How to use Foxta with CPT data?

This document aims to show and clarify how to use Foxta with Cone Penetration Testing data (CPT) for shallow and deep foundations.

Shallow foundations

Foxta is appropriate for the design of shallow foundations, including :

• isolated footings
• spread footings
• slab foundations...

Remember:

• Fondsup allows for the design of rectangular or circular isolated and spread footings.
• Tasplaq handles 3D modeling of pavements or rafts for any geometry and loading on a multilayer soil.
• Tasseldo provides estimations of settlements and stresses in a multilayer soil.

Fondsup

Fondsup allows for the design of rectangular or circular isolated and spread footings based on CPT data.

Parameters tab

If CPT data are available, one has to define the design method as From penetrometer results.

In this case, we recommend to process by measurements.

Soil definition tab

• $q_c$ : tip resistance [kPa]

The import wizard allows for the import a lot of data from clipboard (button Import).

For example, one may define all values in an Excel spreadsheet, select the range of data and then copy to clipboard.

 

Tasplaq and Tasseldo

The Tasplaq and Tasseldo modules require the input of soil elastic modulus ($E_s$) values.

How to determine soil elastic modulus ($E_s$) from CPT data?

In case of an important level of shear strains (raft under localized load, shallow foundation, embankment or slope edge...), Eurocode 7 (NF P 94-261 J.2.2) suggest to use Robertson model:

• If $I_R<2.2$:

  • $\alpha_E=0.015\cdot10^{0.55I_R+1.68}$

• If $I_R>2.2$:

  • $\alpha_E=\frac{Q_T}2$for $Q_T<14$
  • $\alpha_E=11.7$ for $Q_T>14$

In case of important volumetric strains ($\varepsilon_v$) (embankment on soft soil...), Eurocode 7 (NF P 94-261 J.2.3) suggest to use Sanglerat model:

Deep foundations

Here are a few examples of deep foundations which may be designed with Foxta:

• piles (reinforced concrete, steel...)

• micropiles

• barrettes

• pile-raft foundations

• stiff inclusions...

   

Remember:

• Fondprof is used to evaluate the bearing capacity of a single deep foundation.

• Taspie+ handles :

  • the calculation of pile and soil settlements in case of a single deep foundation under axial loading
  • the design of stiff inclusions

• Piecoef+ provides fine calculations for single deep foundations under lateral loading.

• Groupie+ handles 3D calculations for a group of piles connected on top with an infinitely rigid footing.

Fondprof

Fondprof is used to evaluate the bearing capacity of a single deep foundation.

Parameters tab

First of all, define the design method as From penetrometer results** in the Parameters tab.

After, you may chose to process by measurements or by layers. In the case of CPT data, it can be useful to process by measurements.

Layers tab

We recommend to define one soil layer per $q_c$ value from CPT data.

Skin friction

• $q_c$ : tip resistance [kPa]

  the Import wizard allows for the import a large amount of data from clipboard (button Import).

  So, one may define all values in an Excel spreadsheet, select a range and then copy to clipboard.

  The Soil class values have to be imported by digits:

  • 0 for No import
  • 1 for Clays and silts
  • 2 for Intermediate soils
  • 3 for Sands and gravels
  • 4 for Chalks
  • 5 for Marls and marly limestones
  • 6 for Weathered or fragmented rocks

  

• $q_{sl}$ : skin friction [kPa]

  a Wizard may be used to automatically calculate the $q_{sl}$ value for each input of $q_c$ .

  

  $q_{sl}$ wizard available in Layers tab

Tip resistance

The $k_{c,min}$ and $k_{c,max}$ values may be defined by the user.

• $k_{c,min}$ : minimal penetrometric factor for bearing capacity calculation (no unit)

  • $k_{c,min}=0.30$ for clay
  • $k_{c,min}=0.20$ for intermediate soil
  • $k_{c,min}=0.10$ for sands and graves
  • $k_{c,min}=0.15$ for other cases (chalk, marls, altered or fragmented rock)

• $k_{c,max}$ : maximal penetrometric factor for bearing capacity calculation (no unit)

Final penetrometric factor for bearing capacity calculation will be obtained from the following expression:

In case one wants to impose a $k_c$ value, one has to define : $k_{c,max}=k_{c,min}=k_c$

a Wizard may be used to calculate automatically $k_{c,min}$ and $k_{c,max}$ values for each soil categorie.

Pile types are detailed on the user interface.

Taspie+

Taspie+ handles :

• the calculation of pile and soil settlements in case of a single deep foundation under axial loading
• the design of stiff inclusions

Reaction laws

Skin reaction law

$q_s$ value corresponds to $q_{sl}$ value obtained using FondProf.

In the case of CPT data, $K_t$ may be evaluated with the following expression:

• The lower values correspond to granular soils (sands...)
• The higher values correspond to fine soils (clays...)

Tip reaction law

Stiffness diagram for lateral skin and tip resistance

$q_p$ value is obtained from FondProf at the bottom level of deep foundations.

In the case of CPT data, $K_p$ may be evaluated by following expression:

• The lower values correspond to granular soils (sands...)
• The higher values correspond to fine soils (clays...)

Layers tab

We consider a model composed by 3 soil layers.

Soil/pile tab

One has to define mobilisation law for the skin friction and the the tip resistance point by point.

We will consider 3 layers, each one associated to a $q_c$ value. For each layer, a skin reaction law and a tip reaction law must be defined.

How to determinate input values for skin reaction law?

• Point n°1 :

  $$\left(y;q_s\right)=\left(\frac{q_{sl}}{2K_t};\;\frac{q_{sl}}2\right)$$

• Point n°2:

  $$\left(y;q_s\right)=\left(\frac{3q_{sl}}{K_t};\;q_{sl}\right)$$

How to determinate input values for tip reaction law?

• Point n°1 :

  $$\left(y_p;q_p\right)=\left(\frac{q_{b}}{2K_q};\;\frac{q_{b}}2\right)$$

• Point n°2:

  $$\left(y_p;q_b\right)=\left(\frac{3q_b}{K_q};\;q_{b}\right)$$

Piecoef+

Piecoef+ provides fine calculations for single deep foundations under lateral loading.

Lateral reaction law

The lateral reaction law has to be chosen depending on the nature of the loads applied onto the deep foundation.

How to determinate input values for lateral reaction law?

• Law with 2 plateaux when:

  • Permanent loads applied at pile head are dominant, with:
  $$\left\{\begin{array}{l}k_s=\beta\frac{q_c}{2B}\\p_{max}=\frac{q_c}{\beta_1}\end{array}\right.$$
  • Short term loads applied at pile hear are dominant, with:
  $$\left\{\begin{array}{l}k_s=\beta\frac{q_c}B\\p_{max}=\frac{q_c}{\beta_1}\end{array}\right.$$

• Law with 3 plateaux when:

  • Very rapid accidental loads applied at pile hear are dominant, with:

    $$\begin{array}{ccc}\left\{\begin{array}{l}k_{s1}=\beta\frac{q_c}B\\p_1=\frac{q_c}{\beta_1}\end{array}\right.&and&\left\{\begin{array}{l}k_{s2}=\beta\frac{q_c}{2B}\\p_2=\frac{q_c}{\beta_2}\end{array}\right.\end{array}$$

  • Loads due to transverse earth pressure are dominant, with:

    $$\begin{array}{ccc}\left\{\begin{array}{l}k_{s1}=\beta\frac{q_c}{2B}\\p_1=\frac{q_c}{\beta_1}\end{array}\right.&and&\left\{\begin{array}{l}k_{s2}=\beta\frac{q_c}{4B}\\p_2=\frac{q_c}{\beta_2}\end{array}\right.\end{array}$$

Parameters tab

Choose manual input with 2 plateaux or 3 plateaux depending on the nature of the loads applied on the deep foundation.

Soil/pile tab

Groupie+

Groupie+ handles 3D calculations for pile group topped with a rigid footing and loaded with a torsor (6 components: 3 forces and 3 moments) taking into account the stiffness of the foundation and of the soil.

The same reaction laws than Taspie+ and Piecoef+ are implemented in Groupie+, so one may refer to previous sections to define all input parameters.

Soils tab

If CPT data are available, one should define the reaction curve with Point-by-point input.

Soil reaction laws may be defined through the provided wizards (input parameters are described in previous sections) :

• Lateral reaction law

• Friction reaction law

• Base reaction law: