Study on Osterberg Cell Test for Large-scale Pile
Pile testing is a fundamental part of foundation design, recent social demands urge pile foundation technology ensure the precise prediction of the performance of the piles, so, the precise prediction of the bearing capacity is the most important thing which requires accurate and reliable test technique. Thus, a number of the pile load test such as static load test, dynamic load test, statnamic load test and Osterberg cell test have been used in practice at present. Of all, Osterberg cell test has been used increasingly over the past decade or so, and it has several outstanding merits such as great testing load capacity, simplicity and its reliability, which lead to the widespread use especially for large-scale pile. It has successfully settled the reaction system which does not solved by the conventional static load test.
Osterberg cell test has successfully applied in Zhaoyuan Songhua River Bridge and Longhua Songhua River Bridge in North China. Three pile have been tested for each bridge .The test piles were 2000mm RC pile with lengths between 74 and 80 meters. Osterberg cell test was performed at pier 11(11-3,11-11)and pier 12(12-7)of Zhaoyuan Songhua Bridge and pier 1(1-1,1-2,1-3)of Longhua Songhua Bridge. With the soil pressure cells and strain gauges instrumented, the relationships of the shaft force-level, skin friction-level. Skin friction-displacement and load –settlement have been obtained. The results of the load-settlement are shown as follows:
In the Osterberg cell test, the skin friction resistance and pile base bearing capacity are mainly obtained through in –situ test ,in which the results interpretate the comprehensive interactions of the pile-soil system. In order to further the research on the self-balanced experimental mechanism, this paper studied the reactions between the soil and piles with the FEM, in which we applied the Drucker-prager elasticity-plasticity criterion to the analysis. The figure2 shows the FEM computational main stress characteristic.
In the FEM analysis of the two projects, the Bridge of Longhua Songhua River adopted the joint elements to simulate the interactions procedure of the pile-soil contact surface, but the Zhaoyuan Songhua River Bridge didn’t. The result of the FEM indicated that the displacements values of the pile head, pile bade and upper and lower parts of the load cell are relatively approximate to that of the Osterberg cell test, especially when we apply the joint elements to simulate the contact interface of pile-soil ,which shows that the joint elements are more reasonable to reflect the pile-soil interactions, and presents the reasonable interpretation of the transition mechanism of the stress. The results of FEM and Unterberg cell test are compared as table1.
Table 1 Displacement of the key points between FEM method and Osterberg cell test (unit:mm)
Displacement of the key point Zhaoyuan Songhua River Bridge (11-11) Longhua songhua River bridge (1-3)
FEM Unterberg FEM Unterberg
Pile head 25.89 40.00 42.8 34.79
Pile base 5.38 8.79 4.23 4.25
Upper of the cell 38.67 41.94 43.58 39.36
Lower of the cell 9.47 14.54 5.07 6.72
Ground displacement around the pile head were monitored when Osterberg cell test was carried out ,Figure3 shows the figure of the monitoring. From the monitoring points displacements, we can draw a conclusion that when the pile head displacement arrives at the biggest the project permits, the surface displacements around the pile head only influence the fields lesser than 5m in radius, decreasing with the increment of the distance(refer to Fig.5). The main points where displacements obviously occurred concentrate in the light range of the pile ,this reflects the pile-soil interaction mechanism from a side view, namely the pile-soil sliding friction zone mainly concentrates on its boundaries.
Table 2 Displacement of the monitoring points (mm)
Displacement of the pile head Displacement of monitoring points
1.3m 2.05m 2.5m 3.75m 4.7m
34.79 5.07 1.85 1.75 1.07 0.41
Viewed from the Osterberg cell test, the counterforce loaded on upper and lower part of the load cell is equivalent. The ultimate bearing capacity is decided only when both the skin friction and the pile base bearing capacity reach their limits. At present, during in-situ pile testing, the key point of deciding the ultimate bearing capacity is to locate the balance position correctly (namely the load cell position). The prediction of the balance position primarily comes from the geological conditions and the friction resistance of each layer. During the in-situ testing, it is obvious that the balance position is higher than normal, that is, the upper part displacement of the load cell exceed a lot than of the lower part, therefore, the test results always do not reflect the actual limit bearing values. Accordingly, the position of the load cell must be determined upon the geological conditions in practice: If the geological conditions of the supporting strata appear good (such as inlay rock piles), it is better to directly install the load cell under the pile bottom. On the conditions that the friction resistance at the upper pile appears smaller than the bottom, the limit bearing capacity of the pile could be obtained successful by applying counter forces on the pile head. Previous projects indicate that the experimental precisions are fairly similar to that of general static loading test. When the skin friction resistance appears greater than the bearing capacity of the pile base, the load cell should be placed at a certain position where the sum skin friction and the pile base bearing capacity equal to the skin friction of the upper part, that is, the genuine “self-balance”. But the location of balance is very hard to determined, and that is what the self-balance test system lies in. There is another way to solve the “self-balance” position that is to locate the load case according to pile foundation bearing capacity similarity testing results. If the limit bearing capacity is required and the end bearing capacity appears smaller than the skin friction, the limit skin friction could be obtained by expanding the pile bottom and settling the load cell on the expanded head, but the end capacity influenced by the expanded head should be considered also.
Viewed from the bearing situation of loading piles, its limit state of bearing capacity is restricted by the destruction of the pile base, and its load-settlement behaviors is linked to the destroy pattern of the pile base. Generally, when the bearing strata is dense sand, silt and hard clay, the end pile dose the action as integral destruction, on the contrary, it performs partially destruction if the overburden strata is hard, and when there is weak subjacent bed, it performs as punching shear destruction. If the bearing strata are mediate sand, silt, high and/or low compression clay, the end pile act as the pierce-destruction. As to saturated clay, if added rapid loading, there is not enough time for the soil to contract in volume, so the shear surface expands, which therefore results in the integral or the shear destruction, and the shear destruction surface is similar to the “pear-shaped” around the pile ending. The cases above could be reflected in the Q-s curve as follows: there are some features such as obvious sharp-change inflexion, settlement gradient sudden increasing and obvious destruction point., and the failure is named the “sharp destruction”. As to the super-long large diameter piles, since the pile ending receives a intensive constrained force from the strata, its destruction forms are not obvious, and its settlement gradient always performs slightly to a constant beyond the critical value, which is named the “gradual destruction”. Under the vertical loads, the strata usually act as vertical compression deformation. When the bearing strata are compressed, the area of the tensile stress occurs around the pile ending. At the same time, fan-shaped crackle appears under the large loading. But it is difficult to from a constitutive sliding plane or integral shear destruction plane with large deformation. The ground settlement has conformed this. As mechanism of the pile-soil system is so sophisticated that further research should be done to interpret the behavior of the pile-soil and its yielding destruction pattern.
研究Osterberg静载荷试桩法测试大型桩
桩基检测是基础设计的一个基本组成部分，根据设计的要求桩基检测要准确，因此，精确的测试承载力是最重要的事，这需要准确和可靠的检测技术。因此，一些桩荷载试验，如静载试验，动态负载测试, 目前,statnamic桩承载力测试技术与Osterberg静载荷试桩法已用于在实践中，在过去的十年中Osterberg静载荷试桩法已用于越来越多的工程，它有几个优秀的优点，如强大的测试负载能力，简单性和可靠性，从而导致广泛使用，尤其是大型桩。它已成功地解决了传统的静载试验反应体系不成功安放问题。
Osterberg静载荷试桩法已成功地应用在在中国北方肇源松花江大桥和龙华松花江大桥。每个桥均以做过三个桩以上的测试都以通过。测试桩是2000mm直径长度在74到80m之间。Osterberg静载荷试桩法是在肇源松花江大桥码头11 （ 11-3,11-11 ）和码头12 （ 12-7日）及龙华松花江大桥码头1 （ 1-1,1-2,1-3 ）。与土压力，电池和应变计仪器，与土压力，电池和应变计仪器，关于竖向力和水平力。获得桩的位移，荷载及沉降。荷载-沉降结果显示如下：
在Osterberg静载荷试桩法，表面摩阻力和桩基础承载力，主要是通过原位测试，把这种结果理解为全面互动的桩土体系。在为了进一步研究对自身平衡的实验机制，本文研究的反应之间的土壤和桩与有限元法，在这方面，我们采用了德鲁克-普拉格弹塑性标准分析。图2显示的有限元计算的主要应力特性。
在有限元分析的两个项目，龙华松花江大桥通过联合分子模拟的互动程序，桩土的接触面，但肇源松花江大桥没有。结果，有限元分析指出，在Osterberg静载荷试桩法测试表明，桩顶没有连接时，桩基和上，下部分的负载相对近似表示，尤其是当我们用联合物连接模拟接触界面的桩土时表明联合物能更合理的反映桩土相互作用，能更合理的解释压力的传递机制。结果，有限元方法和Osterberg静载荷试桩法相比，见表1。
对桩顶部分地面位移进行了监测，当osterberg静载荷试桩法进行，图显示的数字监测。从监测点的位移，我们可以得出一个结论，当桩顶位移到达最大的允许值时，表面上的位移靠近桩顶部分不仅会影响五米半径范围内，降低与增量的距离（参考图5 ）。主要点位移发生明显集中的一系列的桩，这反映了桩土相互作用的机理，从一个侧面观，即桩土滑动摩擦区主要集中在其边界。
Table 1 Displacement of the key points between FEM method and Osterberg cell test (unit:mm)
Displacement of the key point Zhaoyuan Songhua River Bridge (11-11) Longhua songhua River bridge (1-3)
FEM Unterberg FEM Unterberg
Pile head 25.89 40.00 42.8 34.79
Pile base 5.38 8.79 4.23 4.25
Upper of the cell 38.67 41.94 43.58 39.36
Lower of the cell 9.47 14.54 5.07 6.72
Table 2 Displacement of the monitoring points (mm)
Displacement of the pile head Displacement of monitoring points
1.3m 2.05m 2.5m 3.75m 4.7m
34.79 5.07 1.85 1.75 1.07 0.41
从Osterberg静载荷试桩法，上，下部分的荷载反作用力是相等的。极限承载力是决定只有当表面的摩擦和桩基础承载力达到极限。目前，在现场试桩期间，极限承载力是找到正确的平衡位置的关键点（即荷载作用的位置）。检测平衡点的位置，主要来自地质条件和摩擦阻力。期间，在现场测试，这是很明显，平衡点的位置是高于正常位置，即是上部分位移的荷载超过了很多的较低部分，因此，测试结果总是不反映实际极限承载力的情况。因此，荷载位置确定后，地质条件，在实践中：如果地质条件的持力层良好（如嵌岩桩） ，最好是在桩底下直接安装称重传感器。在桩上部侧摩阻力似乎小于底部端摩阻力时，桩顶反作用力能极大提高桩的极限承载力。以前的实验表明，该实验的精度是相当类似于一般静载试验。当侧摩擦阻力大于桩基础桩基础的承载力时，称重传感器应放在侧摩擦力和桩基础承载力之和等于上层侧摩擦力的地方，即是真正的“自我平衡” 。但平衡位置是很难确定的，而这正是自我平衡测试系统的关键所在，还有另一种方式可以解决“自我平衡”的位置问题，就是根据桩的地基承载力相似的测试结果来放置荷载。如果要求用极限承载力，并且桩端承载能力比侧摩阻力小时，有限的侧摩檫力可以通过对装底扩径和桩顶放置承重传感器来获得，但也要考虑由于桩顶扩大引起的桩端承载力变化。
从桩受荷时来看，其极限状态的承载力是受制于桩基础的破坏，其受荷下沉与桩基础形式的破坏有联系。一般而言，当承重层是密砂，淤泥和硬粘土，桩端会表现出疲劳破坏，相反，如果覆盖层非常硬，它会表现出部分破坏，当有软弱基床，它会表现出冲击剪切破坏。如果持力层是中沙，淤泥，高和/或低压缩粘土，桩端会表现出片状破坏。对饱和粘土来说，如果快速增加荷载，土壤没有足够的时间凝聚，所以剪切面扩大，导致全部或部分剪切破坏，另外剪切破坏表面像桩端周围“梨型“面一样。上述例子可以在下面的质量分数曲线上反映出来：有一些明显特征，例如拐点急剧变化，沉降值突然增加并且有明显的破坏点，这种破坏称为为“快速破速”。对于超长的大直径桩，由于桩端受到土层密集的压力，其破坏形式并不明显，其沉降值经常表现出略微超过临界值，这种称为“持续破坏“。在竖向荷载下，各土层通常表现出垂直压缩变形。当受力层被压缩时，桩周围表现出桩应力集中的现象。同时，在大负荷下出现扇形裂纹。但也很难从大变形变成连续滑裂面或整体剪切破坏。地面沉降证明了这一点。桩土系统是如此的复杂，应该做进一步的研究来解释桩土系统的形式及其破坏模式。
