<p><p><strong>Reference(s):</strong></p><p><em>Broms (2003)<br>Bruce (2001)<br>CDIT (2002)<br>EuroSoilStab (2002)<br>Hodges et al. (2008)<br>Jacobson et al. (2003)</em></p><p><strong>Method Summary</strong></p><p>Laboratory mix trials (bench scale testing) are an essential process in any deep mixing project. The purpose of conducting mix trials is to determine which stabilizing agent(s) and in what quantity will produce the desired soil improvement results. Broms (2003) lists stabilizing agents, including lime, Portland cement, gypsum, granulated blast furnace slag, fly ash, and coal ash. A variety of stabilizing agents may be applied to mix trials; however, CDIT (2002) notes that Portland cement and Portland blast furnace slag cement Type B are the most commonly used stabilizing agents in wet DMM applications. Broms (2003) identifies unslaked lime and Portland cement, often in combination, as the most commonly used agents in dry DMM applications.</p><p>Mix trials should be conducted on soil samples representative of all major soil types and conditions identified by subsurface investigations at the proposed DMM site. Soil for lab mix trials may be collected by standard means (e.g., tube and piston samplers), with emphasis placed on the proper representation of all major soil types and sufficient amount of soil collected to prepare numerous samples for lab testing.</p><p>Both Japanese and Scandinavian mixing and test sample preparation procedures have been standardized through CDIT (2002) and EuroSoilStab (2002), respectively. Although a standardized U.S. procedure has not been implemented, factors including mixing device, mixing time, mold dimensions, sample compaction, curing time, and curing conditions should be detailed when designing a lab mix trial program. Jacobson et al. (2003) and Hodges et al. (2008) provide recommended lab mixing procedures for the dry and wet method, respectively. They illustrate that differences in mixing procedures can yield significant variation in strength. Broms (2003) suggests that the lab mix trials be conducted in two stages. The first stage determines preliminary shear strengths for different stabilizers. The second stage determines the increase in shear strength over time for the most promising mix designs. Curing times of prepared samples range from 3 to 90 days, although most design is based on the 28-day strength. Ample time should be provided for curing and testing of samples prior to the start of production DMM work.</p><p>Typically, unconfined compression tests are performed on stabilized samples, but other tests including triaxial (UU or CU) and permeability tests may be performed on samples.</p><p>Bruce (2001) indicates that, due to inherent variations between the lab mixing procedure and actual field mixing, strength values of stabilized soil samples provide only an “index” of actual strengths expected in field applications.</p><p>It may also be noted that the lab mix trial process may be performed independently by both the owner and the contractor. The owner’s objectives in performing lab mix trials are to confirm the feasibility of using deep mixing for the proposed application and to aid in the design process. The contractor’s purpose in performing lab mix trials is to determine a mix design that will reliably meet project specifications. In this sense, the lab mix trials performed by the contractor are a means of quality control.</p><p><strong>Accuracy and Precision</strong></p><p>Arguably, lab mix trials are a part of the design process rather than QC/QA, depending on who conducts them. They serve as a starting point for predicting field strength of the improved soil and are vital in assessing the potential for strength improvement of the soil to be treated in the field. Lab testing is not a precise prediction of field parameters but provides an index of the field parameters. Common shear strength tests for deep mixing applications include unconfined compression and, sometimes, UU or CU triaxial tests.</p><p><strong>Adequacy of Coverage</strong></p><p>If each soil type to be improved at a particular site is included in the lab trials, then the lab trials apply to the entire deep mixing project. In this sense, lab mix trials are extremely valuable, not only in determining the feasibility of the deep mixing project, but in predicting the level of improvement in the field. However, some method(s) of post-construction verification must still be performed.</p><p><strong>Implementation Requirements </strong></p><p>Retrieving samples of soil from the site makes use of standard boring equipment. Guidelines exist for the type and quantity of binder based on the soil type. A basic lab mixing and testing program would be straightforward to implement, although there would be considerable cost for the sampling, mixing, and testing. Time required for curing and testing of samples creates a need for careful planning for proper implementation.</p><p><strong>General Comments</strong></p><p>Laboratory mix trials are an essential component of any deep mixing project, regardless of the application. The lab mix trials verify the design strength is achievable and confirm the selection of the binder type. The laboratory mixing efficiency and energy applied is generally higher than the field mixing efficiency and energy. There are also differences in the curing conditions between prepared samples in the lab and production columns in the field. Therefore, the lab mix trials provide an index of the expected field strengths and additional field verification is required.</p></p>