CLEARWATER COUNTY GEOTECHNICAL HAZARD STUDY DESCRIPTION OF GEOTECHNICAL TERRIAN UNITS

GTU 2 – Loess and loess over basalt Ql, Qp, Qlcr Qld, QTlbr, QTlsr

The relatively flat portions of the County are generally covered by silty and clayey loess. The Lewiston basin has calcareous wind deposited silt, Ql, from 5 to 20 feet thick on top of relatively flat basalts. The Palouse Formation, Qp, comprises the Palouse Hills to the north and the loess can be up to 200 feet thick. The Qlcr geologic unit is loess, colluvium and residuum on gentle dip slopes of basalt and Qld is 1 to 6 feet of loess mantling duripan or lime and silica cemented angular basalt clasts.

To the east the QTlbr geologic unit represents generally 1 to 6 feet of loess over basalt which has weathered to clayey saprolite. QTslr represents thin deposits of loess over transported and residual sediments.

In general loess is a fine-grain wind deposited sandy, silty, clayey soil with a low dry density. It has clay cementation that makes it strong and stable when dry. However, the soil is highly compressible due to it’s low density and unstable when wet. It can become wet naturally generally resulting in landslides. Development can also cause the soil to become wet or saturated resulting in large settlements under applied loads and/or loss of bearing capacity. Natural landslides occur on slopes steeper that about 20% and under high moisture contents, generally higher that 18%.

Slope - Slopes are flat (0-10%) to sloping (10-20%). However some slopes exceed 20% and slopes should be checked for each site.

Ground Water – Regional ground water is generally deep but thin clay layers in the loess can cause local perched water tables. It is these water tables that cause slope instability due to high local moisture contents.

Erosion - Erosion potential is high on both natural and made ground.

Soils – Loess is wind deposited soil that varies from sand to silt to clay. Generally to soil becomes more fine grain toward the east. The clay acts as a cementing agent that provides strength to the sand and silt particles. However, loess is subject to collapse and loss of strength upon wetting. Thickness of the loess varies from up to 200 feet in the uplands west of the County near Genesee to about 6 feet near the eastern county boundary.

Landslide Potential - Subject to shallow landslides on steep slopes and deep landslide on more gentle slopes. Landslide potential is generally associated with natural water contents over 18 percent. Therefore all made slope should be protected from surface water infiltration.

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Earthwork – Loess is strong and, without water infiltration, can stand in vertical slopes. Steep slopes are sometimes desirable because they decrease surface area for water infiltration and erosion. Earthwork should be accomplished in dry weather.

Roadways – Loess has poor road support capabilities, generally requiring a minimum of 8 inches of crushed rock surfacing and good surface drainage. Fill slopes should not exceed 2H:1V. Cut slopes can be vertical depending on the natural water content of the soil to be cut.

Foundations – Foundation support capabilities of this soil is good except when wet. Loess tends to collapse and cause large settlements or complete loss of support. Developments should include adequate surface and foundation drainage to prevent soil saturation.

Septic Systems – Septic or wastewater systems will not work because loess is relatively impermeable. Septic systems and surface ponds may cause collapse of loess.

Site Specific Studies - Loess soils are variable and subject to collapse and loss of bearing capacity. Subdivisions and large structures should require a Site Improvement Permit. All of the steep slopes should be classed as Special Areas of Concern. The Soil Engineering and Engineering Geology Report should specifically address landslide potential from the proposed development including cuts and fills, location of building and location of septic or other wastewater disposal systems. Site development should be designed by a civil engineer and a geotechnical or geological engineer licensed in Idaho.

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