The Geology of Water Supply and Building Things

Water is vital to human life. Early settlements developed along rivers and other places where water was easily avalable. Now that we have a far larger population, and reasons to live in places where there isn't an easy supply of water, we have to be able to find it, collect it and get it to where it is needed.


What the Syllabus Says - and the Details

Hydrogeologists study the geological factors affect the siting of reservoirs and dams. 

The reservoir is the lake, the dam is the wall holding the water back.

These factors are..

Permeability of bedrock - if the rock is permeable because it is porous or has joints through it then the water will flow away through the rock. Not much point building a dam then! Metamorphic rock are generally good for putting reservoirs on as they are highly compacted.

Stability of bedrock - if the rock you're building the dam on isn't stable then the dam will move. Big flood! Bad! Very bad - lots of compensation claims to pay. Metamorphic rocks tend to be stable as they are normally very strong.

Dip of strata - take your inclinometer and go measure the dip...

If the rocks are dipping from the reservoir to the dam - that's bad. Don't build the dam. The weight of water will push it down the dip of the rocks so the dam basically slides away. More floods! More compensation claims! If the rock dips from the dam towards the reservoir - that's good. The weight of the water will only push the dam harder into the bedrock.

Faults - if the faults are under the reservoir they'll only matter if they provide a route for water to flow away through. If the faults are under the dam then they could be forced to move by the weight above - oops!

Joints - these are a pain. (Joint-pain?) Under the reservoir they let the water out. under a dam they would cause the rock to be to weak so the dam could move and collapse.

Geotechnical geologists also need to consider these features when; designing the foundations for buildings, deciding how steep to make a road or railway cutting, or when designing a tunnel. 
Dipping rock layers can slide off each other! Rocks with joints can collapse under an increase in load. If the structure of the rock means that it is likely to move when it is loaded with a building then they will ask for designs with 'ties' that attach beds of rock together. With tunnels the problem is that beds might move when you make the hole. If the beds slide into the tunnel you won't have a tunnel anymore.
If the rocks are permeable then that can cause tunnels to fill up with water - they would need to design a concrete liner to keep the water out. Water flowing through permeable rocks can cause cutting walls to slide and collapse.

Dams and reservoirs are expensive - even when they work. It's often better to find a suitable aquifer (a porous or permeable rock, situated below the level of the water table so the spaces in it will be flooded, that we can pump water out of). If the aquifer rock outcrops on the surface

then it will fill up when it rains - water will drain down into it. Notice that the aquifer needs to be between two impermeable layers of rock; these are called aquitards or confining beds.

If the surface outcrop is above where we want to remove the water from then we may be able to just dig a well and the pressure of the water will force the water to rise up it.

If you're really lucky, your aquifer will outcrop at a lower point than where it is filling up. If this happens you'll have the cheapest water source of all - a spring. The aquifer leaks its water onto the surface.

Of course - if you take too much water out of the aquifer it may collapse. That will cause all sorts of problems...