SOIL MOISTURE AND TEXTURE EFFECTS ON MOLE DRAIN STABILITYAND ON FORCE REQUIREMENT FOR INSTALLATION — A LABORATORY

STUDY

T. G. Sommerfeldt

Research Station, Agriculture Canada, Lethbridge, Alberta T1J 4B1.

Received 13 May 1982, accepted 15 November 1982

Sommerfeldt, T. G. 1983. Soil moisture and texture effects on mole drain stability and on force requirement forinstallation — a laboratory study. Can. Agric. Eng. 25: 1^1.

The effects of soil moisture and texture on the force requiredto install mole drains and the stability of the drains afterinstallation were studied in the laboratory, andthe results were used to explain differences in mole drain performanceat twofieldsitesin southern Alberta. Foreffective durable drains, the soil should contain sufficient clay to be cohesiveand plastic (texture: clay loam or finer) and the moisture contentshould be at or above that of the plastic limit (50% offieldcapacity) duringinstallation. The forcerequired to install the drainsdepended on both textureand moisture. Belowthe plastic limit, less force was required in the loam than in the finer-textured soils, while at moisture levels above theplastic limit more force was required in the loam than in the finer-textured soils. Failure of the moles to function asdrains in a clay loam at one field location and to function in a clay loam at another field location is attributed to theirrespective soil moisture content at time of installing the moles.

INTRODUCTION

Much of the estimated 100 000 ha ofnonirrigated and 50 000 ha of irrigatedland that has become waterlogged and saline in southern Alberta could benefit fromsubsurface drainage. In some of theseareas, inexpensive mole drainage may befeasible.

Mole drainage has been used successfully in England since the 18th century(Hudson et al. 1962). There, the effectivelifeof mole drains varies fromdays to decades, and averages 10-15 yr (Nicholson1946). In the U.S.A., mole drainage hasnot been used extensively except in organic soils where at best it was only temporary (Fouss and Donnan 1962). Withthe introduction of corrugated plastic tubing and new methods for rapid installation, interest in mole drainage has diminished in North America (Raadsma 1974).

Moles often fail to function as drains.Childs (1942) in a generalized statementindicated that unsuitable soil conditionssuch as sand pockets and dry soil at thetime of moling are causes for failures. Herecognized organic matter and clay as factors that add the required stability to thesoil, yet some soils with 60% clay contentwere unstable.

In an investigation of several soilsdrained by moles, Hudson et al. (1962)found that the clay content varied from 26to 50%. They reported that clay contentalonewas not a satisfactory guide for molestability as clays vary in properties depending on different clay minerals.

Hobbs and Laliberte (1967), in a studyof shallow moles for irrigation, reportedthat the soil tended to shatter rather thancompact when the soil moisture content

was below 50% of the available moisture

range. Raadsma (1974) reported that thesoil in which the mole drain is placedshould contain sufficient moisture to becohesive and plastic to allow the channelsto be shaped without cracking or sealingof the channel walls. He did not identifywhat the minimum moisture contentshould be.

At one test in southern Alberta, moledrains failed almost immediately after installation (Rapp 1968). Failure was attributed to collapse of the moles. In anotherexperiment (Sommerfeldt, unreported),the discharge of mole drains was greater11 yr after installation than it was in thefirst year. The soil texture class at bothsites was clay loam. Also, at the site thatfailed, the power required to install thedrains was greater and the soil was drierthan at the other site (Paziuk, pers. com-mun.).

The power requirement to install moledrainage is a majorcost, thoughcomparedwith other drainage options the cost of installing mole drainage is relatively small.The power requirement decreases with increasing soil moisture (Hobbs and Laliberte 1967; Norum and Gray 1970). However, Norum and Gray specify that theirresults applied within the moisture rangeof 0.15-0.23 for the sandy loam,0.10-0.28 for the loam, and 0.11-0.25 forthe clay loam (dry weight basis). Theyalso reported that less power is requiredin sandy soil than in clayey soils.

The objectives of this laboratory studywere to determine the effects of soil moisture and texture on the stability of moles,and on the relative power requirement fortheir installation. The results are used to

CANADIAN AGRICULTURAL ENGINEERING, VOL. 25, NO. 1, SUMMER 1983

explain differences in mole drain performance at two field sites in southern Alberta.

MATERIALS AND METHODS

Materials

The torpedo used to form the moles was19 mm in diameter and 50 mm long. Acable (2-mm diam.) was attached to thebullet-shaped nose to pull the torpedothrough the soil. (In the field machine, thetorpedo is horizontally attached to the endof a shank that extends to the surfacewhere the force is applied to pull the torpedo through the soil.)

The soil bins used were 30 cm squareand 33 cm deep. A port, 33-mm diam. and80 mm above the base (at center), wascentrally located at each end. The torpedowas pulled through these ports.

A hand-powered winch, fastened to oneend of a 3-m long bench, was used to pullthe torpedo through the soil. A bracketheld the soil bin at the other end of thebench while the torpedo was pulledthrough the soil. A spring-scale with a cursor to indicate the maximum amount of

force applied while pulling the torpedothrough the soil was installed in the linkage between the winch and the torpedo.

Three subsoils were used for this experiment, a loam (L, 39.4% sand and18.5% clay), a clay-loam (CL, 40.5%sand and 32.9% clay), and a clay (C,15.5% sand and 66.0% clay). In preparation for the tests the soils were air-driedand crushed to pass an 8-mm sieve.

Methods

The force required to pull the torpedothrough the soil was determined in triplicate oneach soil at six soil moisture levels: AD (air

dry), 0.25 FC, 0.50 FC, 0.75 FC, FC, and 1.25FC(FC = moisture content at 20 kPa suction).To obtain these moisture levels, the amount ofsoil needed to fill a bin was spread out on thebench and the required amount of water wassprinkled on it. This was thoroughly mixed,sealed in a plastic bag for 1 wk, and stirreddaily by rolling the bag.

The bulk density of each soil for all moisturelevels was approximately 1.5, 1.4, and 1.3 forthe L, CL, and C, respectively. (The bulk density of the CL and C in the field would be inthe order of 1.45 and 1.4. These densities were

impossible to attain by the method used.)These bulk densities were obtained by repeatedly dropping the bin, containing the requiredamount of equilibrated soil, from a height of50 mm onto a solid block until the depth of soilin the bin was 30 cm. These densities were

easily obtained when the soil moisture was0.50 FC or less. Above 0.75 FC it was increas

ingly difficult to obtain the desired bulk densityas the soil moisture content increased.

Before the bin was filled with soil, the cablewas suspended with clamps horizontallythrough the centers of the ports. After the bulkdensities were established, the cable at thefront of the bin was attached to the scale, whichwas attached to the cable from the winch. At

the rear of the bin the torpedo was attached tothe cable. When the linkage was completed,the clamps holding the cable in place were removed. The torpedo was then steadily pulledthrough the soil at a rate of 60 mm/sec. Theforce as indicated by the cursor was recorded.

The channel left by the torpedo was examined for stability. To determine the durabilityof the moles, the bin was dropped three timesfrom a height of 50 mm onto a solid block andthe channel was re-examined for stability.

The stability of the moles in the soils at FCmoisture level was further evaluated by leaching water (continuous head, 1 cm) through thesoil into the channels and out through the portsfor as long as 14 days. Durability of the channels was determined visually.

RESULTS

Stable moles were only obtained at soilmoisture levels of 0.50 FC or more (Table

I). At the 0.50 FC moisture level, themoles in all three soils remained fullyopen but, when dropped, those in the Lsoil collapsed. At 0.25 FC some cohe-siveness was evident in that the moles re

mained partially open behind the torpedobut they all collapsed upon dropping. Inthe AD condition, there was no evidenceof cohesiveness as the moles collapsed immediately behind the torpedo. At soilmoisture levels greater than 0.50 FC, inall soils, the moles remained fully openand stable behind the torpedo, except inthe C at 1.25 FC — the moles flowed shut

after standing for a few minutes. When thesoil bins were dropped, the moles in theL soil collapsed at all moisture levels. Inthe CL and C soils they retained theirshape at the 0.75 FC level and were deformed at the FC level and at the 1.25 FC

level in the CL.

The mole walls in the L soil had a

smooth, dry, compacted appearance thatbecame more evident with increased soil

moisture. At 1.25 FC mole walls acquireda somewhat vitreous appearance and,when dropped, they shattered.

In the CL and C soils, the mole wallsat the 0.50 FC level of moisture had a

scaly rough appearance that became morepolished as the soil moisture increased.The scaliness, created by friction drag onthe torpedo, was evident at all moisturelevels but it decreased with increasing soilmoisture levels.

When subjected to leaching, the molesin the L soil collapsed within 1 h after thewater was applied. However, the moles inthe CL and C soils remained fully openand operative for the 14-day duration ofthe test.

The force required to pull the torpedoin the CL and C soils increased with in

creasing soil moisture to 0.50 FC (Fig. 1)and then decreased with increasing moisture level to FC. For the C soil there was

continued decrease in force to 1.25 FC.

Force in the L soil increased generallyfrom AD to 1.25 FC with deviations at

0.25 FC and 0.75 FC.

The force required remained generallyconstant while pulling the torpedo in itsentire travel through the soil.

At moisture levels of 0.50 FC and less,the required force in the CL and C soilsexceeded that in the L by 1-152% (Fig.1). When the moisture levels exceeded0.50 FC, the required force in the CL andC was 10-78% of that required in the Lsoil.

By the absence of visible fracture lines,it seemed that soil displacement duringformation of the mole was radial rather

than in one direction, such as vertical. Thedepth of mole, below the soil surface, relative to the torpedo diameter evidently exceeded critical depth at which vertical lifting of the over-burden by the torpedowould occur.

DISCUSSION

Clay content of the soil was an important factor in this experiment. It affectedmole stability, soil plasticity, and forcerequired to form the mole depending onthe soil moisture content. The sand con

tents of the L and CL soils were similar,but the clay content of the L was aboutone-half that of the CL. The moles in the

L collapsed when subjected to droppingand leaching treatments and the soil wasweakly to non-plastic. The moles in theCL soil were stable under the droppingand leaching treatment and the soil wasplastic. The difference in responses is attributed to clay content. The clay contentin the L was not sufficient to provide thecohesiveness necessary to form stablemoles. According to Kohnke (1968), soilswith less than 14-16% clay do not exhibitplasticity in any moisture range. Further,the amount of clay necessary to make, a

TABLE I. VISIBLE CONDITIONS OF MOLES IN L, CL, AND C SUBSOILS AT DIFFERENT SOIL MOISTURE LEVELS AFTER FORMING ANDAFTER DROPPING (A DISTANCE OF 50 mm) THREE TIMES

Soil

moisture

level

Condition of mole after

forming dropping

ADt Collapsed0.25 FC 35% open Collapsed0.50 FC Open, smooth surface Collapsed

0.75 FC Open, smooth surface Collapsed

FCt Open, smooth surface Collapsed-fragmentsdislodged

1.25 FC Open, but cracked Collapsed-shattered

tAir dry.tMoisture content at 20 kPa suction.

CL

Condition of mole after

forming dropping

Collapsed35% open CollapsedOpen, rough surface Open, stable

Open, some surface Open, stablesmear

Open, rough surface. Open, deformedsoil adhered to torpedoOpen, surface smear Open, deformed

Condition of mole after

forming dropping

Collapsed75% openOpen, some surfacesmear

Open, surface smear

Wet, plastic

Open, flowed shut onstanding

CollapsedOpen, stable

Open, stable

Open, deformed

CANADIAN AGRICULTURAL ENGINEERING, VOL. 25, NO. 1, SUMMER 1983

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SOIL MOISTURE LEVEL

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Figure 1. Force required to pull torpedo in a loam (L), a clay loam (CL) and a clay (C) at sixsoil moisture levels: AD (air dry), 0.25 FC, 0.50 FC, 0.75 FC, FC and 1.25 FC (FC:20 kPa suction).

than in the finer-textured soils. Above

0.50 FC our data are inverse to theirs.

When the soil moisture level was in the

plastic range less force was required in theC and CL soils than in the L soil.

The deviations in force required to pullthe torpedo through the L soil are attributed to soil characteristics. Standard de

viations around each of the data pointswere small, discounting the question ofdata overlap. The L soil was the subsoilof a fine sandy loam surface soil and probably had many of the characteristics of itscoarser-textured overburden. Its clay content was undoubtedly borderline for exhibiting plasticity. Seemingly, the moisture content at 0.25 FC and 0.75 FC was

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ideal for reducing resistance to the torpedo.

The clay seemed not only to give plasticity and cohesiveness to the soil but italso acted as a lubricant when the moisture

level exceeded the plastic limit level.These laboratory results indicate that,

when installing mole drains, the soilshould contain sufficient moisture to be

plastic, that is 0.50 FC or wetter, and contain sufficient clay (CL texture or finer) tobe plastic and cohesive for forming stablemoles. As the moisture level increases

above the plastic limit, the power requirement for installing the drains will decreaseaccordingly in plastic soils.

These laboratory results explain whyone mole drain system in the field failedand the other functioned well. The two

mole drain systems were installed insouthern Alberta, both in clay loam soilat drain depth (about 75 cm). One with a36.5% clay content failed immediately(Rapp 1968). The other, with 29.0% clay,had a greater discharge 11 yr after installation than during the first year of operation under similar conditions (Fig. 2). Thepower requirements for installing themoles in the site that failed greatly exceeded those at the site that gave years ofservice (Paziuk, pers. commun.). Apparently, the important difference in the twosystems was the soil moisture content atthe time of installation. The soil at the site

where the mole failed was drier and, basedon these results, the moisture content wasbelow the plastic limit, while at the successful site it was equal to or greater thanthe plastic limit.

Mole drains are less costly to install

soil plastic depends on the type of clay andon the relative amounts of sand and silt.

The minimum moisture level of 0.50FC for the moles to remain open after formation, and the reductions of force required to pull the torpedo through with soilmoisture contents at greater than 0.50 FC,are indications that 0.50 FC was near thelower plastic limit (Baver et al. 1972). Theplastic limit extended to the 1.25 FC levelfor the CL soil, but it seems that the upperplastic limit was exceeded and the liquidlimit was reached in the C at 1.25 FC.

In the CL and C soils the relationshipof force required to pull the torpedo andsoil moisture was similar to that of shear

value and soil moisture reported by Baveret al. (1972). In other words, the amountof force required in each case increasedwith soil moisture to a maximum at theplastic limit and then decreased with increased soil moisture. However, the ratesof change in force were not the same. Similarly, within the plastic range these datasupport the findings of Hobbs and Lali-berte (1967) and Norum and Gray (1970)in that the force required decreased withincreasing soil moisture. Norum and Grayspecified that their data were applicablewithin moisture ranges of 0.15-0.23 forsandy loam, 0.10-0.28 for loam and0.11-0.25 for clay loam. These moistureranges appear to have been in the plasticrange for the loam and clay loam soils andin the liquid range for the sand.

With regard to textural effect, Norumand Gray (1970) found the force requiredincreased with increasing fineness of thesoil. Below 0.50 FC these results are in

the same order as theirs, in that less forcewas required in the coarser-textured soil

1969 1971 1973

YEAR

J_ _L

1975 1977 1979

Figure 2. Mean mole drain discharge immediately after floodings to bring the water table tothe soil surface, over an 11-yr period.

CANADIAN AGRICULTURALENGINEERING, VOL. 25, NO. 1, SUMMER 1983

than other forms of subsurface drainage.Thus, it would be economically feasibleto re-install them periodically should thesystem fail. Mole drainage may also beuseful in conjunction with subsurface tubedrainage in a slowly permeable soil. Themoles would serve as a collecting network, conducting water to the tube drains.

ACKNOWLEDGMENT

The suggestions and assistance of D. E.Campbell, B. J. Lamond and N. Paziuk arerecognized and appreciated.

REFERENCES

BAVER, L. D., W. H. GARDNER, and W.R. GARDNER. 1972. Soil physics. 4th ed.John Wiley and Sons, Inc., New York.

CHILDS, E. C. 1942. The mechanics of moledrainage. Emp. J. Exp. Agric. 10: 169-181.

FOUSS, J. L. and W. W. DONNAN. 1962.Plastic-lined mole drains. Agric. Eng. 43:512-515.

HOBBS, E. H. andG. E. LALIBERTE. 1967.Mole irrigation. Can. Agric. Eng. 9: 1-2.

HUDSON, A. W., H. G. HOPEWELL, D. G.BOWER, and M. W. CROSS. 1962. Thedraining of farm lands. Massey College,Palmerston, North, New Zealand.

KOHNKE, H. 1968. Soil physics. McGraw-

Hill Book Co., New York.NICHOLSON, H. H. 1946. The principles of

field drainage. Cambridge University Press,London.

NORUM, D. I. and D. M. GRAY. 1970. Un-lined mole lines for irrigation. Trans. ASAE(Am. Soc. Agric. Eng.) 13: 661-663, 668.

RAADSMA, S. 1974. Current drainage practices in flat areas of humid regions in Europe. Pages 115-143 in J. van Schilfgaarde,ed. Drainage for Agriculture. No. 17, AgronSeries, Am. Soc. Agron., Inc., Madison,Wise.

RAPP, E. 1968. Performance of shallow subsurface drains in glacial till soils. Trans.ASAE (Am. Soc. Agric. Eng.) 11:214-217.

CANADIAN AGRICULTURAL ENGINEERING, VOL. 25, NO. 1, SUMMER 1983