Inside this Issue

Technical Corner Importing Soil Data Discussions with.. Dr. F. C. Townsend, P.E. Professor, University of Florida Program Status Make sure you have the most current version   FB-MultiPier v4.08   FB-Deep v1.21

Contact BSI Bridge Software Institute University of Florida PO Box 116580 Gainesville, FL 32611 Online: bsi.ce.ufl.edu Email: bsi@ce.ufl.edu Fax: (352) 392-3697

Welcome to the Spring issue of the BSI Newsletter!

In this issue, Dr. Frank C. Townsend shows how to convert pressuremeter data into P-Y curves for use with FB-MultiPier or FB-Deep in the Discussions article. In the Technical Corner we explain how to use the new import soil import feature to copy soil layer data from existing projects with FB-MultiPier.

The articles "Technical Corner" and "Discussions" are open for input from all readers. If you have a topic that you think should be discussed, let us know. Did you create a great model with features that you want to share? Everyone is welcome to submit articles for possible inclusion in subsequent issues. Please contact BSI at BSI@ce.ufl.edu with your ideas.

With the release of FB-MultiPier v4.07, BSI added a new feature that will allow users to import soil properties from existing input files. This was requested by many users who have previously had to manually edit their input files to do this.

In the past, users had to manually copy the soil header from one input file into another. This introduced the possiblity of user error and required some knowledge of the input file structure. With v4.07 the process is automated so that the user only has to select the file with the desired soil data. The program will always select the full data set and copy it correctly without any chance of error.

On the Soil page there is a new label 'Import Existing Soil Data' and button 'Import'. Clicking this button will bring up a file chooser dialog. Use this to select the input file that contains the soil data you want to import and click 'Open'. The program will automatically update the Soil Edit window and all the soil displayed in the Soil page.

In addition to the import option we added a 'copy' feature that will allow you to copy the properties from a single layer in the current file. This will allow the users to quickly create a soil set when the same soil is used repeatedly by eliminating the repeated steps of entering the soil properties.

In the Soil Edit window, right click any layer and you will see a new option, 'Copylayer'. Highlight this option and the submenu will activate, showing a list of the current soil layers. When you choose one of these layers the program will automatically copy that layer's properties and overwrite the properties of the layer that is currently selected. The Soil Edit window will automatically update to show these changes and the new properties can be viewed under the soil layer's model edit information.

Be careful when using these features as both of these processes completely overwrite the existing soil data in the current input file and there is no way to recover the overwritten information.

Based on data reduction from Robertson et al. 1995 [1] a set of P-y curves can be created from pressuremeter data as long as the following 8 values are available:
A) Distance from Pressuremeter Control Unit to ground surface (m or ft)
B) Distance from ground surface to tip or Pressuremeter probe (m or ft)
C) Length from center of probe to tip (mm or in)
D) Distance from ground surface to center of membrane (m or ft)
      Note: Ground water table elevation does not affect pressure corrections as total pressure is what is used.
E) Total length of pressuremeter membrane (mm or in)
F) Initial circumference of pressuremeter membrane (mm or in)
G) Diameter of pile to be installed in the field (mm or in)

Given corrected pressuremeter data:
   

Point	P (bar)	V (cm3)
1	1.5	10
2	2	20

etc…

It will be convenient to convert any quantity such as pressure or length to the units you wish the P-y curves to be expressed as. In other words, convert the bars to psi or KN/mm2 before doing any calculations.

Initial radius of probe:
R_P = Initial Circumference of Probe / 2π

Initial volume of probe:
V_P = π * R_P² * Length of Membrane

Determine P in units of Force/Length:
Robertson et al.[1] suggested a method that uses the results of a pushed-in pressuremeter to evaluate P-y curves of a driven displacement pile. They multiplied the pressure component of the PMT curve by an α factor to obtain the correct P-y curve. This factor is applied to the pressure component of the PMT expansion curve. The critical depth is assumed to be four pile diameters as shown below. Robertson et al. reduced α near the surface assuming that the response is affected by the reduced vertical stress. To obtain the p-y curve (Fig. 1), the pressuremeter curve is translated to the lift off pressure that is equivalent to the initial lateral stress around the pile. The stress is multiplied by the pile width and the strain component (ΔR/R) is multiplied by the pile half width. For small strain conditions (ΔR/R) is equal to (ΔV/2V).

A reduction factor, α, (Fig. 1) is applied to the P for shallow depths.

If Depth from Ground Surface to Center of Membrane / Pile Diameter > 4
Then α = 1.5
Else α = 1.5 * Depth from Ground Surface to Center of Membrane / 4 * Pile Diameter
Thus P = (Corrected Pressure from PMT)*(Pile Diameter)*(Reduction Factor)

Determine Y in units of Length

Y = ((Corrected Volume from PMT) / (2 * Initial Volume)) * (Pile Diameter / 2)

Example: Given a fully corrected Pencel pressuremeter curve shown below, the procedure is as follows:

1. Extend unload point A back to P = 0.0 using sequential 2 reload points as a straight line quide
2. Readjust V= 0 where P=0; i.e., we have shifted the origin to where the unload curve would have been. This shift mitigates hole disturbance effects. Although Volume has been readjusted, we do not recalculate the radius, but use R₀.
3. Select 10 "good" points along the reload curve as the 10 points to represent the p-y curve. (Most programs only allow 10 points to describe a p-y curve)

Numerical Data Reduction - Table 1 presents the corrected volume and pressures displayed graphically in Figure 2.

Table 2 below presents the 10 selected data points to use for the p-y curves. The "new zero" volume = 47.91, and consequently the values of V₀ are less this amount.

First the P units are changed from bars to PSI (1 bar = 14.4858 psi).

Then P = (pressure)*(pile diameter) * (reduction factor α), and
Y = ((Corrected Volume from PMT) / (2 * Initial Volume)) * (Pile Diameter / 2)

Example for point #4 of table 2.
P = (6.276bars * 14.4858 psi/bar)(30in)(α = 1.5) / (1000 lbs / kip) = 4.091 kips/in
y = (2.111cc)(30in/2 / (2*16.221in³)(2.54³) = 0.06in

Table 3 below shows the points for the calculated Py curve (Fig. 4).

References:
[1] Robertson, P.K., Campanella, R.G., Brown, P. T., Grof, I., and Hughes, J. M., "Design of Axially and Laterally Loaded Piles Using In Situ Tests : A Case History," Canadian Geotechnical Journal, Vol. 22, No. 4, pp. 518-527, 1985.