Soil Investigation Testing

Services: Soil Investigation & Testing

picture of soil testing in progressOne of the initial steps in the design process for a subsurface sewage disposal system (septic system) or a stormwater infiltration system is to investigate and test the soils where the proposed system will be located. The type of soils and any evidence of groundwater or shallow bedrock can greatly impact the type of septic or infiltration system that can be used and in extreme cases whether or not the soils are even suitable for a septic or infiltration system. Due to the importance of the soil testing results in developing a cost effective design, Arthur H. Howland & Associates, P.C uses our in-house soil scientist on all septic system and infiltration system projects to perform and accurately document all results from soil investigations and tests.

Deep Observation Tests

A soil investigation typically starts with the excavation of deep observation pits at key locations of the site.  These test pits, which are commonly dug using a backhoe or excavator, are typically two feet wide and dug to a depth of seven feet or to ledge rock (whichever occurs first).  The purpose of these observation pits is to record the soil profile and determine the limiting factors for the design.  This is important because different layers or horizons in the soil each influence water and effluent differently.  The color, texture, composition, and consistency of the horizon are recorded.  Other important factors such as root depth, groundwater, and mottling are all recorded.   This information is used to determine the depth of the most restrictive layer which is then used as the basis for the design calculations.  The most common restrictive layers are seasonal high groundwater, glacial till or hardpan layers, and ledge rock.  These different restrictive layers inhibit movement of effluent and water and need to be accounted for in any successful design. Deep observation pits are always backfilled after the soil profiles are recorded to prevent safety risks.

Percolation Tests

The second part of the soil investigation is the percolation test. The test, which is performed in a post hole that is approximately six inches wide with a varying depth, is used to measure how quickly water dissipates from the hole.  The depth of the percolation test is determined by the depth of the restrictive layer.  A pre-soak is first performed by adding twelve inches of water and allowing the test hole to either go dry or run for a period of one hour.  The purpose of the pre-soak is to allow sufficient soil-water contact time. During the pre-soak, swelling clays that may be present in the soil will expand resulting in reduced void spaces within the soil. Sufficient presoaking also allows the advancing capillary wetting front, which controls the rate of water flow in unsaturated soils, to move far enough away from the test hole so that an apparent equilibrium flow rate is achieved. Following the pre-soak, the percolation test hole is then refilled to twelve inches of water.  The water level is then measured at ten minute intervals to determine the rate that the water is absorbs into the ground.  The final measurement at the end of one hour is used to determine the percolation rate and is expressed as the amount of time it takes for the water level to drop one inch.  The percolation test hole is also backfilled after the completion of the test.

Permeability Tests

In situations where more precise information regarding a soil’s ability to transmit water is required, a permeability test can be conducted.  These tests are typically conducted as part of a hydraulic analysis for sites with large design flows or on properties with marginal soils or lack of space.  During this test, a permeability tube sample is collected from a deep observation pit and brought back to our office for falling head permeability testing.  During a falling head permeability test, the soil tube sample is placed vertically in a bed of sand and filled with water.  The water level is measured over time and several tests are performed for repetition.  This information is used to determine the hydraulic properties of the soil.

Coordination with Regulatory Agencies

In addition to our staff soil scientist, a health representative will be present during the soil investigation of a site to observe and record the findings of the different soil tests.  In most residential situations, the health representative is a local sanitarian appointed by the town.  In situations where large flows are anticipated, a representative of the Department of Environmental Protection will witness the deep observation pit tests.

If you are in need of soil testing for your property, contact us at (860) 354-9346 to discuss how Arthur H. Howland & Associates, P.C can provide cost effective, project specific solutions that fit your needs.

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