Development of a methodology for predicting the bearing capacity of rooted soft soil

A. Pirnazarov, Machine Design, KTH Royal Institute of Technology, Stockholm, Sweden, abpirnaz@kth.se

U. Sellgren, Machine Design, KTH Royal Institute of Technology, Stockholm, Sweden, ulfse@kth.se

B. Löfgren, Swedish Forestry Research Institute Skogforsk, Uppsala, Sweden, bjorn.lofgren@skogforsk.se

The mechanical properties of soil are most important factors to treat in route planning for forest harvesting. The forest soil is usually a very complicated and sensitive and it consists of large areas of sand and clay with embedded stones and rocks, several root layers of significant importance to the bearing capacity, and there are also large areas of marshland with very low bearing capacity.

The cut-to-length (CTL) method, which is widely used in forestry, is based on two heavy and articulated working machines (10-40 tons), so called harvesters and forwarders,  with six to eight wheels mounted on bogies. Thinning with the CTL method is a harvesting operation in order to regulate the density of the stand for improved future growth, but the forestry machines cause soil disturbances and damages on the tree roots. When designing new types of forestry machines, but also when doing route planning for present machines, there is a great need for better predictions of the bearing capacity and trafficability of the soil in the harvesting area, and the rutting and root damages caused by the operations. 

The tensile strength of tree roots contributes significantly to the bearing capacity of forest soil. The mechanical properties of such non-homogeneous soil is very difficult to measure with the Bevameter technique.  For that reason, cone penetrometers are widely used to measure the mechanical soil properties that are required for wheel-soil interaction mobility and trafficability predictions. But a cone penetrometer cannot completely characterize rooted  soil. To investigate the different factors that affect rooted soil trafficability are important. Thus, test rigs that can be used to characterize the effects from individual roots and root layers on the bearing capacity of soil in a laboratory setting is highly needed.

This paper presents a recently developed laboratory rig for testing the bearing capacity of rooted soil and a method on how to transform the test data into a soil model that can be used for dynamic simulations of forestry machine-soil interaction. Matlab is the key tool used to analyze and transform the test data into a soft soil model to be used in simulations with Adams.

The applicable conference tracks where the paper will fit are indicated in the table below:

Track	Description	Highly applicable	Applicable	Not applicable
1	Soil and terrain modeling and characterization	X
2	Planetary rovers and mobile robotics			X
3	Tires, wheels, and tracks modeling	X
4	Agricultural and earthmoving equipment		X
5	Operation on soils, vegetation, snow, and ice			X
6	Vehicle dynamics, mobility, and safety		X
7	Terramechanics		X