低温多年冻土地基大直径钻孔灌注桩未回冻状态承载性质试验研究

EXPERIMENTAL STUDY OF BEARING CHARACTERISTICS OF LARGE-DIAMETER CAST-IN-PLACE BORED PILE UNDER NON-REFREEZING CONDITION IN LOW-TEMPERATURE PERMAFROST GROUND

多年冻土地基由于大直径钻孔灌注桩桩身混凝土水化热造成桩周冻土融化,而桩体混凝土灌注初期桩土体系不具备冻结强度,导致钻孔灌注桩初期承载力很低。研究低温多年冻土地基大直径钻孔灌注桩未回冻状态的承载力和变形性质,可为工程施工工期安排提供技术依据。结合青藏铁路索南达杰特大桥工程进行低温多年冻土大直径钻孔灌注桩地温测试及现场静载试验,为此设置桩土界面测温孔(SB)、桩侧测温孔(SC,距离桩壁30 cm)以及未受施工扰动的基准地温孔(JZ)。获得夏季灌注桩混凝土入模温度为11℃,不同龄期桩土体系的地温分布,并分析桩土体系的回冻过程。测试数据表明:混凝土灌注完成30 d以后,桩顶至地表下2 m为正温,地表下2 m到桩底桩身表面均为负温,在-0.43℃^-1.26℃范围内变化;灌注50d以后,桩土界面地温逐渐降低,为-1.0℃^-1.85℃,与未受扰动天然地基地温相比,桩土体系尚未完全回冻。同时进行不同地温条件下基桩的现场静载试验,分析竖向承载力、变形及桩侧摩阻(或冻结力)分布特性。当加载到最大荷载(7 600 kN)时,桩顶竖向位移达到4.93 mm,卸载后未恢复的变形为1.01 mm,说明低温多年冻土地基钻孔灌注桩在未完全回冻状态下(试桩龄期30 d)基桩具有较高竖向承载力,且变形量小。

For the large-diameter cast-in-place concrete bored piles in low-tempera^re permafrost ground, a large amount of heat produced by concrete hydration would result in thawing of permafrost soil around the pile, and thus the interface between the pile and the soil would not have enough freezing strength, making it impossible the pile to reach the designed bearing capacity at an early stage after the construction. Thus, the bearing and deformation features of the piles under a non-refreezing state were studied. A series of in-situ ground temperature tests and static loading tests were performed for the pile foundations of the Sonam Dargye Bridge located at the Qinghai-Tibet railway during its construction. The pile-soil-surface thermometer holes, pile-side thermometer holes（30 cm from the pile skin） and reference thermometer holes to measure the ground temperature undisturbedby construction, denoted by SB, SC, and JZ, respectively, were set up to obtain the corresponding temperatures. The temperature distributions for the pile-soil system at various ages were obtained when the piles were cast in summer and the initial concrete temperature was 11 ~C. The refreezing process of the pile-soil system as well as the distribution characteristics of the vertical bearing capacity, vertical deformation and skin friction（or freezing force） of the pile under various ground temperatures were analyzed. Test results show that 30 days after the construction of the pile, the temperature along the pile skin is negative（approximately -0.43℃ - -1.26 ℃） except in the section 0 - 2 m below the ground surface; 50 days after the construction of the pile, it becomes a bit lower（approximately-1.0 ℃ - -1.85℃）. Compared with the original undisturbed state, the pile-soil system is not fully refrozen. When a maximum vertical load of 7 600 kN is applied, the vertical pile-head displacement is 4.93 mm while the residual deformation after unloading is 1.01 mm. It shows that the pile has a high vertical bearing capacity and a small deformation under a non-fully refreezing condition with the pile age of 30 days.