# Stevens¶

## Introduction¶

The Stevens method of SRD is implemented according to Stevens et al (1982). It is based upon the unplugged and plugged capacities and can be applied in sands, clays and rocks. It uses a series of skin friction and end bearing factors to derive upper and lower bound soil resistance to driving results.

## General¶

Lower and upper bound values of SRD are calculated for both unplugged (coring) and plugged pile conditions. In the unplugged situation internal and external skin friction is used, combined with annular end bearing. In plugged driving skin friction is mobilised only on the external wall and the end bearing area is equal to the full area of the pile.

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

 Lower Bound Coring $$SRD = 1.5 Q_s + Q_a$$ 1.5 is the standard Stevens Factor for Unplugged SRD - LB Skin Factor Upper Bound Coring $$SRD = 2.0 Q_s + Q_a$$ 2.0 is the standard Stevens Factor for Unplugged SRD - UB Skin Factor Lower Bound Plugged $$SRD = Q_s + Q_p$$ Upper Bound Plugged $$SRD = 1.3 Q_s + 1.5 Q_p$$ For sand and rocks 1.3 is the standard Stevens Factor for Plugged SRD - Skin Factor - Sand (UB) 1.5 is the standard Stevens Factor for Plugged SRD - End Factor - Sand (UB) $$SRD = Q_s + 1.67 Q_p$$ For clay

Where:

 $$Q_s$$ is the total outside shaft capacity (kN) $$Q_a$$ is the total end bearing resistance on the pile annulus (kN) $$Q_p$$ is the total end bearing resistance on the total cross sectional area of the pile (kN).

## SAND¶

The unit shaft friction and unit end bearing are calculated similarly to the SANDAPI_ axial method. The shaft capacity, $$Q_s$$, is obtained through intergration over the outside shaft area. Contrary to the SANDAPI_ axial method, the Stevens method uses by default a $$K$$ value of 0.7.

## CLAY¶

The unit shaft friction is calculated using the CLAYAPICOMM_ axial method, however it is then mulitplied by a factor $$Fp$$ which is defined by:

\begin{align}\begin{aligned}F_p = 0.5 \cdot (OCR)^{0.3}\\OCR = \left( \frac{s_u}{s_{u,NC}} \right)^{\left( \frac{1}{0.85} \right)}\\s_{u,NC} = \sigma_v \cdot (0.11 + 0.0037.PI)\end{aligned}\end{align}

Where:

 $$s_u$$ is the undrained shear strength of the clay; $$s_{u,NC}$$ is the undrained shear strength of the clay if normally consolidated; $$PI$$ is the plasticity index. It is not necessary to enter the liquid limit and plastic limits to derive the plasticity index if an OCR value is entered directly. $$\sigma_v$$ is the effective overburden pressure; $$OCR$$ is the overconsolidation ratio. The OCR can be calculated using the plasticity index, however OPILE will also accept an optional override value of the OCR to be input;

Unit end bearing in clay is calculated according to:

$q = s_u \cdot N_c$

where:

 $$N_c$$ is the dimensionless bearing capacity factor, taken as $$9$$.

Note

The default factor of 1.67 used to predict the plugged upper bound end bearing in cohesive soils corresponds to calculating the unit end bearing with a bearing capacity factor $$Nc$$ equal to $$15$$ instead of the default $$9$$.

## ROCK¶

According to Stevens et al (1982) driving piles into rock is anticipated to severly fracture the rock layers and reduce the rock to a granular material. Therefore the unit skin friction for piles driven in rock layers is calculated assuming sand parameters.

For poor to fair quality rock unit end bearing is limited to values given for granular materials. For more competent rock the unit end bearing is calculated according to:

$q = u \cdot N_u$

Where:

 $$u$$ is the compressive strength of the rock $$N_u$$ is the dimensionless bearing capacity factor, typically equal

The standard of default Stevens factors used to combine shaft friction and end bearing into an SRD can be customised in OPILE in the SRD Soil Input