AbstractAlthough slump testing has been used extensively to assess the workability of conditioned soil in shield-driven tunnelling, this traditional evaluation method still requires many subjective descriptions. This study developed a novel and objective index to assess the workability of conditioned coarse-grained soil. A series of slump tests was performed on gravely sands with different conditioning parameters. The test results suggested that foam conditioning was suitable only for sands (lacking fine grains) with low water contents. Inspired by the observation that appropriately conditioned soils usually form a platform with a certain diameter on the top of slumped specimens, a novel evaluation index is proposed to evaluate the workability of conditioned coarse-grained soil. In contrast to the traditional method of using slump value to assess conditioned coarse-grained soil, a prominent advantage of this method is that it eliminates the need to judge whether water or foam bleeds from slumped specimens, and thus it is easier to implement on construction sites. The results of verifications with theoretical analysis and field applications on a construction site confirmed that this new method is practically effective for evaluating the conditioning state of coarse-grained soil in mechanized tunnelling with earth pressure balance (EPB) shield machines. The proposed F index provides a simple but quantitative evaluation perspective for the workability of conditioned soils, and thus is of significant engineering value.