The funny thing about erosion is that if we don’t know what we’re looking for, we can walk right by it. This can be especially true when it comes to noticing processes so incremental that they’re hard to see.
On the other hand, almost everyone understands geologic erosion. This is the natural wasting of slopes that eventually creates the “good bones” of scenery we enjoy. “Oh yeah,” says a friend, “erosion produced the Grand Canyon. Why would anybody want to be in a professional group that wants to stop erosion?” He has a point. Geologic erosion is going on all around us in myriad subtle ways, creating remarkable landscapes over time. It can seem contrary that the processes creating these landscapes are incremental for the most part, and we might walk right past most of them without noticing that they are going on.
One area of dynamic change is the soil itself, which is a good reason for ESC professionals to know something about the genesis and behavior of soils at their project site and in its vicinity. “But how exciting can dirt be?” challenges my friend. “Can’t people just do the erosion control without having to study up on dirt?”
Actually, many erosion control ordinances for construction are based on the assumption that all soils are highly erodible. This allows the BMPs to be applied evenly to all soils rather than being tweaked for different levels of erosion hazard. For the most part, this makes erosion prevention and sediment control practices accessible to all who will be working with the site, without respect to their education or resources. This is the positive side of getting into erosion control without getting into soils.
But having a particular understanding of specific soils has an upside, because it can illuminate a whole world of site dynamics that have the potential to affect our projects. We know that erosion’s mechanical and chemical processes are fueled by gravity and solar energy (or lack of it) and operate at the interface between the earth and the atmosphere. We know that these processes-landslides, glaciation, pothole grinding, solution of limestone, and all the other ways that water, gravity, and climate work together-transfer earth materials from highlands to lowlands, gradually lowering the earth’s surface to sea level. “OK, so to do erosion control, people have to study ancient history?” my friend asks.
Well, not exactly, because the same processes that operated historically at the earth’s surface are operating today. This means we can look at active examples of local, climatically, and geologically idiosyncratic erosion and apply the understanding of these dynamics to local projects and regional land and resource planning. If we can get our arms around the rates and intensities at which these processes are operating, we can build value into our projects. I tell my friend that it’s like starring as the brawny sleuth in an environmental thriller.
Stalking the Bad Guys of Shrink and Swell
Early in his career with the Soil Conservation Service, soil scientist Dick Kover had an experience with a dynamic soil he describes as “one of the great horror pictures in soil taxonomy.” The site was a subdivision in Thousand Oaks, CA; the time was the early ’60s. “They had the worst of all conditions in this subdivision,” he recalls, “a soil high in montmorillonitic clays-the bad guys of shrink and swell.”
The soil, a vertisol also known as “black gumbo,” “adobe clay,” and “seven-hour clay” (because of the limited window of time in which it is dry enough to be ploughed, but not too dry), is common in grasslands with dry summers and wet winters, explains Kover. Vertisols are found in areas of California in the Mediterranean climatic zone and in some parts of coastal Texas and the other Gulf States that have distinct wet and dry seasons. They are rare in both cold and warm deserts. Vertisols can be associated with the water-laid sediments of dry lakebeds and other low valley locations. They can also form on siltstone, clay stone, and volcanic bedrock that have weathered to clay.
Where vertisols occur in grasslands, these clay soils are unusual because they have no distinct horizons and possess 1-3% organic matter. Both these characteristics reflect the soils’ dynamic nature, a feature that can go unnoticed by the casual observer or one not familiar with the site on which it occurs. Among the clays, montmorillonite has the capacity to hold on to the highest volume of water. This accounts for its gains in volume when wetted. By the same token, it loses volume when it dries out. Thus, as this soil gains and loses water, it shrinks and swells. “There is constant motion going on,” says Kover. Polygonal cracks open up, sometimes 2-3 in. in width. He quips that he once lost three golf balls down such cracks in a single game on a course near Berkeley, CA.
When drying starts and cracks begin to open, organic material falls into the cracks, along with granular soil material from the surface. When the soil is wetted and swelling occurs, these materials are incorporated into the soil at depth, which explains why these soils do not have distinct horizons in their profiles. As wetting and drying alternately occur, these soils churn, so Kover figures his golf balls eventually will pop out at the surface.
The problem comes when development is located in areas of high shrink-swell soils. The soils have the potential to crack concrete slabs, shear water and gas lines, break sidewalks, and bend roads. Kover witnessed these impacts at the boarded-up subdivision in Thousand Oaks, and they made a huge impression on him. But if the potential for these problems is known, he says, it is easy to design around them.
Compiling a Dossier on Mystery Soils
To discover the potential quirks of soils on private sites, the first stop for information should be the local office of the soil and water conservation district or university extension service to see what soil survey information is available. The Natural Resources Conservation Service (formerly the USDA Soil Conservation Service) does field mapping of soils and publishes a soil survey for each county in the nation. The published surveys are storehouses of useful information about the genesis and behavior of local soils, their engineering properties, and the crops and native vegetation they support. The information is free and comes with aerial photos of the county at a scale of about 1:24,000 on which soils series have been mapped. This scale roughly matches the scale of 7.5-minute topographic maps. A cursory look at the soil survey can often signal whether additional soils or geotechnical investigation is needed.
The overarching principle to apply when developing shrink-swell soils, says Kover, is to get foundations down below the active layer of the soil, which is usually within the first meter or so beneath the soil surface. If excavating to get below the active layer, it’s important to keep water from seeping into the inactive layer from both natural and human-made sources such as groundwater, septic seepage lines, roof drains, or dry wells. The excavated area is backfilled with gravel, on which a foundation can be set or a concrete slab poured. Other approaches include driving piles to get structural support from beneath the churning layer or using more reinforcing steel in the concrete slabs. In any case, the cost of finding out about a site’s soils and providing the appropriate extra engineering is relatively cheap compared to the expense of having to board up a subdivision and walk away from it.
Tailing the Invisible: Wind and Extreme Cold
Wheat farmers of the Palouse and northern Midwest have long grappled with problems of freeze-thaw that put some districts at tremendously high risk of erosion. The rolling topography of much of this zone is derived from geologic, or background, erosion of fine mineral sands and silts that blanket older landforms. These fines were blown in from regions whose landscapes were pulverized by continental glaciers or deluged by glacial outwash. In short, a lot of finely ground rock material was swept out of glaciated areas by post-Pleistocene winds and deposited in a thick veneer on the landscapes of adjoining regions.
These relatively young, unweathered, wind-blown mineral soils tend to lack the complement of clays that make the soil cohesive. So right from the get-go, such soils have a high hazard of erosion. To this recipe for erosion, add steep slopes and a propensity for soils to freeze in mountainous country and at latitudes greater than about 45°. It could be easy to think that the potential hazard of soil freezing would be low for well-drained sandy and silt-loam soils. But, says Richard Dierking, retired chief of the soil survey classification and mapping branch of the former USDA Soil Conservation Service, water can and does collect in the less permeable layers of some of these soils and in depressions on sideslopes and in other low areas. The amount and location of groundwater, the microrelief of the landscape, and the nature of the subsoil and buried landforms are important factors in determining the hazard of freezing.
And finally, aspect, or the direction a slope faces, is a critical factor in determining the erosion hazard of a low-cohesion soil subject to freeze-thaw. A south- or east-facing, saturated, frozen soil will receive the full brunt of the sun’s rays early in the day. As it thaws, the soil will become more saturated and begin to move. If it’s on a slope, it will move faster, explains Dierking. “This can be quite dynamic,” he remarks. “During spring thaw, the whole thing can become one big soup bowl. If you’ve got a slope, you’ve got a problem.”
Soils that warm up more quickly in springtime can sometimes be identified on aerial photographs. “You see more erosion on the south slopes than on the northeast slopes,” he says. Sometimes there are topographic indicators of this on contour maps as well. In some locations, the vegetation associations of north- and south-facing slopes reflect these local differences in soil and climatic conditions and can be indicators of soil dynamics to be expected when any soil-disturbing project is planned.
Scheduling the Operation
For development projects likely to be driven by time, money, and scheduling, Dierking notes that it can be cost-effective to plan earth-disturbing work in loess soils to avoid the spring-thaw period. It can pay to look at the local soil survey and talk to the local agronomist, planner, and soil surveyor about site soils to forestall any surprises.
Getting set up properly for overwintering is critical for development in the Palouse, according to James Carley, natural resources consultant and president of the Washington Society of Professional Soil Scientists. Soils may freeze hard in early winter, then be covered by snow. The problem comes when the warm chinook winds kick up in January, typically during the time of deepest snow pack. Sometimes accompanied by warm rain, the chinook causes rapid snowmelt over frozen ground. “This takes that topsoil off, and down the stream it goes,” says Carley.
To stave off this nightmare and to protect downstream wetlands and water resources from being overwhelmed by sediments, it is important to seed disturbed areas to grasses as soon as possible. Carley notes that wheat grasses and hard fescue do well in the Palouse, where loess soils and their susceptibility to erosion are a fact of life.
For people developing single lots, Carley points out that there are striking economic reasons why it’s essential to think about what’s underfoot. On one soil, the landowner might be able to put in a traditional septic system for about $2,500, but next door, soil characteristics might require a sand-based pressure-bed system costing six times as much. This is not the kind of information most people want to hear after they’ve bought the land and are well on the way to making their dream home come true.
Sleuthing for Hidden Clues of Subsurface Ground Movement
Another dynamic condition can exist beneath the soil surface that neither single-lot nor multilot developers want to discover by surprise, states Tim Blackwood, geotechnical engineer for GeoEngineers of Portland, OR. This is a deep-seated landslide that can occur at a very low angle and move an entire area, yet might not appear to be a slide. Such slides may go for many years with little or no movement but can be reactivated by one of a number of factors. The most common factor is generally an extended period of above-normal rainfall that effectively weakens a contact zone between materials of different permeability or strength. But rainfall alone is not always a direct factor, says Blackwood, who mentions several deep-seated slides in the arid Great Basin region of the western United States, where groundwater plays a key role.
Deep-seated slides are common in the Pacific Northwest, notes Blackwood, because volcanism, glaciation, hillslope wasting, and the work of rivers and regional winds have created landscapes in which sediments, lavas, cemented gravels, wind-blown soils, and other materials of contrasting permeability and strength are interbedded. Groundwater moving through more permeable materials can cause excess groundwater pressure at a contact zone, resulting in translational movement that can leave a house straddling a void, sever utility pipes and wires, or demolish a road. If the slide has a large areal extent, portions of it might be moving during one time period while other portions appear to be relatively stable. The trick is to identify the extent of the slide, even though a good portion of it is relatively flat and appears motionless.
The Importance of Thorough Homework and Fieldwork
A good first approach is to look for topographic indicators on a 7.5-minute quadrangle. Features to watch for, suggests Blackwood, are a steep head scarp, a hummocky body, and an oversteepened toe that often ends at a stream. Contour lines in the body of the slide are often irregular, reflecting deranged drainage. In some settings there will be several somewhat parallel drainages through the body that might wander away from and back to each other, eventually joining. In other conditions, drainage might be undeveloped. However, none of these features may be distinguishable on the topo if the contour intervals are too large for this detail to be represented or if tree canopy is present. Topographic maps are based on aerial photos, and if the cartographer could not see the ground in the source photo, the nature of the microtopography might not be represented on the resulting map.
But nothing substitutes for going to the field to have a look. At the first level of reconnaissance, geotechs are looking for the gross morphologic indicators: head scarp, body, and toe. Landscape changes-such as slumps, sags, and drainage irregularities-are noted. The locations of springs are marked. Bowed or jackstrawed trees are jotted down. Geological information, such as outcroppings, is mapped. The field crew may measure some cross-sections to delineate the geometry of the slope or landslide. Blackwood estimates that perhaps as much as 80% of landslide problems can be identified with this first level of information review and field reconnaissance. “The remaining 20 percent gets 80 percent of the analytical effort.”This effort goes into subsurface explorations and instrumentation to determine what is happening with geology and groundwater at depth, how the earth is moving, and to what extent.
Calling in the Reinforcements
Today, says Blackwood, most municipalities rely on engineering standards contained in the Uniform Building Code for earthwork on large sites. But a lot of smaller sites can fall through the gaps as a result of the lack of a regulatory instrument requiring geotechnical investigation. However, he says, this is slowly changing. Many communities are beginning to use zoning overlays that identify areas of potential landslide hazard. In general, development is not allowed in such areas or is subject to rigorous requirements for geotechnical investigation and engineering design. These requirements can go a long way toward protecting landowners and developers from expensive surprises.