Investigation of the Effect of Rake Angle on the Draft Requirement of Ripping in a Sandy Clay Soil through Field Experiments and Computer Modelling

John Ndisya


In this study, the effect of rake angle of a ripper plough on the draft requirement in a sandy-clay soil was investigated; the study was based on field experiments, laboratory tests and numerical simulation. Field experiments were conducted at the University of Nairobi field station in Upper Kabete, Kenya located at 1o15’S and 36o44’E at an altitude of about 1940 meters above sea level. The soil at the location is a humic Nitisol belonging to the class of sandy-clay. Field experiments featured the Factorial Completely Randomized Block design to eliminate experimental error. The plot was sub-divided into four blocks to eliminate variation due to distance and slope; the treatments were randomly assigned to the blocks and replicated four times.

The numerical model was developed using the principles of the discrete element method embedded in a commercial code.  EDEM Academic™ software was applied to model the draft requirement to ripping using ripper tines of rake angles; 30, 45, 60 and 75 degrees. The model was calibrated using the results of soil physical tests that included; moisture content, bulk density, sieve analysis, shear strength, cone index and angle of repose. The integrity of the model was validated statistically using empirical data from field tests; the statistical methods utilised included; measures of central tendency, the One Sample Kolmogorov-Smirnov test, analysis of variance, the student t-test and multi-linear regression. The validated model was applied to predict the effect of the rake angle on the draft requirement; the forces arising due to particle and boundary contact during simulation were calculated using an inbuilt contact constitutive relation and displayed using the model’s inbuilt query feature.

Statistical analysis indicated that the rake angle significantly influenced the value of the draft requirement in the sandy-clay soil at the 95% level of confidence. The value of the draft requirement was found to decrease from the rake angle of 30 degrees to attain a minimum value at 45degrees then increased through the rake angle of 60degrees to attain a maximum at the rake angle of 75 degrees; the rake angle of 45 degrees was thus found to give the minimum amount of the draft requirement while the rake angle of 75 degrees was found to give the maximum amount of the draft requirement.


Rake angle, draft requirement, discrete element method, computer model, Kenya

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