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Circular Concrete Cap with Helical Piles - Design Example

In this example we will design a helical pile (Steel pipe PP4.5x0.337 with 3 helixes - 12in, 14in and 16in diameter), installed in  medium to dense sands, and soft to stiff, overconsolidated clays.

A. Project Description - Model Parameters

 

Table 1 and Table 2 below present the soil properties and stratigraphy that has been estimated and used within the DeepFND software. Table 3 presents the pipe section properties. The water table is at El: -5ft.

 

Table 1: Soil properties.

Soil Layer

Soil Type

General properties

φ’

(deg)

c’ or Su
(psf)

γ

(pcf)

ELOAD

(ksf)

ERELOAD

(ksf)

e50

Sub. Mod.
(pci)

F

Soft Sand

30

0

120

300

900

-

30

O1

Soft Clay

-

400

105

120

360

0.02

-

V

Stiff Clay

-

1500

125

600

1800

0.005

-

S1

Medium Sand

34

0

125

400

1200

-

50

S2

Dense Sand

36

200

130

600

1800

-

100

 

Table 2: Stratigraphy.

Soil Layer

Elevation (ft)

OCR

Ko

F

0

1

0.5

O1

-7

1

0.562

V

-14

2.5

0.839

S1

-22

1

0.441

S2

-31

1

0.412

 

Table 3: Helical Pile Properties.

Pile Type

Helical Pile (Steel Pipe with Helixes)

Pile Section

PP4.5x0.337

Steel Material

Fy = 80ksi

Pile Depth (Total)

39 ft

Helixes

3 Plates (D=12, 14 and 16 in), Thickness: 0.5in

 

B. Modeling with DeepFND/HelixPile

Our software programs DeepFND and HelixPile are identical. They share the same user-friendly interactive interface, and they include the same analysis options. The only difference is that DeepFND can do the lateral and vertical design of all pile types (helical and non-helical). On the other hand, HelixPile includes only helical pile sections. This example was created using the Helical Pile component of the DeepFND software, but the exact same options should be followed to create the model in HelixPile.

In DeepFND software, we should define initially the soil properties of all soils according to the geotechnical report, the model stratigraphy, the pile head loads and the pile initial depth and structural section.

  • Define soil properties and stratigraphy:

From the General tab of DeepFND we can select the option “Edit Soil Type Data”. In the dialog that appears, we can modify the existing soils database or add new soils, and then for each one of them, we have to define the general soil properties, the soil model and the lateral soil properties. The soil parameters can be defined manually, or with the use of the software SPT estimator or local parameter estimation tools. 

Figure: Edit Soil Type Data Dialog.

  • Define stratigraphy:

From the General tab of DeepFND we can select the option “Edit Boring”. In the dialog that appears, we can define the top of the soil layer elevation and the soil type for each soil layer.

Figure: Edit Soil Layers Dialog.

  • Define pile section and initial length:

In DeepFND we have to define the pile type, structural section (pipe section and helix configurations) and original depth. Later, based on the analyses results, we can choose to optimize the pile section and the pile embedment. The required pile length can also be calculated by the software. We have to double-click on the pile and define the pile parameters in the dialog that appears. By pressing “Edit” on this dialog, we can define the pile type and the pile structural section.

 

Figure: Define pile dimensions and data dialog.

Figure: Define pipe section and helix configurations.

C. Pile Cap Options and Pile Configuration in DeepFND/HelixPile

The pile cap can be created with the Pile Cap Wizard within our programs. Through user-friendly dialogs we can define several parameters like the pile cap shape, the pile positions and direction and the external load magnitude and position on the pile cap. In this case, we will design a 3ft thick concrete circular pile cap (8ft Radius), supported by 17 helical piles. An 800 Kips vertical load and two lateral loads (a 60 Kips load along the  local X axis and a 50 kips load along the local Y axis) are applied on the pile cap centroid. The following figures present the pile cap dimensions and the loads applied on the cap centroid and the generated model (top view) with the axial load marked at the centroid respectively.

Figure: Pile Cap Dimensions and Load at Centroid.

Figure: Pile Cap with Helicap Piles - Top View.

  • Pile Locations and Generated 2D Cut Section:

In this case, 17 helical piles (PP4.5x0.337 with 3 helixes) have been distributed to the pile cap. The following figures present the piles layout (pile center local x and y coordinates) and the generated model (2D section – side view) respectively.

Figure: Pile Locations - Local X and Y Coordinates.

Figure: Generated Model - 2D Section Side View - Middle Cut Section.

D. Pile Group and Pile Cap Analysis and Results

Since the model is ready, we can choose to calculate the design section. After the analysis is succeeded, we can review several calculated results on the model area, both on the side view, and on the pile cap top view. The software displays the calculated graphs and shadings in detail on the model area. We can also select to display the 3D Model View of our pile cap with all installed piles, where we can also display the calculated graphs and shadings.

 

Figure: Pile Moment Diagrams – X-Axis (Middle Pile Row)

Figure: Pile Displacement Diagrams (Edge Pile Row).

Figure: Pile Cap – Moments Shading (X-Direction).

Figure: 3D Model – Pile Cap Settlements and Pile Moment Diagrams.

Presentations_DeepFND_Software.png

 

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