7/30/2019 A Procedure for the Sampling and Testing of Large Soil Cores

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N O T E S 589A PROCEDURE FOR THE SAMPLING AND

TESTING OF LARGE SOIL CORES1

K. K. W A T S O N A N D S. J . LE E S 2Abstract

The significance of acquiring reliable data on the soil-watercharacteristics of field soils is discussed in relation to the inputrequirements of computer-based numerical models of the un-saturated flow process. A specification outl ining th e conditionsto be fulfilled in sampling and testing a large soil core is thendetailed. E quipm ent used in extracting a 40-cm-diam soil coreis described together with relevant laboratory instrumentation.

Additional Index Words: soil water, soil water pressure, un-disturbed soil core.

TH E I N C R E A S I N G USE of de te rmini s t i c catchment modelsin hydrologic s tudies highl ights the need for reliablei n fo rm a t ion on the soil water characteristics of the near-surface horizons o f fie ld soils. Detailed i nve s t i ga t i ons of theinfi l t rat ion behavior o f field soils us ing inf i l t rome te r s havebeen common for the past 30 to 40 years (Horton, 1940;W ilm, 1943; Sc hiff, 1953; Watson, 1958).3 However, wi thsuch studies, the emphasis has been centered mai n l y on thesurface flux condition with little attention be ing paid to thepattern o f w a t e r movement in the prof i l e ; in addi t ion , it hasalways been di f f i cul t to ensure that one- d im ens iona l f lowconditions have prevailed. Accordingly, the s tudies haveprovided little deta i led knowledge o n t h e t i m e - d e p e n d e n twater storage characteristics of the prof i l e and the soil-w a t e r r edi s t r ibut ion regime. With the advent of computer-based numerical models, w h i c h are now able to monitor thesoil-water cond i t i ons i n a fie ld profi le under i n t e r m i t t e n tsur face f lux cond i t ions (Lees an d Watson, 1975)4, i t hasbecome necessary to have reliable i npu t data on the soil-water characteristics of field soils.A n experimental arrangement f o r de t e rm in ing the hys-teresis an d i nf i l t r a t ion- redi s t r ibut ion charac te r i s t i c s o f f ieldsoils has been described b y Watson et al. (1975). In thestudy an undisturbed monolith of soil of hexagonal crosssection and area 1.5 m2 was isolated f r om the s u r r ound i ngsoil. The base of the monolith was u n d i s t u r b e d , thus prov id -in g cont inui ty wi th th e unde r l y i ng soil. Stabil i ty problemsin the soil-water pressure measurements w e r e encountereddue to the temperature sens i t iv i ty of the t ens iome te r -pre s -sure transducer system used. However, un i t s can be de-signed (e.g. Watson, 1967) to overcome th i s problem thusallowing accurate water content and soil water pressuremeasurements for the fie ld soil to be made under na t u r a l

1 Cont r ibut ion f rom th e School o f Civil Enginee r ing, T heUn iv . of New S outh W ales , Kensington, N.S.W . , A ustral ia. Re-ceived 6 Dec. 1974. Approved 23 Dec. 1974.2 Associate Professor an d Research Fellow, respectively.3 K. K. W atson. 1958. In f i l t ra t ion s tudies with par t icu la r r e f-erence to infi l t rometer exper iments on a small rural catchment .M.E. thesis , Un iv. of New South W ales .4S. J. Lees and K. K. W atson . 1975. The use of a dependentdomain model of hysteres is in numerical soi l water s tudies .W ater Resour. Res. (In press)

environmental conditions. The main disadvantages of theapproach lie in the d i f f i c u l t y o f monitoring t he w a te r bal-ance of the monolith by direct means and the necessity oft ak i ng sophisticated and expensive equipment to the field(often in reasonably remote areas) to make the necessarymeasurements. If a ful ly instrumented caravan were avai la-ble, this latter problem could be sa t i s factor i ly overcome;h o w e v e r , considerable periods in the fie ld w ould berequ i r ed .

Materials and MethodsThis Note describes an alternative to the in situ measurement ,

namely the recovery of a large soil core from the field and thetesting of the hydrologic p roper t ies of the core in the laboratory.The design specificat ion for such an al ternat ive may be sum-marized as follows:(a) A pro fi le depth of approxim ately 100 cm is required inth e soil core to give a significant characterization of the near-surface horizons.(b) T he area of the core should be large enough to "averageout" some of the local soil inhomogeneities.(c ) T he mass of the core an d container should be l imited toapproximately 350 kg to faci l i tate t ranspo rt to the laboratory.(d ) A mobile an d inexpensive means of extract ing th e coreis required.(e ) T he core should be ins t rumented with nondestruct ivewater-content measuring equipment and with rapid-responsesoil-water pressure sensors so that any hysteresis in the soilwater characteristics can be measured, together with th e move-ment of water under the infi l t rat ion, redis t r ibut ion, and evapo-ration processes.( f ) To prevent the development of a water table condi t ionin the core fol lowing intermit tent infi l t rat ion-redis t r ibut ionsequences, a controllable lower boundary is required to main-ta in an unsaturated condi t ion at the base and thus s imulate inan approximate manner the condi t ions of the natural profi le .(g) T he core should be supported b y a weighing mechan ismso that the water balance can be cont inuously monitored,(h) W here s t rat i ficat ion of a fine over coarse mater ial is welldefined in the profi le , ins t rumentat ion faci l i t ies should beavailable to monitor poss ible wet t ing front ins tabi l i ty (Hil land Parlange, 1972) and to measure an y resultant pressuredifferential that m ay exist across a horizon tal section.A ' s tudy of the above specificat ion indicates that the maindisadvantages are the imposed base boundary condi t ion and theimpossibil i ty of s imulat ing accurately in the laboratory theenvi ronmental changes to which natural soi ls ar e subjected. In

any par t icular project these disadvan tages mus t be balancedagainst those stated earlier for the field monol i th me thod whendec id ing on the technique to be used.In the present s tudy a seam-welded mild s teel pipe of wallth ickness 4.8 mm and internal diameter 40.2 cm was used asthe cyl indr ical container for sampling the field profile. A 115-cm length of this pipe was machined at each end and then gal-vanized . The pipe was dr i l led and tapped in 45 posi t ions downit s length for the later inser t ion of t ens iome te r s an d the rmo-couple psychrometers ; brass plugs were screwed into the tappedholes. Lugs were welded to the out s ide c i r cumfe rence of thepipe near the top and the bot tom, pr ior to galvanizing. T hebot tom lugs were used for at taching a 5-cm deep cut t ing edge.T he procedure used in obta ining th e sample was to force th esampling cyl inder into th e soil in depth inc rement s of 5 to 7 cm.Befo re each incremental inser t ion of the c y l i nde r th e soil w ascu t away f rom around the cyl inder to a depth of 5 to 10 cmbelow the bot tom of the cut t ing edge, so that when the cyl in derw as forced in to th e soil a smal l amount of soi l only had to bepared away by the cut t ing edge. This reduced the force requiredto insert the cylinder and ensured that there was minimum dis-t u rb a n c e to the sample. Par t icular care had to be taken to makesure that th e cylinder w as driven in vertically an d that no hori-

7/30/2019 A Procedure for the Sampling and Testing of Large Soil Cores

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590 SOIL SCI. SOC. A M E R . PROC. , VO L. 39, 1975Pulleys

Steal cylinder

Fig. 1Sketch of portable driving equipment.zontal loading w as applied that could fracture th e core at theleading edge. Cores should be obtained when the soil is moistor wet. If the soil water content is low, it is advisable to irrigateth e sampling area prior to sampling.T he system used fo r driving th e cylinder into the soil issketched in Fig. i. It was both portable and inexpensive, andproved to be effective. The driving "hammer" comprised acylinder of lead encased in a steel jacket of mass approximately100 kg. This was dropped from a height of 3 to 4 m onto alarge block of hardwood (approx. 50 cm by 40 cm by 30 cm)which rested on the top edge of the cylinder. The lead massmoved through guides in a vertical steel frame that was held ina stable position by four guys attached to the top of the frame.A small winch with a brake attachment was used to lif t the massvi a a wire rope an d pulley system.W h en th e cylinder ha d been forced into the soil to give a coreof the required depth (100 to 110 cm) the lead mass was re-moved and the wire rope was attached to the lugs near the topedge of the cylinder. The dropping mechanism could then beused to lift the core and its cylinder. A sm all tension was ap-plied to the wire rope to take the weight of the cylinder, and thesoil beneath the cutting edge was cut through. The cylinder wasthen lifted approximately 10 cm. This allowed a 50-cm diammild steel plate to be inserted under th e soil core. T he plate w asbolted to the cylinder so that the core would not slide out of thecylinder during lifting. The winch was then used to lift thecylinder and core on to the tray of a truck for t ransport to the

laboratory. The low cost of the cylinders would allow severalsamples to be taken in this manner over a ca tchment area. E x-perience has indicated that 1 day is required to obtain eachsample.It is not intended in this Note to describe in detail the labora-tory instrumentation; however, a brief descript ion is appropriatein view of the specif ication o utlined earlier .The water content was measured using the two-tube gammaat tenuat ion method with a 3 mCi Cs-137 source in one stain-less-steel tube and the scintillation detector in the other. Thesource and detector were both heavily collimated to give awater-content sampling area of 5 mm-depth and 2 cm-width.The tubes were spaced 15 cm apart. The soil-water pressurew as measured using 20 tensiometer-pressure t ransducer unitsconnected to a data acquisition system. The tensiometer cer-amics were 1 cm in diame ter and 2 cm long epoxied to b rasstubing and inserted horizontally into the sample. Provision wasalso made for insert ing up to 19 thermocouple psychrometersin the sample. A d efined lower bo undary condit ion wasachieved by using a specially manufactured 1-bar ceramic plate12 mm thick and 40 cm in diameter. This was epoxied into arecess in a plated mild steel housing and bolted to the bot-tom of the cylinder after the cutting edge had been removed.An appropriate suction (say 5 0 0 cm of water) was then main-tained on the water in the air-tight reservoir behind the plate.The water balance was measured by supporting the entire as-sembly on three accurate load cells an d connec ting these to thedata acquisition system.

DiscussionThe system described in this Note has proved to be a con-

venient and satisfactory method for determining the hydro-logic characteristics of those field soils w h i c h exhibit onlymoderate swelling behavior. However, the di f f icu l ty ofassessing satisfactorily the non-Darcy flow through rootcracks and worm holes still remains. The surface f lux meas-urements validly include these f low components but beneaththe surface the situation is very complex due to the occur-rence of a three-dimensional distribution of water in i t ia tedfrom a random series of surface entry points.

AcknowledgmentsT he financ ial assistance of the W ater Research Found ation ofAus t ra l i a is acknowledged w ith thanks.