(This is one in a series of articles on the investigative methods used in forensic engineering)<\/p>\n
Japanese tunnel collapse<\/strong><\/p>\n The recent highway tunnel failure in Japan (Ref. 1) reminded me of the difficulty in reliably determining the physical, chemical,\u00a0and mechanical properties of foundation soils and rocks.<\/p>\n These properties are used in design and construction and\u00a0must be determined for all\u00a0earth and earth-supported structures resting on or in the ground. \u00a0Earth is made up of soil, rock, and water.<\/p>\n Design and construction of the tunnel relied on the properties\u00a0of the rock the tunnel was in.\u00a0 The tunnel would be a rock structure, a structure formed\u00a0of or in rock.<\/p>\n Media reports are that the concrete lining of the tunnel collapsed\u00a0after the anchor bolts corroded and gave way\u00a0– more specifically,\u00a0possibly\u00a0the heads of the bolts corroded and rusted.\u00a0 The concrete lining held in the place by the failed anchor bolts would then fall to the floor of tunnel and the\u00a0vehicles there.<\/p>\n Questions?<\/strong><\/p>\n Unlikely, is the answer\u00a0to both questions considering the nature of a tunnel.<\/p>\n Difficulty carrying out reliable engineering investigations<\/strong><\/p>\n A forensic engineering investigation of the cause of a failure or a complete collapse, where the initial hypotheris is that the cause lies in the ground, would check if the physical, chemical,\u00a0and mechanical properties were reliably determined.\u00a0 This checking during a forensic engineering\u00a0investigation would experience similar difficulties to that during a field investigation for original design purposes.<\/p>\n Cause of difficulty<\/strong><\/p>\n The difficulty in reliably determining the physical, chemical,\u00a0and mechanical properties of foundation soils and rocks is due to the heterogeneous nature of\u00a0the ground beneath the structure.<\/p>\n Soil and rock\u00a0are construction materials\u00a0like the timber, concrete, and steel, for example,\u00a0that are elsewhere\u00a0in a structure.\u00a0 The difference is that where concrete and steel are very uniform – the same throughout,\u00a0soil and rock are very non-uniform – not the same throughout; heterogeneous.<\/p>\n The design properties of steel, for example, are selected from a book taken off a shelf in the design office.\u00a0 The properties are\u00a0very reliable.\u00a0 The properties of\u00a0foundation soils and rocks\u00a0must be determined for each construction site by means of field and laboratory testing.\u00a0 The properties as determined can\u00a0be quite reliable or quite unreliable, and everywhere in between.<\/p>\n I learned a long time\u00a0ago when practising in England to expect the unexpected when dealing with the ground.<\/p>\n A review of foundation failures in England found that many were due to inadequate determination of the properties of the foundation soils and rocks.<\/p>\n What reliability depends on<\/strong><\/p>\n The reliability of\u00a0the properties\u00a0determined for\u00a0foundation soils and rocks\u00a0depends in part on:<\/p>\n Examples of variable reliability in engineering investigation<\/strong><\/p>\n In the case of the tunnel in Japan, the\u00a0surface of the construction site\u00a0would be a mountain.\u00a0 How do you reliability and thoroughly determine the properties of the rock along the alignment of\u00a0a tunnel beneath a mountain?\u00a0 There are methods but the costs are very high.\u00a0 There are less\u00a0costly methods but the reliability is much lower.<\/p>\n The rock is investigated in advance of the working face of the tunnel during construction.\u00a0 But this doesn’t give as reliable data on the properties of the rock above and near the crown of the tunnel.<\/p>\n A highway is a linear structure like a tunnel.\u00a0 The surface of the\u00a0highway site is relatively\u00a0level.\u00a0 Determining reliable\u00a0physical,\u00a0chemical,\u00a0and mechanical properties is easier by comparison to a tunnel.<\/p>\n Mountains are sometimes formed by the upthrusting of rock formations from below.\u00a0 The mountains on the west coast of\u00a0North America were formed that way.\u00a0 This\u00a0uplifting distorts and\u00a0fractures the rock – introduces greater heterogeneity into the rock formation.\u00a0 There is likely to be greater\u00a0variability\u00a0in\u00a0the physical, chemical,\u00a0and mechanical properties along the tunnel alignment because it is beneath a mountain.<\/p>\n The ground surface at construction sites in Truro and also on the valley floor of the Annapolis Valley in Nova Scotia are level but the foundation soils must be expected to be quite variable.\u00a0 This because\u00a0of how they were formed in water and beneath glaciers.<\/p>\n Nature of field testing and the inherent uncertainty<\/strong><\/p>\n Field testing at construction sites to determine physical, chemical,\u00a0and mechanical properties is characterized by testing\u00a0at discrete points.\u00a0 The judgement call for the professional engineer is how close or far apart those points should be.\u00a0 Then interpreting and extrapolating the data from the test points to all the soil and rock inbetween the discrete points – the great mass of soil and rock compared to the very small\u00a0amount of soil that is field tested.\u00a0 Then\u00a0assigning physical, chemical, and mechanical properties to these construction materials.<\/p>\n Because of\u00a0the uncertainty inherent in this process,\u00a0engineering reports include a section on the limitations of the\u00a0engineering investigation.\u00a0 The section states that if changed conditions are encountered during construction – changed with respect to those reported,\u00a0the engineer who did\u00a0the field tests must be contacted.\u00a0 He is\u00a0given an opportunity to review\u00a0his interpretation and extrapolation of\u00a0the data at the discrete field test points, and the properties he assigned to the mass of soil and rock for design purposes.<\/p>\n Forensic engineering investigation of foundation failures\u00a0is burdened by this same inherent uncertainty.\u00a0 In spite of this, a forensic investigation would be more thorough and reliable if for no other reason but to avoid making the same mistake twice<\/p>\n References<\/strong><\/p>\n 1. Globe and Mail, December 4, 2012.<\/p>\n <\/p>\n","protected":false},"excerpt":{"rendered":" (This is one in a series of articles on the investigative methods used in forensic engineering) Japanese tunnel collapse The recent highway tunnel failure in Japan (Ref. 1) reminded me of the difficulty in reliably determining the physical, chemical,\u00a0and mechanical … Continue reading \n
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