Technical solutions to reduce the risk of subsoil compaction: effects of dual wheels, tandem wheels and tyre inflation pressure on stress propagation in soil

The use of heavy machinery is increasing in agriculture, which induces increased risks of subsoil compaction. Hence, there is a need for technical solutions that reduce the compaction risk at high total machine loads. Three field experiments were performed in order to study the effects of dual wheels, tandem wheels and tyre inflation pressure on stress propagation in soil. Vertical soil stress was measured at three different depths by installing probes into the soil horizontally from a dug pit. In one experiment, also the stress distribution below the tyre was measured. Beneath the dual wheels, vertical stresses at 0.15 and 0.3 m depth were lower between the two wheels than under the centre of each wheel, despite the gap between the wheels being small (0.1 m). At 0.5 m depth, vertical stress beneath the wheels was the same as between the two wheels. The stress interaction from the two wheels was weak, even in the subsoil. Accordingly, measured stresses at 0.3, 0.5 and 0.7 m depth were highest under the centre of each axle centre line of tandem wheels, and much lower between the axles. For a wheel load of 86 kN, tyre inflation pressure significantly affected stress at 0.3 m depth, but not at greater depths. Stress directly below the tyre, measured at 0.1 m depth, was unevenly distributed, both in driving direction and perpendicular to driving direction, and maximum stress was considerably higher than tyre inflation pressure. Calculations of vertical stress based on Boussinesq's equation for elastic materials agreed well with measurements. A parabolic or linear contact stress distribution (stress declines from the centre to the edge of the contact area) was a better approximation of the contact stress than a uniform stress distribution. The results demonstrate that stress in the soil at different depths is a function of the stress on the surface and the contact area, which in turn are functions of wheel load, wheel arrangement, tyre inflation pressure, contact stress distribution and soil conditions. Soil stress and soil compaction are a function of neither axle load nor total vehicle load. This is of great importance for practical purposes. Reducing wheel load, e.g. by using dual or tandem wheels, also allows tyre inflation pressure to be reduced. This reduces the risk of subsoil compaction.     

Subsoil compaction by heavy sugarbeet harvesters in southern Sweden: III. Risk assessment using a soil water model

Due to its persistence, subsoil compaction should be avoided, which can be done by setting stress limits depending on the strength of the soil. Such limits must take into account soil moisture status at the time of traffic. The objective of the work presented here was to measure soil water changes during the growing period, use the data to calibrate a soil water model and simulate the soil susceptibility to compaction using meteorological data for a 25-year period. Measurements of soil water content were made in sugarbeet (Beta vulgaris L.) from sowing until harvest in 1997 on two sites classified as Eutric Cambisols in southern Sweden. Sampling was carried out at 2-week intervals in 0.1 m layers down to 1 m depth, together with measurements of root growth and crop development. Precompression stress of the soil at 0.3, 0.5 and 0.7 m depth was determined from uniaxial compression tests at water tensions of 6, 30, 60 and 150 kPa and adjusted as a logarithmic function of the soil water tension. Soil water content was simulated by the SOIL model for the years 1963–1988. Risk calculations were made for a wheel load of 8 t and a ground pressure of 220 kPa, corresponding to a fully loaded six-row sugarbeet harvester. Subsoil compaction was expected to occur when the major principal stress was higher than the precompression stress. The subsoil water content was very low in late summer, but increased during the autumn. At the end of August, there was practically no plant available water down to 1 m depth. There was in general good agreement between measured and simulated values of soil water content for the subsoil, but not for the topsoil. In the 25-year simulations, the compaction risk at 50 cm depth was estimated to increase from around 25% to nearly 100% between September and late November, which is the period when the sugarbeet are harvested. The types of simulation presented here may be a very useful tool for practical agriculture as well as for society, in giving recommendations as to how subsoil compaction should be avoided.     

Persistence of soil compaction due to high axle load traffic. I. Short-term effects on the properties of clay and organic soils

Short-term effects of high axle load traffic on soil total porosity and pore size distribution were examined in field experiments on a clay (Vertic Cambisol) and an organic soil (Mollic Gleysol) for 3 years after the heavy loading. The clay soil had 48 g clay (particle size less than 2 μm) per 100 g in the topsoil and 65 g per 100 g in the subsoil. The organic soil consisted of well-decomposed sedge peat mixed with clay below 0.2 m depth down to 0.4–0.5 m and was underlain by gythia (organic soil with high clay content). The experimental traffic was applied with a tractor-trailer combination in autumn 1981. The trailer tandem axle load was 19 Mg on the clay and 16 Mg on the organic soil. There were three treatments: one pass with the heavy axle vehicle, with wheel tracks completely covering the plot area, four repeated passes in the same direction, and a control treatment without experimental traffic. During loading, the clay was nearly at field capacity below 0.1 m depth. The organic soil was wetter than field capacity.

One and four passes with the high axle load compacted both soils to a depth of 0.4–0.5 m. On the clay soil the total porosity was reduced by the heavy loading nearly as much as macroporosity (diameter over 30 μm) to 0.5 m depth. On the organic soil, macroporosity was reduced and microporosity (under 30 μm) increased in the 0.2–0.5 m layer by the heavy loading. Total porosity did not reveal the effects of compaction on the organic soil. The compaction of the clay below 0.1 m persisted for 3 years following the treatment despite annual ploughing to a depth of 0.2 m, cropping and deep cracking and freezing. Likewise, in the subsoil (below 0.2 m) of the organic soil, differences in pore size distribution persisted for a period of at least 3 years after the heavy loading.     

Soil stress as affected by wheel load and tyre inflation pressure

The relative importance of wheel load and tyre inflation pressure on topsoil and subsoil stresses has long been disputed in soil compaction research. The objectives of the experiment presented here were to (1) measure maximum soil stresses and stress distribution in the topsoil for different wheel loads at the same recommended tyre inflation pressure; (2) measure soil stresses at different inflation pressures for the given wheel loads; and (3) measure subsoil stresses and compare measured and simulated values. Measurements were made with the wheel loads 11, 15 and 33 kN at inflation pressures of 70, 100 and 150 kPa. Topsoil stresses were measured at 10 cm depth with five stress sensors installed in disturbed soil, perpendicular to driving direction. Contact area was measured on a hard surface. Subsoil stresses were measured at 30, 50 and 70 cm depth with sensors installed in undisturbed soil. The mean ground contact pressure could be approximated by the tyre inflation pressure (only) when the recommended inflation pressure was used. The maximum stress at 10 cm depth was considerably higher than the inflation pressure (39% on average) and also increased with increasing wheel load. While tyre inflation pressure had a large influence on soil stresses measured at 10 cm depth, it had very little influence in the subsoil (30 cm and deeper). In contrast, wheel load had a very large influence on subsoil stresses. Measured and simulated values agreed reasonably well in terms of relative differences between treatments, but the effect of inflation pressure on subsoil stresses was overestimated in the simulations. To reduce soil stresses exerted by tyres in agriculture, the results show the need to further study the distribution of stresses under tyres. For calculation of subsoil stresses, further validations of commonly used models for stress propagation are needed.     

Modern forestry vehicles and their impacts on soil physical properties

“Close-to-nature forest stands” are one central key in the project “Future oriented Forest Management” financially supported by the German Ministry for Science and Research (BMBF). The determination of ecological as well as economical consequences of mechanized harvesting procedures during the transformation from pure spruce stands to close-to-nature mixed forest stands is one part of the “Southern Black Forest research cooperation”. Mechanical operations of several typical forest harvesting vehicles were analysed to examine the actual soil stresses and displacements in soil profiles and to reveal the changes in soil physical properties of the forest soils. Soil compaction stresses were determined by Stress State Transducer (SST) and displacement transducer system (DTS) at two depths: 20 and 40 cm. Complete harvesting and trunk logging processes accomplished during brief 9-min operations were observed at time resolutions of 20 readings per second. Maximum vertical stresses for all experiments always exceeded 200 kPa and at soil depths of 20 cm for some vehicles and sequences of harvesting operations approached ≥500 kPa. To evaluate the impacts of soil stresses on soil structure, internal soil strengths were determined by measuring precompression stresses. Precompression stress values of forest soils at the field sites ranged from 20 to 50 kPa at soil depths of 20 cm depth and from 25 to 60 kPa at soil depths of 40 cm, at a pore water pressure of −60 hPa. Data obtained for these measured soil stresses and their natural bearing capacities proved that sustainable wheeling is impossible, irrespective of the vehicle type and the working process. Re-occurring top and subsoil compaction, increases in precompression stress values in the various soil horizons, deep rut depths, vertical and horizontal soil displacements associated with shearing stresses, all affected the mechanical strengths of forest soils. In order to sustain naturally “unwheeled” soil areas with minimal compaction, it is recommended that smaller machines, having less mass, be used to complete forest harvesting in order to prevent or at least to maintain currently minimal-compacted forest soils. Additionally, if larger machines are required, permanent wheel and skid tracks must be established with the goal of their maximum usefulness for future forest operations. A first step towards accomplishing these permanent pathways requires comprehensive planning with the Federal State Baden-Württemberg. The new guideline for final opening with skid tracks (Landesforstverwaltung Baden-Württemberg, 2003) proposes a permanent skid track system with a width of 20–40 m.     

轮式和履带式车辆行走对农田土壤的压实作用分析

由履带式行走机构代替轮胎被认为是减缓大型农业车辆对土壤压实的有效手段之一。与轮胎相比,履带具有更大的接地面积,能够有效减小车辆对土壤的平均压力。然而履带与土壤接触面间的应力分布极不均匀,应力主要集中在各承重轮下方,履带减缓土壤压实的能力是目前有待研究的问题。该研究通过在土壤内埋设压力传感器,测试比较了相近载质量的轮胎和履带式车辆作用下,0.15和0.35 m深度土壤内的最大垂直及水平应力,同时研究了车辆行驶速度对土壤内垂直及水平应力大小的影响。基于土壤压实分析模型计算了轮胎和履带压实的0.1~0.7m深度土壤内的最大垂直及水平应力分布。通过对0.15和0.35 m深度的土样进行室内测试,比较了轮胎和履带式车辆压实对土壤透气率、先期固结压力及干容重大小的影响。结果表明,履带相比较于轮胎,能够减小土壤内的垂直及水平应力,但垂直应力的减小量比水平应力大;轮胎对0.15和0.35m深度土壤作用的平均最大垂直应力分别约为履带的2.2及2.0倍,而平均最大水平应力仅分别约为履带的1.2及1.1倍。轮胎作用下的最大垂直及水平应力在表层土壤内明显大于履带,但两者的应力差值随着土壤深度的增加逐渐减小,分别在0.7和0.4 m深度时无明显差别。轮胎和履带压实作用下,0.15和0.35 m深度土壤内的垂直及水平应力均随车辆行驶速度的增加而减小,履带作用下的应力减小速度大于轮胎。履带作用下0.15和0.35 m深度内土壤的透气率均明显小于轮胎,但土壤的先期固结压力及干容重无显著区别。研究结果为可为农业车辆行走机构的选择及使用提供参考。     

Modelling compaction of agricultural subsoils by tracked heavy construction machinery under various moisture conditions in Switzerland

In recent years, agricultural land in Switzerland has been increasingly used as temporary access ways for heavy machinery in road and pipeline construction operations. The Swiss soil protection law requires that measures are taken to prevent soil compaction in such operations, but gives no criteria to determine tolerable loads. We studied the compaction sensitivity of a loess soil (Haplic Luvisol) at different soil moisture conditions in a field traffic experiment and by a numerical model on the computer using finite element analysis. Two plots, one wetted by sprinkling and one left dry (no sprinkling), were traversed by heavy caterpillar vehicles during construction of a large overland gas pipeline. Compaction effects were determined by comparing precompression stresses of samples taken from trafficked and non-trafficked soil. A finite element model with a constitutive relation, based on the concept of critical state soil mechanics, was used to interpret the outcome of the field trials.

We found significantly higher precompression stresses in the trafficked (median 97 kPa) compared with the non-trafficked (median 41 kPa) topsoil of the wet plot. No effect was evident in the topsoil of the dry plot as well as in the subsoils of the wet and the dry plot. The observed compaction effects were in agreement with the model predictions if the soil was assumed to be partially drained, but disagreed for the wet subsoil if fully drained conditions were assumed. Agreement between model and experimental results also required that the moisture dependence of the precompression stress was taken into account.     

Langjährige Verdichtungswirkung durch unterschiedliche Achslasten auf einem Löß-Ackerstandort

Long-term compaction effects on loess derived soils by distinct axle loads Field traffic may cause subsoil compaction of arable land and can deteriorate growing conditions of plants. In a case study the state of compaction of two adjacent fields on loess derived soil (field A and field B) was examined, which belong to two neighbouring farms. Within the past 20 years the maximum axle loads on both fields differed greatly (4 Mg and 8.9 Mg). Both fields were compared with a bordering ridge under permanent grass, which had not been loaded mechanically in recent years. The aim of this study was to evaluate the state of compaction as affected by the impact of vehicular field traffic. It was found that in the depth range of a traffic-pan in field A (about 40 cm) the penetration resistance was higher than in the corresponding depth under grass, but substantially lower than in field B. Bulk density and air capacity are similarly different between locations. The vertical compressive stress as a function of soil depth was calculated for the maximum axle loads that occur on both fields under wet conditions. For the 40 cm depth on field A stress values were near 60 kPa, but on field B the values were about 130 kPa. The loading stresses, acting on the soil during one season, were assessed from the weight of the vehicles and the travel distance per area. The accumulated stress was by 17% higher on field B than on field A. On field A the compactive stress of loading ended at about 40 cm depth. On farm B, however, with much higher axle loads during sugarbeet harvest, the compactive stress extended to about 70 cm soil depth. This case study demonstrates that the state of compactness of agricultural fields will be strongly dependent on the intensity of vehicular traffic, which comprises axle load as well as time and frequency of passages.     

Rules of thumb for minimizing subsoil compaction

Subsoil compaction is persistent and can affect important soil functions including soil productivity. The aim of this study was to develop recommendations on how to avoid subsoil compaction for soils exposed to traffic by machinery at field capacity. We measured the vertical stress in the tyre–soil contact area for two traction tyres at ca. 30‐ and 60‐kN wheel loads on a loamy sand at field capacity. Data on resulting stress distributions were combined with those from the literature for five implement tyres tested at a range of inflation pressures and wheel loads. The vertical stress in the soil profile was then predicted using the Söhne model for all tests in the combined data set. The predicted stress at 20 cm depth correlated with the maximum stress in the contact area, tyre inflation pressure, tyre–soil contact area and mean ground pressure. At 100 cm depth, the predicted vertical stress was primarily determined by wheel load, but an effect of the other factors was also detected. Based on published recommendations for allowable stresses in the soil profile, we propose the ‘50‐50 rule’: At water contents around field capacity, traffic on agricultural soil should not exert vertical stresses in excess of 50 kPa at depths >50 cm. Our combined data provide the basis for the ‘8‐8 rule’: The depth of the 50‐kPa stress isobar increases by 8 cm for each additional tonne increase in wheel load and by 8 cm for each doubling of the tyre inflation pressure. We suggest that farmers use this simple rule for evaluating the sustainability of any planned traffic over moist soil.     

Persistence of soil compaction due to high axle load traffic. II. Long-term effects on the properties of fine-textured and organic soils

The long-term effects of high axle load traffic on soil structure were investigated in three field experiments. Two of the experiments were located on fine-textured mineral soils (Vertic Cambisol). The clay soil had 48 g clay (particle size less than 2 μm) per 100 g in the topsoil and 65 g per 100 g in the subsoil, and the loam soil had clay contents of 30 g and 42 g per 100 g in the topsoil and subsoil, respectively. One experiment was located on an organic soil (Mollic Gleysol) consisting of well-decomposed sedge peat mixed with clay from 0.2 to 0.4–0.5 m depth, and underlain by gythia (organic soil with high clay content). In the autumn of 1981, one pass and four repeated passes with a heavy tractor-trailer combination compacted the soils to 0.4–0.5 m depth. The trailer tandem axle load was 19 Mg on the clay and 16 Mg on the other soils.

For 9 years after the experimental traffic, the main crops grown were spring cereals. During this time, the maximum axle load applied during field operations was 5 Mg and the maximum tyre inflation pressure was 150 kPa. The clay and loam froze to 0.5 m depth for 6 and 2 years, respectively. During several growing seasons all three soils dried and cracked. In the ninth year after the loading, soil penetrometer resistance, saturated hydraulic conductivity (K_sat), macroporosity and number and area of cylindrical biopores were measured and the visual structure of the soils examined.

Compaction in the plough layer was alleviated by ploughing and natural processes, whereas in the subsoil the effects of the compaction were still measurable, in all experiments, in the ninth year after the high axle load traffic. In the clay soil in the 0.3–0.5 m layer and in the organic soil in the 0.28–0.4 m layer, the penetrometer resistance was 22–26% greater and the soil structure more massive in the plots compacted with four passes than in the control plots. In the 0.4–0.55 m layer in all soils, the loading with four passes decreased K_sat by 60–98% and macroporosity (diameter greater than 300 μm) by 37–70%. In the fine-textured mineral subsoils, cylindrical biopores were found in all treatments. The trend of the results was, however, for biopores to be fewer in compacted than in control plots.     

Compaction of agricultural and forest subsoils by tracked heavy construction machinery

Precompression stress has been proposed as a criterion for subsoil compression sensitivity in regulations, limiting mechanical loads by vehicles, trafficking on agricultural and forest soils. In this study we investigated the applicability of this criterion to the field situation in the case of tracked heavy construction machinery. ‘Wet’ and ‘dry’ test plots at three different test sites (soil types: Eutric Cambisol and Haplic Luvisol under crop rotation and Dystric Cambisol under forest) along an overland gas pipeline construction site were experimentally trafficked with heavy tracked machines used for the construction work. The comparison of samples taken from beneath the tracks with samples taken from non-trafficked areas beside the tracks showed that no significant increase in precompression stress occurred in the subsoil. Comparing calculated mean and peak vertical stresses with precompression stress in the subsoil, only little compaction effects could have been expected. Precompression stress was determined by the Casagrande procedure from confined uniaxial compression tests carried out in the laboratory on undisturbed samples at −6 kPa initial soil water potential. Dye tracer experiments showed little differences between flow pattern of trafficked and non-trafficked subsoils, in agreement with the results of the precompression stress, bulk density and macroporosity measurements. The results indicate that Casagrande precompression stress may be a suitable criterion to define the maximum allowable peak stresses in the contact area of a rigid track in order to protect agricultural and forest subsoils against compaction.     

侵蚀坡耕地土壤强度变化特征

Undisturbed soil cores were taken from different slope positions （upslope, backslope and footslope） and soil depths （0-15, 20-35 and 100-115 cm） in a soil catena derived from Quaternary red clay to determine the spatial changes in soil strength along the eroded slope and to ewluate an indicator to determine soil strength during compaction. Precompression stress, as an indicator of soil strength, significantly increased from topsoil layer to subsoil layer （P 〈0.05） and was affected by slope position. In the subsoil layer （20-35 cm）, the precompression stress at the footslope position was significantly greater than at the backslope and upslope positions （P 〈0.05）, while there were no significant differences at 0-15 and 100-115 cm. Precompression stress followed the spatial wriation of soil clay content with soil depth and had a significant linear relationship with soil porosity （r^2 = 0.40, P 〈 0.01）. Also, soil cohesion increased with increasing soil clay content. The precompression stress was significantly related to the applied stress corresponding to the highest change of pore water pressure （r^2 = 0.69, P 〈 0.01）. These results suggested that soil strength induced by soil erosion and soil management wried spatially along the slope and the maximum change in pore water pressure during compaction could be an easy indicator to describe soil strength.     

Compaction of an Eutric Cambisol under heavy wheel traffic in Switzerland — field data and modelling

Heavy agricultural machinery can cause structural degradation in agricultural subsoils. Severe structural degradation impedes plant growth. Therefore, compaction must be limited to layers that can be structurally reclaimed and remoulded with reasonable effort by tillage. The purpose of this study was to investigate the impact of a single pass with a sugar beet harvester on the soil properties of an unploughed Eutric Cambisol. Field measurements and laboratory testing were carried out in Frauenfeld, Switzerland. In addition 2D calculations of strain, stress and subsequent compaction were conducted using a three-phase (soil skeleton, pore water, and air) model for unsaturated soil incorporating a recently developed constitutive law. Model data were compared to the field measurements. Due to the pass of the machinery, the soil was compacted down to a depth of at least 0.15 m and at most 0.25 m. This compaction was indicated by an increase in soil bulk density and pre-consolidation pressure as well as by a decrease in total porosity and macroporosity. The surface displacement measured in the field was consistent with the calculated model data. The calculated and measured stresses at depths of 0.35 and 0.55 m stand in good accordance with each other, whereas at a depth of 0.15 m the pressure measured in the field exceeded the calculated pressure. In this study, we show the degree of compaction due to heavy wheel traffic and the suitability of a model approach to describe compaction processes.     

Pipeline right‐of‐way construction activities impact on deep soil compaction

A 762‐mm‐diameter pipe 1,886 km long was installed to transfer crude oil in the USA from North Dakota to Illinois. To investigate the impact of construction and restoration practices on long‐term soil productivity and crop yield, vertical soil stresses induced by a Caterpillar (CAT) pipe liner PL 87 (475 kN vehicle load) and semi‐trailer truck (8.9 kN axle load) were studied in a farm field. Soil properties (bulk density and cone penetration resistance) were measured on field zones within the right‐of‐way (ROW) classified according to construction machine trafficking and subsoil tillage (300‐mm‐depth tillage and 450‐mm‐depth tillage in two repeated passes) treatments. At 200 mm depth from the subsoiled surface, the magnitude of peak vertical soil stress from trafficking by the semi‐truck trailer and CAT pipe liner PL 87 was 133 kPa. The peak vertical soil stress at 400 mm soil depth appeared to be influenced by vehicle weight, where the Caterpillar pipe liner PL 87 created soil compaction a magnitude of 1.5 greater than from the semi‐trailer truck. Results from the soil bulk density and soil cone penetration resistance measurements also showed the ROW zones had significantly higher soil compaction than adjacent unaffected corn planted fields. Tillage to 450 mm depth alleviated the deep soil compaction better than the 300‐mm‐depth tillage as measured by soil cone penetration resistance within the ROW zones and the unaffected zone. These results could be incorporated into agricultural mitigation plans in ROW construction utilities to minimize soil and crop damage.     

Risk assessment of subsoil compaction for arable soils in Northwest Germany at farm scale

The physically defined concept “precompression stress (Pc)” is presented at farm scale, including two operation methods in order to define precaution and critical values for the legislation and executive level according to the German Soil Protection Law. The first step is the prevention of subsoil compaction in general by the definition of the mechanical strength of soils, which is defined by the Pc. This Pc value is used as the precaution value, to ensure site-adjusted land use. The second step is to predict the change of soil functions after exceeding the Pc and furthermore to assess if critical values (test and action values) caused by subsoil compaction are reached or already exceeded. Criteria for the definition of critical values by subsoil compaction concerning crop production are discussed in order to also establish such values in the European Soil Framework Directive. The “Pc” concept, which includes predicted and regionalized “Pc”-maps, was verified on a research farm in the weichselian moraine landscape in Northern Germany for areas resistant or susceptible to soil deformation at the given water content throughout the year. Furthermore, the stress-dependent changes of the air capacity after exceeding the Pc was predicted by pedotransfer functions and linked with the farm soil map. As an additional proof for the validity of the Pc concept, a field experiment on a Stagnic Luvisol was also conducted in order to measure the stress distribution up to 60 cm depth using the Stress State Transducer (SST) system at two different wheel loads (3.3 and 6.5 Mg) using a tractor-pulled mono-wheeler. According to the effective soil strength, the wheel load should not exceed 3.3 Mg at field capacity to avoid subsoil compaction.     

Seasonal dynamics in wheel load‐carrying capacity of a loam soil in the Swiss Plateau

Subsoil compaction is a major problem in modern agriculture caused by the intensification of agricultural production and the increase in weight of agricultural machinery. Compaction in the subsoil is highly persistent and leads to deterioration of soil functions. Wheel load‐carrying capacity (WLCC) is defined as the maximum wheel load for a specific tyre and inflation pressure that does not result in soil stress in excess of soil strength. The soil strength and hence WLCC is strongly influenced by soil matric potential (h). The aim of this study was to estimate the seasonal dynamics in WLCC based on in situ measurements of h, measurements of precompression stress at various h and simulations of soil stress. In this work, we concentrated on prevention of subsoil compaction. Calculations were made for different tyres (standard and low‐pressure top tyres) and for soil under different tillage and cropping systems (mouldboard ploughing, direct drilling, permanent grassland), and the computed WLCC was compared with real wheel loads to obtain the number of trafficable days (NTD) for various agricultural machines. Wheel load‐carrying capacity was higher for the top than the standard tyres, demonstrating the potential of tyre equipment in reducing compaction risks. The NTD varied between years and generally decreased with increasing wheel load of the machinery. The WLCC simulations presented here provide a useful and easily interpreted tool to guide the avoidance of soil compaction.     

Prevention of traffic-induced subsoil compaction in Sweden: Experiences from wheeling experiments

Abstract

In this paper we describe the susceptibility of Swedish subsoils to compaction and discuss strategies for prevention of traffic-induced subsoil compaction against the background of experiences from wheeling experiments conducted in Sweden during recent years. The susceptibility of Swedish subsoils to compaction must be considered high because subsoils are often wet during field operations and machinery with high wheel loads is used. The risk of subsoil compaction could be reduced by technical solutions, such as the use of dual and tandem wheels instead of single wheels, low tyre inflation pressure or tracks. However, each of these solutions has its limitations. Results from several wheeling experiments on different soils indicate that residual deformations occur even when the applied stress is lower than the precompression stress. Hence, soil compaction could not be avoided completely by limiting the applied stress to the precompression stress.     

Soil compaction and stress propagation after different wheeling intensities on a silt soil in South-East Norway

The objective of this study was to evaluate the effect of wheeling with two different wheel loads (1.7 and 2.8?Mg) and contrasting wheeling intensities (1x and 10x) on the bearing capacity of a Stagnosol derived from silty alluvial deposits. Soil strength was assessed by laboratory measurements of the precompression stress in topsoil (20?cm) and subsoil (40 and 60?cm) samples. Stress propagation, as well as elastic and plastic deformation during wheeling were measured in the field with combined stress state (SST) and displacement transducers (DTS). We also present results from soil physical analyses (bulk density, air capacity, saturated hydraulic conductivity) and barley yields from the first two years after the compaction. Although the wheel loads used were comparatively small, typical for the machinery used in Norway, the results show that both increased wheel load and wheeling intensity had negative effects on soil physical parameters especially in the topsoil but with similar tendencies also in the subsoil. Stress propagation was detected down to 60?cm depth (SST). The first wheeling was most harmful, but all wheelings led to accumulative plastic soil deformation (DTS). Under the workable conditions in this trial, increased wheeling with a small machine was more harmful to soil structure than a single wheeling with a heavier machine. However, the yields in the first two years after the compaction did not show any negative effect of the compaction.     

Influence of single passes with high wheel load on a structured, unploughed sandy loam soil

In a field experiment, a sandy loam was subjected to single passes with a sugar beet harvester at two different soil water potentials. Different hopper fillings resulted in ground contact pressures of 130 kPa (partial load) and 160 kPa (full load) underneath the tyre. Bulk density, macroporosity (equivalent pore radius >100 μm), penetrometer resistance, air permeability and pre-consolidation pressure were measured within and next to the wheel tracks at depths of 0.12–0.17, 0.32–0.37 and 0.52–0.57 m. Furthermore, the soil structure at two horizons (Ahp 7–24 cm, B(C) 24–38 cm) was visually assessed and classified.

The moist plot responded to a wheel load of 11.23 mg (160 kPa) with an increase in bulk density and pre-consolidation pressure as well as with a decrease in air permeability and macroporosity at a depth of 0.12–0.17 m. With a wheel load of 7.47 mg (130 kPa) on the moist plot and with both wheel load levels on the dry plot, only slight changes of the soil structure were detected. At a depth of 0.32–0.37 and 0.52–0.57 m, the measurements did not indicate any compaction. An ANOVA indicates that the factor “soil water potential” and the factor “wheel load” significantly influence the bulk density at a depth of 0.12–0.17 m. No interactions occurred between these two factors. The wheel traffic on the test plot had no effect on the yield of winter wheat planted after the experimental treatment.

Bulk density, macroporosity and pre-consolidation pressure proved to be sensitive to detect compaction because they varied only slightly and are easy to measure. In contrast, the standard deviation of air permeability is large. The soil structure determined visually in the field confirms the values measured in the laboratory. The results of the penetrometer resistance measurements were not explainable.     

Deep tillage and traffic effects on subsoil compaction and sunflower (Helianthus annus L.) yields

The main function of deep tillage is to alleviate subsoil compaction, but how long do the benefits of this technique remain? Traffic on loose soil causes a significant increase in soil compaction. Subsoiling and chisel plowing were carried out at 450 and 280 mm depth, respectively on a compacted soil in the west Rolling Pampas region of Argentina. The draft required, physical soil properties, root growth, sunflower (Helianthus annus L. Merr.) yield and traffic compaction over the subsequent two growing seasons were measured. Cone penetrometer resistance was reduced and sunflower yields increased following deep tillage operations. Subsoil compaction caused changes to the root system of sunflower that affected shoot growth and crop yields. Although subsoiling and chiseling had an immediate loosening effect, it was evident that after just 2 years, when traffic intensity was >95 mg km ha⁻¹, re-compaction and settling had occurred in the 300–600 mm depth range.