compaction of restored soil by heavy agricultural machinery—soil physical and mechanical aspects

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Soil & Tillage Research 93 (2007) 28–43

Compaction of restored soil by heavy agricultural

machinery—Soil physical and mechanical aspects

B. Schaffer *, W. Attinger, R. Schulin

Institute of Terrestrial Ecology, ETH Zurich, Universitatstrasse 16, 8092 Zurich, Switzerland

Received 19 August 2005; received in revised form 6 February 2006; accepted 21 March 2006

Abstract

As construction and open-cast mining activities continue to expand on fertile agricultural land, the removal and subsequent

restoration of soil to be re-used for plant growth has become an increasingly important issue in soil protection. A key factor for the

success of soil restoration is that the soil is allowed to develop sufficient mechanical strength to withstand the stresses involved in the

intended type of land use. The objective of this study was to investigate the effects of the first use of heavy agricultural machinery on the

physical and mechanical properties of a restored soil after the period of restricted cultivation (as prescribed by current guidelines), when

the soil is re-submitted to normal agricultural management. We performed two traffic experiments on a soil which had been restored

according to current guidelines 4 years before the beginning of the study. In the first year of the study, a combine harvester passed two

times across the wetted experimental area, and in the following year 10 times. Two passes along the same tracks caused only weak

compaction effects, mainly reducing coarse porosity. In contrast, after 10 passes, deep ruts had formed, and coarse porosity was

drastically reduced down to the subsoil. Confined uniaxial compression tests revealed an increase in precompression stress and a

decrease in the slope of the virgin compression line, i.e. the compression index, after 10 passes. However, precompression stress was

still much lower than the exerted soil stresses at the corresponding soil depths, indicating that due to the short duration of the wheel

loadings equilibrium conditions were not reached in the traffic experiments and that further compaction would have occurred with

additional passes. The decrease in compression index found after 10 passes may be due to the practice that samples are pre-conditioned

to a specified water tension for the oedometer tests. The results show that loads may exceed precompression stress for short durations

even in a restored soil which is still far from having re-gained normal strength without serious damage. Thus, the use of precompression

stress as a criterion for traffickability was on the safe side in preventing damage to the ecological quality of the soil by compaction, even

if the concept did not fully apply to the field reality of the mechanical stress conditions.

# 2006 Elsevier B.V. All rights reserved.

Keywords: Soil restoration; Soil compaction; Precompression stress; Compression index; Bolling probe pressure

1. Introduction

Large areas of fertile land are temporarily used for

construction and open-cast mining purposes or perma-

nently destroyed by building. In the course of these

activities, large quantities of soils are excavated. In

* Corresponding author. Tel.: +41 44 633 61 43;

fax: +41 44 632 11 08.

E-mail address: [email protected] (B. Schaffer).

0167-1987/$ – see front matter # 2006 Elsevier B.V. All rights reserved.

doi:10.1016/j.still.2006.03.007

many cases these soils are later restored at the original

site as in many open-cast tunnelling, pipeline construc-

tion and gravel exploitation projects or used for soil

restoration at a new site. In the Canton of Zurich

(Switzerland), for instance, 50–60 ha of agricultural

land are presently restored per year for re-cultivation of

closed gravel pits and other landscaping activities. This

corresponds to 0.07–0.08% of the total agricultural area

of the canton (about 75 000 ha). Thus, restoration of soil

to be re-used for plant growth has become an

increasingly important issue in soil protection.

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http://dx.doi.org/10.1016/j.still.2006.03.007

B. Schaffer et al. / Soil & Tillage Research 93 (2007) 28–43 29

Excavation, transport and repacking disrupt the

structure of a soil and cause a rearrangement of clods,

aggregates and particles. This leads to mechanical

destabilization and an increased risk of soil compaction.

The risk depends on many factors, including soil wetness,

texture and stabilization by plant roots, as well as on the

care with which the soil is handled (Schroder et al., 1985).

The increased risk of compaction does not end with the

rebuilding of the soil. Freshly restored soils have a low

degree of aggregation and are very susceptible to

compaction (Lebert and Springob, 1994). Over the

years, strength will redevelop (Schneider and Schroder,

1991) due to physical (Voorhees, 1983; Bullock et al.,

1985), chemical (Dexter et al., 1988) and biological (Von

Albertini et al., 1995) regeneration processes. The

formation of aggregates is considered to play a key role

in this process (Horn, 1983, 1988; Baumgartl and Horn,

1991; Lebert and Horn, 1991).

It is important for the success of a soil restoration that

the soil is allowed to regain sufficient mechanical

stability before it is used again for agriculture. Undue

handling during cultivation operations within the first

years after restoration may easily damage or even

completely destroy the weak soil structure and thus

reduce water conductivity (Logsdon et al., 1992;

Arvidsson, 2001) and air permeability (Horn, 1986;

Gysi et al., 1999). This in turn may have negative

ecological (Soane and van Ouwerkerk, 1995) and

economical (e.g. yield losses: Hakansson and Reeder,

1994) consequences. On the other hand, economic

interest in the re-use of restored soil for crop production

creates pressure to minimize the time of restricted

cultivation. Responding to this conflict between

economic interests and the imperative of sustainability,

guidelines like those of Zwolfer et al. (1991), VSS

(2000), BUWAL (2001) and FSK (2001) have been

issued. These guidelines only allow very restricted land

use for at least three to four years after restoration in

order to avoid compaction by excessive mechanical

stresses. The guidelines are based on practical

experience and to some degree represent a compromise

between land use and soil protection interests.

The high compaction risk of restored soils calls for

preventive measures. The regeneration of mechanical

stability after disturbance depends on soil properties

and on the way how the soil is repacked and

subsequently cultivated (Lebert, 1991; Davies and

Younger, 1994). Therefore a stability criterion is needed

that is directly related to the mechanical properties of

restored soils. The concept of precompression stress has

been applied to agricultural soil mechanics e.g. by Horn

(1981) and Kirby (1991a). Precompression stress is

determined from compression curves (void ratio versus

logarithm of normal stress) obtained by confined

uniaxial compression tests (Koolen, 1974). Concep-

tually, it indicates the maximum stress a soil has been

submitted to before under given conditions (Kirby,

1991a; Veenhof and McBride, 1996). According to the

conventional concept of precompression stress, defor-

mation is elastic (reversible) at stresses below and

plastic (irreversible) above precompression stress.

Compression above precompression stress occurs along

the virgin compression line (VCL). The slope of the

VCL, i.e. the ‘‘compression index’’ (CI) according to

Larson et al. (1980), is inversely related to soil stiffness.

Upon unloading, the maximum applied stress becomes

the new precompression stress, and upon reloading, the

soil is further compressed along the original VCL when

the applied stress exceeds this new precompression

stress. Thus, the concept implies that precompression

stress increases during compaction, whereas CI remains

unaffected.

Its conceptual features make precompression stress

attractive as a criterion for the limit up to which a soil

may be loaded without irreversible damage to its

ecological functions, i.e. as an indicator of ‘‘ecological’’

trafficability. The latter should be distinguished from

what may be called ‘‘technical’’ trafficability, for which

e.g. the empirical ‘‘California Bearing Ratio’’ (CBR,

see e.g. Porter, 1950), which is an index of the shear

strength of a soil (Turnbull, 1950), is widely used.

Despite the advantage of the precompression stress

concept that it directly relates soil stress to strength,

there are a number of uncertainties when using

precompression stress determined by laboratory tests

as an indicator of the maximum stress experienced by

the soil in situ. First, the concept presupposes that

stress–strain conditions in soil samples during uniaxial

compression tests are comparable to those in undis-

turbed soil subjected to mechanical load in the field. In

particular, the concept relates to equilibrium conditions

(i.e. sufficiently long exposure to a load, slow increase

in load) and laterally confined expansion. These

conditions may often not be sufficiently fulfilled in

field situations. As a consequence, the (time-dependent)

stress tensors describing the stress distribution below a

moving wheel (e.g. Abu-Hamdeh and Reeder, 2003)

and in an uniaxial compression test (Koolen, 1974) can

be quite different.

Second, precompression stress is conditional on the

soil moisture status and the drainage conditions, which

are usually not the same in laboratory tests and the field.

Third, the concept refers to conditions with no changes

in the structural arrangement of soil particles (i.e.

B. Schaffer et al. / Soil & Tillage Research 93 (2007) 28–4330

turbation, kneading or crack formation) other than

vertical displacement. Fourth, the procedure by

Casagrande (1936), which is usually applied to

determine precompression stress, is suited to define

precompression stress when the change from purely

elastic to plastic deformation occurs rather abruptly.

However, in unsaturated soils, the transition is often

very gradual (e.g. Arvidsson and Keller, 2004), and

plastic deformation is likely to take place even below

the stress defined by the Casagrande procedure (Keller

et al., 2004). Finally, sampling, sample trimming (Muhs

and Kany, 1954), sample dimensions (Muhs and Kany,

1954; Koolen, 1974), as well as the experimental testing

method (e.g. Keller et al., 2004) will influence the

results.

The concept of precompression stress was tested for

validity in several field studies on land with undisrupted

subsoil. Gysi et al. (1999) found that after a single

vehicle pass, precompression stress was increased to

approximately the maximum soil pressure where soil

stability was exceeded. Semmel and Horn (1995)

reported that intensive trafficking during several years

increased precompression stress in comparison to soil

under conservation tillage. Keller et al. (2002) observed

distinct permanent soil displacement at locations where

the vertical stress exerted by the machine exceeded the

precompression stress and much less, but still measur-

able, residual displacement under conditions where the

applied load did not exceed the precompression stress.

Arvidsson et al. (2001) found a good agreement

between the calculated depth of compaction and the

depth to which vertical soil displacement was observed.

Also Berli et al. (2004) showed that expected and

measured compaction after trafficking with tracked

machinery corresponded well. However, we are not

aware of any field studies in which the precompression

stress concept was tested for restored soils. Thus, the

objective of this study was to investigate the effects of

the first use of heavy agricultural machinery on the

physical and mechanical properties of a restored soil

and to determine whether compaction effects were in

agreement with the concept of precompression stress as

outlined above.

We performed two traffic experiments on a restored

soil at the end of the period of restricted cultivation

prescribed by guidelines, after which the soil is

submitted to normal agricultural management for the

first time. During the first experiment a fully loaded

combine harvester made two passes and during the

second experiment 10 passes over wetted areas of

restored land. We measured soil pressure during the

passages of the machine and determined mechanical

and physical soil properties of samples taken from

trafficked and non-trafficked locations.

2. Material and methods

2.1. Test site

The traffic experiments were performed at a restored

site in the north-western part of Switzerland near

Solothurn (78330E, 478120N). The soil of the site had

been restored in autumn 1999 for agricultural cultiva-

tion above a highway tunnel which had been

constructed in open-cast fashion. The original soil

was a Eutric Cambisol. Restoration operations had been

allowed only under conditions where the water potential

of the soil was below �6 kPa. The soil had been

repacked in stripes of 6 m width using a crawler

excavator which never trafficked the freshly deposited

soil (Rohr and Schuler, 2004). The restored soil

consisted of a 40 cm layer of former topsoil material

above a 70 cm layer of former subsoil material. The

subsoil was built on a 40 cm thick drainage layer,

consisting of gravel-rich sand, which in turn was built

on top of an impermeable bottom layer (artificially

compacted excavation material consisting of clay and

sandstone), separating the restored soil from the tunnel

roof.

2.2. Management and preparation of the test site

A few days after the soil had been put in place in

1999, radish (Brassica oleara) was sown manually. In

the following spring (2000), after the topsoil had been

mulched, grubbed and harrowed, a grass mixture (UFA

323 Gold) including clover and alfalfa (Medicago

sativa) was sown. In late spring the grass was mown and

left on site. All these operations were conducted using a

Steyr 968 tractor with a total weight of 3985 kg. The

maximum additional weight of tillage equipment on the

tractor was approximately 650 kg. This corresponds to

mean ground contact pressures of 28–33 kPa (static

load). In the following 3 years, the site was mowed two

times (2001) to three times (2002 and 2003) a year,

using the same Steyr 968 equipped with a front mower.

The mean contact pressure of the tractor during these

operations was 33 kPa. The grass was harvested either

for silage by means of a Jaguar crop chopper of 6830 kg

with a mean contact pressure of 48 kPa or as hay after

drying on-site. For transportation a trailer was used that

weighed 4000 or 6000 kg when fully loaded with hay or

silage, respectively. The resulting mean ground contact

pressures varied between 113 and 170 kPa under the


Fig. 1. Layout of the two traffic experiments (TE1: 2 passes; TE2: 10 passes) with sampling and measuring locations of TE1 (N1: non-trafficked; T1:

trafficked; T1a: trafficked, additional sampling under the second track; B1: Bolling probes; TM1: tensiometers) and of TE2 (N2: non-trafficked; T2:

trafficked; B2: Bolling probes; TM2: tensiometers).

Table 1

Soil water potentials at time of trafficking (averaged values; standard

errors in parentheses)

Traffic

experiment

Sub-plot Soil water potential (kPa)

0.15–0.20 m

depth

0.30–0.35 m

depth

TE1 1 �2.9 (0.2) �2.8 (0.3)

2 �4.2 (0.4) �3.2 (0.3)

TE2 1 �3.0 (0.1) �2.4 (0.1)

2 �2.8 (0.1) �2.1 (0.1)

trailer and between 37 and 42 kPa under the tractor.

From summer 2003 until the end of our experiments in

autumn 2004, manual mowing was the only operation

performed on the test site.

During the whole period soil water potentials were

monitored using tensiometers. Operations on the land

were only allowed when the soil water pressure was

below �15 kPa in the topsoil and below �10 kPa in the

subsoil (Rohr, 2002). Equipment with ground contact

pressures higher than 60 kPa was allowed to operate

only when the soil was definitely drier than that, i.e.

when the respective tensiometer readings gave more

negative values than these limits. Thus, cultivation fully

conformed with Swiss guidelines (VSS, 2000;

BUWAL, 2001; FSK, 2001).

In autumn 2004, after the traffic experiments had

been completed, the soil was ploughed to a depth of

0.20 m, and cultivation was changed from grassland to

crop rotation. Adopting conventional terminology, we

refer to the upper 0.20 m as ‘‘topsoil’’ and to the soil

below as ‘‘subsoil’’.

2.3. Traffic experiments

The first of the two traffic experiments (denoted as

TE1) was conducted in summer 2003 and the second

(denoted as TE2) in summer 2004. Their design is

depicted in Fig. 1. TE1 consisted of two passes,

representing a moderate loading as it may occur within

a field during normal cultivation. TE2 consisted of 10

passes. It represents the situation of high loading as it

may occur at the edge of a field, where traffic

concentrates and manoeuvres are carried out.

Because we were interested in the effects of

trafficking under realistic worst case conditions the

two sub-plots were irrigated several times in the days

before the experiments. After the last irrigation, the

water was left to drain and to redistribute in the profile

for 1 day before the soil was trafficked. Tensiometer

readings were taken immediately before the passages

with the combine harvester. Soil water potentials were

similar in both traffic experiments (Table 1). They

ranged between �3 and �4 kPa in the topsoil and

between �2 and �3 kPa in the subsoil.

2.4. Machine properties

For both traffic experiments we used the same

combine harvester, model Claas Dominator 76, fully

loaded and equipped with a mower bar (Fig. 2). The

total weight of the machine was 9730 kg. The combine

harvester had V-shaped Goodyear Traction Sure Grip

front tyres (dimensions 23.1–26) and Dunlop rear tyres

with longitudinal ribs (dimensions 12.5/80–18). Tyre

inflation pressure was around 120 kPa in both experi-

ments. As both the front and rear tyres were slightly

flattened on the ground, the tyre–soil contact areas were

approximately of rectangular shape. Length and width

of the contact areas were determined on hard ground

using a measuring tape. Mean ground contact pressure,


Fig. 2. Combine harvester used for the traffic experiments.

i.e. the ratio between wheel load and contact area

(length � width of the respective tyres), was around

120 kPa under the front tyres and 200 kPa under the rear

tyres (Table 2). Shape and dimensions of the contact

areas did not vary visibly in the course of the traffic

experiments.

The rear axle of the machine was slightly narrower

than the front axle. Consequently, the tracks of the front

and rear tyres were slightly displaced relative to each

other and the total track width was larger than the width

of the individual tyres (Fig. 1 and Table 2). The speed of

the vehicle was approximately 0.2 m s�1 in both

experiments.

2.5. Measurements and parameter determinations

Soil pressure during the vehicle passes was measured

by means of Bolling probes (Bolling, 1987) installed at

a depth of 0.32 m directly under as well as 0.13 m to the

left and to the right of the centre line of the front tyre

track (denoted as centre, inner and outer probes with

regards to the centre of the vehicle axle). Pressure

readings were taken at a frequency of 32 s�1 by means

of a data logger.

Samples were collected within 1–4 days after

trafficking from two sub-plots at sampling locations

below and beside the tracks, as depicted in Fig. 1.

Samples were taken from the topsoil at 0.12–0.22 m

depth and from the subsoil at 0.27–0.37 m depth (with

respect to the soil surface of the respective sampling

locations after trafficking) using sharpened steel

cylinders with a volume of approximately 1 l (diameter

Table 2

Characteristics of the fully loaded and equipped combine harvester used fo

Wheel load

(kg)

Width of contact

area (m)

L

a

Front tyres 3455 0.52 0

Rear tyres 1410 0.27 0

10.8 cm, height 11.0 cm). Due to the formation of ruts,

it was not possible to take the samples from the

trafficked locations at exactly the depths corresponding

to the specified sampling depths before displacement by

wheeling. However, topsoil and subsoil of the recon-

stituted profile were quite homogeneous and the

differences between the sampled layers were small,

indicating that vertical gradients were almost negli-

gible. As the results show, the differences between the

two also remained small after trafficking. Thus, any

error that may have originated from taking all samples

at the same depths with reference to the ‘‘modified’’ soil

surface (i.e. also in the ruts after trafficking) was

considered tolerable. Twelve samples were taken per

sub-plot, depth and treatment, resulting in 96 soil

samples per experiment. After TE1 we also sampled

five cores per sub-plot and depth under the second track

(total of 20 samples from T1a locations in Fig. 1) in

order to check if the effects under the two tracks were

comparable. These cores were sampled 11 weeks after

the first traffic experiment. Therefore, they were not

included in the evaluation and statistical analysis of the

trafficking effects.

The soil samples were stored at a temperature of 4 8Cuntil they were analyzed for coarse porosity, fine-to-

intermediate porosity, bulk density, precompression

stress and CI. The samples were water-saturated,

weighed, and subsequently desorbed to a water

potential of �6 kPa (value with respect to the centre

of the sample) by applying a hanging water column.

Then they were weighed again. The drained pore

volume was interpreted as the coarse porosity that is

drained at field capacity (AG-Boden, 1982; Berli et al.,

2004).

Thereafter we used the samples conditioned to

�6 kPa water potential to conduct confined uniaxial

compression tests as described by Berli et al. (2004).

The cylinders with the samples were built into the

compression cells of oedometers. The samples were

subjected to 16 steps of increasing pressure from 4 to

600 kPa. Each compression step lasted for 1800 s. In

general, this time allowed for drained compression

conditions. Only at very high pressures, some samples

were not sufficiently drained. This resulted in a decrease

r the traffic experiments

ength of contact

rea (m)

Contact area

(m2)

Ground contact

pressure (kPa)

.55 0.2860 119

.25 0.0675 205


of the slope of the VCL. When this occurred, the

respective measurements were not taken into account in

the analysis. Precompression stress was determined

numerically from the resulting compression curves

according to the procedure of Casagrande (1936) using

an Excel spreadsheet. The bisector of the horizontal and

tangent through the point of maximum curvature was

calculated, and the VCL was determined by fitting a

regression line to the linear part of the compression

curve at high stresses. The relevant points were chosen

by visual inspection. Precompression stress was

calculated from the intersection of the bisector and

the VCL, and the CI from the slope of the VCL.

After compression, the samples were dried at 105 8Cfor at least 6 days and weighed again. The pore volume

desorbed between �6 kPa water potential and oven-dry

state was interpreted as fine-to-intermediate porosity.

The oven-dry weight was also used to calculate bulk

density (Blake and Hartge, 1986). Finally, the dried

samples were taken out of the steel cylinders and wet-

sieved, using a 2 mm sieve. The coarse fraction was

dried at 105 8C and weighed. We used the mean gravel

density of the samples to calculate the volumetric gravel

content.

To avoid that heterogeneities in the spatial distribu-

tion of the coarse fraction within the test site would add

additional variance to the data, we subtracted its volume

and mass from the respective values of the bulk soil

and calculated the coarse porosity CPfe, the fine-

to-intermediate porosity FIPfe, and the bulk density

BDfe with respect to the residual mass and volume, i.e.

to the fine earth fraction:

CPfe ¼ VCP=ðVSample � VcfÞ (1a)

FIPfe ¼ VFIP=ðVSample � VcfÞ (1b)

BDfe ¼ ðmSample � mcfÞ=ðVSample � VcfÞ (1c)

where the subscript fe denotes the fine earth fraction,

VCP and VFIP are the volumes of the coarse pores and

fine-to-intermediate pores, VSample and msample are the

sample volume and mass, and Vcf and mcf are the

volume and mass of the coarse fraction. (We performed

the same analyses as we did with these fine-earth related

Table 3

Texture, organic matter and gravel content of the soil material (averaged valu

Sampling locations Sanda (kg kg�1) Silta (kg kg�1)

Topsoil N1, T1, N2, T2 0.498 (0.022) 0.319 (0.011)

Subsoil N1, T1, N2,T2 0.514 (0.016) 0.314 (0.009)

a Determined with the pipette method.b Measured as weight loss after oxidation by H2O2.

measurements also with the respective measurements

including the coarse fraction, and obtained very similar

results, but with less statistical significance.)

2.6. Statistical analysis

For the evaluation and statistical analysis of the

trafficking effects, arithmetic means of the measured

values were determined, separately for each sampling

location and depth (i.e. N1, T1, N2, T2 locations, cf.

Fig. 1), resulting in four replicates of non-trafficked

reference locations (two sub-plots each in 2003 and

2004) and in two replicates each of the moderately (2

passes) and strongly trafficked (10 passes) soil per depth

(cf. Fig. 1).

Compaction effects due to trafficking were analysed

by two-way analysis of variance with depth and number

of passes as independent variables, followed by a

Fisher’s least significant difference post hoc analysis to

compare differences between group means of the

dependent variables. Main effects of the independent

variables and their interaction as well as the differences

between the group means of the dependent variables

were considered to be significant if the probability level

was 0.05 or less.

3. Results

3.1. Soil properties

Table 3 gives the soil texture, organic matter and

gravel content of the experimental site. The texture was

a loam according to US soil taxonomy (USDA, 1997).

The organic matter content of the test site was around

0.02 kg kg�1. It was slightly lower in the subsoil. The

average gravel content was about 10% by volume, the

mean gravel densities were 2.31 g cm�3 in the sampling

locations of TE1 and 2.28 g cm�3 in the sampling

locations of TE2. Whereas gravel content was highly

variable within the test site, texture, organic matter

content and also the soil physical and mechanical

properties of the non-trafficked locations were quite

homogeneous within the test site. Nonetheless, some

es of the sampling locations, cf. Fig. 1; standard errors in parentheses)

Claya (kg kg�1) Organic matterb (kg kg�1) Gravel (m3 m�3)

0.183 (0.011) 0.022 (0.003) 0.097 (0.010)

0.173 (0.007) 0.019 (0.004) 0.094 (0.019)


Fig. 3. Tracks after 2 passes (left) and 10 passes (right).

sampling locations differed significantly from the

others, in the topsoil with respect to texture (P <0.05), gravel content (P < 0.001), coarse porosity

(P < 0.01), fine-to-intermediate porosity (P < 0.001),

bulk density (P = 0.001) and precompression stress

(P < 0.01), and in the subsoil with respect to texture

(P < 0.05), gravel content (P < 0.001), coarse porosity

(P < 0.05), fine-to-intermediate porosity (P < 0.001),

bulk density (P < 0.01) and precompression stress

(P < 0.01) (Kruskal–Wallis test). These differences

indicate a spatial variability which is typical for restored

soils.

3.2. Visible compaction effects in the field

The passages of the combine harvester left clearly

visible ruts in both experiments (Fig. 3). The ruts of the

two passes of the first experiment were of moderate

depth (2–6 cm), consisting mainly of the impressions

Fig. 4. Example of the time course of the smoothed Bolling probe pressure,

the end of the pass by the front tyre, t1 and t2, and by the rear tyre, t3 and t4t1 � t � t2), and the rear tyre, SPi,r(t) (for t3 � t � t4), of the combine harvest

baselines of BPPi(t) before the passes of the front and rear tyre and at the end

(i = 1, 2 for TE1 and i = 1, 2, . . ., 10 for TE2).

from the tyre lugs. The 10 passes of the second

experiment produced ruts of up to 10 cm depth, and at

certain locations they were even deeper. The soil

structure of the trafficked sub-plots was not visibly

altered by the two passes of TE1. It remained crumbly in

the upper 10 cm and blocky below. The 10 passes of

TE2 destroyed these aggregates and led to a dense

cloddy or even massive, coherent structure down to the

bottom of the opened profiles, i.e. to at least 40 cm

depth below the ruts.

3.3. Soil pressure during trafficking

To reduce random scatter, the Bolling probe pressure

data were smoothed by taking the running median of

five subsequent values for each point in time. Fig. 4

shows an example of the smoothed time course of the

Bolling probe pressure measurements, BPPi(t). Index i

denotes the number of the pass (i = 1, 2 for TE1 and

BPPi(t) (- - -), its first derivative ( ) used to define the beginning and

, and the additional soil pressure exerted by the front tyre, SPi,f(t) (for

er (—). Pi;f (for t < t1), Pi;r (for t2 < t < t3) and Pi;e (for t4 < t) are the

of the vehicle pass, respectively. Index i denotes the number of the pass


Fig. 5. Peak pressures exerted by the combine harvester at 0.32 m depth vs. number of passes under the front tyre (left) and rear tyre (right) during

TE1 (2 passes) and TE2 (10 passes). Averaged values of the measurements of the centre (*, ~), inner ( , ) and outer probes (*, ~) (with regard

to the centre of the vehicle axle). Triangles represent the measurements in TE1, circles the measurements in TE2.

Fig. 6. Relative changes in the baselines of BPPi(t) directly before

ðDPi;fÞ and after ðDPi;eÞ pass i, vs. number of passes during TE1 and

TE2. +: increase; �: decrease. Averaged values of the measurements

of the centre (*, ~), inner ( , ) and outer probes (*, ~) (with

regard to the centre of the vehicle axle). Triangles represent the

measurements in TE1, circles the measurements in TE2.

i = 1,2, . . ., 10 for TE2). All time courses of the

recorded Bolling probe pressure during wheeling were

very similar in shape: narrow, high peaks with a steep

increase and an equally steep decrease in pressure. The

precise beginning and end of the passes were

determined, separately for the front and rear tyre, as

the points in time where the first derivative of BPPi(t) in

time started and ceased to differ from zero, respectively.

In the example given in Fig. 4 the pass of the front tyre

started at t1 and ended at t2, while the pass of the rear

tyre started at t3 and ended at t4.

The additional soil pressure exerted by the combine

harvester was calculated, separately for the front tyre

and rear tyre (Fig. 4), as

SPi;fðtÞ ¼ BPPiðtÞ � Pi;f ;

for pass i of the front tyre ðt1 � t � t2Þ(2a)

SPi;rðtÞ ¼ BPPiðtÞ � Pi;r;

for pass i of the rear tyre ðt3 � t � t4Þ(2b)

where BPPi(t) is the smoothed Bolling probe pressure

during pass i, and Pi;f and Pi;r are the baselines of

BPPi(t) for the two tyres (Fig. 4). Due to drifts in

BPPi(t) between the passes of the front and rear tyres,

Pi;f and Pi;r were usually different. Fig. 5 shows the

changes in peak pressures, i.e. the maximum values of

SPi,f(t) and SPi,r(t) with the number of passes. Soil

pressure was generally higher in the centre line of

the tracks than on the outer and inner sides of them

during the passes of the front tyres. The values recorded

during the passes of the rear tyres varied greatly, as the

rear tyres were not only narrower, but also slightly

displaced relative to the front tyres and, consequently,

also relative to the probes. Therefore the outer probes

recorded much lower values than the other probes.

Despite these variations there is a clear trend of increas-

ing peak values with number of passes under both tyres.

In TE2, the peak pressures increased by a factor of about

1.5–1.6 from the first to the last pass of the front tyre and

even more (1.6–1.9) under the rear tyre. During TE1 no

increase was found under the front tyre. Under the rear

tyre the peak values of the second pass were between

1.1 and 2.9 times higher than those of the first pass.

Fig. 6 shows how the baselines of the Bolling probe

pressure, recorded directly before and after pass i,


differed from the hydraulic pressure in the Bolling

probes before the first passage (in %):

DPi;f ¼ ðPi;f � P1;fÞ=P1;f � 100%;

before pass i of the front tyre(3a)

DPi;e ¼ ðPi;e � P1;fÞ=P1;f � 100%;

after pass i of the rear tyre(3b)

where P1;f , Pi;f and Pi;e are the baselines of BPPi(t) of

pass 1 and i, respectively (Fig. 4).

After a large initial increase of the baselines in TE1,

they decreased in the probes of the centre line, but did

not distinctly change in the other probes. In TE2, except

for the initial increase in the inner probes, the baselines

of BPPi(t) decreased steadily with the number of passes.

After the 10 passes in TE2, they were 13–21% lower

than at the beginning of the experiment.

Fig. 7. (a) Coarse porosity, (b) fine-to-intermediate porosity and (c) bulk dens

passes (*). Each line represents a separate sampling location (cf. Fig. 1). Gr

the T1a locations.

3.4. Traffic-induced changes in soil physical

properties

Multivariate two-way analysis of variance revealed

that the investigated soil physical and mechanical (see

next chapter) properties were significantly altered by

trafficking (Wilks’ Lambda, P < 0.005), but did not

significantly vary with depth. There was no significant

interaction between trafficking and depth. This means

that compaction effects did not significantly differ

between the two sampling depths.

As already indicated by the ruts and the deformation

of soil structure below the tracks, the 10 passes of the

second experiment caused much stronger effects on

bulk density and porosity than the two passes of the first

experiment (Fig. 7 and Tables 4 and 5).

Two passes reduced coarse porosity by 12–13%

(P < 0.05), 10 passes even by up to almost 50%

(P < 0.001) (Fig. 7 and Tables 4 and 5). A strong effect

ity of non-trafficked (*) and trafficked locations after 2 ( , ) and 10

ey triangles refer to samples taken after TE1 under the second track in


Table 4

Coarse porosity, fine-to-intermediate porosity, bulk density, precompression stress and compression index of the non-trafficked and trafficked

sampling locations (2 and 10 passes) and differences in percentage between trafficked and non-trafficked sampling locations (relative to the values of

the non-trafficked locations)

Number of

passes

Sampling

locations

Fine earth fraction Samples including gravel

Coarse porosity

(m3 m�3)

Fine-to-intermediate

porosity (m3 m�3)

Bulk density

(g cm�3)

Precompression

stress (kPa)

Compression

index

Averaged values of the sampling locations (standard errors in parentheses)

Topsoil 0 N1, N2 0.168 (0.007) 0.329 (0.004) 1.33 (0.01) 43 (3) 0.222 (0.009)

2 T1 0.146 (0.005) 0.347 (0.002) 1.33 (0.01) 43 (3) 0.221 (0.006)

10 T2 0.086 (0.008) 0.383 (0.003) 1.42 (0.01) 46 (3) 0.148 (0.006)

Subsoil 0 N1, N2 0.158 (0.006) 0.330 (0.002) 1.35 (0.01) 43 (2) 0.217 (0.008)

2 T1 0.139 (0.005) 0.339 (0.004) 1.38 (0.02) 42 (3) 0.206 (0.010)

10 T2 0.098 (0.009) 0.371 (0.004) 1.43 (0.01) 50 (4) 0.153 (0.005)

Differences between trafficked and non-trafficked sampling locationsa (%)

Topsoil 2 T1, N1, N2 �13 5 0 �1 �1

10 T2, N1, N2 �49 16 7 6 �34

Subsoil 2 T1, N1, N2 �12 3 2 �2 �5

10 T2, N1, N2 �38 12 6 16 �30

a +: higher values in the trafficked sampling locations, �: lower values in the trafficked sampling locations.

on the coarse pores was expected, as they are generally

reported to be more sensitive to compaction than finer

pores (Horn et al., 1995).

The decrease in coarse porosity was partially

compensated by an increase in the fine-to-intermediate

porosity (Fig. 7 and Table 4). A similar conversion of

coarse pores into fine-to-intermediate pores due to

compaction has also been reported by Gupta et al.

(1989), Alakukku (1996) and Richard et al. (2001).

After the two passes in 2003, the fine-to-intermediate

porosity in the trafficked locations was 3 and 5% higher

than in the non-trafficked locations (not significant), and

after the 10 passes in 2004, it was 12–16% higher in the

trafficked than in the non-trafficked locations

(P < 0.001, Table 5).

As coarse porosity was decreased more than fine-to-

intermediate porosity increased, the net effect on bulk

density was also an increase (6–7%) after 10 passes

(P < 0.01) (Fig. 7 and Tables 4 and 5). Similar effects

Table 5

Statistical significances of traffic-induced changes in the soil properties (F

Compared

groups

Sampling

locations

Fine earth fraction

Coarse

porosity

Fine-to-interme

porosity

0 vs. 2 passes T1, N1, N2 P < 0.05

0 vs. 10 passes T2, N1, N2 P < 0.001 P < 0.001

2 vs. 10 passes T1, T2 P < 0.001 P < 0.05

Blank fields indicate P > 0.05.

on bulk density were reported e.g. by Gameda et al.

(1984), Schjønning and Rasmussen (1994) and

Arvidsson (2001). The effects of the two passes of

TE1 on bulk density were weak. In the topsoil, no

difference between trafficked and non-trafficked loca-

tions was found, while in the subsoil, bulk density was

increased by 2%, but this increase was not significant

(Tables 4 and 5).

The additional samples taken after the two passes of

TE1 in the T1a locations (cf. Fig. 1) showed the same

effects as the other samples taken after TE1 (Fig. 7).

3.5. Traffic-induced changes in soil mechanical

properties

The 10 passes of the second experiment also had

much more pronounced effects on the soil mechanical

properties than the two passes of the first experiment

(Figs. 8 and 9 and Tables 4 and 5).

isher’s least significant difference post hoc analysis)

Samples including gravel

diate Bulk density Precompression

stress

Compression

index

P < 0.01 P < 0.001

P < 0.05 P < 0.001


Fig. 8. Mean compression curves in the topsoil (left) and in the subsoil (right) of non-trafficked (*) and trafficked locations after 2 ( ) and 10

passes (*).

Fig. 8 shows the averaged compression curves for

the eight sampling locations of the two experiments.

No distinct effects on the curves were observed after

two passes. However, the 10-pass experiment strongly

reduced the initial void ratio (i.e. the void ratio at

1 kPa normal stress of the compression curves) and

changed the shape of the compression curves. The

transition zone between elastic and plastic behaviour

was slightly shifted to higher normal stresses in the

compression curves of the samples from the 10-time

trafficked locations, as compared to those from the

non-trafficked locations. The VCLs of both the

topsoil and the subsoil converged with increasing

normal stress. This was particularly distinct in the

subsoil, where all curves had very similar void ratios

at 600 kPa normal stress. They seemed to coalesce

into a single point at a void ratio of about

0.45 m3 m�3. Similar convergence was also reported

Fig. 9. (a) Precompression stress and (b) compression index of non-trafficke

line represents a separate sampling location (cf. Fig. 1). Grey triangles refer t

by Veenhof and McBride (1996) and Culley and

Larson (1987).

The corresponding effects on precompression stress

and CI are shown in Fig. 9. After two passes, the

precompression stress was not changed; due to spatial

variability of the test site it was even slightly lower in

the trafficked than in the non-trafficked locations. Ten

passes increased precompression stress by 6% in the

topsoil and by 16% in the subsoil (Table 4). However,

the changes were not significant (Table 5). Two passes

decreased CI slightly by 1–5%, whereas 10 passes

decreased CI highly significantly (P < 0.001, Table 5)

by about one-third (Fig. 9 and Table 4).

The additional samples taken from the T1a locations

(cf. Fig. 1) again revealed the same effects as the

samples taken in the T1 locations (Fig. 9).

In summary, heavy trafficking (10 passes) led to a

strong alteration in the soil pore space and the

d (*) and trafficked locations after 2 ( , ) and 10 passes (*). Each

o samples taken after TE1 under the second track in the T1a locations.


Fig. 10. Relationship between soil physical and mechanical parameters of all samples (n = 187) of non-trafficked (*) and trafficked locations after

2 ( ) and 10 passes (*). (a) Precompression stress vs. initial void ratio, (b) compression index vs. initial void ratio, and (c) precompression stress vs.

compression index, and linear regressions of all samples (—).

mechanical properties, while after ‘‘light’’ trafficking (2

passes), the effects were much less pronounced, and

mainly coarse porosity was reduced (Table 4). This

highlights the value of coarse porosity as an indicator

for soil compaction.

3.6. Relationships between mechanical properties

and void ratio

Fig. 10 shows relationships between the mechanical

properties and initial void ratio and the results of linear

regression analysis. Here the initial void ratio refers to

the total volume of the samples including gravel, as the

latter has an important effect on the mechanical

properties and cannot be removed without destroying

the structure of the samples. The relationships between

the mechanical properties and the initial void ratio were

not affected by trafficking (Fig. 10). Precompression

stress decreased (R2 = 0.21) and CI increased

(R2 = 0.43) as void ratio increased. Accordingly

precompression stress decreased with increasing CI.

The relationship was weak though (R2 = 0.12). The

positive correlation between CI and initial void ratio

means that the VCL became steeper with increasing

initial void ratio, or in other words, the soil became

stiffer with increasing initial mass density. At high

stresses the VCLs converged to approximately the same

points (Fig. 8).

4. Discussion and conclusions

4.1. Soil pressure during trafficking

The peak pressures recorded under the centreline of

the track during the first pass by the front tyre (60 kPa in

TE1 and 76 kPa in TE2) were similar to the pressure of

about 54 kPa found by Gysi et al. (1999) during the pass

of a sugar beet harvester of 132 kPa mean contact

pressure under moist conditions.

One reason for the progressive increase in peak

pressure (Fig. 5) might have been the formation of the

ruts, because this decreased the distance between the

load (i.e. tyres) and the Bolling probes, so that increased

pressures were recorded. This explanation was found to

be in good agreement with calculated values using the

stress propagation equation of Frohlich (1934). In

addition, also a modification in stress propagation with

number of passes might have occurred. Horn et al.


(1994), Semmel and Horn (1995) and Horn (2003)

reported that repeated wheeling during a short time

period at constant water content induced an increase in

the ratio between vertical and horizontal stress, which is

equivalent to an increase in the concentration factor.

This effect may have been due to the increase in

wetness, as the degree of water saturation increases with

compaction and the concentration factor increases with

the water content of a soil (Sohne, 1953, 1958; Horn,

1983). The increase might also have been due to the

structural deterioration below the ruts. Unstructured

soils generally have a higher concentration factor than

structured soils (Horn, 1988). Similar to our findings,

Horn et al. (1994) found that the absolute magnitudes of

the mean normal stress and the vertical stress increased

with the number of passes, whereas Semmel and Horn

(1995) reported that the absolute magnitudes decreased

with the number of passes.

The fact that the baseline of BPPi(t) returned to lower

values after a pass than where it had been before (Fig. 6)

means that the probes expanded to slightly larger

volumes after the load was removed from pass to pass. It

is possible that due to increased compactness with each

subsequent pass the soil around the probes carried more

of the weight of the overlying soil, so that the probes

were slightly unloaded. An increase in water saturation

during compaction would have caused a decrease in

water suction. This may have decreased the resistance

of the soil against expansion of the probes, allowing the

probes to expand.

4.2. Traffic-induced changes in soil properties

At the time of the traffic experiments in the fourth

and fifth year after restoration, the precompression

stress averaged 43 kPa on the test site (Table 4).

Compared to literature values of 30–125 kPa for upper

layers of restored soils of different ages (Schneider and

Schroder, 1991; Lebert and Springob, 1994) and of 10–

250 kPa for upper layers of undisrupted agricultural

soils (Lebert, 1989; Quasem et al., 2000), the soil at the

considered site was still rather weak. The moderate

effects on soil porosity and bulk density after two passes

in our first experiment are similar to those that have

been found in corresponding depths in agricultural soils

of similar stability after a single pass by sugar beet

harvesters (Gysi et al., 1999; Arvidsson, 2001). The two

passes had similar effects on bulk density and stability,

and distinctly stronger effects on soil porosity than

several passes by tracked construction machinery with

low ground contact pressures on agricultural soils of

higher stability (Berli et al., 2004). The restored soil of

our study behaved similar to an undisturbed soil of low

stability. It could withstand moderate loads, but was

strongly compacted when heavily trafficked.

To check if soil pressure exerted by the machine

during the experiments could have exceeded the

precompression stress of the test site, we calculated

the stresses expected under the centre of the tyre,

separately for the front and rear tyre, using the stress

propagation equation given by Frohlich (1934). As

explained above, the tyre–soil contact area was assumed

to be rectangular. Furthermore, we assumed that the

vertical stress was evenly distributed over this area. The

resulting average ground contact stresses are given in

Table 2. We are aware that in contrast to our assumption,

stress is usually unevenly distributed below tyres (e.g.

Gysi et al., 2001) and maximum stresses were estimated

to be as high as 1.5–2 times the mean contact stress

(Koolen and Kuipers, 1983). However, as we had no

information on the stress distribution over the tyre–soil

contact areas of the combine harvester used in our

experiments, we chose the assumption of even

distribution for several reasons: (i) it was the least

arbitrary choice, (ii) with increasing depth, propagated

stresses depend less and less on deviations of the

assumed from the actual contact stress distribution, and

(iii) we could assume that if the calculated stress

exceeded the precompression stress, then the latter

would have been exceeded quite certainly also in the

experiments. A concentration factor of 5 was chosen to

account for the wet soil conditions.

For the depth of 0.17 m, the calculated vertical stress

was 118 kPa under the front tyre and 156 kPa under the

rear tyre (including 3 kPa vertical stress of the overlying

soil). At 0.32 m depth, the respective values were 99 and

83 kPa (including 5 kPa vertical stress of the overlying

soil). The magnitude of these values is in reasonably

good agreement with the peak values of the measured

pressures at 0.32 m depth (Fig. 5). The calculated

maximum stresses all clearly exceed the precompres-

sion stress found in the non-trafficked locations

(Table 4). Under equilibrium conditions, thus strong

soil compaction should have occurred, given that the

soil was actually wetter in the traffic experiments

(Table 1) and hence weaker than that in the confined

uniaxial compression test, and that our calculations will

have underestimated rather than overestimated the

‘‘true’’ vertical soil stresses. However, distinct compac-

tion was observed only in the second experiment with

10 passes, and also in this experiment precompression

stress increased only to 46–50 kPa (Table 4), which is

considerably less than the maximum stress applied to

the soil.


We believe that the main reason for this discrepancy

is that conditions were far from equilibrium. In our

experiment, trafficking stress lasted for 6–8 s under the

front tyre (from t1 to t2 in Fig. 4) and for 5–6 s under the

rear tyre (from t3 to t4) during a single pass. Soils exhibit

higher strength and are less compacted if a load is

applied for a short time only (Ghezzehei and Or, 2001;

Fazekas and Horn, 2005). This explains why Horn and

Hartge (1990) found that precompression stress

increased less at high than at low wheeling speeds

and always remained below the values of ground

contact pressure. The hypothesis that equilibrium

conditions were not reached is further supported by

the cumulative nature of the observed compaction

effects. Each pass produced some additional deforma-

tion, from which the soil did not recover until the next

pass. In the course of 10 passes, these effects

accumulated to a significant compaction effect.

Cumulative effects due to repetitive trafficking have

been found in various other studies, as reviewed by

Hakansson and Reeder (1994). Finally, also the

progressive increase of peak stresses indicates that

equilibrium conditions were not reached even after 10

passes. Thus, we conclude that further compaction

would have occurred with additional passes.

The discrepancy between the observed changes in

precompression stress and the (conservatively) calcu-

lated stresses may in addition be due to the differences

in the stress situation experienced by a sample volume

of soil under a moving wheel in the field and in a

confined uniaxial compression test in the laboratory, as

pointed out already before (see Section 1). According to

critical state soil mechanics, however, we should have

expected the soil to yield under a bulk stress that was

even lower than precompression stress if the samples

were subjected to larger shear stresses in the field

experiment than generated during oedometer tests (cf.

Hettiaratchi, 1987; Kirby, 1989).

An unexpected effect at first glance was the decrease

in CI. According to the concept of precompression

stress CI should not be affected by compaction, i.e.

exceedance of the precompression stress. Indeed,

various authors have shown experimentally in labora-

tory compression tests that CI remains unaffected if

samples are subjected to subsequent cycles of loading

and unloading at constant water content (e.g. Stone and

Larson, 1980; Kirby, 1991a), and Kirby (1989) found

that artificial soil samples, which had been reconstituted

at the same water content, but to different initial void

ratios, tended to have the same VCL (and thus CI) upon

compression. Most of the field work on compaction by

trafficking only looked into effects on precompression

stress, but not on CI (e.g. Semmel and Horn, 1995; Gysi

et al., 1999; Berli et al., 2004). One exception is the

study of Culley and Larson (1987). In accordance to our

findings, these authors found that CI was lower in

trafficked than in non-trafficked locations and that it

decreased with increasing bulk density irrespective of

the experimental trafficking. The effect may be at least

partially due to the commonly adopted practice that

samples are conditioned to a specific initial water

tension for water-unsaturated compression tests (e.g.

Horn, 1981; Culley and Larson, 1987) and not to a

specific water content. If soil is compacted and then

conditioned to the same water potential as samples of

the uncompacted soil, as it is the case in field

experiments like ours, this means that the compression

tests are not run at the same water content for

compacted and control samples, because water reten-

tion characteristics are changed by the compaction.

Whether this fully explains the observed decrease in CI

remains to be shown. The literature on the dependence

of CI on water content is conflicting. Whereas Larson

et al. (1980) and Kirby (1991b) did not find any changes

in CI due to variation in water content, Leeson and

Campbell (1983), Hettiaratchi (1987) and O’Sullivan

and Robertson (1996) found that CI strongly depended

on water content.

In summary, our results show that loads may strongly

exceed precompression stress for short durations

without causing serious damage even on a rather weak

soil with undeveloped structure, but that sub-critical

compaction may rapidly accumulate to substantial

effects if such loadings are repeated frequently. This

means that the concept of precompression stress was not

adequate to precisely predict the resistance of the

restored soil investigated in this study against compac-

tion under the applied loads and experimental condi-

tions. Nonetheless, we believe that precompression

stress provides a useful parameter on which to base limit

values in regulations designed to prevent compaction of

restored soil, because it was on the safe side with a large

margin of security.

Acknowledgments

We are very grateful to the following persons for the

permission to conduct the experiments on the test site,

for the great technical support in the field and the

information about the restoration and the subsequent

cultivation operations of the experimental site (in

alphabetical order): Franz Borer, Dieter Fux, Martin

Keller, Christian Ledermann, Werner Rohr, Peter

Schuler, Gaby von Rohr, Urs Zuber. The work was


partially funded by the Swiss Agency for the Environ-

ment, Forests and Landscape (BUWAL/SAEFL) and by

the Environment Protection Agency of the Canton of

Solothurn, Switzerland.

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compaction of restored soil by heavy agricultural machinery—soil physical and mechanical aspects

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