A large-scale (10MW) ground-mounted solar farm will be established in North-East NSW, on land. You have been tasked with the design of the foundations of the photovoltaic mounting system, depicted in Fig. 1 below.
Each segment of the mounting system is going to be founded on two free-headed closed-end hollow steel pipe piles. The design (factored) ULS loads applied at the head of each pile type (A) have been determined by the Mechanical Engineer, and are provided in the Table 1 that follows.
Table 1. Resultant factored loads applied at the head of each pile type, at the ground surface.
Pile type Vertical force N (kN) Horizontal force Q (kN) Bending moment M (kNm)
A 15 18 21.6
The available data for the geotechnical design of the piles comprise one piezocone CPTu in situ test, three K0-consolidated undrained triaxial compression tests and one constant-rate-of-strain consolidation test, all performed on undisturbed samples retrieved with a fixed-piston sampler. The results of these tests are depicted in Figs. 2 and 3, that follow. The physical properties (Atterberg limits etc) of the soil samples have also been measured in the laboratory, and these are depicted in Fig. 3. The groundwater table at the time of the geotechnical investigation was found at -0.4m from the ground surface.
Tasks that you must complete include:
- Determine characteristic values of the geotechnical parameters that are required to estimate the long- and short-term axial bearing capacity, and the short-term lateral load capacity of the pile type that has been assigned to you.
- Determine the required pile diameter, section thickness and length so that the pile type that has been assigned to you satisfies the bearing capacity requirements against axial compressive loads for long- and short-term conditions, and the lateral load capacity requirements for short-term conditions only. You must use Hand calculation, considering simplifying assumptions where required.
What you must deliver:
- Executive summary reporting the hollow steel pipe pile length, external diameter and section thickness you determined for your pile type
- table with the description of your geotechnical unit(s) and the characteristic geotechnical parameters required for your long- and short-term pile capacity calculations.
- short note explaining how you determined these geotechnical parameters (max one page including figures)
- A list in bullet form with all the assumptions you have made
- A calculation sheet with your pile axial bearing and lateral capacity calculations. All the formulas that you used must be included in the calculation sheet, or properly referenced. Only the calculations for the final pile length/diameter/section thickness reported in the executive summary are required.
On top of the assumptions that you will make, you must consider the following:
- The loads provided in Table 1 are factored. You should not apply any additional factors on the loads.
- The weight of the pipe piles should be considered in the calculations. The weight is not included in the provided compressive loads.
- Do not consider any uplift loads in the calculations.
- Do not ignore the contribution of the first 1.5D of length of the pile shaft to the axial friction resistance. This is a special project, and provisions will be taken to avoid disturbance during pile driving.
-The basic geotechnical reduction factor should be taken equal to φgb=0.5, both for axial bearing and lateral resistance.
-Do not need to consider any SLS design criteria (pile settlement and lateral deflection). Design must satisfy only the ULS design criteria.
-the mobilised soil friction angle can be determined from undrained triaxial tests as φ′=sin-1(3M)/[(6+M)] with M=q/p'. q is the deviatoric stress and p' is the mean effective stress (see Fig. 3)
-Consider the yield stress of the pipe pile steel to be 250MPa.
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