根据不同桩长对比试验优化设计桩持力层的研究

 通过宁波某工程不同桩长桩的静载试验，对比分析钻孔灌注桩在两个不同持力层深度时桩底注浆与不注浆的承载力和沉降特性。研究结果表明，选择第8土层——粉砂土作为持力层，桩长为55 m，并采用桩底注浆技术能显著地提高桩基承载力和减少桩顶沉降；选择第13土层——含黏土的砂砾层为持力层，其下为基岩，虽然能有效地防止桩端刺入破坏，但由于桩长为88 m，施工时需要穿越第8层厚度达20 m左右的粉砂层，施工难度很大，施工质量难以保证。这易导致桩侧泥皮和桩端沉渣厚度较大，从而降低单桩承载力，使实测桩基承载力达不到设计要求。从定量的角度进行技术经济分析，优化选择桩基持力层，在确保工程桩安全的前提下尽可能降低成本和方便施工，最终选择粉砂土层为持力层。

STUDY ON OPTIMIZATION DESIGN OF PILE BEARING LAYER ACCORDING TO CONTRASTIVE TESTS WITH DIFFERENT PILE LENGTHS

The behaviors of bearing capacity and settlement of bored piles in two different depth bearing layers with/without end post-grouting are studied contrastively through static load tests for piles with different lengths in a certain engineering practice of Ningbo City. The study results show that if the 8th soil stratum—silty soil is selected as bearing layer，the pile length is 55 m and employing pile end post-grouting can increase the bearing capability and decrease the pile top settlement remarkably；although selecting the 13th soil stratum—sandy gravel embedded with clay as bearing layer，where there is bedrock below，it can prevent pile end from plunge failure effectively. Therefore the pile length is 88 m and it is very difficult to penetrate the silty sand stratum during installation，which is about 20 m in thickness. Therefore，the construction quality is hard to control. This makes mudcakes around piles at pile ends too thick，resulting in decreasing of the bearing capability of single pile and failure to meet design requirements. The technical economic analysis is made quantitatively and the selection of pile bearing layer is optimized. In order to guarantee the safety of engineering piles，it should reduce the cost as minimum as possible and make pile installation convenient. The silt soil stratum is finally selected as the bearing layer.