CIVL4201/CIVL6201 S1 2024 ASSIGNMENT 1 Calculation of short and long-term settlements of geostructures Part 1 (15/30) The geotechnical design of the cylindrical steel water storage tank (diameter D = 5 m, height H = 8 m) schematically depicted in Figure 1 has been based on one exploratory borehole, and relevant in situ tests performed for this purpose. The simplified log of the borehole and the geotechnical parameters used in the design, interpreted from seismic dilatometer (SDMT) tests, are shown in Figure 2. During the geotechnical investigation the groundwater table level was found at the ground surface. Considering the hydrogeological regime of the area, one can reasonably assume that the groundwater table level does not vary with time. The water tank has been founded on a flexible concrete pad footing. The footing, which diameter is equal to the diameter of the tank (D = 5 m), is resting on the natural ground surface. A surveying marker was placed at the edge of the footing (Figure 2), immediately after construction was completed. Subsequently, the tank was filled up to the rim with water. Filling of the tank took 1 week to complete, while settlement was monitored frequently during and after filling, for a total period of 100 days. Results of the surveying campaign are shown in Figure 3. The owner of the project is concerned by the surveying measurements, as settlement of the tank's footing has already exceeded 80 mm, and does not appear to be stabilising. The design criterion set by the owner is that total settlement at the edge of the footing cannot exceed 100 mm during the service life of the pipe fittings, which is 10 years. If the settlement exceeds this limit, the piping and relevant connections will have to be replaced, at additional cost to the owner. The owner of the project is planning to lodge a claim against the consultant that designed the tank and has engaged you, as independent consultants, to provide them with an estimate of the settlement that will develop at the edge of the footing after 10 years from the end of construction. The owner is particularly reluctant to proceed with additional geotechnical investigations, and has asked you to use only the existing geotechnical data shown in Figure 2 (and the surveying measurements of course) to inform your prediction. 1. Use PLAXIS to determine the settlement of the edge of the flexible pad footing at 10 years after construction of the tank. Assume that the tank is filled with water throughout the whole service life of the fittings (10 years), and that the weight of the tank walls and of the concrete footing are negligible, compared to the weight of the stored water. List all the assumptions that you have made in your report, and present your analysis model in sufficient detail, bearing in mind that your report will probably be used in a formal arbitration process, and will be scrutinised. 2. After you have completed Task 1, and you have back-calculated (with reasonable accuracy) the required soil permeability value(s), estimate again the settlement of the tank at 10 years after the end of construction, using this time only hand calcs and Terzaghi's consolidation theory, while assuming 1-D conditions. List all the additional assumptions/simplifications that you had to adopt to use 1-D settlement theory, and compare your prediction against the prediction you have obtained with PLAXIS. If the predictions are different, provide a brief explanation (no more than 3-4 lines) discussing the reasons behind the difference, and identify which prediction is the most reliable one. Measured settlement Surveying marker Flexible pad footing D = 5 m H=8m Figure 1. Schematic of the water tank. Borehole log 0 m (Foundation level) -0.5 m Interpreted soil layers and measured parameters Loose silty sand (permeable, alluvial crust) y=16kN/m³ E'=10MPa v'=0.25 Soft silty clay y=14kN/m³ E'=1.5MPa v'=0.25 -14 m End of borehole Very dense gravelly sand (Fully permeable material) Figure 2. Geotechnical data retrieved from an exploratory borehole drilled in the vicinity of the water tank, and from in situ testing. 0 -0.01 -0.02 -0.03 -0.04 -0.05 -0.06 -0.07 -0.08 -0.09 at marker (m) End of construction Tank filling period -0.1 -0.11 -0.12 0.077m at 7 days 0.082m at 100 days 0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 Time since beginning of tank filling (days) Figure 3. Available surveying measurements. Part 2 (15/30) A high-rise tower is planned to be built near a heritage building, which was constructed of unreinforced masonry in the early 20th century. The masonry building is shallowly founded on the surface of thick, loose sand deposits, which compressibility properties have been measured during an earlier geotechnical investigation, and are shown in Figure 4. The 30m-thick sand layer is underlaid by the sandstone bedrock, which is practically incompressible. The natural groundwater table is found at the ground surface. Dewatering, that will take place during excavation of the tower's basement, will result in lowering of the groundwater table in the vicinity of the heritage building. It is well-known that lowering of the groundwater table will result in settlement of nearby buildings, in addition to the settlement due to their self-weight. The owner of the heritage building has presented to the developer of the tower a structural report stating that the maximum acceptable settlement at the edge of the building (Point E, Figure 4) is Uy,E = 20 mm, and the maximum acceptable settlement at the middle of the building (Point M, Figure 4) is Uy,M = 22 mm. The structural engineer also found that the stress applied to the building's foundation under serviceability conditions is equal to 72 kPa. The developer has engaged you to: 1. Estimate, using PLAXIS, the settlement that has already taken place at E and M due to the self-weight of the building only, before lowering of the water table. 2. Calculate, using PLAXIS, the maximum allowable drop in the water table D (see Figure 4) so that the total settlement at E and M (due to the self-weight of the building and due to lowering of the water table) does not exceed the criteria set by the owner of the heritage building. The accuracy required for the estimation of D is ± 0.5 m. 3. Using the value of D that you have found from Task 2 above calculate, this time using hand calcs only, the uniform settlement of the building due to lowering of the water table only. Some assumptions that you can adopt to simplify your PLAXIS model preparation and your hand calcs include: The building's out-of-plane width is much larger compared to its length, L = 30 m. The building's stiffness can be represented by uniform elastic parameters E = 1 GPa and V₁ = 0.2. These parameters account for the unreinforced masonry's elastic properties, but also for openings, windows, and other building features. Dewatering will result in the groundwater table dropping uniformly in the vicinity of the building i.e., the groundwater table level will remain horizontal, as shown in Figure 4. List in your report any additional assumptions that you have made, and present your analysis model in sufficient detail, to allow for the owner's consultant to check your simulations and calculations. In addition, briefly (2-3 lines) discuss any differences between the prediction of uniform settlement you have obtained via hand calcs, and the corresponding results of your PLAXIS model. Unreinforced masonry building L = 30 m E₁ = 1 GPa V₁ = 0.20 H = 6 m Initial water table Loose sand deposits 15 m Uy,E y = 15 kN/m³ Uy,M D = ? E' = 15 MPa v' = 0.30 30 m Water table during dewatering, Sandstone (incompressible) Figure 4. Outline of Part 2