Mole Channels Research Articles

Hydraulic Characterization of Permeable Materials Used for Filling Mole Drain Channels in Structurally Unstable Soils

Highlights Permeable materials can be used to fill mole drains and provide support to the cavity walls in structurally unstable soils. Permeable materials extend the lifespan and improve the hydraulic performance of mole drainage systems. Darcy’s law for nonlinear porous flow explained the hydraulic performance of the permeable materials used in this study. A numerical method was established to assess the suitability of permeable materials of unknown hydraulic performance. Design guidelines were developed to assist in the planning of filled mole drainage systems installed in structurally unstable soils. Abstract. Mole drainage in soils with low permeability is a cost-effective technique for managing soil drainage. Following installation, the satisfactory performance of the system will depend on having sufficient soil fissure development extending from the soil surface to the roof of the mole channel and mole channel stability to avoid its collapse. Permeable materials such as river-run gravel and crushed stone can be used to fill mole channels to prevent channel collapse in halomorphic and high-silt content (e.g., 50%–70%) soils and extend the lifespan of the system. The objectives of this work were to: (i) characterize a range of permeable materials that can be used to fill mole drains and provide support to the cavity walls in structurally unstable soils; (ii) verify the applicability of Darcy’s law to explain the hydraulic performance of such materials, and (iii) develop a set of design and planning guidelines aimed at improving the performance of mole drainage systems that are filled with permeable materials. Five permeable materials were characterized, namely: three types of river-run gravels, crushed baked mud bricks, and crushed stone. The materials were characterized for their mechanical and hydraulic properties, and the applicability of Darcy’s law was experimentally determined. Small- and medium-sized gravels are easy to handle and apply during field installation due to their shape and flowability properties and the reduced risk of clogging when placed in the mole channel. However, the hydraulic performance of these materials would not allow for system designs longer than about 100 m, which therefore would limit their utilization in practice. Large-sized gravel offered an intermediate solution with overall satisfactory hydraulic performance. Crushed brick and crushed stone offered good alternatives to gravel, allowing for longer (e.g., >120 m) system designs for all combinations of drain spacing and drain gradient. However, their angularity, poor uniformity, and size range distribution can compromise the efficiency of handling and delivery in current mole plough systems. For all materials, the hydraulic conductivity (Kh) decreased with an increase in the hydraulic gradient. For a given hydraulic gradient, Kh was dependent on aggregate size, such that a larger D50 value led to increased Kh. The nonlinear performance of Darcy’s law was verified for all materials, and potential equations were fitted to represent such relationships, all showing acceptable fits to the nonlinear models (P<0.001). A linear relationship (P<0.05) between the parameter m of the potential model and total porosity was established. The set of equations developed by this study will enable estimating Kh based on the aggregate size distribution or as a function of the total porosity of the material. Crushed brick and crushed stone could provide a cost-effective solution for the installation of filled mole drain systems in structurally unstable soils. Keywords: Darcy’s law, Extreme rainfall, Halomorphic soils, Hydraulic conductivity, Reynolds number, Silty soils.

Modeling Performance of a Tile Drainage System Incorporating Mole Drainage

Abstract. Mole drain performance is known to vary temporally and spatially due to variations in soil properties, installation conditions, mole channel integrity, and weather patterns. In fine-textured, low-permeability soil profiles, moles can be installed to supplement an underlying tile drain system. However, moles are often not included in such designs. The objective of this modeling study was to investigate the performance impacts of variations in mole integrity and design in such a soil profile during a range of rainfall event scenarios. A finite element software package (SEEP/W) was used to model a field site having (system 1) subsurface tile drains (0.9 m depth, 15 m spacing) with gravel aggregate(10 to 50 mm) and intersecting mole drains (0.6 m depth, 1.4 m spacing). The field site was subjected to a pedological survey to characterize the soil profile, while an on-site weather station and end-of-pipe flowmeters provided rainfall and discharge data from which the model could be calibrated. The calibrated model showed close agreement between modeled and observed subsurface discharge in the validation period (coefficient of mass residual = 0.12, index of agreement = 0.94, model efficiency = 0.74). The model was then used to evaluate the impact of three alternative designs: tile drains only, a common practice in similar soils (system 2); a design similar to system 1 but with the saturated hydraulic conductivity (Ks) of the mole-drained layer decreased to mimic a reduction in mole drain integrity and effectiveness (system 3); and a design similar to system 1 but with Ks of the mole-drained layer increased to mimic improved soil disturbance and fissuring during installation (system 4). These systems were analyzed using the calibration (event A) and validation (event B) rainfall events as well as two notional rainfall scenarios: a “fixed rainfall” scenario (event C) with a rainfall rate of 2 mm h-1 applied to all systems for 50 h and a “historical rainfall” scenario (event D) with annual (30 year) average daily values for the area (taken as the average monthly totals divided by the number of days per month) applied over a year. Results showed that the modeled designs exhibited similar relative behavior in all simulated rainfall scenarios. Systems 1 and 4 consistently outperformed systems 2 and 3 in terms of average and peak discharge and water table control capacity. Across rainfall events, system 2 (without mole drains) was the least effective and was seen to decrease drain discharge by an average of 63% and reduce mean water table depth by an average of 72% relative to systems 1 and 4. Results showed the importance of mole channels in supplementing tile drainage on fine soils, as well as the importance of mole integrity for optimal performance. Such a tool could provide decision support in the drainage system design process and assess the implications of design variations on cost, expected performance, and likely returns to the landowner by estimating seasonal variations in drainage discharge and water table position. Identifying and characterizing the major soil types on a farm through soil profile pedological descriptions and collation of real soil physical and meteorological data is essential to prescribe appropriate drainage designs and prioritize areas for drainage installation in light of technical feasibility and cost estimates. With high-resolution data, the software can be calibrated for other drainage system and climate change scenarios. Keywords: Mole drainage, Rainfall, SEEP/W, Simulation, Soil physical properties, Subsurface drainage.

Mole drain performance in a clay loam soil in Ireland

There is uncertainty regarding the suitability of some soils for mole drainage. Soils with questionable potential for the formation of stable mole channels generally have <45% clay and >20% sand, and gravel mole drainage is an alternative. However, this technique is unattractive due to its relative cost (2014 price: €1500–2800 ha−1 versus €125–300 ha−1). The performance of conventional mole drains and gravel mole drains was compared on a clay loam soil in the south of Ireland over 12 rainfall events. The installation dates of mole drainage treatments were staggered (January versus July) to assess how soil moisture status during installation affected performance. Treatments were (A) undrained control, (B) mole drainage installed in January 2011 (suboptimal), (C) mole drainage installed in July 2011 (optimal) and (D) gravel mole drainage installed in July 2011. Gravel mole drains were consistently more efficient in removing excess water than mole drains. Mean total drain flow (DF) during rainfall events was greater (P < .05, standard error 0.56 mm) in gravel mole drains (7.4 mm) than any mole drainage treatment irrespective of installation conditions (B [3.8 mm] and C [4.2 mm]). Initially, DF volumes exceeded overland flow (OF) in all treatments (Events 1–4, mean total OF was 0.47 [±0.08] of mean total DF). From Event 5 onwards, this relationship changed with OF being greater than DF (mean total OF was 2.02 [±0.41] of mean total DF). Results showed that across treatment replicates, the water table depth was consistently shallower in control plots. Mole drains were effective for a period of approximately 2 years. Although gravel mole drains were more effective, they were also subject to significant deterioration during the study. Installation conditions had no effect on mole drain performance. The results suggest that on this site conventional mole drainage repeated every 2 years could provide adequate drainage at low cost.

Evaluation of tephra for removing phosphorus from dairy farm drainage waters

Research was conducted in the Manawatu region, New Zealand, to investigate the ability of Papakai tephra to remove phosphorus (P) from dairy farm mole and pipe drainage waters. The capacity of this tephra to adsorb P was quantified in the laboratory using a series of column experiments and was further evaluated in a field study. In a column experiment, the P adsorption capabilities of 2 particle size factions (0.25–1, 1–2 mm) of Papakai tephra were compared with that of an Allophanic Soil (Patua soil) known to have high P adsorption properties. The experiment used a synthetic P influent solution (12 mg P/L) and a solution residence time in the columns of c. 35 min. By the end of the experiment, the 0.25–1 mm tephra removed an estimated 2.6 mg P/g tephra at an average P removal efficiency of 86%. The 1–2 mm tephra removed 1.6 mg P/g tephra at an average removal efficiency of 58%. In comparison, the Patua soil removed 3.1 mg P/g soil at a P removal efficiency of 86%. Although, the Patua soil was sieved to 1–2 mm, this size range consisted of aggregates of finer particles, which is likely to have contributed to this material having a higher P adsorbing capacity. A field study was established on a Pallic Soil, under grazed dairy pastures, to compare drainage water P concentrations from standard mole and pipe drainage systems (control) and drainage systems incorporating Papakai tephra. The 2 tephra treatments involved filling mole channels with 1–4 mm tephra (Mole-fill treatment) or filling the trench above intercepting drainage pipes with ‘as received’ tephra (Back-fill treatment). Over an entire winter drainage season, the quantity of total P (TP) lost from the control treatment drainage system was 0.30 kg P/ha. The average TP losses for the Mole-fill and the Back-fill treatments were 45% and 47% lower than the control treatment, respectively.

Phosphorus losses to surface waters following organic manure applications to a drained clay soil

The effects of annual applications of farm manures (pig slurry (PS), broiler litter (BL) and cattle farmyard manure (FYM)), liquid digested sewage sludge (LDS) and inorganic phosphorus (P) fertiliser on P concentrations and losses in tile drain flow from a calcareous clay soil were studied over four winter drainage seasons. The site was under arable cropping in South Eastern UK and had been intensively underdrained in autumn 1994. The tile drainage system comprised of plastic pipes covered with gravel backfill and supplemented by 2 m spaced mole channels. Application of PS in November 1994, 4–6 weeks before the onset of winter drainage, resulted in high concentrations of dissolved P (up to 10 mg l −1) and total P (TP) in drain flow (up to 75 mg l −1). TP losses following application of PS over this first drainage season (1155 g ha −1) were increased four-fold compared to a control receiving no P (277 g P ha −1). The majority of the increased loss occurred in the first drainage event due to the rapid transport of the PS through the macropores created in the soil following the installation of tile and mole drains 1 month before the slurry was applied. Application of BL, FYM, LDS and inorganic P fertiliser at maximum recommended rates did not significantly increase P losses in any drainage season, nor did the PS in subsequent years. This study supports current guidelines on good agricultural practice, which recommends that liquid farm manures should not be applied to recently drained clay soils to avoid direct contamination and P enrichment of the drainage water.

Improving mole drainage channel stability in irrigated areas

Mole drains in flood irrigated agriculture can rapidly fail due to high flows of irrigation water entering the mole channel through the soil cracks formed during the moling process. Currently mole drains are formed using a straight leg mole plough that results in rapid irrigation water flow to the mole drain. The use of an angled leg mole plough to reduce the direct inflow of irrigation water and produce more stable mole channels was investigated. The leg of the angled leg mole plough comprised an upper vertical section to which an angled section carrying the mole foot was attached at a 30° angle. The trials were carried out on two contrasting clay soils in a flood irrigated area. One of the soil types was structurally stable on wetting, the other unstable. The quality of the mole channels formed at installation using the straight leg plough was good, but using the angled leg plough were only moderate due to some instability problems with the equipment. This was caused by the mole foot pitching and thus forming an oval channel. After installation, irrigation was applied to the mole channels to ascertain their stability on wetting. The angled leg moles proved more stable than the straight leg moles in the unstable soil. This was due to reduced water flow rates into the channel causing less erosion and to the prevention of the leg slot opening up directly into the mole channel, through shrinkage, during dry periods. The latter prevented significant soil wash and ingress into the channel during the following irrigation. This improved stability is of significant importance in that it may allow the adoption of mole drainage on sodic and swell/shrink soils where previously mole drainage would have been ineffective. In the structurally stable soil, the stability of the angled leg moles was found to be slightly worse than those installed with the straight leg plough. This was probably due to inadequate soil packing in the channel roof during installation. The trial results indicate that mole channels installed with angled leg plough have the potential for much greater stability on sodic and swell/shrink soils than moles installed with current straight leg mole ploughs. Before this potential can be fully achieved further development of the mole ploughing technique is required to ensure that high quality moles are consistently formed at installation. Suggestions are made for future improvements to the mole plough and the moling technique.

Phosphate Losses through Field Drains in a Heavy Cultivated Soil

AbstractLosses of total and molybdate‐reactive phosphate (MRP) were measured for 4 yr in water flowing from interconnecting mole and pipe drains under 0.24 ha plots of a heavy cracking clay soil. The molybdate‐unreactive phosphate (MUP) obtained by difference appeared from the results to be mainly phosphate carried on suspended soil material. The losses of all three categories of phosphate were closely related to the cumulative drainflow, with the same relations covering three of the four years, designated the synoptic years. Losses were much greater in the other year, probably because phosphate was applied after, rather than before, plowing and to very wet soil. The ceiling to annual total phosphate loss, assuming 500 mm drainage, was 0.4 kg ha−1 in the synoptic years, with 76% of the loss occurring as MUP. It was about 1 kg ha−1 in the high‐loss year, with 88% as MUP, implying that MUP comprised 96% of the extra loss that year. Halving the phosphate application lessened the loss of total phosphate but not conclusively, and the decrease in MRP loss was not significant. Restricting and thereby delaying drainage lessened losses of MUP, probably because suspended material carrying it was allowed to settle. Increasing the spacing between mole channels from 2 m to 4 m increased the losses of MUP but not MRP. This probably happened because to meet a flow pathway connected to a mole channel, rainwater had to travel further horizontally and collected more phosphate‐containing soil material.

Simulation studies of mole drain characteristics

A laboratory study was carried out to assess the suitability of clay soils for moling and to evaluate the performance of mole drains constructed under wide variety of initial soil moisture and bulk density conditions by simulating the field situations in a Hale-Shaw model. The soil properties at the time of construction of mole channels significantly affected the performance of mole drains, channel stability, development of cracks and fissures and the force required for moling. At an initial soil moisture content of 20 per cent (v/v) and bulk density of 1.4 g/cm3 the drainage performance was quite satisfactory At low density (1.25 g/cm3 it was difficult to construct the mole channels due to too loose packing. Also, deformation in mole shapes were predominant when the same were subjected to saturation. At higher moisture content no major cracks and fissures developed to act as the secondary channels for carrying water to the mole drains.

Shallow subsurface drainage in an irrigated vertisol with a perched water table

Waterlogging and salinity are reducing the productivity of irrigated agriculture on clay soils in south east Australia. We compared five drainage treatments: (1) undrained control (Control); (2) mole drains (Mole); (3) mole drains formed beneath gypsum-enriched slots (GES) (Mole + GES); (4) shallow pipe drains installed beneath GES (Shallow Pipe); (5) deep pipe drains (Deep Pipe). The experiment was set out on a vertisol and our measurements were made during the growth of an irrigated onion crop. Over the 3 months before the spring irrigations commenced, the perched water table on the Control was less than 400 mm below the soil surface for 27% of the time, whereas the shallow drainage treatments (Treatments 2, 3 and 4) reduced this time to less than 4%. During the irrigation season, the perched water table on the Mole + GES treatment rose above 400 mm for 3% of the time. The perched water table on the Mole treatment was above 400 mm for 14% of the time, compared with 19% of the time on the Control. The Deep Pipes were less effective in reducing the depth to the perched water table, both before and during the irrigation period. Mole drains increased the gas-filled porosity above the drains. However, the gas-filled porosity remained below reported levels for optimum root growth. Although the drains effectively drained excess water, and lowered the water table, the hydraulic gradient was insufficient to remove all of water from the macropores. Gypsum enriched slots above the mole drains increased the gas-filled porosity in the slots but the drainable porosity in the undisturbed soil appeared to be inadequate for optimum root growth, even though some drainage occurred near the slots. Discharge from the shallow drainage treatments averaged 58 mm for each irrigation, and was considerably more than the amount required to drain the macropores. The mole channels were in reasonably good condition at the end of the irrigation season, with at least 70% of the cross-sectional area of the channel open. Shallow subsurface drains increased onion yield by about 38%. For each day the water table was above 400 mm, the yield declined by 0.23 tonnes per hectare. Farmer adoption of shallow subsurface drainage will depend on the long-term economic benefits (influenced by the longevity of the mole channels and yields response) and the need to develop more sustainable management practices.

Movement of pesticides to surface waters from a heavy clay soil

AbstractA field experiment at Cockle Park, Northumberland on a clay loam soil (Dunkeswick series) cropped with winter wheat investigated the effects of drainage and season of application on pesticide movement. Isoproturon, mecoprop, fonofos and trifluralin were applied in two consecutive seasons at normal agricultural rates to three hydrologically isolated plots each of 0.25 ha. Two of the plots were mole‐drained and the third was an undrained control. Surfacelayer flow and drainflow from each plot were monitored at 10‐min intervals. Samples of flow were analysed for pesticides to evaluate transport of applied chemicals from the site. Despite widely differing properties (Koc 20–8000 ml g−1, t1/2 10–60 days), all four pesticides were found in surface‐layer flow and mole drainflow from the site. Maximum concentrations of pesticides in flow ranged from 0.1 to 121 μg litre−1 (aqueous phase) and &lt; 0.2 to 48 μg litre−1 (particulate phase). Over two contrasting seasons, total losses of pesticides in flow followed total amounts of flow and were approximately four and five times larger, respectively, in 1990/91 than in 1989/90. The maximum loss occurred from the undrained plot and was 2.8 g isoproturon (0.45% of that applied). Total losses of autumn‐applied pesticides from an undrained plot were up to four times greater than losses from a mole‐drained plot. Mole drainage decreased movement of pesticides from this slowly permeable soil by reducing the amount of surfacelayer flow. Maximum concentrations of mecoprop and isoproturon in drainflow were 10–20 times larger following spring application than after application in autumn. Bypass flow down soil cracks was an important process by which pesticide was lost from the site, with transport to the drainage system via mole channels (55 cm depth) after less than 0.5 and 6.7 mm net drainage in the two winters.

Observations of water movement to and within mole drainage channels

Dye tracers were used to record the time of travel for water movement from the soil surface to mole drains installed in a clay soil. For two events it is shown that travel times are of the order of 1·0 to 1·25 h, representing an effective transmission rate close to 12 m/d. Flow velocities within moles channels are typically of the order of 0·5 m/min. These observations can be used to explain the rapid response of mole drains as the consequence of direct movement of water from the surface to the mole channels.

Mole plough and soil factors influencing the hydraulic connection between mole channels and the permeable backfill placed over pipe drains

A mole plough in operation, tends to move soil into, and a quantity of backfill out of, the permeable backfill placed in the pipe drain trench. This movement can affect the efficiency of the hydraulic connection between the mole channel and the backfill and may shorten the life of the backfill zone for future mole drainage. Soil and backfill displacements generated by a range of mole plough foot and expander geometries working under different field conditions are reviewed and the situations where serious problems are likely to arise are identified. Important influencing factors include expander shape and size, the width of the backfill zone, soil moisture conditions and the relative degrees of consolidation in the soil and backfill materials. Operational recommendations are made to minimize displacements and to enable satisfactory hydraulic connections to be achieved.

A laboratory method for investigating the stabilization of mole channels

A method has been devised to allow production of model mole channels in blocks of soil under laboratory conditions. Subsequently, the channels were subjected to a combination of treatments including wetting and drying of the whole soil block and the passage of water down through the soil to the channel and along the channel itself. Erosion from within the channel was monitored by measurement of sediment yield and the structure of the channel was itself examined by use of an endoscope. Mole channels produced with polyacrylamide, had smoother walls initially than those without and also after subsequent testing by water flow. Additionally, channels treated with polyacrylamide, also produced less sediment than untreated channels. Such models were used to study the beneficial effects of polyacrylamide on mole channel stability but could also be used to study other aspects of mole channel production and stability under varying controlled conditions.

Efficiency of subsoiling and subsurface drainage in heavy alluvial soils of the G.D.R.

The durability of mole channels, disruption zones and backfills of drain trenches was observed, and the traction requirements of rigid tines were determined, during long-term field trials on alluvial clay soils. Mole channels formed above the water-table and having an effective top depth of ≥ 0.55 m will be ascertainable as cavities over 3–7 years, and thereafter as zones of increased horizontal permeability. In disruption zones, improved water permeability at the depth of ≥ 0.45 m continued for more than 54 months. After 20 years, backfilled soil tends to be less permeable than the adjacent undisturbed soil. The causes of differences in durability are analysed and discussed in the paper. The results are used to determined an efficient installation method. It is recommended to apply pipe drainage with permeable backfill together with mole loosening (Maulwurflockerung). This combination should be adapted to local field conditions.

Mole plough grade control

The potential advantages in terms of draught force, grade accuracy and subsequent mole channel stability of floating as compared with scrubbing beam mole plough systems are outlined, together with the grade control principles for each system. Following field experimental work, the design requirements for a successful floating beam system are identified together with the need for a grade smoothing device to reduce the effect of tractor pitching on channel grade. The design of a grade smoother is described. Details are provided about its superior performance when used in association with a floating beam plough, in comparison with traditional scrubbing beam and direct hitched floating beam systems. Methods of using the new system to grade mole channels on level soil surfaces are outlined.

Subsurface Drainage Studies in a Fine Textured Soil with Impaired Permeability

ABSTRACT DRAIN flow rates were measured from subsurface drains having gravel backfill and mole channels installed above the drain pipes to enhance profile drainage and water movement to the drains. The gravel backfill increased the peak flow rates. The mole channels did not improve water movement to the drains. Saturated hydraulic conductivity measurements revealed a soil layer at the 0.30 to 0.50 m depth having low hydraulic conductivity that restricted downward movement of water.

Effect of mole submergence on the life of mole channels

It has been suggested that submergence of mole channels in field drainage systems will cause their premature collapse and lead to a failure of the system. Limited experience on Field Drainage Experimental Unit's sites has not supported this view. An experiment was set up on a clay soil with a good moling potential, and where a previous moling had lasted up to 5 years. In a pumped drainage scheme submergence was examined to determine if it was a cause of mole channel failure. Isolated plots were subjected to short (usually less than 3 h) and long periods (up to 20 days) of submergence. The hydrological performance and the condition of the mole channels was monitored for 5 years. Short periods of submergence did not appear to have a substantial effect on the mole drainage system and a 4–5-year life of the mole channels could be expected. Longer periods of submergence did not cause any immediate deterioration but the gradual collapse of the roof and side walls, which was the normal mode of failure, became more accentuated. This effect reduced the life of the mole channels to 3–4 years, although 3 years after moling the watertable control of the submerged plots was substantially better than similar adjacent undrained land.

Effects of soil management practice on soil physical conditions affecting root growth

Cultivation and drainage are two major management techniques used to modify soil physical conditions for crop growth. In England and Europe ploughing increased the volume of macropores (pores of diameter >50 μm) but markedly reduced their continuity between topsoil and subsoil compared with direct drilling using a triple disc drill. In Australia, during the first year after pasture, tine cultivation to 100 mm had no greater adverse effect on pore continuity than had direct drilling using a tine drill. The importance of macropores for effective land drainage was evident from the reduced efficiency of mole channels in the presence of a cultivation pan. Root growth and activity were related to the volume of macropores but were impaired when the continuity of those pores was reduced or, as in undrained land, they became water-filled.

A technique for the non-destructive monitoring of subsurface drains

A simple borescope technique for the non-destructive monitoring of mole channels and pipe drains is described. Requirements and specifications for a suitable borescope and ancillary equipment are discussed as well as practical considerations for field use. The fully portable equipment is shown to be successful in both long and short term evaluations of subsurface drain performance.

A heuristic model of soil water regimes in clay soils in the presence of mole drainage

Dye tracers were used to record the time of travel for water movement from the soil surface to mole drains installed in a clay soil. For two events it is shown that travel times are of the order of 1·0 to 1·25 h, representing an effective transmission rate close to 12 m/d. Flow velocities within moles channels are typically of the order of 0·5 m/min. These observations can be used to explain the rapid response of mole drains as the consequence of direct movement of water from the surface to the mole channels.