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In a series of field experiments we measured the stresses in sand beneath the rubber belt of a prototype belted tractor and beneath single front and dual rear tyres of a radial ply type equipped tractor, using earth stress cells installed beneath the belt or tyres. The stress in the sand increased with increasing axle load for both belt and tyres. The peak vertical stress beneath the belt was about 6 times the average stress calculated as total weight divided by total area of belt. Depending on axle load, the stresses near the soil surface were similar beneath belts and tyres, whereas at 35 to 45 cm depth the stresses beneath tyres exceeded those beneath belts. Stresses beneath individual wheels on the track bogey could be distinguished near the soil surface, but at 35 to 45 cm depth only the average stress could be identified.

In a series of field experiments we measured the stresses in sand beneath single front and dual rear tyres of a John Deere 8300 tractor, using earth stress cells installed beneath the tyres. The stress in the sand increased both with increasing tyre inflation pressure (constant axle load) and with increasing axle load (constant tyre pressure). Analysis of the experiments using a finite element model showed that the stresses in the top 60 - 70 cm of soil depended mainly on the tyre / soil contact pressure; the influence of the lugs was particularly important. Stresses deeper than that depended mainly on the total load applied to the surface, increasing with increasing axle load. The model was used to simulate compaction in clay soils. Compaction predicted beneath the tyres depended on the strength of the soil, which in turn depends on the moisture content. However, tyre / soil contact pressure and axle load were still important.