Background In the granite distribution area of Longling section of China-Myanmar oil and gas pipeline, the slope soil erosion of pipeline operation slope is very serious. The previous treatment experience shows that the conventional soil and water conservation measures have poor effect on the fully weathered granite backfill soil. For this, pertinence treatment measure research needs to develop. The shear strength of fully weathered granite backfill soil is affected by its moisture content and relative density. Optimizing themoisture content and relative density of backfill soil to improve the shear strength of soil is an important measure to control the erosion on the slope of Longling section of China-Myanmar oil and gas pipeline.

Methods In this paper, the fully weathered granite backfill soil from the pipe trench of the China-Myanmar oil and gas pipeline in Longling county was selected as the test soil. The moisture content and relative density were measured by ring knife method. The shear strength parameters were determined by direct shear test with quadruple direct shear apparatus. Slow shear method was selected according to specification. The slope erosion modulus was measured by artificial simulated rainfall test. Two groups of soil samples were equipped according to the interval of 5% moisture content and 0.2 relative density, respectively, and the effects of moisture content and dry density on shear strength were analyzed.

Results 1) With the dry density of 1.30 g/cm³ soil samples as a benchmark, when the soil moisture content is 5%, 10%, 15%, 20% and 25%, the cohesion C is 9.15, 13.67, 20.33, 8.87 and 3.63 kPa, respectively, and the internal friction angle φ is 36.83°, 33.11°, 34.59°, 34.85°and 32.42°, respectively. The 15% moisture content is the optimal moisture content.2) Taking the soil sample with 15% water content as a reference, the cohesion C of soil with density ratio at 0.06, 0.27, 0.51, 0.72 and 0.83 are 20.00, 20.33, 26.75, 26.96 and 31.12 kPa, respectively, and the internal friction angle φ are 31.77°, 34.59°, 36.97°, 39.07° and 41.88°, respectively. When the relative density of the soil increases from 0.06 to 0.83, the cohesion and internal friction Angle of the soil increase by 53% and 18%, respectively. When the relative density is 0.50, the improvement effect of soil shear strength parameters is the best.3) The results of artificial simulated rainfall experiments show that the slope soil erosion modulus is related to the shear strength of soil, especially negatively correlated with cohesion.

Conclusions The results show that the moisture content of fully weathered granite backfill soil increases, and the cohesion increases first and then decreases rapidly, and the internal friction angle decreases gradually, under the same dry density. Under the same moisture content, the cohesion and internal friction angle increase with the increase of relative density. Under the same moisture content, increasing the relative density of soil can effectively improve the anti-erosion ability of soil.