# Cohesionless

The following properties are required or available for all cohesionless soil layers:

- Reduction Factors (%) – percent decrease in computed unit resistance for layer
- Skin Resistance Loss
- End Bearing Loss

- Unit Resistance Limits – maximum value of unit resistance
- Skin Friction Limit
- End Bearing Limit

The following parameters are required for each method as described below. For more information see **Bored Capacity Theory Manual.**

## Ks-Delta/NQ Method

Skin friction parameters:

- Skin Friction Angle –specify or compute from SPT Table as described in
**Interpretation of SPT N Values from Table**section - OCR - over consolidation ratio
- Ks/Ko - ratio between soil-pile and at-rest coefficient of lateral earth pressure
- Delta/Phi (δ/ϕ') - ratio between soil-pile interface and soil friction angle

The ultimate unit skin (frictional) resistance (*f**sult*) is computed as follows:

Where:

End bearing parameters:

- End Bearing Angle - specify or compute from SPT Table as described in
**Interpretation of SPT N Values from Table**section - Nq - bearing capacity factor inputted or computed automatically based on end bearing friction angle

*Nq* may be found from various literature where one of most common is from Berezantzef presented by Polous and Davis (1980) as shown below. This graph is used for the **Auto** *N**q* option. It is advised to decrease the entered angle of internal friction by 2 to 3 degrees before using the graph. See the theory manual for more details about *N* *q* computation.

The bearing capacity factor, Nq from Berezantzef.

The ultimate unit end (base) resistance (q_{b,ult}) is computed as follows:

Where:

## SPT-AASHTO Method

Skin friction and end bearing parameters:

- SPT Table - specify SPT N values with depth

The ultimate unit skin (frictional) resistance (*f** _{s,ult}*) in kPa units is computed as follows:

The ultimate unit end (base) resistance (*q** _{b,ult}*) in kPa units is computed as follows:

See **Interpretation of SPT N Values from Table** section for more details.

## SPT-User Factor Method

Skin friction and end bearing parameters:

- SPT Table - specify SPT N values with depth

Skin friction parameters:

- a and b parameters

The ultimate unit skin (frictional) resistance (*f** _{s,ult}*) is computed as follows:

fs,ult = a + b N

End bearing parameters:

- c and d parameters

The ultimate unit end (base) resistance (*q** _{b,ult}*) is computed as follows:

q_{b,ult} = c + d N

See **Interpretation of SPT N Values from Table** section for more details.

## Beta/NQ Method

Skin Friction Parameters:

- Beta (
*β*)

The ultimate unit skin (frictional) resistance (*f** _{s,ult}*) is computed as follows:

Where:

End bearing parameters:

- End Bearing Angle - specify or compute from SPT Table as described in Interpretation of SPT N Values from Table section
- Nq - bearing capacity factor inputted or computed automatically based on end bearing friction angle

*Nq* may be found from various literature where one of most common is from Berezantzef presented by Polous and Davis (1980) as shown below. This graph is used for the **Auto** *N**q* option. It is advised to decrease the angle of internal friction by 2 to 3 degrees before using the graph. See the theory manual for more details about *N* *q* computation.

The bearing capacity factor, Nq from Berezantzef.

The ultimate unit end (base) resistance (*q** _{b,ult}*) is computed as follows:

Where:

## Interpretation of SPT N Values from Table

### AVERAGING SPT N VALUES

For skin segments, the SPT N value is taken at the depth of the segment. Therefore, each skin segment has an assigned SPT N value. In the case of Ks-Delta method, each skin segment will then have its own computed friction angle. For the end bearing segment, such as in the Nq method, the N value is the average from one diameter above the tip depth to two diameters below the tip depth. For non-circular piles, the shortest dimension of the pile at the tip depth is used instead of the diameter for determining the range of values in the average. A weighted average is used based on the N value and its depth.

### FRICTION ANGLE FROM SPT VALUES

There is an option to specify SPT N values with depth in order to calculate the friction angle for some methods mentioned above. RSPile uses the relationship between standard penetration test values and angle of internal friction (*ϕ' *) as presented by Peck, Hanson, and Thornburn (1974).

Note that the entered N values are used without correction to obtain N_{60}.