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The development of gravel mole drainage
J. ugvic. Engng Res. (1985) 32, 143-151
The Development
of Gravel Mole Drainage J. MULQUEEN*
A form of gravel mole drainage, in which clean gravel aggregate of 1&20mm, viz. passing 20 mm and retained on 10 mm square mesh, is filled into a mole channel, was developed as a solu-
tion to the problem of failure of mole drains because of unstable channels. The development and testing programme for the system is described. The results of field trials show that gravel mole drains perform equally well as conventional mole drains where the latter are stable and they continued to perform well in soils where mole drainage failed because the mole channel became filled with soil. The oldest drains are still working after 17 years and the system is in commercial use. 1.
Introduction
Gravel mole drains are essentially mole drains filled with gravel. This gravel is used to forestall structural failure and closure of the mole cavity in unstable soils. Mole drainage has been recommended for heavy soils of low permeability which require close drain spacing.“” It is recognized that mole drains have a longer life in soils with high clay and low sand and ironstone grit’ content. But it is also recognized that clay content alone is not a satisfactory guide to whether or not mole drains will have a long life. =g3More recently, Rycroft and Thorburn assessed the water stability of clay aggregates as a method of predicting suitability of soil for moling. At the present time however, no entirely satisfactory method exists for predicting the stability of mole drain channels and mole drainage by its nature is only a temporary measure and must be repeated.5 There are many soils which require the close drain spacing and soil loosening which mole drains permit, but in which the mole drains themselves are unstable. These soils are widely distributed in Ireland where they are mainly glacial tills derived from Old Red Sandstone, some Carboniferous limestones and Namurian shales.6 The drain spacing and minimum hydraulic conductivity requirements on these soils can be calculated from a nomogram supplied by Toksjz and Kirkham’ based on potential flow theory. Assuming that the soil below the plane of the mole is practically impervious, results show that drain spacings of l-l.2 m are required when the hydraulic conductivity of the soil slab above the mole drain invert is about 0.5 m/day. Where the hydraulic conductivity of the soil slab above the mole is less than 0.5 m/day, it must be brought to this level by loosening. This can be achieved by drawing the mole plough at spacings of l-1.2 m as above in many agricultural soils or by loosening with rippers at closer spacings in hard and compacted soils.8 Mole channels must remain open over their length to discharge the daily inflow of water which may amount to 500-1000 mm rainfall annually.“ The mechanism of mole drain failure is incompletely understood, but structural collapse has been observed in sandy and stony soils and a gradual reduction in the bore of the mole, leading to its closure, observed in some silty and clayey soils, is thought to be due to creep flow. Some mole drains on slopes have also been found blocked near their junction with collector drains. These blockages have been caused by deposition of soil eroded from upstream sections. Attempts have been made to stabilize the mole wall with a plastic liner, g,1o but costs so far limit installations at the spacings required except on some deep well structured soils where drain spacings of 3 m or greater suffice. This paper describes the development of an alternative solution to the problem of breakdown of mole drains, by feeding in permeable gravel to support the wall. At the same time this gravel permits water to flow through at the required velocity. While the system has been developed ‘Agncultural Recwed
Institute.
20 February
Ballinrobe, 1984; accepted
Co. Mayo,
Ireland
in revised form 24 September
1984
143 0021-8634/85/060143+09%03.00/0
@ 1985 The British Society
for Research
in Agncultural
Engineenng
144
THE
DEVELOPMENT
OF GRAVEL
MOLE
DRAINAGE
between 1965 and 1970 and is in commercial use,’ ‘I*’ 2 it has not been technically described so far although some aspects have been analysed.13*14 At about the same time, a system of gravel drains for peat was developed by Burke and McCormack.’ 5 2.
Development
of the system
The first trials were carried out using a hopper and chute suspended on the rear frame of a Ransomes C59 mole plough. Several chutes to accommodate varying sizes of gravel from coarse sand (2 mm) to gravel (30 mm) were constructed. These were interchangeable and could be slid along rails welded to the body and locked in position. The height of aggregate fill in the drain was controllable by a sliding door on the rear of the chute and, in the limit, the height could be brought up to ground level. The shank of the mole plough was made 6 mm wider than each chute by welding steel plates on the section above the mole in contact with the soil. The hopper was sized to take 0.5 t of gravel and it was fed both manually and by front end loader. The shank was surface hardened to minimize wear in very abrasive siliceous soils. All drains were drawn at a uniform depth of 450 mm and were filled with various gravel aggregates to heights varying from 50 mm to ground level. Drains at different spacings were also observed. Where drains were filled to ground level, it was found difficult, in the brittle topsoil, to make the drain a uniform rectangular cross section filled with gravel aggregate uncontaminated with soil. Long drains filled to ground level with gravel and spaced at 5 and 10 m on steep slopes tended to have the gravel scoured out by high flow rates caused by surface run-off in heavy storms. Moreover, drains with varying heights of sand were frequently found with back pressure indicating that the flow rate through sand was too slow to evacuate quickly the water taken up from the soil. These findings along with the satisfactory performance of partially filled drains and the requirements for close spacings and soil loosening calculated from theory’ led to the development essentially of a mole drain filled with gravel. 3.
Sizing of the drain in relation to the type of gravel
The flow of water through a full flowing gravel drain can be expressed as: Q=kl
Ai,
..
where Q is the quantity discharged per unit time, k, is flow rate per unit gradient through the gravel aggregate, A is the cross sectional area of the drain, i is the hydraulic gradient assumed equal to the slope of the drain. This equation assumes that the discharge (Q) is uniform along the length of the gravel mole channel. In practice the discharge in a gravel drain increases from zero at the upstream end to Q at the downstream end where it discharges into a collector drain. Because the flow is nonuniform, a discharge of 1.75 Q could also be considered appropriate for equal hydraulic gradients in accordance with drain pipe design practice. In this paper discharges are calculated on the basis of uniform flow. The flow rate is designated by k, since it is known it is not a constant as in the Darcy equation.16 Reynolds numbers calculated for 20 mm and 10 mm gravel at gradients of 0.02% and 25%, were 41 and 8 17 for 20 mm gravel and 9 and 243 for 10 mm gravel respectively. These data were calculated on the assumption of 20 and 10 mm tubes and they suggest that the flow is likely to be turbulent. ‘6 The discharge requirements of the gravel are: Q=slR,
..
where s is the drain spacing, Eis the drain length, R is the design rainfall rate. Equating (1) and (2) gives k,Ai=slR.
.(3)
145 -
5
c
2
0.1
I
I
I
I
I
I
I
I
0.2
0.5
I
2
5
IO
20
50
Gradlent,
100
%
Fig, 1. Hydraulic conductivity of two single sized gravels in relation to hydraulic gradient;‘h,‘7 O-O, 10 mm gravel
A-A,
20 mm gravel;
For gravel 10-20 cross
a given field slope (i), drain spacing (s) and climate (R), the problem is to select a suitable aggregate, drain cross sectional area and length. After flow tests in a-permeameter, mm washed gravel or broken stone aggregate was selected as this could give an economical sectional area and meet the flow requirements (Fig. 1). For the following values: k, = 50 000 m/day i= 0.50, A = 0.0056 m*, s = 1a2m and R = 0.025 m, the maximum permissible length of gravel drain is 466 m without back pressure. In practice, much shorter lengths of drain (l&100 m) are drawn because of changes in slope and the occurrence in the soil of stones and boulders which change the cross sectional area of the drain. The width of the drain is fixed at 75 mm and the height of gravel fill can be varied by a door on the rear of the chute. 4.
The gravel plough machine
After the form of the drains and drainage had evolved, further tests were carried out to improve the design of the plough. These tests showed that the orifice of the simple chute could easily be blocked with soil (Fig. 2). Because of its very small contact area with the ground a little topsoil entering the chute could block it. This was overcome by lengthening the orifice of the chute in contact with the soil to 0.3 m. To prevent soil entering the chute as the plough entered the ground it was found that the gravel must begin to flow at the orifice as the mole touched the
146
THE
DEVELOPMENT
OF GRAVEL
I
MOLE
DRAINAGE
\
Fig. 2. Orl$ce ofprototype gravelplough; the orifice (a) is regulated by the diagonal sliding door (b) on chute (c); this chute is mounted behind shank (d) which carries mole (e)
ground. A hydraulically operated door was necessary to start and stop the flow of gravel. It was also found that output could be greatly enhanced if the machine could be hydraulically taken out of the ground and also if the machine was mounted on a three-point linkage where the fields were short. Accordingly, these improvements were incorporated in the design of a commercial gravel plough (Fig. 3). In this plough a frame (a) comprised of two hollow steel sections 150 mm apart is welded to a tool bar (b) suitable for mounting on the three-point linkage of a tractor. This frame carries a shaft (c) to which are welded two depth control wheels (d). To the rear of the wheel shaft the frame carries a rolling disc coulter (e) which is loaded by a coil spring (f). At the rear of the frame is mounted the mole shank (g) which carries a 75 mm mole (h). The shank carries a 1 t capacity hopper (i) mounted so that it can swing sidewards and facilitate movement around boulders. This hopper carries a hydraulically operated shutter (j) to start and stop the flow of gravel and a chute (k) equipped with a door (1) to regulate height of gravel in the mole. The hopper is loaded with gravel by a belt conveyor from a gravel cart. 5.
Field testing of gravel mole drainage
Field tests were carried out on grassland on farmers’ fields at 16 sites to test the performance of gravel mole drainage compared with mole drainage under a variety of soil and hydrological conditions. Plots varied from 0.5-1.0 ha in size and sites were located on soils where mole drains were known to have become blocked from structural failure. These soils were derived mainly from decalcified limestone with much chert as described by Walshls and from Silurian and Devonian sandstones and shales and were spread over three counties. These drains were excavated to observe if soil or roots of grasses could clog the permeable gravel and therefore obstruct the flow of water. Notes were also made on the state of wetness and suitability for traffic of drained and undrained ground. No significant movement of soil fines into the gravel or root growth in the gravel was noted over a 334 year period and land drained by gravel mole drains was dry while adjoining ground was wet. No erosion of the invert of the channel was noted on steep slopes where mole erosion was known to occur. In 1972 arrangements were made by the Northern Ireland Agricultural Trust, the Northern Ireland Department of Agriculture, Mr J. P. Prunty (a land drainage contractor) and the Irish
J. MU1 *QUEEN
147
148
THE DEVELOPMENT
OF GRAVEL
MOLE
DRAINAGE
Agricultural Institute to test the acceptability of the system to farmers and to carry out an experimental evaluation of the system at the Department’s Castle Archdale Experimental Husbandry Farm (County Fermanagh). The soils on this farm are derived from Old Red Sandstone, have 3&40% of fine sand and a hydraulic conductivity in the range 1.10m21.10m4 m/day. ‘I’While hydraulic conductivity measurements showed that this soil needed mole drainage as it needs the soil loosening and close spacing which mole drainage permits, textural analysis suggested that the high fine sand content might lead to early closure of the mole channels. In these experiments gravel mole drainage at 450 mm depth and 1.5 m spacings were compared with mole drainage at similar depth and spacing, pipe drainage at 6.3 m centres and a control. Measurements included drain flow rate, water table levels, soil surface ratings for animal and machine traffic and grass yields. ‘I’ In the early years, both gravel mole and mole drains gave similar results for drainflow, water table control and ground surface ratings. Gravel mole drains were as good as moles where the channel of the latter remained open and were superior to mole drains where these tended to break down.lg For example, from 18 September 1979 to 19 November 1979, with a rainfall of 241.9 mm, where the moles were stable, gravel mole, mole and pipe drains removed 84, 83 and 54% of the rainfall respectively. This experiment showed that gravel moles were as good as mole drains when judged by the three parameters of drainflow, water table levels and soil surface ratings for trafficability in a site where mole drains remained open and in good condition. Both gravel mole and mole drain treatments were much better than the pipe drain and the no drain treatments which were impassable to machinery in wet weather. Although the treatments were not statistically replicated, previous uniformity of measurements on similar sites had suggested that a difference of 56% in discharge between gravel mole as compared with pipe drains was unlikely as the difference in discharge between plots was only 6% for a run-off of 119 mm over one month.8 Analyses8 also showed that the higher the discharge the less the difference for similar antecedent moisture conditions with differences as low as 1.3% for one month with 115 mm run-off when two rain storms contributed to most of the run-off. The effect of gravel mole and mole drains on the water table in the early years in a site where drains tended to be unstable is shown in Table 1. This table shows that (in the early years) both gravel mole and mole drains consistently maintained the average water table below 0.3 m, while the control no drainage treatment had an average high water table and was frequently waterlogged in wet weather. As in the first experiment, there appeared to be little difference between gravel mole and mole drains and both were superior to the no drainage treatment. In later years, the mole drains deteriorated and had to be redrawn while the gravel mole drains continued to TABLE 1
Mean water table levels, 1 cm below soil surface, 19761980 on a site drained 1975 at Castle Arcbdalels Month January February March April August September October November December Mean
Control
Gravelmole
Mole
13 19 16 28 38 22 16 14 11 20
34 37 36 43 49 36 36 33 31 37
30 35 32 41 48 34 34 30 37 35
149
J. MULQUEEN
provide good drainage. In these experiments the gravel mole drains have continued to provide good drainage for almost 10 years up to the present time. These experiments, along with the results of tests on farmers’ fields, indicate that gravel mole drainage is an effective drainage system on soils requiring close drain spacing and soil loosening, where mole drains have a short life. Additional testing has shown that gravel mole drains can provide an effective drainage system for sportsfields. In making sportsfields, cut and fill earthworks often result in overcompaction of the soil. This overcompacted soil must be loosened by heavy duty rippers where it is very hard as the gravel plough may not enter it. The field can then be drained by gravel moles at l-1.2 m spacings. If the field is subsequently compacted in the surface layers by play or machinery it can be loosened by subsoiler or ploughs to raise the hydraulic conductivity to at least 0.5 m/day. This additional testing also revealed two sites with soft silty soils where neither gravel mole nor mole drains provided satisfactory drainage. These soils are derived from a small isolated outcrop of Clare shales which comprise the lowest beds of the Namurian shale formation. Field tests showed that loosening did not raise the hydraulic conductivity of the soft soil slab overlying the gravel mole sufficiently. Hydraulic tests on the gravel moles themselves showed that these drains could discharge the design quantities of drainage water if it could enter them sufficiently fast. Conventional mole drains were found filled with soft soil in a slurried condition apparently as a result of creep flow.
6.
Materials and costs
Gravel mole drainage is much more costly than conventional mole drainage. Assuming a 75 mm diameter mole, 1 m3 of gravel will fill 226 m of drain. If the drains are spaced at 1.2 m there are 8333 m drain/ha requiring 36.9 m3 of gravel/ha. Some provision must also be made for the loss of gravel at stockpiles and arising from spillage and this is estimated at lo%, bringing the gravel requirements to 40.6 m3/ha. Assuming a bulk density of 1.55 t/m3, the gravel requirements are 63 t/ha. The machinery required to carry out gravel mole drainage consists of a tracked or heavy duty wheeled tractor equipped with a gravel plough, at least one and preferably two gravel carts fitted with side delivery conveyor belts and drawn by tractors and a loading shovel at the gravel stockpile. Average output is estimated at 1.4 ha/day and annual output at 140 ha/year for 100 working day season. The costs of the system depend largely on the frequency of the collector drains and on the cost of the gravel for the mole. Assuming a spacing of 60 m for collector drains, there are 167 m/ha collector drain and an additional 50m main drain is estimated to be required. At a cost of &1.40/m these drains cost &304/ha. At &4/t, gravel for the mole costs &252/ha. The machinery listed above is estimated to cost f460/day to operate and the cost/ha is &328. The total cost of the gravel mole drains is estimated at &580/ha and the total cost of drainage at &884/ha. Assuming a life of 20 years and an interest rate of 12% the annual cost of the drainage is (884)(0.13388) or &118/ha. This is also the benefit which must be equalled or exceeded to pay for the investment.
7.
Conclusions
The work to-date has shown that an insert of 10-20 mm clean washed gravel or broken stone aggregate can forestall the closure of mole channels in unstable soils. The gravel serves to support the wall, minimize erosion of the mole invert and, at the same time, allow a sufficiently rapid discharge of drainage water. Experiments and field tests showed that no appreciable silting up or root growth took place in the gravel. Discharge and water table measurements showed that gravel mole drains continued to provide good control of the water table in soils where mole drains had a short life. This control of the water table was also reflected in improved surface
150
THE
DEVELOPMENT
OF GRAVEL
MOLE
DRAINAGE
ratings for trafficability. In soils of low hydraulic conductivity, the soil slab over the gravel mole must be loosened to increase its hydraulic conductivity to a minimum value of 0.5 m/day. The gravel plough can loosen and crack this slab if it is drawn when the soil is relatively dry or it can be done independently by a ripper or subsoiler. At present no reliable guide is available as to when to use gravel mole drains or conventional mole drains. There are some known soils where conventional mole drains have a long life and there are others such as fine sandy and gritty soils where gravel mole drainage is clearly called for. But between these limits there is a large band of soils where uncertainty exists. Field tests showed that conventional mole drains have a short life in many soils derived from Silurian and Devonian rocks, from siliceous cherty limestone and from certain Carboniferous shales. Following a field testing programme with a prototype machine, problems such as proneness of the chute to clogging were identified and these were largely eliminated when a commercial machine was designed and built. So far this machine has worked well. The system is much more expensive than conventional moling. Materials and machinery costs have been estimated and an annual cost of El 18/ha was arrived at. Acknowledgements
The author gratefully acknowledges the support and help of the many agencies and persons who made this development possible. In particular grateful thanks are due to the former Northern Ireland Agricultural Trust and the late Philip Falconer of that agency, the Department of Agriculture of Northern Ireland, Mr J. P. Prunty, a land drainage contractor in County Fermanagh, and Mr T. Fisher of Messrs Fisher Engineering, Ballinamallard, County Fermanagh who built the commercial plough, to Messrs P. Butler, P. Finnerty and T. Gleeson of the Agricultural Institute for technical help and advice and MS E. Mellett for typing the manuscript. REFERENCES
Nicholson, H. H. The Principles of Field Drainage. Cambridge: Cambridge University Press, 1942 Hudson, A. W.; Hopewell, H. G.; Bowler, D. G.; Cross, M. W. The Draining of Farm Lands. New Zealand: Massey College, 1962. M&a, Z. The influence of the mineralogical composition of clay fraction on @e-time of mole drains, pp. 177-181. Proceedings of the Symposium on Hydrology and Technical Problems on Land Drainage, 1966 Rycroft, D. W.; Thorburn, A. A. Water stability tests on clay soils in relation to mole draining. Soil Sci., 1974 117(S) 306310 Rohey, 0. E. Some mole drainage experiments in Michigan. Agric. Engng., 1928 9(4) 115-l 17 Mulqueen, J.; Gleeson, T. Some relationships of drainage problems in Ireland to solid and glacial geology, geomorphology and soil types. Land Drainage, pp. 11-31. Rotterdam; A.A. Balkema, 1982 Toksiiz, S.; Kirkham, D. Graphical solution and interpretation of a new drain spacing formula. J. Geophys. Res., 1961 66(2) 509-516 Burke, W.; Mulqueen, J.; Butler, P. Aspects of the hydrology of a gley on u drumlin. Ir. J. agric. Res., 1974 13(2) 215-229 Busch, C. D. Low cost subsurface drainage. Agric. Engng., 1958 39(2) 92-93,97, 103 Ede, A. N. Floating-beam moleplough trials. Farm Mechanisation, 1961 (August) 274275 O’Neill, D. G. Gravel tunnel drainage at Castle Archdule. Agric. in N. Irel. 1977 52(2) 40-45 Kellett, A. J. GraveIjZIed mole channels. FDEU Tech. Rep. 7912 1980, Ministry of Agriculture Fisheries and Food Kirkham, D.; Selii, M. S. Theory of a rectangular gravel envelope in drainage design. Soil Sci. Sot. Am. Proc., 1973 37 517-521 Mulqueen, J.; Harrington, D. Some applications of physics in the development of a gravel filled mole drain. Physics in Industry, pp. 5 15-5 17, Oxford: Pergamon Press, 1976 Burke, W.{McCormack, P. Gravel drainsforpeat. Ir. J. agric. Res., 1969 8(2) 285-287 Scheidegger, A. E. The Physics of Flow through Porous Media. Toronto: University of Toronto Press, 1963
J. MULQUEEN
151
” Courtney, J. V. The hydraulic characteristics of gravel used in gravel tunnel drainage. Special study report National College of Agricultural Engineering Silsoe, 1978, Unpublished I8 Walsh, M. Soils. County L&trim Resource Survey. Dublin: An Foras Taluntais, 1973 ls O’Neill, D. G. Annual Progress Reports on Research and Technical Work. Castle Archdale Experimental Husbandry Farm, Irvinestown, County Fermanagh, 1974-1982
The Development
of Gravel Mole Drainage J. MULQUEEN*
A form of gravel mole drainage, in which clean gravel aggregate of 1&20mm, viz. passing 20 mm and retained on 10 mm square mesh, is filled into a mole channel, was developed as a solu-
tion to the problem of failure of mole drains because of unstable channels. The development and testing programme for the system is described. The results of field trials show that gravel mole drains perform equally well as conventional mole drains where the latter are stable and they continued to perform well in soils where mole drainage failed because the mole channel became filled with soil. The oldest drains are still working after 17 years and the system is in commercial use. 1.
Introduction
Gravel mole drains are essentially mole drains filled with gravel. This gravel is used to forestall structural failure and closure of the mole cavity in unstable soils. Mole drainage has been recommended for heavy soils of low permeability which require close drain spacing.“” It is recognized that mole drains have a longer life in soils with high clay and low sand and ironstone grit’ content. But it is also recognized that clay content alone is not a satisfactory guide to whether or not mole drains will have a long life. =g3More recently, Rycroft and Thorburn assessed the water stability of clay aggregates as a method of predicting suitability of soil for moling. At the present time however, no entirely satisfactory method exists for predicting the stability of mole drain channels and mole drainage by its nature is only a temporary measure and must be repeated.5 There are many soils which require the close drain spacing and soil loosening which mole drains permit, but in which the mole drains themselves are unstable. These soils are widely distributed in Ireland where they are mainly glacial tills derived from Old Red Sandstone, some Carboniferous limestones and Namurian shales.6 The drain spacing and minimum hydraulic conductivity requirements on these soils can be calculated from a nomogram supplied by Toksjz and Kirkham’ based on potential flow theory. Assuming that the soil below the plane of the mole is practically impervious, results show that drain spacings of l-l.2 m are required when the hydraulic conductivity of the soil slab above the mole drain invert is about 0.5 m/day. Where the hydraulic conductivity of the soil slab above the mole is less than 0.5 m/day, it must be brought to this level by loosening. This can be achieved by drawing the mole plough at spacings of l-1.2 m as above in many agricultural soils or by loosening with rippers at closer spacings in hard and compacted soils.8 Mole channels must remain open over their length to discharge the daily inflow of water which may amount to 500-1000 mm rainfall annually.“ The mechanism of mole drain failure is incompletely understood, but structural collapse has been observed in sandy and stony soils and a gradual reduction in the bore of the mole, leading to its closure, observed in some silty and clayey soils, is thought to be due to creep flow. Some mole drains on slopes have also been found blocked near their junction with collector drains. These blockages have been caused by deposition of soil eroded from upstream sections. Attempts have been made to stabilize the mole wall with a plastic liner, g,1o but costs so far limit installations at the spacings required except on some deep well structured soils where drain spacings of 3 m or greater suffice. This paper describes the development of an alternative solution to the problem of breakdown of mole drains, by feeding in permeable gravel to support the wall. At the same time this gravel permits water to flow through at the required velocity. While the system has been developed ‘Agncultural Recwed
Institute.
20 February
Ballinrobe, 1984; accepted
Co. Mayo,
Ireland
in revised form 24 September
1984
143 0021-8634/85/060143+09%03.00/0
@ 1985 The British Society
for Research
in Agncultural
Engineenng
144
THE
DEVELOPMENT
OF GRAVEL
MOLE
DRAINAGE
between 1965 and 1970 and is in commercial use,’ ‘I*’ 2 it has not been technically described so far although some aspects have been analysed.13*14 At about the same time, a system of gravel drains for peat was developed by Burke and McCormack.’ 5 2.
Development
of the system
The first trials were carried out using a hopper and chute suspended on the rear frame of a Ransomes C59 mole plough. Several chutes to accommodate varying sizes of gravel from coarse sand (2 mm) to gravel (30 mm) were constructed. These were interchangeable and could be slid along rails welded to the body and locked in position. The height of aggregate fill in the drain was controllable by a sliding door on the rear of the chute and, in the limit, the height could be brought up to ground level. The shank of the mole plough was made 6 mm wider than each chute by welding steel plates on the section above the mole in contact with the soil. The hopper was sized to take 0.5 t of gravel and it was fed both manually and by front end loader. The shank was surface hardened to minimize wear in very abrasive siliceous soils. All drains were drawn at a uniform depth of 450 mm and were filled with various gravel aggregates to heights varying from 50 mm to ground level. Drains at different spacings were also observed. Where drains were filled to ground level, it was found difficult, in the brittle topsoil, to make the drain a uniform rectangular cross section filled with gravel aggregate uncontaminated with soil. Long drains filled to ground level with gravel and spaced at 5 and 10 m on steep slopes tended to have the gravel scoured out by high flow rates caused by surface run-off in heavy storms. Moreover, drains with varying heights of sand were frequently found with back pressure indicating that the flow rate through sand was too slow to evacuate quickly the water taken up from the soil. These findings along with the satisfactory performance of partially filled drains and the requirements for close spacings and soil loosening calculated from theory’ led to the development essentially of a mole drain filled with gravel. 3.
Sizing of the drain in relation to the type of gravel
The flow of water through a full flowing gravel drain can be expressed as: Q=kl
Ai,
..
where Q is the quantity discharged per unit time, k, is flow rate per unit gradient through the gravel aggregate, A is the cross sectional area of the drain, i is the hydraulic gradient assumed equal to the slope of the drain. This equation assumes that the discharge (Q) is uniform along the length of the gravel mole channel. In practice the discharge in a gravel drain increases from zero at the upstream end to Q at the downstream end where it discharges into a collector drain. Because the flow is nonuniform, a discharge of 1.75 Q could also be considered appropriate for equal hydraulic gradients in accordance with drain pipe design practice. In this paper discharges are calculated on the basis of uniform flow. The flow rate is designated by k, since it is known it is not a constant as in the Darcy equation.16 Reynolds numbers calculated for 20 mm and 10 mm gravel at gradients of 0.02% and 25%, were 41 and 8 17 for 20 mm gravel and 9 and 243 for 10 mm gravel respectively. These data were calculated on the assumption of 20 and 10 mm tubes and they suggest that the flow is likely to be turbulent. ‘6 The discharge requirements of the gravel are: Q=slR,
..
where s is the drain spacing, Eis the drain length, R is the design rainfall rate. Equating (1) and (2) gives k,Ai=slR.
.(3)
145 -
5
c
2
0.1
I
I
I
I
I
I
I
I
0.2
0.5
I
2
5
IO
20
50
Gradlent,
100
%
Fig, 1. Hydraulic conductivity of two single sized gravels in relation to hydraulic gradient;‘h,‘7 O-O, 10 mm gravel
A-A,
20 mm gravel;
For gravel 10-20 cross
a given field slope (i), drain spacing (s) and climate (R), the problem is to select a suitable aggregate, drain cross sectional area and length. After flow tests in a-permeameter, mm washed gravel or broken stone aggregate was selected as this could give an economical sectional area and meet the flow requirements (Fig. 1). For the following values: k, = 50 000 m/day i= 0.50, A = 0.0056 m*, s = 1a2m and R = 0.025 m, the maximum permissible length of gravel drain is 466 m without back pressure. In practice, much shorter lengths of drain (l&100 m) are drawn because of changes in slope and the occurrence in the soil of stones and boulders which change the cross sectional area of the drain. The width of the drain is fixed at 75 mm and the height of gravel fill can be varied by a door on the rear of the chute. 4.
The gravel plough machine
After the form of the drains and drainage had evolved, further tests were carried out to improve the design of the plough. These tests showed that the orifice of the simple chute could easily be blocked with soil (Fig. 2). Because of its very small contact area with the ground a little topsoil entering the chute could block it. This was overcome by lengthening the orifice of the chute in contact with the soil to 0.3 m. To prevent soil entering the chute as the plough entered the ground it was found that the gravel must begin to flow at the orifice as the mole touched the
146
THE
DEVELOPMENT
OF GRAVEL
I
MOLE
DRAINAGE
\
Fig. 2. Orl$ce ofprototype gravelplough; the orifice (a) is regulated by the diagonal sliding door (b) on chute (c); this chute is mounted behind shank (d) which carries mole (e)
ground. A hydraulically operated door was necessary to start and stop the flow of gravel. It was also found that output could be greatly enhanced if the machine could be hydraulically taken out of the ground and also if the machine was mounted on a three-point linkage where the fields were short. Accordingly, these improvements were incorporated in the design of a commercial gravel plough (Fig. 3). In this plough a frame (a) comprised of two hollow steel sections 150 mm apart is welded to a tool bar (b) suitable for mounting on the three-point linkage of a tractor. This frame carries a shaft (c) to which are welded two depth control wheels (d). To the rear of the wheel shaft the frame carries a rolling disc coulter (e) which is loaded by a coil spring (f). At the rear of the frame is mounted the mole shank (g) which carries a 75 mm mole (h). The shank carries a 1 t capacity hopper (i) mounted so that it can swing sidewards and facilitate movement around boulders. This hopper carries a hydraulically operated shutter (j) to start and stop the flow of gravel and a chute (k) equipped with a door (1) to regulate height of gravel in the mole. The hopper is loaded with gravel by a belt conveyor from a gravel cart. 5.
Field testing of gravel mole drainage
Field tests were carried out on grassland on farmers’ fields at 16 sites to test the performance of gravel mole drainage compared with mole drainage under a variety of soil and hydrological conditions. Plots varied from 0.5-1.0 ha in size and sites were located on soils where mole drains were known to have become blocked from structural failure. These soils were derived mainly from decalcified limestone with much chert as described by Walshls and from Silurian and Devonian sandstones and shales and were spread over three counties. These drains were excavated to observe if soil or roots of grasses could clog the permeable gravel and therefore obstruct the flow of water. Notes were also made on the state of wetness and suitability for traffic of drained and undrained ground. No significant movement of soil fines into the gravel or root growth in the gravel was noted over a 334 year period and land drained by gravel mole drains was dry while adjoining ground was wet. No erosion of the invert of the channel was noted on steep slopes where mole erosion was known to occur. In 1972 arrangements were made by the Northern Ireland Agricultural Trust, the Northern Ireland Department of Agriculture, Mr J. P. Prunty (a land drainage contractor) and the Irish
J. MU1 *QUEEN
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THE DEVELOPMENT
OF GRAVEL
MOLE
DRAINAGE
Agricultural Institute to test the acceptability of the system to farmers and to carry out an experimental evaluation of the system at the Department’s Castle Archdale Experimental Husbandry Farm (County Fermanagh). The soils on this farm are derived from Old Red Sandstone, have 3&40% of fine sand and a hydraulic conductivity in the range 1.10m21.10m4 m/day. ‘I’While hydraulic conductivity measurements showed that this soil needed mole drainage as it needs the soil loosening and close spacing which mole drainage permits, textural analysis suggested that the high fine sand content might lead to early closure of the mole channels. In these experiments gravel mole drainage at 450 mm depth and 1.5 m spacings were compared with mole drainage at similar depth and spacing, pipe drainage at 6.3 m centres and a control. Measurements included drain flow rate, water table levels, soil surface ratings for animal and machine traffic and grass yields. ‘I’ In the early years, both gravel mole and mole drains gave similar results for drainflow, water table control and ground surface ratings. Gravel mole drains were as good as moles where the channel of the latter remained open and were superior to mole drains where these tended to break down.lg For example, from 18 September 1979 to 19 November 1979, with a rainfall of 241.9 mm, where the moles were stable, gravel mole, mole and pipe drains removed 84, 83 and 54% of the rainfall respectively. This experiment showed that gravel moles were as good as mole drains when judged by the three parameters of drainflow, water table levels and soil surface ratings for trafficability in a site where mole drains remained open and in good condition. Both gravel mole and mole drain treatments were much better than the pipe drain and the no drain treatments which were impassable to machinery in wet weather. Although the treatments were not statistically replicated, previous uniformity of measurements on similar sites had suggested that a difference of 56% in discharge between gravel mole as compared with pipe drains was unlikely as the difference in discharge between plots was only 6% for a run-off of 119 mm over one month.8 Analyses8 also showed that the higher the discharge the less the difference for similar antecedent moisture conditions with differences as low as 1.3% for one month with 115 mm run-off when two rain storms contributed to most of the run-off. The effect of gravel mole and mole drains on the water table in the early years in a site where drains tended to be unstable is shown in Table 1. This table shows that (in the early years) both gravel mole and mole drains consistently maintained the average water table below 0.3 m, while the control no drainage treatment had an average high water table and was frequently waterlogged in wet weather. As in the first experiment, there appeared to be little difference between gravel mole and mole drains and both were superior to the no drainage treatment. In later years, the mole drains deteriorated and had to be redrawn while the gravel mole drains continued to TABLE 1
Mean water table levels, 1 cm below soil surface, 19761980 on a site drained 1975 at Castle Arcbdalels Month January February March April August September October November December Mean
Control
Gravelmole
Mole
13 19 16 28 38 22 16 14 11 20
34 37 36 43 49 36 36 33 31 37
30 35 32 41 48 34 34 30 37 35
149
J. MULQUEEN
provide good drainage. In these experiments the gravel mole drains have continued to provide good drainage for almost 10 years up to the present time. These experiments, along with the results of tests on farmers’ fields, indicate that gravel mole drainage is an effective drainage system on soils requiring close drain spacing and soil loosening, where mole drains have a short life. Additional testing has shown that gravel mole drains can provide an effective drainage system for sportsfields. In making sportsfields, cut and fill earthworks often result in overcompaction of the soil. This overcompacted soil must be loosened by heavy duty rippers where it is very hard as the gravel plough may not enter it. The field can then be drained by gravel moles at l-1.2 m spacings. If the field is subsequently compacted in the surface layers by play or machinery it can be loosened by subsoiler or ploughs to raise the hydraulic conductivity to at least 0.5 m/day. This additional testing also revealed two sites with soft silty soils where neither gravel mole nor mole drains provided satisfactory drainage. These soils are derived from a small isolated outcrop of Clare shales which comprise the lowest beds of the Namurian shale formation. Field tests showed that loosening did not raise the hydraulic conductivity of the soft soil slab overlying the gravel mole sufficiently. Hydraulic tests on the gravel moles themselves showed that these drains could discharge the design quantities of drainage water if it could enter them sufficiently fast. Conventional mole drains were found filled with soft soil in a slurried condition apparently as a result of creep flow.
6.
Materials and costs
Gravel mole drainage is much more costly than conventional mole drainage. Assuming a 75 mm diameter mole, 1 m3 of gravel will fill 226 m of drain. If the drains are spaced at 1.2 m there are 8333 m drain/ha requiring 36.9 m3 of gravel/ha. Some provision must also be made for the loss of gravel at stockpiles and arising from spillage and this is estimated at lo%, bringing the gravel requirements to 40.6 m3/ha. Assuming a bulk density of 1.55 t/m3, the gravel requirements are 63 t/ha. The machinery required to carry out gravel mole drainage consists of a tracked or heavy duty wheeled tractor equipped with a gravel plough, at least one and preferably two gravel carts fitted with side delivery conveyor belts and drawn by tractors and a loading shovel at the gravel stockpile. Average output is estimated at 1.4 ha/day and annual output at 140 ha/year for 100 working day season. The costs of the system depend largely on the frequency of the collector drains and on the cost of the gravel for the mole. Assuming a spacing of 60 m for collector drains, there are 167 m/ha collector drain and an additional 50m main drain is estimated to be required. At a cost of &1.40/m these drains cost &304/ha. At &4/t, gravel for the mole costs &252/ha. The machinery listed above is estimated to cost f460/day to operate and the cost/ha is &328. The total cost of the gravel mole drains is estimated at &580/ha and the total cost of drainage at &884/ha. Assuming a life of 20 years and an interest rate of 12% the annual cost of the drainage is (884)(0.13388) or &118/ha. This is also the benefit which must be equalled or exceeded to pay for the investment.
7.
Conclusions
The work to-date has shown that an insert of 10-20 mm clean washed gravel or broken stone aggregate can forestall the closure of mole channels in unstable soils. The gravel serves to support the wall, minimize erosion of the mole invert and, at the same time, allow a sufficiently rapid discharge of drainage water. Experiments and field tests showed that no appreciable silting up or root growth took place in the gravel. Discharge and water table measurements showed that gravel mole drains continued to provide good control of the water table in soils where mole drains had a short life. This control of the water table was also reflected in improved surface
150
THE
DEVELOPMENT
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MOLE
DRAINAGE
ratings for trafficability. In soils of low hydraulic conductivity, the soil slab over the gravel mole must be loosened to increase its hydraulic conductivity to a minimum value of 0.5 m/day. The gravel plough can loosen and crack this slab if it is drawn when the soil is relatively dry or it can be done independently by a ripper or subsoiler. At present no reliable guide is available as to when to use gravel mole drains or conventional mole drains. There are some known soils where conventional mole drains have a long life and there are others such as fine sandy and gritty soils where gravel mole drainage is clearly called for. But between these limits there is a large band of soils where uncertainty exists. Field tests showed that conventional mole drains have a short life in many soils derived from Silurian and Devonian rocks, from siliceous cherty limestone and from certain Carboniferous shales. Following a field testing programme with a prototype machine, problems such as proneness of the chute to clogging were identified and these were largely eliminated when a commercial machine was designed and built. So far this machine has worked well. The system is much more expensive than conventional moling. Materials and machinery costs have been estimated and an annual cost of El 18/ha was arrived at. Acknowledgements
The author gratefully acknowledges the support and help of the many agencies and persons who made this development possible. In particular grateful thanks are due to the former Northern Ireland Agricultural Trust and the late Philip Falconer of that agency, the Department of Agriculture of Northern Ireland, Mr J. P. Prunty, a land drainage contractor in County Fermanagh, and Mr T. Fisher of Messrs Fisher Engineering, Ballinamallard, County Fermanagh who built the commercial plough, to Messrs P. Butler, P. Finnerty and T. Gleeson of the Agricultural Institute for technical help and advice and MS E. Mellett for typing the manuscript. REFERENCES
Nicholson, H. H. The Principles of Field Drainage. Cambridge: Cambridge University Press, 1942 Hudson, A. W.; Hopewell, H. G.; Bowler, D. G.; Cross, M. W. The Draining of Farm Lands. New Zealand: Massey College, 1962. M&a, Z. The influence of the mineralogical composition of clay fraction on @e-time of mole drains, pp. 177-181. Proceedings of the Symposium on Hydrology and Technical Problems on Land Drainage, 1966 Rycroft, D. W.; Thorburn, A. A. Water stability tests on clay soils in relation to mole draining. Soil Sci., 1974 117(S) 306310 Rohey, 0. E. Some mole drainage experiments in Michigan. Agric. Engng., 1928 9(4) 115-l 17 Mulqueen, J.; Gleeson, T. Some relationships of drainage problems in Ireland to solid and glacial geology, geomorphology and soil types. Land Drainage, pp. 11-31. Rotterdam; A.A. Balkema, 1982 Toksiiz, S.; Kirkham, D. Graphical solution and interpretation of a new drain spacing formula. J. Geophys. Res., 1961 66(2) 509-516 Burke, W.; Mulqueen, J.; Butler, P. Aspects of the hydrology of a gley on u drumlin. Ir. J. agric. Res., 1974 13(2) 215-229 Busch, C. D. Low cost subsurface drainage. Agric. Engng., 1958 39(2) 92-93,97, 103 Ede, A. N. Floating-beam moleplough trials. Farm Mechanisation, 1961 (August) 274275 O’Neill, D. G. Gravel tunnel drainage at Castle Archdule. Agric. in N. Irel. 1977 52(2) 40-45 Kellett, A. J. GraveIjZIed mole channels. FDEU Tech. Rep. 7912 1980, Ministry of Agriculture Fisheries and Food Kirkham, D.; Selii, M. S. Theory of a rectangular gravel envelope in drainage design. Soil Sci. Sot. Am. Proc., 1973 37 517-521 Mulqueen, J.; Harrington, D. Some applications of physics in the development of a gravel filled mole drain. Physics in Industry, pp. 5 15-5 17, Oxford: Pergamon Press, 1976 Burke, W.{McCormack, P. Gravel drainsforpeat. Ir. J. agric. Res., 1969 8(2) 285-287 Scheidegger, A. E. The Physics of Flow through Porous Media. Toronto: University of Toronto Press, 1963
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” Courtney, J. V. The hydraulic characteristics of gravel used in gravel tunnel drainage. Special study report National College of Agricultural Engineering Silsoe, 1978, Unpublished I8 Walsh, M. Soils. County L&trim Resource Survey. Dublin: An Foras Taluntais, 1973 ls O’Neill, D. G. Annual Progress Reports on Research and Technical Work. Castle Archdale Experimental Husbandry Farm, Irvinestown, County Fermanagh, 1974-1982