A Kick In The Buttress

Buttressing

Imagine a retaining wall in sloping ground.  Upslope there is soil.  Downslope there is a piece of artificially created flat ground.  The whole point of having the wall is to prevent landslip.

The soil on one side of the wall and the fresh air on the other create an imbalance of forces.  One way to create a balance is to make the wall tapered, like a dam.  A cheaper way is to make the wall straight and add occasional tapered thickenings.  A well-designed buttress can save a lot of money on materials by adding strength sufficient for the task.

Now imagine that repairs are in progress.  The wall has been strengthened with wooden props either side of a decaying buttress and the buttress has been removed.  During the night some vandals knock away the props.  The builders arrive in the morning to find a whole section of wall collapsed under a pile of soil.

Now imagine that funds dry up just after the props are put in place.  The site is secured and left, in the expectation that the props will hold out until funds are available.  But there is a major road nearby.  Every time a truck goes by, the ground shakes.  The shaking causes the props to move in microscopic steps.  The steps get bigger over time.  There comes a day when a truck goes by and the whole wall section collapses.  This is called catastrophic failure.

How to deal with such a problem?  Simple.  You determine the rate of movement of the props due to traffic and you determine the acceleration of that rate.  You predict the likely time of collapse.  You then work out a safety factor, for example dividing the projected time to collapse by four.  That gives you the maximum safe time between inspections of the props and probable need for remedial action.

Mutual buttressing

mutual buttressing and transverse compression