Recent Question/Assignment

CIVL6201 S1 2020
This assignment is for students enrolled in CIVL6201
Bearing capacity of shallow and deep foundations
due date & time: 11 Jun 2020 5:00pm
Part 1 (15/100)
The construction of a bridge in an area where generally poor geotechnical conditions prevail will require the use of two cranes. The crane’s outriggers are square, of dimensions 1.00m x 1.00m and according to its manufacturer each caries a maximum vertical load of 250kN.
During a site visit the project manager realised that an extensive working platform (area about 500m2 approximately) is required to safely transfer the vertical load from the outriggers to the natural soil. Therefore a geotechnical investigation was performed specifically to obtain the soil parameters required for the geotechnical design of the working platform. This investigation comprised: i) excavation of a trial pit to identify the stratigraphy of the site, and retrieve samples for laboratory index testing, and ii) performance of a field vane shear sounding in the prevailing uniform clay layer, up to a depth of 6m, following AS 1289-1997 provisions. The results of this geotechnical investigation are shown in Figure 1 below.
You are assigned with the task to design the working platform, using PLAXIS, so that bearing capacity requirements are satisfied. The contractor advised that they have access to large quantities of selected coarse-grained material from a nearby quarry, and therefore they do not wish to use geogrids or other working platform configurations. You must provide them with:
- The thickness of the working platform. The minimum required thickness needs to be estimated with “civil engineering accuracy” e.g. rounded to the nearest decimeter.
- Recommendations regarding the physical and mechanical properties of the working platform material, compaction requirements, excavation of unsuitable materials if required etc. Your recommendations should be compatible with the assumptions of your analysis.
Present your input data, main assumptions, analysis methods, key results and recommendations in a short report.
i. Do not consider any geotechnical reduction factors or load factors.
ii. Assume that the outrigger pads are rigid and rough. To ensure stability, the pads must be embedded at a depth 0.3m into the working platform material.
iii. There are no serviceability (settlement) criteria for working platforms
iv. Assume the groundwater table is on the natural ground surface.
Figure 1.

Part 2 (15/100)
About 20% of the total cost of offshore renewable energy devices corresponds to the cost of their foundation. The need to minimise CO2 emissions while maintaining energy costs at acceptable levels has led to the development of new foundation elements for wind turbines, wave energy converters etc. One of the offshore foundation solutions that has drawn the interest of contractors and engineers is the use of suction caissons (also known as suction buckets), as the one shown in Figure 2 below. Their main advantages are minimal environmental impact during installation (as driving of large monopiles results in noise and vibrations that disturb sea mammals) and their capacity to carry large lateral wind and wave loads (and bending moments).
In order to prove their efficacy in transferring large vertical loads too, a manufacturer has assigned you with the task to determine with PLAXIS the required external caisson radius to carry a vertical force of Q=900kN in a typical very soft marine clay profile, representative of Australian near-shore conditions. This generic geotechnical profile is shown in Figure 2. The manufacturer mentioned that they want to use steel sheets of thickness t=0.1m, while in order to minimise installation equipment costs the length of the proposed caissons must be D=2m.
Present your input data, main assumptions, analysis methods and key results in a short report. Is the caisson more efficient in transferring vertical loads, compared to a circular footing of the same radius? If so, why? You don’t necessarily have to run additional PLAXIS simulations to answer this rather reasonable question, and help the manufacturer promote their solution to their clients, but your answer must be supported by some (even simplistic) calculations.
i. Do not consider any geotechnical reduction factors or load factors.
ii. Ignore lateral forces and bending moments.
iii. Assume the caisson is “wished in-place” i.e. ignore clay disturbance due to the installation of the caisson.
iv. The radius of the caisson should be estimated with “civil engineering accuracy” e.g. rounded to the nearest decimeter.
v. The eccentricity of the vertical force is zero (see Figure).
Figure 2.

Part 3 (10/100)
The manufacturer is investigating additional ways of improving the performance of suction caissons described in Part 2, including a novel method of treating the steel sheets comprising the suction caisson, to increase the mobilised friction resistance at the caisson-clay interface.
As part of this additional investigation, the manufacturer has tasked you with producing a simple design formula or chart that provides the bearing capacity of a caisson with radius Rext (determined in Part 2) as function of the adhesion factor au, for a range of au values au=0.05 to au=1.00.
In addition, in order to optimise costs, the manufacturer would like to know which surfaces should be treated with this novel method to increase au: the external surface of the skirt (Fig. 3), the internal surface of the skirt, the lid (Fig. 3), or all the above? Your answer must be justified with proper calculations and an explanation of the mechanisms providing resistance to vertical loads.
Figure 3.