Sponsored by ASCE Continuing Education and ASCE's Geo-Institute.

This webinar presents an alternative to classical deep foundations and concrete pile caps for construction over soft soil foundations. It applies to roadways, levees and dikes, storage areas, parking lots and related low-to-medium capacity surface loads. Focus is twofold; a high strength geotextile or geogrid reinforced load transfer platform (LTP), and an underlying deep foundation support system which should not have sharp edges, e.g., a geogrid encased stone column or a geotextile encased sand column is suggested.

The webinar is indeed design oriented. Five different published methods based on limit equilibrium for the LTP will be presented and compared to one another. The comparison is poor. This is followed by examples using the FLAC computer code (based on finite differences) which is a 3-D depiction of the situation. Its results highlight the critical issue of geosynthetic edge stresses when placed against a rigid pile or pile cap. This suggests the possibility of using a deep foundation system which is accommodating to the LTP reinforcement. This can be accomplished using geogrid encased stone columns or geotextile encased sand columns. A 2.4 km long dike construction case history encompassing the various points of the webinar is also illustrated.

Purpose and Background

There are many situations where moderate deformations in dealing with construction on soft foundation soils is acceptable. Rather than using a dense rigid pile group with a reinforced concrete pile cap, this webinar presents a very different alternative which is practical to implement and successful at least for moderate surface loads. Explained and illustrated in this webinar is a high strength geotextile or geogrid reinforced soil mattress, called a load transfer platform (LTP). It is supported by a deep foundation system which can be provided by geogrid encased stone columns or geotextile encased sand columns.

This webinar is design related and compares five published methods based on limit equilibrium. The results vary widely! It then transitions into computer design (FDM and FEM) which illustrates the very high stresses on the geosynthetic reinforcement at the edges of rigid piles and/or their concrete pile caps. In turn, this suggests using a more compliant deep foundation such as the geosynthetic encased soil columns mentioned previously. A case history at the end of the webinar which reclaimed 140 ha of soft soil for an Airbus fabrication factory along the Elbe River in Germany illustrates all aspects of the webinar.

Primary Discussion Topics

• Concept of construction on soft foundation soils using geosynthetic reinforcement of a soil mattress 
• Concept of the support of the mattress with geosynthetic related deep foundations 
• Five design methods for the reinforced mattress and how they compare to one another
• Two computer designs and recommendations for this situation
• Methods of constructing compliant deep foundations (stone or sand) using geosynthetics as encasement 
• A large case history embodying both the reinforced mattress and a compliant pile system

Learning Objectives

• Learn about geosynthetic reinforced LTP
• Understand differences in design methods using limit equilibrium as contrasted to computer codes
• Learn about the danger of high edge stress on the geosynthetic reinforcement of the mattress
• Understand about laboratory testing of high strength geosynthetics and their seams
• Learn about new deep foundation systems using encasing geosynthetics which provide low edge stress accommodation
• Understand how each of the above topics are integrated and deployed in a large case history

Webinar Benefits

• Understand that a geosynthetic-related system for moderate load support over soft foundation soils is available
• Appreciate that major design differences exist in using limit equilibrium methods
• In contrast, learn that computer codes based on finite differences or finite elements result in better understanding of the tensile stresses in the reinforcement
• Understand that alternatives to "hard" (classical) deep foundation systems exist and are based on geosynthetic encasement of granular soils
• Understand elements of the laboratory testing of high-strength geosynthetics which is necessary for proper design
• Appreciate that each aspect of the webinar can be incorporated into a case history as is described and illustrated

Intended Audience

• Federal, state and regional geotechnical and materials engineers 
• Geotechnical and geoenvironmental engineers serving private clients in heavy industries
• Private and municipal owners of all types of civil engineering systems 
• Private and public owners of construction sites with soft foundations soils
• General engineering consulting engineers
• Soils and geosynthetics testing laboratories servicing these organizations
• Manufacturers and representatives of geotextiles 
• Contractors and installers servicing these situations
• Academic and research groups
• Others desiring technically related information on this important aspect of our infrastructure system

Webinar Outline

• Concept and Overview
• Limit State Design of Load Transfer Platform
• Finite Difference and Finite Element Modeling
• Geosynthetic Encased Stone and Sand Columns
• Case History
• Conclusions and Recommendations