摘要
锚杆技术广泛应用于边坡加固、斜坡稳定、深基坑支护、结构加固和硐室稳定等工程中。锚杆的使用状态随着环境的作用和变迁发生着各种变化,如预应力的损失、浆体和钢筋体的腐蚀、断裂等,这些病害极大影响锚杆的工作状态。对锚杆工程进行病害调查、病害机理研究、准确掌握在役锚杆是否处于有效的工作状态是一项十分重要的工作。通过模型试验、数值模拟和理论分析,着重研究了土层锚杆在附加荷载作用下的受力特性和破坏特征、界面脱粘机理、水作用下的受力特性及预应力损失规律、锈蚀及锈胀机理。主要内容如下:
(1)通过全长粘结型锚杆在地面附加荷载作用下的模型试验,研究了土层锚杆的受力特性,试验结果表明:对于边坡内全长粘结型锚杆,滑坡体的下滑力首先作用于锚头和滑面处锚杆,然后通过锚杆和土体间的剪切作用向两侧传递;地面荷载在锚固系统内随时间的作用效果表现为“先增大,后降低”的规律,即地面附加荷载长时间作用对坡体的稳定有较大的影响,而瞬间作用对坡体的稳定性影响较小;坡体内全长粘结型锚杆是一种以受拉为主的“拉弯复合构件”,即在地面附加荷载作用下,中上层锚杆表现为滑面外侧受拉而滑面内侧受弯,下层锚杆则表现为整体受拉状态。
(2)通过集中锚固型锚杆模型试验得出,压力型锚杆的抗滑性能优于拉力型锚杆,在同一坡体内,前者分担的荷载是后者的1.5～2倍左右。在荷载作用过程中,锚杆的受力都会随时间进行调整,坡顶受力逐渐变小,坡底受力逐渐增大。当地面附加荷载与水耦合作用时,滑坡体的范围扩大,锚杆的受力由整体受拉逐渐转变为整体受弯,且上层锚杆的受弯作用最大。
(3)在全长粘结型锚杆试验的基础上建立了锚杆的有限元分析模型,计算得出了各层锚杆在有无地面附加荷载作用下的受力特性。结果表明:无地面附加荷载时,锚杆的受力主要来自于土体自重;当附加荷载作用时,坡体内的应力分布受附加荷载的支配,随附加荷载的增加,整个锚固系统的受力重心逐渐由下层锚杆向上转移,且中间锚杆的作用最大。
(4)采用拉拔试验,分析了土体含水量对土层锚杆极限抗拔力的影响以及预应力损失规律,结果表明:土体含水量越大,锚杆极限抗拔力越小;含水量的突然增大,会使得基本稳定的预应力陡然剧增,然后继续降低。
(5)应用Stang理论,建立了土层锚杆灌浆体与土体界面脱粘的计算模型,推出了土层锚杆灌浆体与土体界面脱粘的理论判据。当锚杆所受的拉力达到临界荷载时,锚固段外端的剪切力也达到临界值,此处界面开裂,发生脱粘现象;之后,最大剪应力随着裂纹的扩展向锚固段远端转移;最后整体发生脱粘,锚固体被拔出。临界荷载与界面粘结强度、剪切模量、锚固体的弹性模量、截面积以及长度有关。
(6)根据Phillips公式,提出了锚杆杆体与灌浆体界面脱粘的裂纹扩展理论。认为,与完整的锚固体相比,开裂锚固体的脱粘速度要大的多;而且,当其中一段或多段独立的锚固体单元发生完全脱粘时,其与土体间相互作用的长度也相应减小,所以极限抗拔力会大大减小。
(7)通过分析土层锚杆的界面特征,提出了杆体与灌浆体界面脱粘的软化边界层理论。对于变形钢筋,当胶着力破坏后,杆体与灌浆体发生相对滑动,肋的斜向挤压力对灌浆体会产生楔的作用,不仅使灌浆体被挤碎,同时使外围灌浆体出现内部斜裂缝及径向裂缝。随着荷载的增大,裂缝逐渐向纵向及外围发展。当径向裂缝向纵深发展并与斜裂缝相交时,该范围内灌浆体抗剪强度则会极大降低,在杆体周围形成一个软化边界层。软化边界层的出现使滑动显著增大,导致钢筋与灌浆体界面的粘结退化。
(8)根据钢筋锈蚀原理,通过分析土层锚杆的工作环境,得出无腐蚀环境中锚杆钢筋锈蚀原因,主要有空浆或少浆、砂浆密度不均匀、锚固体产生裂缝、杆体弯曲与孔壁接触等。应用弹性力学理论,推导出钢筋锈蚀导致砂浆开裂的临界条件。
论文通过试验,并结合数值分析,对附加荷载作用下土层锚杆的受力特性进行了研究,对土层锚杆界面脱粘、锈蚀以及水的作用等典型病害机理进行了分析,此研究成果可供公路边坡及基坑工程乃至其它岩土锚固工程设计、施工以及病害分析与防治参考,是确保锚固工程安全可靠的重要理论。
Bolt is widely used in such engineerings as slope reinforcement, slope stability, deep foundation pit supporting, structural strengtherning, chamber stability, et al.. With different effects and transition of environment, however, the use status of the bolt produces change subsequently, such as prestress loss, corrosion and rupture of slurry and steel etc., and these diseases have great impact on on bolt’s work status. So, it is rather important work to investigate the defects of bolt engineering, research the mechaniim of the defect and analyze the in-service bolt’s work status. In this paper, according to the results from model test, numerical simulation and theoretical analysis, the force characteristics and breakage feature of the anchored bar in soil under imposed load is studied, the interface disbondment mechanics is analyzed, as well as the force characteristics under the effect of water, the loss law of prestress, and the mechanism of rusting and corrosive expanding are discussied.The following contributions have been made:
1. For the full bonded anchor in the slope, the downsliding force firstly loads on the anchor head and the part of anchor at slide face, and then transfers to both sides along the anchor due to shearing action between anchor and soil. The long-term effect of ground load has great influence on the slope stability, and the effect in a very short time is less. A full bonding anchor in slop is a tension and flexural composite member, and tension is the main.
2. In the same slope, the antislip behavious of pressure-type anchor is stronger than that one of tensile-type, and the load shared by the former is about 1.5 to 2.0 times as much as that one of the latter. During the course of loading, with increasing time the force loaded on the upper anchor gradyally decreases, and that one loaded on the lower anchor increases. Under the coupling action between additional load from ground and water, the extent of sliding mass enlarges, the whole stress state of anchor gradually changes from tension to bend, and the compact-bending effect on the upper anchor is most obvious.
3. Based on the model test results of the full bonded anchor, the finite element analysis model of anchor was established. The computing results show that without additional load the stress of anchor mainly results from dead- weight of soil; when the additional load gradually increases, the gravity center of stress on the bolt reinforcement system slowly shifts from lower to upper anchor, and the maximum stress occurs on the the middle part of anchor.
4. The results of pull-out test show that the ultimate pull-out resistance is getting smaller with increasing water content, and the sudden increase of water content causes the pre-stress sudden increase in a short time, and then decrease again.
5. Using Stang’s theory, a calculation model of grouting mass on anchored bar in soil and soil interface debonding is set up. The criterion about grouting mass of anchored bar in soil and soil interface debonding is reduced in the paper. The results show that when the tensile stress of anchor reaches the ctitical value, the shearing force also reaches the ctitical one on anchorage zone out end with cracking interface, and then the maximal shearing stress changes to the anchorage zone out end with crack propagation, and finally the whole anchorage system debonds, the bolted mass is extracted.
6. According to Phillips formula, the crack expansion theory of interface debonding of lanchor grouting mass is put forward. The analysis shows that the debonding velocity of anchor interface with cracks is more quickly than that one of a perfect interface, and when one or more individual anchor element appears to be total debonding, the ultimate pullout capacity significantly reduces because of the decrease of interaction length between the anchor and the soil.
7. According to interface characteristics of the anchored bar in soil, the soften boundary later theory of interfacial debonding between the rod and grouting mass is proposed. For deformed bars, the relative slip occurs between the rod and grouting mass after the adhension is destroied, the oblique extruding force of rib causes wedging action for grouting mass, which makes the grouting mass crushed, and induces diagonal and radial cracks in the grouting mass. Those cracks developes to longitudinal direction and surrounding with increasing load, which results in the decreas in shearing strength of the grouting mass, a soften boundary layer surrounding the rod is formed. In this state, the bonding between steel bar and grouting mass will be degenerated.
8. On the basis of steel corrosion principle, the different working environments of anchor are analyzed. In a no corrosive environment, the main rusting reasons of reinforced bar are no or little mortar, uneven mortar density, cracks in anchoring section, bended bolt-bar, et al.. The critical condition of corrosion expansion crack is suggested according to elasticity theory.
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