J. agric. Engng Res. (1998) 71, 75—80Article No. ag980300

The Relationship among the Pre-compaction Stress, Volumetric Water Contentand Initial Dry Bulk Density of Soil

A. Alexandrou1; R. Earl2

1Department of Farm Machinery and Irrigation, Technological Education Institution of Larissa, 411 10 Larissa, Greece; 2 Silsoe College,Cranfield University, Silsoe, Bedford MK45 4DT, UK

(Received 1 September 1997; accepted in revised form 27 February 1998)

Pre-compaction can occur as a result of a combinationof soil settlement and the effect of field machinery opera-tions and, therefore, provides some insight into the load-ing history of a soil. By limiting stress to below thatwhich determined the pre-compaction, the risk of furtherdamage to the soil through additional compaction can beminimised. Pre-compaction stress can be determined ac-curately using plate sinkage test results; however, thesetests are time-consuming and require specialist equip-ment. An indication of pre-compaction stress, predictedfrom readily determined soil properties, could providea useful measure of the mechanical state of soils for useby advisory and extension staff in the planning andmanagement of agricultural mechanisation systems. Therelationship among pre-compaction stress, volumetricwater content and initial dry bulk density was investi-gated in trials conducted initially under laboratory con-ditions on a sandy loam, and in the field on a sandy loamand a clay soil.

Pre-compaction stress for sandy loam soil was foundto increase with increasing dry bulk density and decreas-ing volumetric water content which is characteristic ofthe dominance of frictional resistance within this soil. Forclay soil, however, increases in pre-compaction stresswere found to be largely independent of dry bulk densitybut closely correlated with decreases in volumetric watercontent which influences the cohesive nature of this soiltype. Relationships among these parameters were statist-ically significant for clay soil in the field and sandy loamsoil under controlled conditions in a soil bin. Similarrelationships, for a range of different soils and conditionscould complement existing soil survey databases to pro-vide indications of the susceptibility of a particular soilseries, in a given condition, to further compaction. Thesedata provide a useful management tool to aid selection of

0021-8634/98/090075#06 $30.00/0 75

the number, size and specification of equipment requiredto complete necessary land work in the time available.

( 1998 Silsoe Research Institute

1. Introduction

Pre-loading of soil is a very common event, due toeither long-term geological history or to a recent mech-anically applied load. For field soils, mechanically ap-plied load is the main cause of pre-loading, leading tocompaction. Soil which has been subjected to a compac-tion process is termed pre-compacted soil and will largelyresist further mechanical loading until that loading ex-ceeds the initial pre-compaction stress. In order to min-imize further compaction caused to soil during agricul-tural operations, it is desirable to limit soil loading toa level below the pre-compaction stress of the soil.1

Pre-compaction stress can be determined using vari-ous tests. Casagrande,2 Horn,3 Burmister4 and Schmer-tmann5 used the results of consolidation tests to developtechniques for calculating pre-consolidation stress.Koolen1 used a uni-axial compression test in order todetermine the pre-compaction stress of a soil, while Cor-dier,6 Schmid,7 Lebert et al.8 and Konijn9 examined theinfluence of soil properties on the pre-compaction stress.In all cases, soil samples were removed from the field andloaded in the laboratory. This practice, unavoidably,increases margins of error through deformation duringsampling, unknown amounts of swelling of the sampleprior to loading and a disregard of the effects of soilstructure through the use of small samples. Alexandrouand Earl10 proposed a technique which proved capable ofidentifying the pre-compaction stress of field soils in situwith good accuracy using plate sinkage test results. Thistechnique involved the use of tractor-mounted plate

( 1998 Silsoe Research Institute

Fig. 1. Determination of the pre-compaction stress of soil usingplate sinkage test results

Table 1Mechanical properties of laboratory and field soils

Sandy loam Sandy loam Clay ( field)(laboratory) ( field)

Particle density 2590kg/m3 2670kg/m3 2620kg/m3Sand content 67% 76% 8%Silt content 20% 12% 15%Clay content 13% 12% 77%Organic matter 3)4% 1)8% 3%

76 A. ALEXANDROU; R. EARL

sinkage equipment and was therefore somewhat cumber-some for general use and results were valid only forparticular soils tested and could not be extrapolated toother situations. Prediction of pre-compaction stressfrom soil properties readily available in soil surveydatabases could provide advisory and extension staffwith a tool for use in the selection of agricultural machin-ery (e.g. tyre sizes, dual wheels, flotation tyres, tandemaxles on trailers and correct ballasting of equipment). Inthis paper, the relationship between pre-compaction stressas determined using the plate sinkage test, volumetricwater content and initial dry bulk density is examined.

2. Materials and methods

2.1. Plate sinkage test

A plate sinkage test consists of a plate in contact withsoil on which a known force is applied, and a mechanismfor monitoring the resulting sinkage. Although platesinkage tests have been used for a long time, plate dimen-sions have not been standardized and researchers haveused many different shapes and sizes. A circular plate of150mm diameter was selected for the purposes of thisexperiment. This was a compromise because a smallersize might not include many peds, and a larger size wouldrequire considerable force to impart sufficient stress to thesoil.11 The same plate was used by Alexandrou and Earl.10

Research workers have carried out plate sinkage testsat various penetration velocities and have found thevariability of the results for different velocities not to besignificant12 below 80 cm/s. The velocity used for deter-mination of pre-compaction stress in Alexandrou andEarl10 (1 cm/s) was used here also.

2.2. Determination of pre-compaction stress

Alexandrou and Earl10 showed that pre-compactionstress can be determined from the results of a platesinkage test (Fig. 1). Soil was loaded to a known stress,e.g. 620 kPa in the case of Fig. 1, to represent pre-com-paction conditions (actual pre-compaction stress) whichresulted in 112 mm of sinkage following some elasticrecovery when it was unloaded. This, in effect, producedan in situ soil sample of known pre-compaction stress(620 kPa) for the purposes of testing a procedure forestimating pre-compaction stress. The soil was reloaded,during which the lower portion of the curve (RL) couldbe considered to be a recompression curve. The upperportion of this curve can be assumed to be a continuationof the previous compression line (PCL). By extending theRL and PCL lines, the point of intersection can be deter-mined and provides a prediction of the pre-compaction

conditions. During recompression (Fig. 1), minimal sink-age occurred ((5mm) when the applied stress was lessthan the pre-compaction stress, however, once this wasexceeded, considerable sinkage resulted. In practice, pre-compaction stress would not be known in advance, butcould be predicted by applying the procedure to the initialportion of the plate sinkage test results to identify the‘‘elbow’’ or discontinuity in the curve. This technique, ini-tially developed in laboratory, has been tested on field soilswith good results. In order to investigate the relationshipbetween pre-compaction stress and other easily determinedsoil properties, further plate sinkage tests were conducted inthe soil bin as well as on sandy and clayey field soils.

2.3. Soil bin tests

Initial tests were conducted on a sandy loam soil,under controlled conditions, in a soil bin of dimensions22m long by 1)7m wide by 1m deep. The results ofa mechanical analysis of the soil are presented in Table 1.A tool carrier moves along the bin on rails to facilitatethe preparation of soil to required specifications. A mast,on the front of the carrier, can be moved hydraulically inthe lateral or vertical direction. This mast was modifiedto carry out plate sinkage tests. The vertical velocity ofthe sinkage plate was governed by a hydraulic controlvalve with vertical load monitored by an extended octa-gonal ring transducer.13 Sinkage was measured usinga linear variable differential transformer (1.v.d.t.).

Fig. 3. The relationship between pre-compaction stress, volumet-ric water content and initial dry bulk density for the sandy loamsoil in the laboratory. n, volumetric water content 9—12%; ],volumetric water content 13—16%; j, volumetric water content19—22%; h, soil not subjected to pre-compaction; —, Regression

lines

RELATIONSHIP AMONG THE PRE-COMPACTION STRESS, WATER CONTENT AND DENSITY OF SOIL 77

The soil was prepared uniformly throughout its depthto given bulk density and water content specificationsusing tools and a roller mounted on the carrier. Initialdry bulk densities ranged from 1)13 to 1)55Mg/m3 withvolumetric water content from 9)8 to 21)9%. In total, 36different preparations of dry bulk density and watercontent were tested. Plate sinkage tests were conductedon each preparation and pre-compaction stress esti-mated from the discontinuity in the initial portion of testresults. The relationship between estimated pre-compac-tion stress, volumetric water content and initial dry bulkdensity was investigated.

2.4. Field work

To facilitate field testing of this technique, a DeutzIntrac 2004 tractor was adapted to accommodate theequipment. A vertical ram was fitted to the rear of thetractor (Fig. 2) and chains were used to transfer weightfrom the tractor to the mounted equipment.

The field experiments were conducted at Silsoe on twosites which had been fallow for a year. A summary of themechanical properties of the two fields soils under test isalso presented in Table 1. Both the topsoil and thesubsoil were tested on a particular occasion to increasethe range of conditions encountered. For the clay andsandy loam soil, the dry bulk density ranged from 0)99 to1)36Mg/m3 and 1)30 to 1)50Mg/m3, with volumetricwater content from 25)1 to 51)6 and 10)2 to 21)2%,respectively. A total of 16 plate sinkage tests were carriedout on each soil. Vertical force and sinkage weremonitored by a 100 kN load cell and l.v.d.t., respectively,and recorded using a 21X Campbell data logger. Thel.v.d.t. acted between the vertical ram and a separatedatum frame placed on the soil surface to eliminatespurious results due to movement of the tractor.

3. Results and discussion

3.1. Sandy loam soil

The relationship between pre-compaction stress andinitial dry bulk density for the laboratory sandy loam ata given volumetric water content is presented in Fig. 3.

Fig. 2. Tractor-mounted plate sinkage test equipment

The data have been divided into categories according totheir volumetric water content. This categorization ofdata unavoidably increases the margin of error parti-cularly close to boundaries. These categories were set atvolumetric water contents of 9—12, 13—16 and 19—22%.

Some of the test results on loose soil over a range ofvolumetric water contents could be approximated bya straight line with an intercept at the origin. For thesesoils, no evidence of previous compaction was found(Fig. 3) which implies that sinkage increases in propor-tion to the applied stress, rather than being restricted forstresses below some critical limit. Initial dry bulk densityfor these conditions was found to be less than1)27Mg/m3. For pre-compacted soil bin sandy loam,pre-compaction stress was found to be directly propor-tional to initial dry bulk density at similar volumetricwater contents, but decreased with increase in volumetricwater content for constant dry bulk density. The results,for constant volumetric water content, can be approxim-ated by an equation of the general type:

ppr"A#BD

B(1)

where ppr

is the pre-compaction stress (kPa), DBthe initial

dry bulk density (Mg/m3), A a soil parameter (kPa) andB a soil parameter (mkN/Mg). A and B were determinedby linear regression analysis. Regression results are pre-sented in Table 2 and Figs. 4(a)—(c).

The relationship among pre-compaction stress, vol-umetric water content and initial dry bulk density for thefield sandy loam soil is presented in Fig. 5. The relation-ships can be approximated by straight lines for datapoints representing pre-compacted soil, however, theserelationships are not statistically significant.

It is difficult to arrange for specific conditions to occurin field situations and the natural variation within thefield necessitated the selection of categories comprising

Table 2Regression analysis results for the relationship between pre-compaction stress, volumetric water content and initial dry bulk

density for the laboratory soil

Volumetric Soil parameterswater Significancecontent % A B R2 level P

9—12 !2630 2120 0)69 (0)0113—16 !2270 1810 0)68 (0)00119—22 !1860 1420 0)98 (0)001

Fig. 4. The relationship between pre-compaction stress, volumet-ric water content and initial dry bulk density, with confidenceintervals, for sandy loam in the laboratory at volumetric watercontents of (a) 9—12%; (b) 13—16% and (c) 19—22%. n, volumetricwater content 9—12%;], volumetric water content 13—16%; j,volumetric water content 19—22%; ——, regression lines; — 95%

confidence intervals

78 A. ALEXANDROU; R. EARL

a wide range of volumetric water contents. This, coupledwith the small number of data points, may be the reasonwhy these relationships are not statistically significant.For sandy soil, there is a tendency for pre-compactionstress to increase as initial dry bulk density increases fora given water content, due to the increased frictionalresistance of denser soil. For constant initial dry bulkdensity, pre-compaction stress of a sandy loam decreaseswith increasing volumetric water content since bondswhich connect the soil particles are weakened as a resultof a reduction of the cohesive forces when more water isabsorbed.

3.2. Clay soil

Results for the field clay soil are presented as ‘‘pre-compaction stress versus volumetric water content’’ inFig. 6, because pre-compaction stress was found to belargely independent of initial dry bulk density but closelyrelated to volumetric water content. The relationship forall data can be approximated by an equation of thegeneral form

ppr"C#Dhl (2)

where hvis the volumetric water content and C and D are

soil parameters (kPa).Linear regression analysis was performed on the ex-

perimental data and C and D were found to be 1240 and!21, respectively, with coefficient of determinationR2"0)83(P(0)001).

For the clay soil, pre-compaction stress decreases asvolumetric water content increases. This is as expectedsince bonds which connect the soil particles areweakened as a result of a reduction of the cohesive forceswhen more water is absorbed. The relationship is largelyindependent of initial dry bulk density reflecting thedominance of cohesion, rather than friction, on soilstrength.

4. Conclusions

This work has demonstrated that indications of pre-compaction stress can be obtained for a sandy loam anda clay soil from measures of volumetric water contentand dry bulk density. For sandy loam soil, pre-compac-tion stress was found to increase with increase in dry bulkdensity and decrease in volumetric water content overa range of densities and water contents likely to be

Fig. 5. The relationship between pre-compaction stress and initialdry bulk density for sandy loam soil in the field for two volumetricwater ranges. ], volumetric water content 10—15%; j, volumet-ric water content 16—17%; m, volumetric water content 21%; h,

zero pre-compacted soil; —, regression lines

RELATIONSHIP AMONG THE PRE-COMPACTION STRESS, WATER CONTENT AND DENSITY OF SOIL 79

encountered in the field. Linear relationships betweenpre-compaction stress and initial dry bulk density, forconstant volumetric water content, derived under con-trolled conditions in a soil bin were found to be highlysignificant. Similar trends were observed for sandy loamsoil in the field, however, these were not statisticallysignificant due to the limited number of data pointsavailable. For very loose soil (initial bulk density lessthan 1)27Mg/m3), no evidence of pre-compaction wasobserved which suggests that under these conditionsthere is no critical stress limit, below which soil deforma-tion can minimised.

For clay soil, pre-compaction stress was found to belargely independent of initial dry bulk density but relatedclosely to the inverse of the volumetric water content.This can be attributed to the dominance of cohesiveforces, which are strongly influenced by volumetric water

Fig. 6. The relationship between pre-compaction stress and vol-umetric water content for clay soil in the field for three densityranges. j, dry bulk density 0)99—1)12 Mg/m3; ], dry bulk density1)23—1)28 Mg/m3; n, dry bulk density 1)30—1)36 Mg/m3; ——,

regression lines; —, 95% confidence intervals

content, on soil strength rather than frictional forces. Therelationship between pre-compaction stress and vol-umetric water content for field soil was found to be highlysignificant for a wide range of water contents.

Procedures developed during this work for predictingpre-compaction stress do, however, rely on the derivationof a functional relationship between pre-compactionstress and these parameters using plate sinkage equip-ment in the field. Data from these tests, on a wide rangeof soils and conditions, could be a useful addition toexisting soil survey databases. This would enable predic-tions of pre-compaction stress to be made from readilydetermined soil properties without the need to carry outplate sinkage tests which are both time consuming andlaborious. Predictions of pre-compaction stress couldassist advisory and extension staff in the planning andmanagement of agricultural systems in terms of the num-ber, size and specification of machines required to com-plete the necessary land work in the time available whilstlimiting risk of soil structural damage.

Acknowledgements

The authors are indebted to the Greek State Scholar-ships Foundation for their financial support during theproject.

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