Soil pressure calculator

Soil pressure calculator OSCAR SEMINAR Soil and Machine Workshop 22.11. 23.11.2011 Jouko Törnqvist VTT Technical Research Centre of Finland

2 Shortly of the pressure calculator Soil pressure calculation of forestry machines on terrains with poor carrying capacity Tyres Tyres with tracks Stand alone programm Simple, analytical empirical soil wheel interaction solution Horizontal, even terrain Stiff frame of the machine Linear elastic, soil model while calculating stress distribution in the soil No unlinearities between the frame-tyre-track-soil -contact No suspension, no transfere of loading between axels etc.

3 Shortly... Classification of the terrain One isotropic layer on stiff bottom Soil model with simple loading repetation model (analytical empirical) Needs parameters to descripe the soil Estimation of rutting depth

4 Background

5 Background to the soil model comes from roads - Lot of measurements from different roads (VTT) Pudotuspainolaitteella määritetty kantavuus, MPa Surface stiffness from falling weight deflectometer 300 250 200 150 100 50 JYRKKÄLUISKAINEN REFLEX-KOKEET Public highways Public and private low volume roads Forest roads Frost smelting tests etc Kuormituksen alussa Kuormituksen keskivaiheessa Kuormituksen lopussa KT78, Ranua ROUTAKOKEET kantavuus määritetty Benkelmann-palkilla 0 0 0,001 0,002 0,003 0,004 0,005 0,006 0,007 Urautumisnopeus mm/ylitys Rutting speed, mm/loading cycle

6 Surface stiffness from light weight falling weight Tien deflectometer, pinnan Loadman kantavuus MPa E 2, MPa Background to the soil model 140 120 100 80 60 40 20 0 Soratien kulkukelpoisuuden / painumakehityksen arviointi kelirikon aikana Forest road with gravel surface ESIMERKKI: Jos tielle sallitaan 10 mm urautuminen ja tien pinnan mitattu Loadman-kantavuus on 90 MPa, voidaan sallia enintään 60 ylitystä (100 kn:n akseleita). mutta jo 40:n akselin ylitys voi huonoissa olosuhteissa aiheuttaan 10 mm urasyvyyden 1 10 100 1000 Kuormituskertaluku N Axles (100 kn) S sall = 10 mm S sall = 20 mm S sall = 40 mm Rut depth Kuva 5.1 Sorapintaisen tien urautuminen liikennekuormituksen vaikutuksesta. Kuva perustuu kelirikon aikana tehtyihin mittauksiin kevyesti rakennetuilla metsäautoteillä.

7 Background information Simulation in scale 1:1 Theory development 3.5 HVS-Hiekka 95% 8% Pysyvä muodonmuutos (%) 3.0 2.5 2.0 1.5 1.0 20 kpa 45 kpa 70 kpa 0.5 In situ measurements 0.0 0 20 40 60 80 100 Mobilisoitumisaste (%) Modelling softwares

8 Principles of machinery equilibrium

9 Tyre contact pressure Two tyre pressures - Full - Lowered pressure Full Lowered Interaction with so called Winkler s theory d s k ~ F = k s (p x =s x c 0 ) The soil tyre interaction is linear: the more the tyre penetrates into the ground, the bigger is the local pressure The tyre will keep its round shape independently of the penetration

10 Friction between tyre and soil No slippage between The tyre and soil! Can be changed = 0 - no drifted force to the ground = 0...0,3 drift forward = 0...-0,3 drift backwards

11 Moment and vertical equiliblium from starting situation Starting situation to equilibrium Contact area at start with iterations Axle loads Winkler springs

12 Basic cases Single tyre f i Double tyres with track f i m f i-1 f f=(f i +f i+1 ) m=(f i -f i+1 ) av i /2 After having the penetration into the soil, simplified Boussineq s theory is used to calculate stresses in the soil pixels s x Contact pressure p x = s x c 0 av i V Rope force iteraration force f i s A i i c 0 V s i f i force f i s A i i c 0 i 1 si 1 Ai 1 c0 s 1 f i s i f i f i-1

13 Equilibrium Asumption 0: f i = f i,0 F M - whole machine s i f i =A i s i c 0 No new A i + Is(f i -f i,0 )/f i Yes Final contact pressures p max Superposition + New spring forces are calculated until error is acceptable = New contact area A i Average penetration s i

14 Software flow chart

15 Flow chart

16 Flow chart

17 Program views

18 Type of machine Starting page Bearing class 3/2010 10.3.2010 Törnqvist, Kurkela & Kärhä 18

19 Type of machines - pick one

20 Fill the detailed information of the machine

21 Bearing classes: forest/peatland into soil parameters Changing the forest parameters into classical soil parameters: Mohr-Coulomb Elastic continuum Plastic deformation

22 Cumulation of rutting depth Pälkäne 0-2 Pysyvä ura, Rut depth, cm -4-6 -8-10 -12-14 -16 0 2 4 6 8 10 Ajokerta Number of rides CASE 1 CASE 2 CASE 3 CASE 4 CASE 7 CASE 8 CASE 10 Rut depth/ 1. rut dept Number of rides

23 Relative rut dept in % (rut depth / thickness of peat layes (<1 m)) Cumulation of rutting depth... Contact pressure / compression strength (=2 c) Plastisoitumissuhde Yield ratio 0.25 0.2 0.15 0.1 0.05 Empirical relation from road research: rutting dept after 1. drive 0 Metsä-malli 0 y = 0.001e 5.2461x 0 0.2 0.4 0.6 0.8 1 1.2 Jännityssuhde Stress ratio ( / failure)

24 Formulation of rut dept with increasing N 1. drive rut drive number effect Normalization against the strength For tracked wheels: j i Empirical correction

25 Results *) = maximum average pressure with a length of 0,8 meter

26 Result to the file

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