福建省滨海典型地层搅拌植桩基础竖向承载特性研究

Vertical Bearing Capacity of Mixing Implanted Pile Foundation in Coastal Typical Soil Layer in Fujian Province

搅拌植入管桩是在水泥土搅拌桩中植入管桩,形成共同受力的一种新型桩基础,目前被广泛应用于黏土、粉砂层中,但在滨海软土下覆强风化岩地层里尚无实践经验。为进一步探究桩端入岩对搅拌植入管桩竖向承载特性的影响,依托国道G324线福清新厝双屿至大沃段公路工程,在现场进行了3根搅拌植入管桩的竖向荷载试验,采用慢速维持荷载法开展桩端持力层为强风化凝灰岩的搅拌植入管桩承载性能研究。试验结果表明:在近海典型地层中,搅拌植入管桩的荷载-沉降曲线呈缓降型变化,并表现出摩擦端承桩的工作特性;桩身上部侧摩阻力将先于下部发挥,具有明显的异步性;同一土层不同深度处,桩侧摩阻力随桩土相对位移变化规律基本一致,但极限侧摩阻力会随深度的增大而呈小幅度增加;当搅拌植入管桩桩端进入强风化凝灰岩时,粉质黏土层处桩侧摩阻力实测值与规范推荐值基本一致,而在强风化凝灰岩地层中桩侧摩阻力则较规范推荐值提高了35%~44%;在桩端阻力计算上,桩端承载力特征值建议采用相关标准规范中混凝土预制桩极限桩端阻力标准值的一半,以弥补在搅拌植入管桩桩端入岩条件下端阻力计算无据可依的不足。

Mixing implanted tubular pile is a new type of pile foundation that implanting tubular pile into cement soil mixing piles by joint load carrying.At present,it is widely used in clay and silt layers,but there is no practical experience in the strongly weathered rock strata under coastal soft soil in Fujian province.To further explore the influence of pile tip penetration into rock on the vertical bearing capacity of mixing implanted tubular piles,relying on the highway project of China national highway G324(road section from Fuqing Xincuo Shuangyu to Dawo),the on-site vertical load tests are conducted for 3 mixing implanted tubular piles.The slow speed load method is adopted to study the bearing capacity of mixing implanted tubular piles,which shows the working characteristics of friction end bearing pile.The test result indicates that the load-settlement curve of mixing implanted tubular pile changes slowly,and shows the working characteristics of friction tip bearing pile in the typical offshore strata.The side friction resistance of upper part of pile body,which has obvious asynchrony,develops before the lower part.At different depths of same soil layer,the variation of pile side friction with relative displacement of pile and soil is basically the same,but the ultimate side friction increases slightly with the increase of depth.When the mixing implanted tubular pile tip enters the strongly weathered tuff,the measured value of pile side friction resistance at silty clay layer is generally consistent with the recommended value of specification,while the pile side friction resistance in the strongly weathered tuff stratum is 35%-44% higher than that of specification.In the calculation of pile tip resistance,the characteristic value of pile end bearing capacity is suggested to adopt half of the standard value of ultimate pile end resistance of concrete precast pile in the relevant standard code,in order to make up for lack of evidence for calculation of end resistance of deep mixing tubular pile when the pile end enters rock.