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Case Study: Clay, bedrock and road design

posted Jan 13, 2018, 5:11 PM by jeffery jim   [ updated Jan 14, 2018, 3:44 PM ]

Everyone posted this photo as a construction marvel and some posted it as a meme which insults local authority or government agency in maintaining road. Yes, some of the authorities and government bodies deserve insults but this construction marvel comes with shortcoming of engineering works.

1. First of all, it actually took a week to repair, not 2 days. Second, it sinks again the second time by inches. What i am trying to highlight here is geotechnical feat and solution for this kind of issues which actually need more than a week in order to restore it through engineering phases.

2. What failed in this case is due to construction issues, geological issues and geotechnical issues. 
(a) the construction of tunnel is the external forces which triggered the whole settlement events which lead to the sinkhole incident.
(b) the major issue played the vital role in triggering this event is due to geological issues where the selection tunnel path or corridor mainly on vertical IP that hits the stratum with interchanging strata of clay and bedrock. It is unknown what kind of bedrock but brief search indicates it is possibly lime and karst formation. The problem with sandstone and lime, is that it is have high bearing capacity in perfect condition but deteriorate fast when in contact with water or carbon. Fractures lead to lowering of bearing capacity or the excursion or ingress of aquifers or artesian ground water to aggravate the whole lime formation.
(c) nevertheless, the clay stratum above this bedrock also triggered another issue. As we know, when water is in contact with clay, there will bound to be saturation and consolidation process. The increment of water table or change of water netting will lead to increase of volume and pore pressure laterally. In event that the lateral pore pressure hit a pinnacle of this limestone formation and further causing fractures which eventually lead to the opening of cavity nearby. During the construction of underground structure, the heavy pumping of water drained the groundwater which dried the clay section and caused it to shear and collapsed into the cavity.
(c1) Liquefaction of sand strata above the clay is another plausible cause or contribution to the subsoil drainage.

3. Since we have discussed about the triggering mechanism, let us look at the proper repair work. The rushing beast mode repair works in 7 days come with risk, where the Mayor was not make known or anticipate to undertake risk. The backfilling material used is a mix of sand and chippings which basically give the best degree of compaction and reduction of air void. Nevertheless, the problem which was unforeseen is the the pore water pressure during the construction (as heavy pumping halted) affected by the reduction of water table until three times the depth in horizontal direction. When ground water dissipated back into equilibrium and react with part or section(s) in correspond to liquefaction phenomenon and reduced the water content in the backfill and thus, cause some settlement to the subgrade.

4. This is a short note and assumptions were made. It is a great case study for those who are interested in geotechnical field. To some, such settlement by inches is small, but for structural designer, settlement causes deflection which resulting to the robustness of the structure itself in doubt. Most high-rise buildings will experience an average of 20mm settlement which is less than an inch with an exception if the settlement is premeditated and covered during the design stage.

5. This is an act of humanity; world class. As for engineering, well... they should have considered the possible settlement by adding inches of overburden. Perhaps, they should have used clay backfill and the raft pile with transition pile approach to ensure settlement is covered. Nevertheless, time is of the essence and this approach will need time. Laymen will never understand that.

** This is an educational article and should be use as it is.