J. agric. Engng Res. (1966) 11 (2) 138-141

A Sampler for Extracting Undisturbed Soil Cores

C. J. BAKER*

1. IntroductionWhile the physical properties of the cores

extracted with hand motivated samplers werereported':" to have been quite adequate for theirintended purposes, the lengths of the cores wereusually limited to a few inches. The applicationof power drives and feed-down mechanismsgreatly increased the scope of the samplers,enabling longer cores to be extracted with little,if any, additional disruption to the core. How­ever, the drives generally involved a relativelyhigh initial cost 3- 5 and would be warranted onlywhere large numbers of samples are required. Amore compact design reported by Starodumov'appears to have an uncomplicated drive system,but the arrangement of the sampling tubes maystill involve considerable cost.

The sampler described herein was designedspecifically to utilize readily available and lowcost materials for the drive, feed-down andsampling mechanisms.

2. DescriptionBasically, the sampler consists of an outer­

augered cylinder with an upper thrust-drive headand a lower removable cutting head, the whole ofwhich revolves coaxially around a non-rotativeinner cylinder and soil cutting ring.

2.1. Primary drive and feed-down mechanism

A modified, commercially available post holeboring machine is employed as the driving unit(Fig. 1, top left). While the method of feed-downand drive provided proved to be satisfactory withthe range of soils encountered, they might not beadequate under some soil conditions. Morerobust and exacting mechanisms (e.g. those ofKelley et al.4 or Wellst-") COUld, no doubt, beemployed with the same sampling tubes resultingin possible improvements in performance, un­doubted improvements in convenience of opera­tion, and an increase in initial costs.

• Massey University of Manawatu, Palmerston North, N.Z.

138

2.2. Outer cylinder and thrust-drive headThe outer cylinder, of nominally 4 in i.d.

galvanized water-piping with a tapered thread(Fig. 2, A), is equipped with an external 20°single­start auger (B) and screws into the thrust-drivehead assembly. This head consists of a 4 in diawater-pipe flange welded to a circular plate andcentral sleeve, the latter attaching to the stubaxle of the post hole boring machine. A ballbearing thrust assembly, with a specially shapedself-centring seat for the upper end of the innernon-rotative tube (C), is provided within thehead, the tube being of 3 in o.d. brass with -h inwalls.

2.3. Cutting head

In Fig. 2 the non-rotative soil cutting ring andinner tube are shown with closely spaced shad­ing, while the head and outer cylinder have morewidely spaced shading. At the junction of eachauger flight with the sharpened lower edge of thecutting head , small steeply angled teeth (0) aresituated. Their purpose is to clear soil quickly inthe vicinity of the cutting tips.

The overall o.d. of the cutting head is 6! in,providing a working clearance of 1\-in for theauger flight of the outer cylinder, the initialflight being a 45° triple start type.

Fig. 1, top right shows the lower portion of theouter cylinder, with the cutting head and soilcutting ring in position.

2.4. Inner tube and soil cutting ring

The inner tube, of seamless brass (bottom,left) is split and rejoined on both sides with brasshinges. On one side the hinge pins are with­drawn when the tube is to be hinged openlongitudinally. The slight flaring at the top endfits the self-centring thrust seat in the drive headand the flaring on the bottom end, housing asmall shoulder, accommodates the upper end ofthe soil cutting ring. With the two locking pins(E) on the top edge of the soil cutting ring fittinginto the bottoms of the slits (F) in the inner tube

C. 1. BAKER

during sampling, the tube and ring become oneunit. Thrust incurred by the soil cutting ring onentering the profile is transmitted to the shoulderin the inner tube and thence to the thrust bearingabove.

In position, the soil cutting ring protrudesbeyond the sharpened lower edge of the cuttinghead (Figs 1 and2) and thus forms a non-rotativeleading cutting edge. It also provides a stableaxis about which the cutting head rotates and anon-rotative transference passage for the corepassing into the inner tube. Additionally, theprotruding portion is equipped with two smallstabilizing wings (H), each with a slight clockwiselead. As each wing travels through undisturbedsoil, any counter-clockwise rotational tendencies

139

imparted to the soil cutting ring or inner tubebecause of bearing friction and limited contactwith rotating components, is positively resisted.

The function of the internal diameter relief inthe leading portion of the soil cutting ring is toform a core small enough to minimize frictionbetween itself and the inner tube, but not so smallthat it would be a loose fit and be in danger ofcrumbling. The desirable extent of this diameterrelief varies with the soil type being sampled andits moisture content; it is at a maximum in moistclays and a minimum in dry, well structuredloamy soils.

The bevel on the leading edge of the soil cut­ting ring extends upwards beyond the sharp edgeof the head. Its purpose is to prevent soil from

Fig. 1. The soil core sampler, cutting head, 36 in core with split inner sampling tube, and partially dissected earthwormchannel at the periphery of a core (x 6)

140 A SAMPLER FOR EXTRACTING UNDISTURBED SOIL CORES

becoming wedged between these two compo­nents, thus increasing the tendency of the innercomponent to revolve with its external counter­part during sampling. It was found that as little

as a quarter-turn of the inner tube assembly wasoften sufficient to disrupt the core structure dur­ing formation. No lubrication is necessary be­tween the core and the tube walls. Where

A

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....

r....-----2 H in- - - - -fJ

~L---~-----------------~_r~~

I..

F c F

-I

/.e..,~-r"""'lfi.l ~ - - - - - - - - - - - - - - - - --Itrv-r/'""""7""U/)

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Fig. 2. Cutting head and soil cutting ring detached fromthe outer auger cylinder and inner tube

C. J. BAKER

workers have reported the need for such lubrica­tion with samplers working in anything butextremely sticky or plastic soils, it is probablethat the soil cutting ring, or similar leadingcutting component cannot have been performingits function correctly. Any disruption, howeverslight, increases the tendency of the soil to bindto the tube walls.

For the soils sampled (which ranged from clayloams to fine, sandy loams) the optimum augerspeed was about 210 rev/min.

3. Removal and transportation of coresTo remove a formed core, the soil cutting ring

is slipped out and the cutting head unlocked andunscrewed from the outer cylinder. The innertube is slipped out and hinged open to expose the36 in long core. Removal is achieved con­veniently by placing a length of semi-cylindricalcardboard newsprint-roll core (split longitudi­nally) over the exposed half of the soil core,inverting the combination and then removing thebrass inner tube altogether (Fig. 1, bottom, left).The other half of the split newsprint core is thenplaced over the soil core and the two halvesbound together with adhesive tape.

4. Determination of the state of disturbance ofcores

4.1. Specific recovery ratio (ratio of length ofcore to depth of hole)

A mean of 0·98 was achieved with the samplerdescribed, which is considered to be satisfactory,in view of the physical limitations of measuringaccurately the core lengths and the hole depths.

4.2. Visual examination4.2.1. EARTHWORM CHANNELS

Occasionally an earthworm channel would bedissected for part of its length by the leadingedge of the soil cutting ring. Close examinationand macro-photography (bottom, right) revealedthat the remaining portion of the channel wasalmost invariably left intact to within nin ofthe periphery of the core, thus providing some

141

evidence of undisturbance, at least in the area ofthe core immediately adjacent to the earthwormchannel.

4.2.2. PIT TESTING

A rectangular pit, 1t X 3 X 2 ft deep wasexcavated and portions of the smooth walls andfloor painted liberally with plastic paint. Afterthe paint had dried, the hole was back-filled andreconsolidated. Cores were taken so that halftheir thickness included the backfill and the otherhalf the undisturbed soil, with the painted surfacebetween the two. By careful picking-away of theback-fill portion from the painted surface it wasestablished that with the sampler described, it ispossible to extract cores, including one from thecorner portion of the pit, without distorting thepainted surfaces. This adds further weight to thebelief that the cores extracted were structurallyundisturbed.

AcknowledgementsGrateful acknowledgement is made to the late

M. W. Cross, Dr C. V. Fife, and D. G. Bowler,of Massey University of Manawatu, for theirassistance and guidance. The author wishes alsoto thank J. S. Tyler and the staff ofthe Engineer­ing Department of the University for technicaladvice and assistance. The post hole borer wasloaned by the Grasslands Division of theD.S.I.R.

REFERENCES

I Powell, E. B. A new soil core sampler. Soil Sci., 1926,21,53

2 Andrews, L. A.; Broadfoot, W. M. The San Dimas soilcore sampler. Soil Sci., 1958, 85 (5) 297

• Wells, C. B. Core samplers for soil profiles. J. agric.Engng Res., 1959, 4 (3) 260

4 Kelley, O. J.; Hardman, J. A.; Jennings, D. S. A soilsampling machine for obtaining two, three and four­inch diameter cores ofundisturbed soil to a depth ofsixfeet. Soil Sci. Soc. Am. Proc., 1947, 12 (1) 85

• Wells, C. B. A description and comments on a power­driven soil profile core sampler. Div. Rep. 5/59,CSIRO,1959

6 Starodumov, Yu, N. Mechanized auger for taking un­disturbed soil samples. Soviet Soil Sci., 1962(7) 770