Study On Borehole Wall Stability And Drilling Fluid In A Soil-rock Mixed Stratum Of Reservoir Landslide

In China's Three Gorges Reservoir area,there is a widely distributed landslide accumulation along the slopes on both banks of the Yangtze River,with a maximum thickness of more than 50m.In general,the content of crushed gravel is between 30?70%.Due to the characteristics of high heterogeneity,discontinuity,looseness,and anisotropy,the soil-rock mixtures of the landslide accumulation are easy to cause borehole collapse,diameter shrinkage,sticking and leakage,resulting in borehole wall instability.During the impoundment period of the reservoir,the landslide accumulation has been in a water-saturated immersed state for a long time.The physicochemical interaction of water and soil under the immersion action will cause the shear strength of the soil-rock mixtures to attenuate,which will increase the deformation of soil around the borehole during hole forming.In addition,it will further aggravate the possibility of drilling instability in this type of formation,increase the difficulty of landslide investigation and coring,and threaten the safe operation of monitoring equipment in the landslide monitoring hole.At the same time,with the development of machinery,materials and other fields,the prevention and control of geological disasters has gradually developed in the direction of lighter,miniaturized,composite and mechanized construction.Bored piles will gradually replace traditional manual digging piles for landslide prevention and control.Drilling instability of the landslide soil-rock mixed stratum caused by the storage of water will directly affect the development of drilling projects.In addition,since the hole formation is the foundation of the pile formation,the quality of the pile is closely related to the quality of the hole formation.The instability of the hole will cause the quality of the bored pile to be difficult to guarantee.The instability of drilling in the soil-rock mixture will severely restrict the promotion and application of small-diameter mechanical bored anti-slide piles in the prevention and control of landslide disasters in reservoirs.Therefore,it is urgent to study the stability of the borehole wall of the soil-rock mixed stratum on the basis of studying the attenuation of the soil shear strength caused by the impoundment of the reservoir landslide,so as to provide suggestions for the safe and efficient construction of the soil-rock mixed stratum.In this thesis,a landslide soil-rock mixture of a certain accumulation in the Three Gorges Reservoir area is taken as the research object,and the attenuation law and attenuation mechanism of the soil-rock mixture shear strength under the action of the impoundment of the reservoir are studied.A three-dimensional numerical model for the analysis of borehole wall stability in the soil-rock mixed stratum was established,and the borehole wall stability was evaluated by the finite difference method.Considering the influence of hole drilling on borehole deformation,taking rotary drilling as an example,based on the self-developed drilling test platform,the deformation characteristics of the soil-rock mixed stratum during the drilling process are studied.Developed a safe and efficient drilling fluid for drilling in a soil-rock mixed stratum,which directly serves relevant projects.The main research contents and results are as follows:(1)The attenuation law and mechanism of shear strength of a reservoir landslide soil-rock mixture are revealed through immersion test.With the increase of immersion time,the shear strength of both decreased significantly.The cohesion(c)and internal friction angle(?)of the fine-grained component of the soil-rock mixture decreased significantly due to immersion.In contrast,the c value of the soil-rock mixture decreased significantly after long-term immersion,but the?value decreased less.The decline rate of the c value of both decreased with the increase of soaking days.After soaking days reached 20 days,the c value basically reached a stable level.The mechanism of shear strength attenuation of the soil-rock mixture under immersion is as follows:immersion promotes cation exchange,ion adsorption,and slow dissolution of calcite minerals in the fine-grained silty clay,finally resulting in particle size refinement(transformation from silt-sized to finer particles),and the cementation between soil particles also decreases with mineral dissolution and particle refinement,which is manifested in the decrease of the c and?of silty clay and the decrease of the overall shear strength of the soil-rock mixture.(2)This thesis reveals the changing law of the borehole wall stability of the soil-rock mixed stratum of the reservoir landslide.Based on the borehole wall stability analysis results,suggestions for safe construction of the soil-rock mixed stratum are put forward.Based on FLAC^3D finite difference numerical method,a three-dimensional numerical model for borehole wall stability analysis of soil-rock mixed stratum is established.The results show that the borehole wall stability of soil-rock mixed stratum decreases with the increase of soaking time,borehole depth,borehole diameter and tripping speed,and increases with the increase of groundwater buried depth.The internal friction angle of the soil-rock mixed stratum was large,the shear strength of soil increased with borehole depth at a high rate,the lateral pressure coefficient was small,and the unloading around the hole was small.After soaking,the block stone and silty clay still have a certain cohesion.Affected by this,although the borehole wall had a certain deformation after the stress was redistributed,the hole wall soil quickly reached a new balance.When the reservoir has been running for 30 days or later,and the groundwater depth is 10m when using clean water to drill a hole with a depth of 30m,and a diameter of 400mm,the maximum lateral displacement of the hole at the hole depth of about 25m is close to 20mm,which may cause the risk of squeezing and burying of drilling tools,casing,and reinforcement cage.Therefore,it is recommended that the open hole working length should not exceed 25m under this situation.The drill bit is drilled to 35m,the drill string combination is?400mm drill bit+?355.6mm drill pipe,the bentonite drilling fluid performance in the hole is:?_m=1.3g/cm³,?_p=0.01538 Pa?s,?₀=4.04Pa,when the drill bit is tripped at the hole depth of 30m,the maximum instantaneous lifting speed should not be greater than 0.6m/s,otherwise it may cause squeezing of the drilling tool;the maximum instantaneous speed of drilling down should not be greater than 0.8m/s,otherwise it will cause instability of the lower borehole wall.(3)Based on the self-developed drilling test platform for monitoring the deformation characteristics of the soil around the borehole,the deformation characteristics of boreholes during the rotary drilling process were studied,and the numerical test results were verified.The results showed that regardless of the simulated groundwater conditions,drilling led to ground uplift.The maximum vertical displacement of the ground increased with the increase of the borehole diameter and decreased with the increase of the distance from the borehole center.Without immersion and 40cm water level,the maximum horizontal radial displacement,and the range of"affected depth"under the drill bit at the end of each drilling stage increased with the increase of drilling diameter.The maximum horizontal radial displacement of each drilling stage appeared above the end of the drilling stage.At the same time,due to the unloading rebound of the upper soil-rock mixture,the maximum radial displacement of each borehole is in its middle and lower part.A negative displacement area appeared in each borehole when immersed in the 80cm water level,indicating that the borehole shrank or even collapsed.The larger the borehole diameter was,the longer the negative displacement area was.The variations of negative displacement area and earth pressure from a negative value to zero can be used as an important basis for evaluating borehole shrinkage and even collapse.In the numerical simulation work,the drilling in the soil-rock mixed stratum studied in this thesis shows the lateral displacement of the borehole wall to the free face,while in the actual construction,there may be block falling,drill sticking,or even hole collapse,which is consistent with the hole collapse phenomenon in the model test,which verifies the rationality of the numerical test results.(4)The laboratory tests of the polymer drilling fluid were carried out,and the fluid flow pattern,aging performance,and mixed silty clay properties were analyzed.The Marsh funnel viscosity of 0.08%polymer is close to that of 6%bentonite,which were95s and 89s respectively.The rheological behavior of bentonite drilling fluid conforms to the Bingham fluid model,while the polymer drilling fluid conforms to the power law fluid model.The apparent viscosity of polymer drilling fluid basically remained unchanged in the early 4?5 days of aging and then showed a downward trend.After 30days of aging,the maximum decline rate was 26.2%.With the increase of the dosage of silty clay,the apparent viscosity of polymer drilling fluid showed a downward trend.When the dosage of silty clay was 10%,the apparent viscosity decreases by67.51%?75.85%at different rotary speeds.(5)A safe and efficient drilling fluid formula for the soil-rock mixed stratum of the reservoir landslide was put forward.Under the condition of ensuring the safe lowering of drilling tools and reinforcement cages,a certain amount of diameter reduction can be allowed in the borehole,and a low-density polymer drilling fluid is formulated.The formula is:water+0.08%PHPA+0.1%?0.5%Na₂CO₃.The drilling test platform established in this article has verified that the formula causes small radial displacement around the borehole,stable borehole wall during drilling,and high drilling quality,which can achieve safe and efficient construction.