Landscape Expansive Soils Like A Pro

Site Inventory Series Logo for the California Supplemental Exam (CSE) for landscape architects.Expansive soils pose problems for site development and landscaping throughout the state. Moving soils can wreak havoc on structure, pavement, and plantings. Fortunately, there are design and construction methods that landscape architects can use to minimize the negative impact of expansive soils.

What are expansive soils?

Expansive soils shrink and swell in response to soil moisture. Dry soils shrink. Wet soils expand exerting a great deal of force on structures and utilities. Swelling soils ca exert up to 15,000 PSI on building foundations which can lead to cracking.

Expansive soils usually contain a large percentage of clay particles that swell when they absorb moisture.

How can I Identify Expansive Soils?

There are a few characteristics of expansive soils that you can use in the field to help identify the presence of expansive soils. Some (but not all) expansive soils will show surface cracks when they are extremely dry. When expansive soils are wet, the usually are very sticky or slick due to the clay content.

There are a couple of more reliable techniques landscape architects use to confirm the presence of expansive soils.

  • USDA Soil Survey
  • Soil Testing

Soil Surveys
Soil surveys contain a wealth of knowledge about the soils found on and around your site. Use the maps in the survey book to identify the soil type(s) found on your site. Next, read the details about you site’s soil swelling potential.

Soil Testing
Soil surveys are reasonably accurate, but a soil sample analyzed by a geotechnical engineer can deliver superior analytical results. Soil testing by the right lab can reveal the soil’s true shrink-swell potential. You can then use this information to design appropriate footings, foundations, and structures that can safely withstand the extreme pressures caused by soil movement.

A geotechnical engineer ca be included as part of your client’s project team if the swell potential is very high. Geotechnical engineers can provide reports about the soil’s swell potential, treatment recommendations, and engineering guidelines that can help landscape architects safely develop a site.

How is Landscape Construction Affected By Expansive Soil?

Site development can take place on expansive soils if precautions are taken to reduce the negative aspects of expanding soils. There are a couple of ways to prepare a site.

Engineered Fills

The most desirable method for new construction is to replace the expansive soil with stable, impervious engineered fill. Typically, three or four feet of expansive soils are removed and subsequently replaced with structural engineered fills. When adequately compacted and protected from the elements, engineered fill creates a stable substrate for development. Consult with a geotechnical engineer to assist in creating the perfect engineered fill.

Soil Treatment
Soil treatment techniques are occasionally used on large sites or transportation projects to stabilize large swaths of land. Cement and lime are two of the more common ingredients used to help reduce the swell potential of expansive soils. However, both of these methods increase the soil’s pH which can limit you choice of plant species. Before adding any chemicals tithe soil, consult with a geotechnical engineer

With proper soil preparation techniques, structures can be safely constructed on expansive soils.

Best management practices for expansive soils

While expansive soils can cause more complications than non-expansive soil’s during site development, there are some practices that landscape architects can use to help minimize soil movement around structures.

Here are some guidelines to help minimize the chance of damaging landscapes and structures from expanding soils.

Positive Drainage
Since water infiltration is the leading cause of soil expansion, it makes sense to grade a site to keep water away from structures. Some experts recommend a finish grade of 10 percent around structures so water flows away from foundations. Maintain constant soil moisture to prevent soils from expanding and contracting.

Prevent Infiltration
Install an impermeable moisture barrier around the perimeter of a structure. It is very important to maintain a constant level of moisture around and under buildings. By maintaining a five-foot barrier around a structure, the shrink-swell cycle is interrupted and damage to structures is less likely to occur. Moisture barriers can be covered with decorative rock or mulches to help the perimeter area blend into the landscape and provide an aesthetically pleasing appearance for site users.

Plant Placement
Plant roots are very effective at pulling water out of soils. Unfortunately, this natural function affects soil moisture levels which can magnify expansive soils shrink-swell cycle. Locate shrubs and groundcovers five feet or more away from structures to minimize changes in moisture.

Trees must be placed farther away from the building foundation to avoid affecting foundation soil moisture levels. Plant trees at least 15 feet away from buildings and structures.

Proper plant placement can minimize harmful changes in soil moisture levels.

Plant Selection and Irrigation Methods
Select plants with low supplemental water requirements to avoid added extra moisture to the soil around buildings from irrigation.

Use efficient, low-volume irrigation near foundation plantings. A well-designed drip irrigation system can effectively apply water to landscape plants and avoid introducing irrigation water into soil near the foundation.

Proper Management = Successful Development

Expansive soils are a real problem that landscape architects encounter in California. Through proper identification, engineering, remediation, and design, expansive soils can be safely developed.

Related Articles:

  1. How to Defeat Expansive Soils

John is a landscape architect who is currently preparing to take the California Supplemental Exam to become licensed in California. He is currently a licensed professional landscape architect in Georgia and Florida. John graduated from California State University, Pomona with a BSLA degree in landscape architecture in 1997 and has extensive practice experience in residential and commercial landscape design.

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