PDR Method – Combined Piled-Raft Foundation Calculator

PDR Method – CPRF Calculator

This calculator implements the simplified Poulos–Davis–Randolph load-sharing/stiffness approach. It is intended for preliminary engineering calculations and checking. It is not a substitute for a project-specific geotechnical investigation, detailed soil–structure interaction analysis, structural design, or the governing Indian Standard/project specifications.

1. Basic Foundation Data

Total service load Q (kN)

Number of piles n

Pile diameter D (m)

Pile length L (m)

Raft length Bx (m)

Raft width By (m)

2. Foundation Stiffness

Enter stiffness values obtained from your selected raft/pile analysis method. Units: kN/m.

Raft stiffness Kr (kN/m)

Pile-group stiffness Kpg (kN/m)

Interaction factor αrp 0 to <1. Use the calculated value below only when the adopted Randolph formulation is appropriate.

3. Optional Randolph Interaction-Factor Calculation

This section calculates αrp from an adopted simplified Randolph-type formulation. Review the formulation and soil parameters for the actual project before using the value.

Average soil shear modulus Gavg (kPa)

G at pile shaft/near pile Gl (kPa)

G at pile base Gb (kPa)

Poisson's ratio ν

Use calculated αrp? No – use entered αrpYes – use calculated αrp

4. PDR Calculation

5. Results

Raft load Qr

–

kN

Pile-group load Qp

–

kN

Raft share

–

%

Pile-group share

–

%

Combined stiffness Kpr

–

kN/m

Average settlement s

–

mm

Load per pile (average)

–

kN/pile

Used αrp

–

–

Calculation Details

Parameter	Value	Unit

6. Equations Used

Raft load fraction:

X = [(1 − αrp) Kr] / [Kpg + (1 − 2αrp) Kr]

Raft load:

Qr = X × Q

Pile-group load:

Qp = (1 − X) × Q

Combined stiffness:

Kpr = [Kpg + (1 − 2αrp)Kr] / [1 − αrp²(Kr/Kpg)]

Average settlement:

s = Q / Kpr

Equivalent radius per pile:

rc = √[Ar / (nπ)]

Adopted simplified Randolph-type influence radius:

rm = [0.25 + ξ{2.5ρ(1−ν) − 0.25}]L

ρ = Gavg / Gl     ξ = Gl / Gb

Interaction factor:

αrp = 1 − ln(rc/rp) / ln(rm/rp)

The interaction-factor formulation is sensitive to the adopted soil stiffness model and definitions. Verify the exact formulation required by the governing reference before using it for a final design.

7. Engineering Checks

Run the calculation to display checks.

Important Limitations

• This tool calculates global load sharing and average settlement only.
• It does not determine individual pile loads for corner, edge and interior piles.
• It does not calculate pile structural capacity, geotechnical ultimate capacity, raft bending, punching shear, one-way shear, contact-pressure distribution, lateral loads or seismic effects.
• The quality of Kr, Kpg and αrp controls the result. They should come from appropriate soil parameters, pile/raft geometry, load-transfer analysis, field testing and/or a validated numerical model.
• For important projects, compare the simplified result with an appropriate 3D soil–structure interaction model and field-test data.