Calculate the theoretical efficiency ratio and the bearing capacity of the pile group, as well as the settlement and the distribution of the axial loads of the piles for the safe load on the group (FS=3).

Computational Geotechnics; Elastic and Elastic-plastic deformation of piles and pile groups

Marking scheme:

• Research & referencing (15%)
• Analysis (25%)
• Computer modelling (25%)
• Results & discussion (25%)
• Presentation (10%)

 

Coursework brief:

Model 1) You are asked to analyse the pile shown in Figure 1, for the parameters shown in Table q, according to the last digit of your student ID. Steps to follow:

• Calculate the bearing capacity of the pile, using an undrained analysis. For the interface parameters, use any relevant source from the
• Model the pile using an axisymmetric model in Plaxis 2D and
  1. Create a load displacement curve for the pile, either by applying incremental displacement up to 0.1D or by applying incremental loading (e.g. every 1 Qu) for an purely elastic analysis, and compare with the results of the elastic solution by Randolph and Wroth.
  2. Compare the previous 2 curves with the load displacement curve for the pile, for an elastic-plastic MC analysis and compare the bearing capacity

• Apply a safe design load on your pile (FS=3) and get the axial load, displacement and interface shear force distributions along the pile, compare with the elastic solution by Randolph and Wroth

For your model, it would be easier to consider the pile weightless, otherwise you will have to subtract the pile weight from the calculation. Comment of the advantages and the implications of that.

Model 2) Your pile above, is now a part of a 3-by-3 pile group, at a spacing s/d given in Table 1. Steps to follow:

• Calculate the theoretical efficiency ratio and the bearing capacity of the pile group, as well as the settlement and the distribution of the axial loads of the piles for the safe load on the group (FS=3).
• Create a plane strain elastic model using an “Embedded Beam Row” element for your piles and a plate element for the pile cap, as shown in Figure 2 and
  1. First de-activate the pile cap and try to get an idea of the pile stiffnesses and interaction factors from Plaxis, by applying loads on piles independently, following an elastic
  2. Activate the pile cap and apply the safe load on the pile group and compare the settlement and axial forces on the piles, with your theoretical

• Add some moment increments, no greater than the one which can cause an axial bearing capacity failure to a pile of the group and compare the rotational stiffness and rotation with your theoretical Also try to identify how the interaction affects the displacements of the extreme piles of the group.

Again, for your convenience, assume piles and pile caps weightless, but comment on the benefits and drawbacks of doing that.

 

Table 1. Modelling parameters

Last digit of student ID	L   m	D   m	Cu1   kPa	L2   m	Cu2 kPa	s/d
0	16	1	60	6	200	3
1	18	0.8	140	8	400	4
2	20	0.5	100	7	300	5
3	22	0.6	80	10	250	6
4	25	0.8	250	5	600	7
5	15	1.2	40	6	500	8
6	17	0.6	70	5	150	9
7	21	0.7	200	7	450	10
8	23	0.8	80	8	320	6
9	24	1	50	10	180	5

 

Figure 1. Single pile to be analysed with an axisymmetric model

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Figure 2. Pile group to be analysed with a plane strain model