An evaluation method of ground improvement by jet-grouting

Abstract

The jet grouting method is frequently used as a mean for ground improvement, especially in shield tunneling, and also in all sorts of foundation treatment in recent years. However, an appropriate evaluation method has not been established yet. Currently, the most common evaluation method is to carry out a uni-axial compression test by using a boring core specimen. However, the tests are carried out only on well-improved soil samples, omitting any badly improved samples. The badly improved soils are omitted because they cannot be put under a uni-axial compression test, making this method flawed, as it does not evaluate the overall improved ground. In order to obtain an accurate evaluation, the badly improved samples must be included in the evaluation process. The actual condition in the improved ground must be grasped for establishment of evaluation criterion. However, the actual condition is hardly known about a layout of improvements or solidification in deep ground. In order to grasp the actual condition around the shield tunnel, the authors investigated the reconstruction site of a shield tunnel collapsed. From the investigation, it was found that the every improvement become almost column-shape and each improvement interlocked each other even in the deep ground. The ultimate purpose of this study is to establish an evaluation criterion for ground improvement by jet grouting, and to propose a new evaluation method. The evaluation criterion is established by adapting a method similar to the rock quality designation (RQD), categorizing the improvement condition into 6 different groups. The evaluation method proposed herein is to categorize a core specimen into the 6 different groups and deriving the ratio of each group by its length. From the categorization, the compressive strength is estimated by using a formula derived for the purpose of this study. The evaluation method was applied to 4 different sites with various ground conditions. 3 sites had the estimated strengths higher than the design strengths, and no problems occurred. However, 1 site had lower compressive strength than the design strength; and a trouble occurred under the construction. If the conventional evaluation method were to be applied (uni-axial compression test on well-improved soil samples), all 4 sites would have met the design criterion. This confirms that the evaluation method proposed in this paper is more accurate, compared to the conventional method. (A). Reprinted with permission from Elsevier. For the covering abstract see ITRD E124500.