Ground Improvement in

Organic Soils

Organic soils and Peat soils (OL, OH, PT) can often turn a simple spread footing foundation design into a more costly deep foundation project. The excessive settlement caused by highly compressible organic soils typically exceeds the performance requirements for standard spread footings and requires that piles be driven through the organics to a firmer, more reliable bearing strata. Geopier Rammed Aggregate Pier elements, Geopier rigid inclusions or a combination of the systems can keep the foundation budget on track when organic or peat soils are encountered.

What is an Organic Soil?

The definition of an organic varies based on the source. Many geotechnical engineers classify any soil with a loss on ignition (LOI) of greater than 5% as organic. ASTM D2847 defines organic silt and organic clay soils (OL, OH) as any soil whose liquid limit after oven drying is 75% lower than its liquid limit when tested without drying. Peat soils (PT) are readily identifiable based on the presence of plant fibers and an organic odor but can also be defined as soils with a loss on ignition of over 30 percent. Federal Highway Research Report (FHWA /IN/JTRP-2008/2) on Classification of Organic Soils recommends that a loss on ignition test be used as the initial screening for organics. Soils with LOI test results of <3% are classified as inorganic, LOI > 15% as organic, LOI > 30% as peat and soils in the LOI = 3% to 15% range be tested further based on their liquid limits per ASTM D2847.

For geotechnical engineers, the presence of organic soils is an initial warning that special precautions should be taken when considering foundations that will be directly supported by organic soil layers.

Why Organic Soils can be a problem?

For geotechnical engineers, the presence of organic soils can be associated with soil properties that include a low specific gravity, high compressibility and potential for large secondary compression or creep,and low shear strength. Organic soils often have a higher natural water content and void ratio than inorganic soils and hence a high susceptibility to compression upon loading.

Therefore placement of new fill or new foundation loads on soil profiles with layers of organic soils can result in considerable immediate settlement, well above acceptable limits. Long term settlement can occur due to creep or secondary consolidation of the soil over time.

Ground Improvement Solutions for Organic Soils

There are three ways ground improvement techniques can be used to support structures at sites with organic soils. The choice of which technique to use will depend on the magnitude of the foundation loads and any new grade raise fill, the thickness and compressibility of the organic layer, the depth to a firm bearing layer and the settlement tolerance of the structure.

Reinforce the Organic Layer - For low loads and relatively thin organic layers, Geopier Rammed Aggregate Pier elements have been used to reinforce organic soils and peat soils in place. A low capacity is used for the aggregate pier which results in a low top of pier stress. In this way bulging of the pier in the thin organic layer is controlled to limit the overall settlement of an individual footing. This can be a good solution for one and two-story structures with relatively thin organic layers (2 to 4 feet thick). Case studies in which Geopier elements have been used to stabilize lightly loaded structures overlying organic soils can be found in the article by Charles Allgood, et. al.

Span the Compressible Organic Layer - When organic layers are relatively thick or of variable thickness, Geopier rigid inclusions such as GP3 Rammed Aggregate Pier elements with Cement Treated Aggregate (CTA) zones or Impact Pier elements with sectional grouting zones (used in caving soils and soils below the water table) can be used to transfer loads through the organic layers. The use of the rigid inclusion zone prevents the piers from bulging in the organic zone and allows the settlement to be controlled more economically than a full depth rigid inclusion.

An example of projects where this technique was used included the Walker Jones School built by Forrester Construction in Washington DC. ECS, the geotechnical engineer, discovered organic soils and peat layers at variable locations in their investigation. Therefore it was decided to implement fully grouted Impact Pier rigid inclusion elements to eliminate any pier bulging. This techniques was also used by Gilbane Construction on the Bergen Justice Center Park Development in Hackensack, NJ, where the geotechnical engineer PS&S discovered a relatively uniform layer of organic soils 5 to 7 feet thick. It was decided that the approximately 40 foot deep Rammed Aggregate Pier elements would be grouted using a sand cement grout in a zone 5 to 15 feet below the footing elevation to span the organics.

Bypass the Organic Layer- When loads are high and organic layers are thick, surrounding soils have low blow counts and there is a competent firm soil or rock layer below, it is often best to bypass the soft organic and natural soils with a Geopier rigid inclusion founded on a firm soil or rock layer. Loads from the structure are transferred by the rigid inclusion to the firm soil or rock layer. If new grade raise fill is required this needs to be considered in the design of the rigid inclusions. Fill placed prior to installation can result in negative downdrag on the inclusions, while fill placed after will result in additional loading that needs to be considered in the designs of rigid inclusion and footing or slab design.

This technique was used by Skanska Facchina JV on the 11th Street Bridge project in Washington, DC. A 35 foot high highway embankment was to be built on top of 30+ feet of soft organic silts and clays overlying a firm sand and gravel layer. Working with JMT and Mueser Rutledge geotechnical engineers, rigid inclusions consisting of GeoConcrete Columns were designed to transfer the embankment loads to the firm layer and control embankment settlement.

Bottom Line

Organic and Peat soils create unique challenges for geotechnical engineers and general contractors in developing building sites. The use of different ground improvement techniques can be used to:

Reinforce organic soils and peat with stiff Geopier Rammed Aggregate Pier elements designed for low capacities to limit stress and bulging in the peat soils;
Span the compressible organic layers using Geopier rigid inclusions; or
Bypass the compressible organic layers with Geopier rigid inclusions and transfer stress to a stiff lower soil layer.

Have an organic soil project? Click here for a project assessment.

Geopier Quality Assurance & Quality Control Training

GeoStructures provides training on Geopier Quality Assurance & Quality Control to engineers and technicians. If your firm is interested in participating, a presentation will be held at your office (early morning or late afternoon) followed by jobsite training.

For more information visit our website.

Need a Project Assessment?

If you have a project that could benefit from the use of Geopier ground improvement visit our website to submit a Project Assessment OR CALL: 410-458-0289.

Past Newsletters

April 2015 - Importance of Having Actual Structural Loads

March 2015 - Use of Ground Improvement for Floor Slab and Mat Foundation Support

February 2015 - Using Rammed Aggregate Pier Elements to Replace Piles or Drilled Shafts

January 2015 - Using Ground Improvement for Uncontrolled and Contaminated Fill Sites

News & Events

Instagram Quarterly Photo Contest

(Due June 30th)

If you work with GeoStructures you are invited to take part in a quarterly contest for clients/partners to showcase photos of current projects where GeoStructures is providing ground improvement (in-progress 4/1-6/30). To participate, upload your project photos to Instagram and include @geostructures along with #geocontestQ2. Please include your first and last name if submitting under a company account. The winner will receive a prize package from GeoStructures.

First Horizontal Piece Is Placed Along the New Tappan Zee Bridge

GeoStructures, Inc. provided support for the west abutment of the new Tappan Zee Bridge. On June 17th a crane hoisted three curved steel girders, the new Tappan Zee Bridge's first horizontal structure. For more information click here.

GeoStructures Annual Summer Picnic

June 11th

Employees and their family gathered at Franklin Park for the Annual Summer Picnic. Kids participated in a hula hoop contest, balloon toss and even took a swing at the pinata while the softball game ended with another victory for team GT Willis. For more photos visit our website.

GeoStructures Receives the ABC Safety Award

Ann Puleo, Safety Manager, accepted the 2015 STEP (Safety Training Evaluation Process) Silver award. STEP Silver designation shows commitment to improving jobsite safety through training and prevention.

In honor of Father's Day take a look at the history and accomplishments of The Father of Geotechnical Engineering - Karl von Terzaghi. To learn more visit our website.

GeoStructures, Inc

413 Browning Court Purcellville, VA 20132

703.771.9844

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