Abstract
Geotechnical processes such as subsidence, seepage, scouring, extreme temperatures as well as geological events pose constructional challenges even despite advancements in design codes, ground investigation techniques and numerical analysis. This paper analyses a case record where subsidence and damages to existing structures occurred adjacent to a location where dewatering works were being carried out for a new pipeline. Initially, the damages were thought to be solely linked to the dewatering effects, but it was still a puzzle as to why the problems only occurred at particular locations and not over the entire dewatered area. To solve the puzzle, forensic ground investigation and geophysics were performed. Based on borehole and CPT results, the ground profile was interpreted and compared with results from two parallel geophysical methods. Both of these revealed that the problem area was part of an old channel that had been dredged decades earlier and reclaimed by filling with loose material. This was corroborated by theoretical calculations of ground subsidence, which was found to be consistent with values of 600–700 mm measured on site. This work therefore demonstrates the importance of thorough consideration of past site history as part of design of new works or analysis of dewatering effects on existing structures.
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Notes
Sample Calculations for line-2.
Considering borehole-8 which was drilled in the structural failure area on line-2:
Depth of groundwater table = 2.80 m, water level after dewatering = 6.00 m, thickness of soil under consideration (H) = 6.00 − 2.80 = 3.20 m.
Relative density (RD) of soil before commencement of dewatering assumed to correspond to the loosest state. After dewatering, RD = 40% as correlated from average of SPT N-values in BH-8 within the zone of water level change.
For RD = 40%, maximum dry density = 16.33 kN/m3 (from correlations), minimum void ratio, emin = (Gγw/γmax) − 1 = (2.65 × 9.81/16.33) − 1 = 0.59.
From settlement theory, phase relationship consideration gives ΔH/H = Δe/(1 + e0).
where ΔH = change in thickness of soil layer, H = thickness of soil layer, Δe = change in void ratio, e0 = initial void ratio = 1 (= emax for loosest state).
Total expected settlement, ΔH = H[Δe/(1 + e0)] = H [(emax − emin)/(1 + emax)]\(=\mathrm{H}\left(\frac{{\mathrm{e}}_{\mathrm{m}\mathrm{a}\mathrm{x}}-{\mathrm{e}}_{\mathrm{m}\mathrm{i}\mathrm{n}}}{1+{\mathrm{e}}_{\mathrm{m}\mathrm{a}\mathrm{x}}}\right)\) = 3200 [(1.00 − 0.59)/(1 + 1)] = 656 mm.
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Vandanapu, R., Omer, J.R. Infrastructure Damage Due to Dewatering Without Considering Land Use History. Geotech Geol Eng 38, 5623–5632 (2020). https://doi.org/10.1007/s10706-020-01340-3
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DOI: https://doi.org/10.1007/s10706-020-01340-3