# Quality Assessment of Dry Soil Mixing Columns in Soft Soil Areas of Eastern China
## 1 Introduction  
Dry soil mixing (DSM) is a kind of ground improvement method, in which binder is delivered to the ground and mixed with soil. The DSM process employs mechanical mixing energy with a specifically-designed tool--stirring and blending the soil to distribute the binder as uniformly as possible. The chemical reactions between soil and binder improve the strength, permeability, dynamic stiffness, and deformation properties of the soil [1; 2; 3]. The origin of the DSM method dates back to an original patent filed in 1967, but the field application really developed in the 1970s. In the 1960s, concurrent studies were conducted in Sweden, Japan, and Finland to investigate laboratory performance to offer direction for field applications [1; 4; 5]. From the mid-1980s to the late 1990s, there was a large uptake in the use of dry soil mixing, specifically in Europe, North America, and Japan [6; 7; 8]. DSM was introduced into China in 1983, and it was first used for soft soil foundation treatment for railway tracks. After that, DSM was widely used for soft soil foundation improvement in China. So far, DSM has been used worldwide in railway engineering, highway building, municipal engineering, port and wharf projects, riverbank projects, and industrial and civil buildings [4; 5; 7; 9; 10; 11; 12].  
Since DSM was first applied, it has been critical to ensure its engineering quality. [PERSON] [13] summarized the guidelines for the design and installation of DSM columns. [PERSON] et al. [14] and [PERSON] et al. [3] combined the DSM method with prefabricated verticaldrains to enhance DSM column quality in deeper soft clay. [PERSON] [15] developed a modified dry mixing method that injected water into the soil during the installation process. By adjusting the water content of the soil, columns of significant strength can be produced, resulting in more homogeneous columns of high quality. The geological conditions were the key to controlling the quality of the DSM columns. Cases of field applications of dry soil mixing in different soils are shown in Table 1.  
Evaluation of soil mixing in the field and in the laboratory has been done before by many researchers [16; 17; 18; 19; 20; 21; 22; 23; 24; 25]. A quality assessment typically includes post construction instrumentation and monitoring, in situ tests, and laboratory tests on field core samples. [PERSON] et al. [26] used a standard penetration test to evaluate and control the quality of DSM columns. [PERSON] [27] presented the results of an international survey conducted for the quality assessment of deep mixed columns. Concerning the number of columns to be tested for QA evaluations, the survey showed that the majority of the respondents recommended the use of 1 out of every 100 columns for quality assessments. [PERSON] [28] proposed a reliability-based analytical design of DSM columns. [PERSON] [5] highlighted that the visual inspection of a core sample was very much important. [PERSON] et al. [29] used artificial intelligence to predict the strength of soil-cement.  
However, the structural behavior and quality of the DSM columns are difficult to predict, due to the existence of considerable uncertainties during the DSM process and the variability in geological conditions. This paper proposes an effective quality assessment method for comprehensive evaluation of the DSM columns. The quality assessment method is introduced first; then the statistical analysis, laboratory test results, and field test results are presented. The quality assessment results of the DSM columns in different soft soil areas and the distribution of quality level are also discussed.