Background  The deep anchoring effect of trees taproot and shrubs taproot on soil of slope can significantly improve the stability of slope. But in the rainfall conditions, whether the plant can play a favorable role in the stabilization of slope is controversial. Therefore, this has caused obstacles to judgement of pull-out force of taproot in rainfall conditions.

Methods  To study the factors that affect the pull-out force of taproot, the Symplocos sumuntia taproot was selected as the research object in this paper, and was used to study on the problem of pull-out force of taproot. Among them, pull-out tests were carried out under 4 soil water content conditions(10%, 15%, 20%, and 25%) and 3 vertical stress conditions(3.06 kPa, 6.13 kPa, and 9.19 kPa), and pull-out tests contains two parts:single pull-out test and cyclic pull-out test. The pull-out test device was designed as in Figure 2 by our own laboratory.

Results  1) The taproot pull-out force increased with the increase of stress on the taproot surface and gradually became stable. The single pull-out test results showed that, taking soil water content of 11.74% of the situation as an example, when the vertical stress was 3.06 kPa, 6.13 kPa and 9.19 kPa, the taproot average pull-out stress was 42.61 Pa, 71.52 Pa and 91.36 Pa. 2) With the increase of soil water content, the taproot pull-out force increased firstly and then decreased. The single pull-out test results showed that, taking vertical stress of 3.06 kPa of the situation as an example, when the soil water content was 11.74%, 16.02%, 21.11% and 26.53%, the taproot average pull-out stress was 42.61 Pa, 61.79 Pa, 70.35 Pa and 63.11 Pa. 3) When the soil was clay, the corresponding soil water content of the greatest pull-out force of Symplocos sumuntia taproot was 22%. 4) In the low state of stress, taproot external organizational integrity had a greater effect on the pull-out force; while in the high state of stress, taproot internal organizational integrity had a greater effect on the pull-out force. Taking soil water content of 16% of the situation as an example, when the vertical stress was 3.06 kPa and 9.19 kPa, the average pull-out stress obtained by the cyclic pull-out test was 40.79 Pa and 101.03 Pa. And the average pull-out stress obtained by the single pull-out test was 61.79 Pa and 109.01 Pa.

Conclusions  Experiments show that the soil properties and water content, root surface stress and integrity will have a significant effect on the pull-out force of taproot. And, as long as the soil water content does not exceed the optimal value, the anchoring effect of the taproot will be fully exerted.