<p><p><strong>Reference(s):</strong></p><p><em>Bruce (2001)<br>CDIT (2002)<br>Garbin et al. (2009)<br>Lambrechts (2005)<br>Navin and Filz (2006c)</em></p><p><strong>Method Summary</strong></p><p>Core sampling is the predominant post-construction quality assurance method in current U.S. practice for strength verification of deep mixed columns. A survey was conducted by Puppala and Porhaba (2005) in which a questionnaire was sent to deep mixing practitioners and researchers in Europe, Australia, and the United States. The survey showed that for field sampling and laboratory testing, the majority of the practioners and researchers preferred core sampling over other means of sampling. Coring is typically performed on selected DMM columns after a curing period of about 28 days. Coring has been successfully applied to both wet and dry DMM projects with high recovery rates. Bruce (2001) explains that several key elements contribute to obtaining good quality samples. These include using experienced drillers and logging engineers, using a large diameter (over 3 inches) triple-tube barrel, and using wireline core drilling. The core bit used should direct the drilling water sideways and outward, away from the sample to prevent erosion of the treated soil (Lambrechts 2005). Down-hole video cameras have been used with continuous recording to help validate the continuity of the mixed column (Garbin et al. 2009). Extracted cores should be examined for continuity, uniformity of soil-binder mixing, and strength. Care should be taken in logging, wrapping, and transporting cores from the field to the lab. Core samples are predominantly tested for shear strength using unconfined compression testing, but UU or CU triaxial tests may also be performed.</p><p>Columns to be cored may be chosen by the owner’s engineer based on observation of the column installation process and review of the column installation logs.</p><p><strong>Accuracy and Precision</strong></p><p>Coring has been used to successfully obtain samples for improved soils of varied strengths in both wet and dry mixing applications. However, debate exists about the accuracy of strength achieved from testing cores. Critics of core sampling note that low quality portions of the column are not recovered or cannot be tested, and therefore, the strengths obtained from core sampling may be higher than the average strength of the column. On the other hand, mechanical damage done to the cores during the coring, retrieval, and transportation process could decrease the strength of the recovered core. Recovered core samples provide a continuous profile along the entire length of the column, and they can be visually inspected for integrity and continuity of the deep mixed soil.</p><p><strong>Adequacy of Coverage</strong></p><p>Typically, continuous core samples are obtained from top to bottom of the cored element. Current Japanese practice for large projects in land applications is for a coring frequency of one corehole per 3,000 m³ of soil mix product (CDIT 2002). Lambrechts (2005) recommends a coring frequency ranging from one corehole per 30 columns to one per 140 columns depending on soil conditions, relevant local experience, and the size of the project. There is a tendency to core at a higher frequency at the onset of a project and decrease the frequency as consistency in meeting required strengths is exhibited. Navin and Filz (2006c) recommend the following:</p><p>“Two approaches are provided, the parcel approach and the approach based on daily production per mixing rig. The shear strength used in design is related to the unconfined compressive strength measured in the lab on specimens obtained from field core samples. In the parcel approach, the project is divided into parcels of about 2,500 ft² in plan area (range of 1,500 ft² to 4,500 ft²). Three column elements are selected within each parcel and the element is cored within 28 days after mixing. 5 specimens from each cored element are tested for approximately 25 ft elements, with an additional specimen for each additional 5 ft in length. Test specimens must meet or exceed the 60%, 80% and 95% strengths based on the coefficient of variation used in project design. In the approach based on daily production per mixing rig, one element is selected at random by the engineer from the elements produced each day by each deep mixing rig.”</p><p><strong>Implementation Requirements </strong></p><p>Having experienced personnel and proper coring equipment is vital to successfully obtain quality core samples (Bruce 2001). Coring can also be time consuming and costly compared to other methods. However, in some projects that involve thick mixtures that are difficult for wet grab sampling, coring can be easier to implement than wet grab sampling.</p><p><strong>General Comments</strong></p><p>Core sampling is a useful QC/QA indicator on any deep mixing project relying on the increased strength of the improved soil. Although debate exists concerning the accuracy of strengths obtained, core samples help confirm the degree of mixing and the mixture strength, as well as provide some confirming information about the column geometry. Core sampling has been used successfully as a post-construction quality assurance method for many deep mixing projects.</p></p>