Axisymmetric centrifuge modelling of deep penetration in sand to model driven piles or the cone penetration test

细长的物体(‘探头’或‘渗透仪’)进入土壤(深达10多米)对岩土工程师来说是最重要的。在地面上建造(长长的圆柱体)并将其‘打入’地下的‘打桩’地基就是这样一个物体的例子，它的直径可能超过一米。例如，它最终可能被用于海上结构的基础，例如风力涡轮机。在另一个极端，土木工程师通过推进直径只有几厘米的圆柱形探头来表征地下很深的土壤。探头在地面上遇到的阻力可以通过“外推”来设计出能够承载给定荷载的桩基。然而，究竟应该如何做到这一点，仍然存在争议，这反映了当探头(或桩)深入地面到相当深时，土壤中到底会发生什么的不确定性。岩土工程中的许多问题都比较适合进行数值模拟。然而，深部侵彻的严格数值模拟是异常困难的，因为该问题涉及大变形和高度非线性行为。然而，最近在开始使用“物理”模型理解基本行为的一些详细方面方面取得了相当大的进展。这种方法的主要缺点是，模型中土壤中的应力比桩基础在地面(比方说)30米深时的应力要小得多，这对土壤的性质有很大影响。这可以用‘土工离心机’来克服(在离心机中，重力加速度被有效地增加，从而在一个小模型中产生更高的应力)。最近，在使用数字摄影来‘跟踪’土层移动时也取得了进展(例如。由于推进的穿透仪)。然而，这项技术还没有在土工离心机模型中与推进的渗透仪一起使用。更具体地说，它还没有与圆柱形贯入仪(如桩或CPT)一起使用。取而代之的是“平面应变”的理想化，它有效地假设了贯入仪在一个水平方向上的范围是无限的。该提议的主要重点是开发一个(半)圆柱形贯入仪和土壤物理模型，以便能够观察到相关的土壤运动。这项技术随后将被用于土工离心机，利用数字摄影技术来跟踪土壤的运动。这一结果将提供与目前用于观察土壤运动的“平面应变”理想化和不允许观察土壤运动的(全)圆柱形贯入仪的基本比较。事实上，这项技术在岩土工程中轴对称(圆形)物理模型的所有领域都有潜在的应用。离心机模型的使用将意味着结果将适用于全尺寸情况(如打入桩)。然后，该技术将用于在土壤运动的同时观察渗透阻力，如应用于局部行为的方面，如探头的“尺度效应”、“分层”土壤剖面的影响，以及探头竖井的粗糙度。

The advancement of a slender object (a 'probe' or 'penetrometer') into soil (to depths of 10s of metres) is of fundamental interest to Geotechnical Engineers. A 'piled' foundation which is fabricated (as a long cylinder) at the ground surface and 'driven' into the ground is an example of such an object, which may be more than a metre in diameter. It may ultimately be used, for example, in the foundation for an offshore structure such as a wind turbine.At the other extreme the 'Cone Penetration Test' (CPT) is used to by Civil Engineers to characterise soils which are at significant depths in the ground by advancing a cylindrical probe which is only few centimetres in diameter. The resistance which the probe encounters in the ground can be 'extrapolated' to design a piled foundation to carry a given load. However, there is still debate regarding exactly how this should be done, reflecting uncertainty of precisely what happens within the soil when a probe (or pile) penetrates the ground to a significant depth.Many problems in geotechnical engineering are relatively well-suited to 'numerical' modelling. However, the rigorous numerical modelling of deep penetration is unusually difficult because the problem involves large deformations and highly nonlinear behaviour. Considerable advances have however recently been made in beginning to understand some detailed aspects of the fundamental behaviour using 'physical' models. The main drawback of such an approach is that the stresses in the soil in the model are much smaller than they would be at (say) 30m depth in the ground for a piled foundation, and this significantly affects the behaviour of the soil. This can be overcome using a 'geotechnical centrifuge' (where the gravitational acceleration is effectively increased so that higher stresses are generated in a small model).Recent advances have also been made in the use of digital photography to 'track' zones of soil as they move (eg. due to an advancing penetrometer). However, this technique has yet to be used in conjunction with an advancing penetrometer in a geotechnical centrifuge model. More particularly, it has not been used with a cylindrical penetrometer (as a pile or CPT would be). Instead a 'plane strain' idealisation has been used, which effectively assumes that the extent of the penetrometer is infinite in one of the horizontal directions.The main emphasis of the proposal is to develop a (half) cylindrical penetrometer and soil physical model which will allow the associated soil movement to be observed. This technique will then be used in a geotechnical centrifuge, utilising digital photography techniques to track movement of the soil. The results will provide fundamental comparison with the 'plane strain' idealisation which is presently used to observe soil movement, and the (full) cylindrical penetrometer, which does not allow soil movement to be viewed. Indeed, the technique would have potential application in all areas of axisymmetric (circular) physical modelling in geotechnical engineering.Use of centrifuge modelling would mean that the results would be applicable to full-scale situations (such as driven piles). The technique would then be used to observe resistance to penetration simultaneously with soil movement, as applied to topical aspects of behaviour such as 'scale effects' for the probe, the influence of a 'layered' soil profile, and the roughness of the probe shaft.