottom. The rise of water table with time is recorded. The data is used to compute permeability (Dunn, Anderson, and Kiefer, 1980; Das, 2002). The same procedure was repeated for the study reported here-in. Additionally two undisturbed samples from each borehole were obtained for subsequent determination of soil permeability in the laboratory. These samples were retrieved using Shelby tubes that were pushed very slowly into the soil to minimize the sample disturbance. Wax was applied over the tubes to conserve moisture. Disturbed samples of soil were also obtained for soil classification. The in-situ spacing of already installed sub surface drains was obtained from as-built drawings of SCARP for subsequent comparison.
Laboratory and Field Test Results
The results of field and laboratory testing are summarized in Table 1. The comparison between old and new values of hydraulic conductivities obtained from the field tests for all the holes is depicted in the bar-chart of Figure 2.
Table 1. Summary of test results
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| Bore hole # |
Pre-project permeability (ft/day) |
Spacing of pipe drains (ft) |
Current field permeability (ft/day) |
New spacing of pipe drains (ft) |
Soil type |
Liquid Limit |
Plastic Limit |
Plasticity Index |
Clay % | Laboratory permeability (ft/day) |
| ||||||||||
| 1 | 8.1 | 440 | 11.01 | 514 | CL | 28 | 18 | 10 | 10 | 0.39 |
| 2 | 2.6 | 230 | 3.8 | 290 | CL | 32 | 16 | 16 | 9 | 0.52 |
| 3 | 5.18 | 330 | 7.76 | 420 | CL | 29 | 15 | 14 | 13 | 0.05 |
| 4 | 7.57 | 410 | 9.51 | 477 | CL | 32 | 14 | 18 | 9 | 0.4 |
| 5 | 5.5 | 350 | 8.9 | 460 | CL | 21 | 9 | 12 | 10 | 0.11 |
| 6 | 2.71 | 260 | 4.32 | 300 | CL | 27 | 18 | 9 | 9 | 0.09 |
| 7 | 2.45 | 220 | 4.18 | 290 | CL | 28 | 15 | 13 | 9 | 0.05 |
| 0 | ||||||||||
| average | 4.9 | 320 | 7.1 | 393 | 28 | 15 | 13 | 10 | 0.23 | |
| ||||||||||
Figure 2. Comparison of old and new K
It can be seen that for some bore holes the new permeability is almost twice the old value and in almost all cases there has been 50% increase in permeability over a period of little over one decade. Since increase in the value of permeability has direct bearing on spacing of drains as per Hooghoudt equation, the new spacings were computed and are presented in Table 2.
Table 2. Comparison of old and new drain spacing
Table 3 presents the increase in drain spacing due to increase in permeability over a period of time. It can be seen that for some of the locations the new computed spacing is more than 30 % the spacing adopted in original design. On the average the new spacing based on increased permeability is 23 % more than the design spacing adopted for Mardan SCARP. It is, therefore, concluded that the drains could have been placed at greater spacing with out adversely affecting their performance. The designers, however, at that time were unaware of increase in permeability with passage of time.
Table 3. Increase in drain spacing
As already stated, samples were also retrieved for laboratory testing. The results of lab permeability testing are reported in Table 1. A comparison of lab and field permeability for the seven locations studied reveals that in all cases field permeability is more than ten times greater than the lab permeability. This can be attributed to the small specimens used for lab testing that are not representative of the overall field strata and due to change in soil structure during sampling. It is speculated that the soil gets compressed as the thin walled Shelby tube is pushed into the soil and as a result its permeability is reduced.
CONCLUSIONS
The following conclusions were drawn from this study.
1. Permeability of water logged clays increases with the passage of time as salts are leached out of the soil.
2. The spacing of the tile drains can be increased at the design stage by 20% in anticipation of increase in anticipation of increase in permeability over time. This will result in substantial Project savings.
3. The permeability values determined in the laboratory are lower than the field values, primarily due to the sample compression during sampling and as such field permeability shall be determined for all important designs.
In addition, following recommendations are made for future studies in this field.
1. Studies should be undertaken to determine the effect of leaching on permeability of water logged clays by conducting long term permeability tests in lab and by determining chemical compositions of leachate.
2. Findings of the present study shall be tested by conducting large scale model tests to further consolidate the results of this study.
REFERENCES
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