Vance Kennedy: Understanding how subsidence works in the valley

There has been considerable publicity regarding subsidence, the settling of the land surface, especially in the southwestern part of the San Joaquin Valley. It can, and does, cause major damage to various structures.

Subsidence is known to be related to the pumping of groundwater from aquifers, but how that happens is not generally understood. This is an attempt to explain, in general terms, what occurs.

First, it is necessary to explain that when any solid is placed in a fluid there is a buoyancy effect. The buoyancy effect is dependent on the mass of the fluid pushed aside by the object. For example, if you put a rock on liquid mercury, the rock would float. That’s because the displaced mercury is so dense that the portion of it pushed aside by the rock is pushed down, causing the rock to float.

When a rock is placed in a pool of water, there is a buoyancy effect. But water is far less dense than the mass of the rock (or mercury), so the rock sinks.

However, if you could weigh the rock while it was submerged in water, you’d find that it weighs less than it weighed before you put it in the water. So, when stream sediment is deposited in water on already-existing sediment, the weight it brings to bear on the underlying sediment is not as much as it would have been if it were above the water in the air. Its weight is diminished by the amount of water displaced.

It’s a difficult concept, but crucial to understanding subsidence.

Over geologic time, sediments have eroded from the mountains and moved to lower elevations by rivers. These sediments range in size from very fine clay particles to gravels. Where you find them depends on the velocity, or carrying power, of the streams that moved them down from the mountains.

When these sediments are deposited independently, they vary widely in how much water they contain. In the case of clays, as much as 80 percent of the volume can be water.

The coarser sediments, such as gravels, contain considerably less water and are much more difficult to compact. As later sediments accumulate on top of these earlier deposits, the added weight compresses the underlying clays, sands and gravels. Because the clays contain so much water, they compact much more than the gravels and sands.

However, as the clays compact, it becomes harder and harder to squeeze out water.

Before man started pumping water from these underlying sediments, the water table was at or near the surface. Because of the buoyancy effect, the water near the surface was partially supporting these sediments.

As the water was pumped away, the water table dropped. As it dropped, the full weight of the sediments above the water table concentrated on the underlying sediments, squeezing more water from those beneath – especially the clays and shales (compressed clay). As clay and shale compressed, the land surface settled. That’s called subsidence. When the water table is raised by recharging the groundwater, the buoyancy effect is reinstated.

But once compressed, the shales cannot recover. So the subsidence is permanent.

There are formulas for estimating this added weight. But the increase in pressure can only be a rough estimate because some sediments are less dense than others, making subsidence hard to predict.

This story was originally published February 12, 2016 at 8:05 PM with the headline "Vance Kennedy: Understanding how subsidence works in the valley."