平底大头沉管灌注桩的沉管性状、变径效应及承载特性

ON THE DRIVING BEHAVIOR, VARIABLE-DIAMETER EFFECT AND BEARING CAPACITY OF DRIVEN CAST-IN-SITU PILE WITH FLAT OVERSIZE TIP

十余年工程应用和试验研究表明在沉管灌注桩中采用预制混凝土平底大头以代替普通圆锥形桩靴的新桩型适合于软土地区具有浅埋的良好持力层地质条件下应用．它的扩底明确可靠，沉管并不困难，施工工艺和工效，与普通沉管桩基本相同。在成层土条件下，这种桩存在“变径效应”，使桩身呈多节状，且在桩下段形成复合扩大头，因而其侧阻与普通沉管桩相比有增无减，端阻大为提高。单桩承载力在较厚（≥６ｍ）粉土、砂士持力层中比普通沉管桩增加１倍以上，在粘性土中增加４０％以上。用桩数量比普通沉管桩减少约１／３至１／２。它的群桩效应轻微，沉降小，稳定快。已在浙江、江苏等地用于各类多层至十余层民用建筑和工业厂房，累计建筑面积数１０万ｍ２；及烟囱、水塔等构筑物。

Results ofseries of field and laboratory tests as well as ten oldyears engineering experience of using the innovation by the first author of the precast concrete FLAT OVERSIZE TIP in driven cast-in-situ pile to replace the conventional conical tip,showthat flat oversize tipped pile is suitable for subsoil conditions having shallow firm bearing stratum and easy to be driven into required penetration,while details of construction technology remain the same as those of conical tipped pile.In case of layered soil deposit a VARIABLE-DIAMETER EFFECT has been found of the flat oversize tipped pile,which results in nodular projections in stiffer soil lqyers along pile shaft,meanwhile the enlarged end portion in bearing stratum forms a composite base enlargement with the precast tip.So,much higher bearing capacity is usually achieved of the flat oversize tipped pile.In bearing stratum of silty and fine sand it is twice that of conical tipped pile,and in clayey soils it reaches 140% of that of conical tipped pile. Since the 1980s flat oversize tipped piles have been successfully applied to buildings up to 13 storeis in Zhe jiang and Jiangsu Provinces etc.in China.Besides,water towers,chimmey stalks and other tarll structures storeys amoutnting to somehundred thousand floor,square meters,and saving1 /3 or 1/2 of piles compared with that of conical tipped type Due to slight group action,settlements of buildings and structures on flat oversize tipped piles are usually as small as 10- 15mm in one year after completion and they stabilize soon