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Solutions for Soft Soils

The site you select for its desirable transportation access may turn out to have less than desirable soil conditions. Highways and rail lines often parallel the flat terrain of river valleys and coast lines. The alluvial soils common to these regimes often contain clays that have never carried loads greater than its own weight.

These “soft soil” sites pose real risks. These include spending too much for a conservatively designed foundation, or spending too little for a foundation that ultimately settles and costs even more in repairs and lost productivity.

The first order of business is to perform a subsurface exploration that effectively characterizes how the soft soil layers will respond. The softest and most critical soils are normally quite sensitive to disturbance. Therefore, even careful drilling and soil sampling procedures often yield a relatively poor picture of the soils’ true nature due to unavoidable sample disturbance. The fragile structure of the soil suffers cumulative damage during sampling, transporting, extruding, and lab testing. Also, soil samples are sometimes not taken continuously throughout the boring to reduce costs. The unsampled zones interject another factor of uncertainty.

To counteract these effects, Terracon provides in-situ testing methods that measure in-place soil properties. The Cone Penetration Test (CPT) method steadily pushes a cone-tipped instrument into the ground while a computer measures tip resistance, side friction, and pore pressure for approximately every inch of penetration. This yields continuous, real-time data from which soil layering, strength, and consolidation history can be immediately interpreted. By performing CPT tests in advance, the resulting soil profiles can be quickly evaluated to determine the most effective locations and depths to obtain soil samples for additional classification and compressibility testing. Terracon also provides the in-situ Vane Shear Test (VST) method to measure in-place shear strength and confirm its correlation with the CPT data.

Foot per foot, CPT testing is considerably less expensive than conventional soil borings and lab testing. Given the better soil characterization and higher confidence achieved, we are often able to show that site conditions are far less severe than would have otherwise been interpreted. Regarding in-situ testing of soft soil sites, the question should not be “can I afford to do it?” but “can I afford not to?”

What happens when the site conditions have been properly characterized and are too soft to support your planned development? You are now faced with choosing from a broad array of deep foundation or ground improvement alternatives, or finding a new site.

These challenging sites are when the local knowledge of Terracon’s project engineers, complemented by the assistance of experienced senior consultants and technical leaders in the firm, can provide real value engineering to guide you to the optimal solution. Through our internal network, we link all of our practitioners to extensive resources and specialized experience, thus making our national expertise readily available to all of our engineers. We also provide continuous training on technological advancements with monthly webcasts, and other continuous learning activities available through our Terracon University.