Evaluation Of The Effect Of EICP-Treated Earthen Site Soil Based On Drilling Resistance

Rock-soil immovable cultural heritage are an important part of cultural heritage,among which the conservation of earthen sites with raw earth as the main building material is particularly important and urgent.The surface anti-weathering reinforcement has been the core issue of earthen site conservation,while the judgment of weathering degree and the evaluation of reinforcement effect are the main difficulties of surface anti-weathering conservation of earthen sites.EnzymeInduced Carbonate Precipitation(EICP)technique has been widely used in geotechnical consolidation,but the same problem of reinforcement effect evaluation and less used in reinforcing the site improvement,while drilling resistance test is an effective quantitative evaluation method widely used in stone,mortar and other reinforcement effects.In this study,aiming at the problem of evaluation of reinforcement effect in anti-weathering protection and EICP reinforcement of earthen sites,the site soil was taken as the research object,and the method of drilling resistance test was used to quantitatively evaluate the reinforcement effect.The influencing factors of drilling resistance test,the relationship between drilling resistance and conventional mechanical indexes,the influencing factors and effect evaluation of EICP reinforcement of site soil were systematically carried out.The research showed that EICP can significantly improve the mechanical properties of earthen sites.Drilling resistance test can be used as an effective method to quantitatively evaluate the reinforcement effect of earthen sites.The research can provide some technical support for the protection and reinforcement of earthen sites.The main research contents and results are as follows:(1)A drilling resistance data processing method suitable for site soils was developed,and the influence law of drilling bit diameter,stepping speed and rotation speed on drilling resistance was investigated.The test data of drilling resistance were processed by box diagram principle combined with loess smoothing method,and then the unit drilling work was calculated as the representative value of drilling resistance.Drilling resistance increases with the increase of drill bit diameter,and the analysis suggested that it is mainly because the larger the diameter the more soil is drilled,the larger the contact area at the head of the drill bit becomes the larger the reaction force of extrusion,the larger the reaction force of anti-cutting and friction force at the side of the drill pipe.With the increase of the stepping speed,the drilling resistance also increases.But the drilling resistance decreases with the increase of the rotation speed.It is positively correlated with the ratio of the stepping speed and the rotation speed,which is mainly related to the amount of soil cut per unit time.The size of drilling resistance is controlled by the stepping speed and rotation speed together.When the stepping speed is small,the effect of the size of rotation speed is obvious,and when the rotation speed is large,the effect of the size of stepping speed is obvious.The lower stepping speed and higher rotation speed should be selected for the site soil,and the larger bit diameter should be selected when the damage to the site soil is as small as possible,so as to obtain the drilling resistance value with certain discrimination and reliability.(2)The correlation between drilling resistance values and conventional mechanical indicators was established and analyzed to verify the applicability of drilling resistance in characterizing the mechanical properties of site soils.A series of tests including drilling resistance testing and conventional geotechnical testing were conducted using soil from Xiaguanying ancient city with different initial dry densities representing different weathering degrees,and the correlation between the indexes was analyzed by Pearson correlation analysis.The results showed that the drilling resistance-drilling depth curve can be divided into four stages: drill contact section,drill entry section,resistance growth section and resistance stabilization section,and the average value of the resistance stabilization section can be used as the representative value of drilling resistance;The value of drilling resistance can reflect the degree of surface weathering of earthen sites,which can be used to quantify the surface weathering strength and thickness of soil sites.There is a good linear relationship between the drilling resistance value and the conventional mechanical indexes(unconfined compressive strength,shear strength,cohesion,surface hardness,compression wave velocity).Pearson correlation analysis showed that there is a significant correlation.Drilling resistance test is an effective method to detect the mechanical properties of site soil.Drilling resistance value is suitable for characterizing the mechanical properties of site soil.(3)The effects of urea concentration,urease concentration,p H,curing temperature and time on the EICP reinforcement effect were revealed.The weights of the above factors were assigned by Analytic Hierarchy Process(AHP),and the suitable conditions for EICP technology to reinforce the site soil were obtained.The urease was taken from the supernatant extracted from the Jackbean,and the prepared specimens were reinforced by a two-phase central drip percolation method with the urease solution followed by the cementing solution.Under the drilling conditions and test methods in this paper,the appropriate reinforcement conditions for the EICP technique were determined in terms of mechanical strength based on the drilling resistance: urea concentration of3.5 M,urease solution: cementing solution = 1:2,p H = 7,and curing temperature of 35°C,curing strength has stopped growing after 2 days.Based on the AHP method,the drilling resistance values under different reinforcement conditions were used as the index layer,and the rate of change of drilling resistance,variance and mean value were used as the criterion layer to establish a progressive hierarchical model for the analysis of reinforcement influence factors based on drilling resistance,and the analysis results showed that urease activity played a key role in EICP reinforcement.(4)The variation pattern of calcium carbonate production and drilling resistance was studied,and the reinforcement depth and effective reinforcement range of EICP under different saturation conditions were quantitatively evaluated.The results show that the reinforcement depth reaches 50 mm at 35 % saturation,and the reinforcement had basically reached the bottom at 50 % and 65 %saturation.The drilling resistance contour graph visually presented the reinforcement effect of the central surface of the reinforcement,the drilling resistance had significantly improved,the drilling resistance gradually decreased from the top of the drip penetration to the bottom surface,and the drilling resistance increased with the increase of saturation.The reinforcement solution penetrated the whole section.The overall reinforcement surface reached 50 mm at 35 % saturation,basically reached the bottom at 50 % saturation,and had achieved overall reinforcement at 65 % saturation.The results of the acid washing method showed a gradual decrease in calcium carbonate generation mainly along the drip percolation direction,and the XRD test verified the generation of calcium carbonate crystals,mainly calcite,and the quantitative XRD analysis further verified this trend.The change of drilling resistance is consistent with the change of calcium carbonate production,which is positively correlated.