Experimental Study On The Consolidation Of The HDD Slurry In The Annular Space After Pipeline Installed

Horizontal Directional Drilling (HDD) is widely being used as an advanced technique to install large-diameter pipelines crossing rivers. There exists annular space after the pipeline is installed in the borehole using this method. The annular space is filled with the mixture of drilling mud and cuttings, whose cross-sectional area can reach up to one square meter approximately, we call it "annular slurry". Annular slurry have high water content, high compressibility and it’s strength is much lower than the in-situ soil.When pipeline installations by HDD involves a watercourse area, annular space may result in several post-construction issues such as settlement or even collapse of the surface above the HDD bore path, penetration destroy in gravelly sand, soil deformation of the nearby embankment, etc, which brings hidden trouble to security and stability of rivers and dams. There is no data about the reinforcement of annular space in pipeline installation cases, meanwhile existing specifications about HDD have not yet called for replacement of annulus slurry after installing pipelies by horizontal directional drilling. For the two reasons given above, it is necessary to conduct research on the laws of seepage and consolidation of annular slurry in large-diameter pipeline installation by HDD, which can supply experimental proof for evaluating the influence on crossing area caused by annular slurry using HDD.Based on the project of Natural Science Foundation of China-Study on the consolidation and deformation of annular mud after pipeline installations using HDD,this paper conducts research on the consolidation and deformation characteristics of annular slurry and its imfluence on the surrounding soil in sand obtaining clay by theoretical analysis, model test and numerical simulation. In view of the current fact of limited data about reinforcement of annular slurry, this paper proposes to apply the grouting technique to the reinforcement of annular space on the basic of the research results of Specification for pipeline crossing by horizontal directional drilling.Firstly, a theorectical analysis of the consolidation mechanism of annular slurry is given, including the state and composition of annular slurry under different formation, consolidation characteristic, consolidation loads and suitable consolidation theory. The annular slurry is a mixture of mud and cuttings, the content of cuttings is subject to stratigraphic conditions and construction process. For the projects with stable stratigraphic condition and completely borehole cleaning, the main composition of annular slurry is bentonite slurry, slurry consolidation is manifested by free water filtration, the strength of mixture increases slowly in this case.On the opposite side, for the projects with unstable stratigraphic condition which may trigger hole collapse, the annular slurry is made up of bentonite slurry,cuttings and soils scattered from the pore-wall,and the strength of mixture increases quickly. The consolidation loads of annular slurry are earth pressure Ps, hydrostatic pressure Pw and soil gravity Pg. The engineering properties of annular slurry is similar to super-soft soil, it has a consolidation characteristic of large deformation and nonlinear. Annular slurry consolidation is a kind of large deformation consolidation question, the large deformation consolidation theory should be used to describe it.Secondly, a research about penetration range and influence on the surrounding soil of annular slurry with stable stratigraphic condition is conducted, on the basis of the theorectical analysis. Based on similarity principle, a model test which can simulate the actual operating conditions is carried, the formation is sand containing clay and the annular slurry is bentonite mud. The model test procedures are:firstly, an iron experiment box located in laboratory filled with sand containing clay was designed to simulate the formation; secondly, a horizontal borehole with annular slurry was drilled by a HDD rig; next is the annular mud consolidation procedure; finally, samples of annular mud are taken from the box, drilling fluid test, soil strength tests and SEM images analysis are performed to obtain the property parameters of the samples. Sampling work includes obtaining disturbed sample periodically and undisturbed sample during the experimental box excavation after seven months.The result of visual observation, geotechnical laboratory tests combined with SEM microscopic analysis showed that the annular slurry mainly consists of bentonite slurry; due to the slurry pressure dissipation and the presence of mud cake, annular slurry consolidation is manifested by free water filtration;the free water filtration causes the increasing of surrounding soil’s water content, decreasing of shear strength, compression modulus; however, the bentonite particles in annular slurry hardly penetrate into the surrounding soils, the potential impact on soils is limited.In the model test, we found that the annular slurry consolidation hardly took place because of the low consolidation pressure. In order to compensated for the failure and research the impact of consolidation force on annular slurry strength further, the annular mud consolidation strength test under consolidation load is carried out. The test result revealed that the consolidation pressure and drainage conditions have a significant effect on the strength of growth in the annular slurry. The consolidation rate and strength of annular slurry and surrounding soil increased significantly when consolidation pressure exists and the drainage condition is good. While the consolidation rate and strength of annular slurry and surrounding soil hardly increased under undrained condition, the annular slurry keeps the state of plastic flow in a long period.In order to analyze the impact of the bentonite in annular slurry on the strength of surrounding soils further, a series of remoulded soils strength tests are designed, the remoulded soils have two kinds,one is soil without slurry,the other is soil containing slurry. The test result show that, the strength of soil containing slurry is higher than the soil without slurry, when they have the same water content. That’s to say, the bentonite in slurry contributes positively to the increasing of soils. The SEM microscopic analysis also revealed that, compared to the the soil without slurry, the density of soil containing slurry has increased.The bentonite particles fill part of void spaces between soil particles, so the density and particle number increase, which can improve the strength of the soil to a certain extent.After a series of tests, the finite element software ABAQUS is used to simulate the model test.When the simulation results about the consolidation and deformation characteristics of annular slurry and its imfluence on the surrounding soil in sand obtaining clay are obtained, they are compared with the model test results.The simulation results show that, the annular slurry penetrated through the entire region in the horizontal direction, which is different from the model test results.The deformation of soil mainly consist of the settlement caused by the stress redistribution of annular slurry, while the creep compliance of soil causd by secondary consolidation is small compared to the settlement.The soil deformation value obtained by numerical simulation is much smaller than the monitored value in model test by one to two orders of magnitude.Finally, considering the current fact of limited data about reinforcement of annular space, grouting technique is proposed to apply to the reinforcement of annular space on the basic of the research results of Specification for pipeline crossing by horizontal directional drilling. The grouting process, grouting materials about HDD post-installation grouting are summarized,also recommendationsfor cases needing grouting process are given.