<p><p><strong>Reference(s):</strong></p><p><em>Bruce et al. (2000)<br>Bruce (2001)<br>Burke and Sehn (2005)<br>Larsson (2005)<br>Terashi and Kitazume (2009)</em></p><p><strong>Method Summary</strong></p><p>Wet grab sampling is one of the two most common quality assurance methods in current U.S. practice to determine the as-constructed strength of deep mixed columns. It should be noted that this verification method is only applicable to DMM projects utilizing the wet method. The sampling process involves inserting a special sampler into a freshly installed column to the desired depth, encasing soil-slurry mixture within the sampler, and returning the sampler to the ground surface. At the ground surface, the treated soil is formed into cylinder molds or cubes. Samples are cured under specified conditions (including temperature, humidity, time, etc.) and then tested in the lab. Wet grab samples are typically tested for shear strength using unconfined compression, UU triaxial, or CU triaxial tests.</p><p><strong>Accuracy and Precision</strong></p><p>Debate exists concerning the accuracy of strengths obtained from wet grab samples. Bruce (2001) notes that the presence of unmixed sections of native soil can prevent the sampler from functioning correctly. Additionally, samples are cured in different conditions than the in-situ deep mixed soil and therefore can yield differing strengths when tested. Some authors note that wet grab samples produce higher unconfined compressive strength results than core samples, while others note an opposite trend (Larsson 2005). Wet grab samples are generally forced through a sieve prior to forming test specimens, and these operations impart additional mixing energy to the test specimens. Burke and Sehn (2005) identify one of the advantages of wet grab sampling is that large numbers of samples can be obtained. This allows for testing of samples from the same location at various ages for better prediction of ultimate strength. They also note that the collection of more samples allows for a more complete statistical evaluation of test data.</p><p><strong>Adequacy of Coverage</strong></p><p>Obtaining a sample is relatively straightforward for mixtures that are relatively fluid. For DMM in plastic clay, the mixtures can be relatively thick, and it can be difficult to obtain wet grab samples. Due to the ease of sampling, a comparatively high percentage of columns can be sampled. Terashi and Kitazume (2009) recommend sampling frequencies in the range of one set of samples per 250 m³ to one set per 1,000 m³ of treated soil, with a minimum of one set per rig per day. Typically, wet grab samples are obtained from three depths within a column, whereas coring extends from top to bottom of the column.</p><p><strong>Implementation Requirements </strong></p><p>The implementation requirements consist of obtaining a proper sampler and training to use it. Obtaining samples is a relatively quick and cost-efficient process for relatively fluid mixtures.</p><p><strong>General Comments</strong></p><p>The usefulness of wet grab sampling is debated, as samples are retrieved from discrete locations, undergo additional mixing energy in the sampling and molding process, and are cured in conditions that differ from the in-situ conditions. However, the ease and low cost of sampling make it useful in obtaining a large number of samples. Wet grab sampling has been used successfully as a strength verification method on many deep mixing projects.</p></p>