Alm & Hamre

Introduction

The ALM method is based upon that published by Alm & Hamre (2001) and uses CPT data. It allows for two sub methods to analyse the soil resistance to driving for SAND and CLAY.

General

The total static resistance is calculated similarly to pile bearing capacity principles. OPILE applies factors on the calculated skin friction to account for the contributions from internal and external shaft friction. The default factor for sand is 1.0 (i.e. the predicted shaft friction accounts for both internal and external shaft friction without futher modification), whereas the default factor for clays is 1.8. In clays OPILE will therefore increase the shaft friction returned by the method by a factor of 1.8. to account for the effect of internal shaft friction (i.e. for clays the Alm & Hamre paper is interpreted) This factor could be increased to the ratio ID/OD (or 2) if one wants to account for full internal and external shaft contributions. Tha factor can be reduced to 1 in order to use the predicted shaft friction as is. Unit tip resistance is only applied to the annular area of the pile (i.e. piles are assumed to drive unplugged).

By default, the upper bound resistance is calculated as 1.25 times the best estimate as suggested by Alm & Hamre (2002). OPILE allows the user to specify any other factor on the SRD input screen.

Shaft capacity and end bearing resistance components are evaluated separately, and then combined as follows:

Lower Bound

\(SRD_{LB} = 1.0 \cdot Q_s + Q_a\)

\(SRD_{LB} = 1.8 \cdot Q_s + Q_a\)

For Sand

For Clay

1.0 is the standard ALM Factor on Sand Friction

1.0 is the standard ALM Factor on Clay Friction

Upper Bound

\(SRD_{UB} = 1.25 \cdot SRD_{LB}\)

1.25 is the standard ALM Upper Bound Factor

Where:

\(Q_s\)

is the predicted shaft capacity

\(Q_a\)

is the total end bearing resistance

The standard ALM Factors can be overwritten by the user in the SRD Soil Input - Other SRD Parameters (when selecting ALM as Main SRD Method):

../_images/image1713472596.jpg

Overriding the default Alm & Hamre parameters

The general formulation of side friction along a pile during driving is:

\[f_s = f_{s,res} + (f_{s,i} - f_{s,res}) \cdot e^{k \cdot (d-p)}\]

Where:

\(f_s\)

is the pile side friction (kPa)

\(f_{s,i}\)

is the initial pile side friction (kPa)

\(f_{s,res}\)

is the residual pile side friction (kPa)

\(d\)

is the depth to the actual layer (m)

\(p\)

is the pile tip penetration (m)

\(k\)

is the shape factor for degradation (-)

The shape factor for degradation is the same for sands and clays, using the following:

\[k = \frac{\Big ( \frac{q_T}{p'_0} \Big) ^{0.5}}{80}\]

Where:

\(q_T\)

is the cone tip resistance (kPa)

\(p'_0\)

is the effective overburden pressure (kPa)

SAND

For SANDS the initial friction, \(f_{s,i}\), is taken as the basic static friction formulation, while the residual friction, \(f_{s,res}\), is taken as 20% of the initial friction:

\[f_{s,i} = K \cdot p'_0 \cdot \tan(\delta)\]

No upper limit on friction is included, however the lateral stress coefficient \(k\) is linked to the cone resistance by:

\[k = \frac{0.0132 \cdot q_T \cdot \Big( \frac{p'_0}{p_a} \Big)^{0.13}}{p'_0}\]

Where:

\(\delta\)

is the constant volume friction angle (degrees)

\(p_a\)

is the reference pressure (100kPa)

\(q_T\)

is the total cone tip resistance from the CPT

In SANDS the unit tip resistance is given by:

\[q_{Tip} = 0.15 \cdot q_T \cdot \left( \frac{q_T}{p'_0} \right)^{0.2}\]

CLAY

For CLAYS the initial friction is taken as the recorded CPT sleeve friction, while the residual friction is a function of the normalized cone tip resistance using the following:

\[f_{s,res} = 0.004 \cdot q_T \cdot \left( 1 - 0.0025 \cdot \frac{q_T}{p'_0} \right)\]

In CLAYS unit tip resistance is taken as 60% of the total cone resistance.

\[q_{Tip} = 0.6 \cdot q_T\]