沉管隧道卵石与碎石垫层力学变形特性对比研究

Comparative study on the mechanical deformation characteristics of pebble and gravel cushion in immersed tube tunnel

国内外沉管隧道先铺基础多采用碎石作为垫层材料,目前尚无采用卵石的先例。卵石和碎石在表面光滑度、排列接触方式、颗粒间天然空隙率等物理特性的差异性将影响其力学性能表现。通过物理模型试验和数值模拟计算,对卵石和碎石垫层的力学变形特性进行对比分析。研究表明:(1)两种材料垫层压缩曲线均呈两阶段反弯曲线变化趋势,相同荷载条件下卵石垫层压缩量较碎石高,总体割线模量较碎石低约30%(。2)垫层厚度由0.8 m变为1 m时,卵石垫层割线模量增加了13.0%,碎石垫层割线模量增加了2.2%;卵石垫层力学变形性能对垫层厚度的变化较碎石垫层更敏感。(3)预压荷载由52.5kPa增加到84kPa时,卵石垫层割线模量增加了23.5%,碎石垫层割线模量增加了7.6%;预压荷载越大,卵石垫层能更早达到拐点从而表现出更稳定的力学性能;增加预压荷载对卵石垫层整体力学变形性能的改善较碎石垫层更明显。(4)随沟宽增大,垫层模量在前期再压缩阶段出现模量提高,而在全加载期内总体表现为模量降低;在全荷载范围内卵石垫层对垄沟尺寸变化的敏感度低于碎石垫层。(5)碎石垫层的整体力学性能优于卵石垫层,但两种垫层材料对结构沉降和受力状态的影响相对有限;在对施工偏差敏感度充分分析的基础上,卵石可替代碎石作为沉管隧道垫层材料。(6)开展卵石级配试验研究,获取最佳卵石级配,是未来研究方向。

Gravel was widely applied as the cushion materials for most foundations of the immersed tunnels, while the pebble as an alternative material has not been used till now. The differences in physical characteristics such as surface smoothness, alignment, contact methods and natural porosity between the gravel and pebble will affect its mechanical performance. In this paper, the mechanical deformation characteristics of pebble and gravel cushion were comparatively studied through physical modelling tests and numerical simulations. The results show that: 1) The compression curves of the two material cushions both show a two-phase anti-bending trend. The compression of pebble cushions is higher than that of the gravel under the same loading condition, whereas the overall secant modulus of pebble is approximately 30% lower than that of the gravel. 2) When the thickness of the cushion increases from 0.8 m to 1 m, a 13% increase in the secant modulus for the pebble cushion is observed, while 2.2% for the gravel. The mechanical deformation properties of the pebble cushion are more prone to be influenced by the cushion thickness than the gravel cushion. 3) When the preload increases from 52.5 kPa to 84 kPa, the secant modulus of the pebble cushion increases by 23.5%, while the modulus of the gravel cushion increases by merely 7.6%. A larger preload load leads to an earlier inflection point for the pebble cushion so as to show more stable mechanical properties. The increase in the preload load can more significantly improve the overall mechanical deformation performance for the pebble cushion than for the gravel cushion. 4) As the ditch width increases, the secant moduli of both cushions decrease. The pebble cushion is less sensitive to the changes in ditch size than the gravel cushion in the full load range. 5) The performance of the overall mechanical properties of the gravel cushion are better than the pebble cushion, but the cushion materials have limited effects on the structural settlement and stress. The pebble material can be used as an alternative cushion material for the immersed tunnels after a full analysis of the sensitivity to construction deviation. 6) Investigations on the optimal pebble grading will be the priority in the follow-up study.