Home & Garden Columns

About the House: A Resident’s Guide to Our Mushy Landscape

By Matt Cantor
Friday November 30, 2007

Welcome to my watershed. I really like it here but it is, basically, a big clay bowl and we’re all salad. 

Some of us get lucky by being up on the edge of the bowl or on one of the ridges on the inside, but most of are not and so it gets wet under our houses. 

This image is intentionally over the top but I want to get you started thinking about this in a larger context. We are in a watershed filled with creeks, springs, aquifers and culverted water-ways. If you put clay soils on top of this system of waterways, you can imagine that you end up with something like your first experience on the potter’s wheel. Everything is slippery and it’s hard to maintain a rigid or fixed form. 

You might imagine that it’s rather hard for a house to remain truly rectilinear, plumb and square when resting on this sort of thing. Add to this the fact that many houses were built on “filled” soils that were brought to the site to create a level surface (or because it was cheaper than hauling off the excess soil from local works such road building) and it’s easy to understand why these houses are so wracked and warped. The filled soils may have seemed stable when they were first installed but the loading of many tons of house combined with a few good rains and, voila, you’ve got Trouble (right here in River City!). 

“Filled” soils compact under load or when water is added and many houses have “differential” settlement (one area has settled more than another) that is attributable, in part, to this effect. 

When contractors started building here in the 1800s, they didn’t pay drainage or soils issues much heed and so many of the houses built up through the early 1900s have settlement which stems from these oversights. By 1940, foundations got much stronger and so could “bridge” over soft spots without settlement to a much greater degree. We also observed better site preparation beginning in this time period and the avoidance of filled soils was one such improvement. 

In short, the soils conditions we find locally (and in many other parts of the globe) require that buildings be able to withstand a certain amount of earth movement and poor drainage. 

Many of these issues are hard to resolve without great sums of cash. However, there is one factor in this scenario that is, at least somewhat, manageable and that is the water. 

Wet soils move more than dry soils.  

We can’t really change the soil we’re on (well you can but, boy, it’s really expensive) but you can keep it dryer. There’s no perfect drainage system but if we endeavor to keep the soil below our houses dry we can slow the movement quite a bit and have more stable, less weirdly shaped homes. 

If you’re on a hillside you have a more complex problem, although your water issue may not be as bad as some that I see in flatter areas. 

If your crouton is located on the side of the salad bowl, it’s working it’s way slowly to the bottom of the bowl. Add more dressing, it will get there faster. If your crouton is on the bottom of the bowl, it’s not moving so fast, although it may be sitting in too much Balsamic Vinaigrette. 

As water softens the soils below hillside homes, they will tend to move downhill more rapidly than they will when they’re dry. Those of us who get to live in the hills are, therefore, living in mobile-homes. Gravity not only pulls our houses downhill, it also applies force “differentially” and many hillside homes show separations or cracks that result from different parts of the house moving in different direction and/or at different rates. 

One cause of differential settlement is that the wetting of soils is never uniform. Even if the soils you rest upon are completely homogenous, they will not be getting wet in a uniform manner because water flows in funny and surprising ways, although some aspects of this are predictable. For example, water will flow down against the back of your house (if your house faces downhill), creating wetter soils there. This can make the back wall settle more than the rest. 

The result of uneven wetting is, often, uneven settlement. As I’ve indicated, this is more true with early foundation than with modern ones due to their breadth and strength. 

There also may be harder soil beneath some parts of your house and regardless of wetting, that part might always be held aloft while other parts drop away. 

Settlement can occur just as easily on soils of uniform strength when some parts are kept much dryer than others. Often the middle of the house is staying dryer and does not settle as much as the edges which are wetted to a greater degree. This is not consistent, though, since some houses have deep portion near the middle (especially hillside homes which are cut away for basements or garages). These houses often exhibit the reverse effect with the middle settling faster than the rest because the middle supports rest upon wetter soils in a depression that holds water. 

Just to make matters all the more confusing, our local clay bed has the disconcerting propensity to rise and fall as it wets and dries. Expansive clay soils will push houses upward as they get wet (because the clay takes on water and holds it) and then lower these structures down as they dry out. This is sometimes referred to as “clay-jacking.” 

This turns out to be a locomotive process when you add gravity. Hillside homes are driven downhill very slowly because every time they rise and fall, they get pushed a little further downhill. 

At this point I feel obliged to stop the drama and say that most of the houses I see are not being affected by these forces enough to require any significant repair. Few houses remain truly square after 6 or 8 decades of being subjected to these effects but, in most cases, these changes can be spackled and ignored. 

So, let’s review. You’re living in a crouton on the side of a bowl of Caesar. 

Remember to ask for the dressing on the side. 

Next time, I’ll explain about how to do that and the solution is French (the drain, not the dressing).