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The National Academies

NCHRP Synthesis 20-05/Topic 04-09 [Final (Synthesis)]

Treatment of Soft Foundations for Highway Embankments
[ NCHRP 20-05 (Synthesis of Information Related to Highway Practices) ]

  Project Data

For the objective appraisal of all applicable construction alternatives, the preliminary planning studies must include special foundation investigations. Right-of-way for some soft foundation construction alternatives may exceed usual requirements, and construction alternatives involving subsoil stabilization by consolidation require surcharge loading periods. The high costs of such additional investigations are, however, offset by the potential savings in construction costs. The applicable construction alternatives include (a) elevated structure, (b) embankment fill supported by piles, (c) excavation of soft soils and replacement by suitable fill materials, (d) subsoil stabilization with or without sand drains, and (e) no treatment whatsoever relying instead on specially detailed field investigations and design studies to achieve uniform settlements. Each alternative can be evaluated based on factors such as construction cost, maintenance, ecological and environmental effects, fill availability, and disposal area availability. Where subsoil stabilization involves vertical sand drains, the type of drain influences the design procedures. Where nondisplacement drains are used, field permeability tests are desirable because they result in a somewhat higher field coefficient of consolidation that will reduce the estimated consolidation time. Field permeability values should be reduced to account for effects of embankment loading. Extensive field instrumentation should be required where subsoil stabilization by consolidation is used. Where subsoil consolidation techniques are used, field test sections are desirable to achieve maximum economy. Field test sections are also useful in assuring the technical feasibility of consolidation techniques where elevated structures or other techniques were preferred. The quality and amount of field inspection are especially important and can be related to the post construction behavior of the types of construction discussed. 

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