Jack Pad
The jack pad is the element placed between the tip of the jack and the ground, enlarging the contact area and so reducing the ground pressure. When the area doubles, the pressure under the same load halves. It is the most practical measure against sinking on soft ground such as soil, sand, grass and hot asphalt.
Pressure is force divided by area. Because the contact area of the jack tip is small, even a moderate load produces a pressure that exceeds what soft ground can take. The pad enlarges this area and spreads the load over a wider piece of ground.
- Material: hard plastic or solid timber of adequate thickness is common. A thin pad bends under load and reduces the area again.
- Size: the softer the ground, the wider the pad must be.
- Placement: the pad must sit horizontally and in full contact; a jack bearing on its edge can tip the pad over.
Besides preventing sinking, the pad also improves ground contact detection: on a jack resting on a hard surface the current rise is clear, while on a jack sinking into mud it stays ambiguous. On long stays, using pads also reduces the loss of level caused by the jack sinking during the day. Do not pull the pads out from under before the jacks are retracted.
The context in which this term is used: How to choose levelling jack capacity