ASRB NET Mechanics of Tillage & Traction (Soil Dynamics): practice questions

Question 1 · medium

Below its critical depth, a narrow vertical tine mainly

1. displaces soil sideways and compacts it rather than loosening it
2. lifts soil in a crescent-shaped wedge up and forward to the surface
3. produces no draft because the soil flows freely around the tine shank
4. cuts a clean slot with no soil disturbance

Question 2 · medium

For a wheel on a saturated clay with φ close to zero, which change raises the maximum soil thrust most?

1. Adding ballast to increase the wheel load and so the normal stress on the soil
2. Raising the tyre inflation pressure
3. Increasing the contact area, for example by fitting dual wheels or tracks
4. Increasing travel speed

Question 3 · hard

Using the Wismer-Luth wheel numeric Cn = CI·b·d/W and MRR = 1.2/Cn + 0.04, find the rolling resistance of a towed wheel (b = 0.4 m, d = 1.5 m, W = 15 kN) on soil with CI = 1000 kPa.

1. 1.05 kN
2. 0.070 kN
3. 0.45 kN
4. 2.10 kN

Question 4 · medium

Why is the front axle of a mechanical front-wheel-drive tractor geared so that its theoretical speed is slightly higher than that of the rear axle?

1. So the front wheels pull rather than brake the tractor, avoiding power circulation and tyre scrubbing
2. So the front wheels slip less than zero and act as brakes on slopes, holding the tractor back when descending
3. To reduce the steering effort at headlands
4. To transfer weight from the rear to the front axle

Question 5 · hard

Why is dynamic rearward overturning more dangerous in practice than a calculation based on static equilibrium would suggest?

1. Static analysis overstates the CG height
2. Dynamic overturning can happen only on downhill slopes
3. Static analysis ignores the drawbar pull entirely, whereas in practice the pull is what holds the front end down on the ground
4. When the rear wheels cannot rotate, axle torque reaction rotates the body about the rear axle faster than an operator can react

Question 6 · medium

Why does wet soil compact more than dry soil under the same wheel load?

1. Water adds its own weight to the soil, which increases the applied load under the wheel and so the stress transmitted
2. Wet soil has higher shear strength
3. Water lubricates particle contacts and lowers pre-compression stress, so less stress causes plastic deformation
4. Wet soil transmits stress less deeply

Question 7 · hard

A field was subsoiled in autumn but the pan re-formed within one season. Which is the most likely reason?

1. Subsoiling increases the clay content of the pan
2. Subsoiled soil has higher pre-compression stress than before
3. The pan formed from the fertiliser applied
4. Loosened soil was trafficked, especially when wet, and recompacted

Question 8 · medium

Why should operators avoid breaking the hardpan when working lowland rice fields?

1. The hardpan stores fertiliser
2. The hardpan has no effect on water or traction
3. The hardpan provides the firm layer for traction and reduces percolation losses of ponded water
4. Breaking the hardpan increases the soft layer's cohesion, making puddling slower and the mud too stiff

Question 9 · hard

A tractor has GTR = 0.546 and MRR = 0.079 at 12 % slip (from the Brixius equations). What is the tractive efficiency?

1. 85.5 %
2. 88.0 %
3. 75.3 %
4. 41.1 %

Question 10 · medium

What is the effect of increasing the lug height of a cage wheel in a deep puddled layer?

1. Pull usually falls to zero because tall lugs float
2. It has no effect on pull, because only rim width and wheel diameter matter in deep puddled soil where lugs cannot grip
3. Lugs reach firmer soil and engage more soil, generally raising pull at the cost of more mud lifted and more power
4. It reduces sinkage to zero