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# Finding the phi angle for layer

## Finding the phi angle for layer

(OP)
Hello everyone

I have this problem for Foundation Engineering. When determining the allowable downward capacity, I need to calculate the toe bearing capacity. To calculate it I used the following formula:

q'_t = B*gamma*N_gamma + sigma'_ZD * N_q*

To calculate the N_q* and N_gamma, I need to find the phi angle for each layer. The N_60 count for each layer is given to me. But I'm confused on how do I need to find the phi angle if N60 is given for one layer of soil ?

### RE: Finding the phi angle for layer

There are "cook book" charts relating N to angle of friction. Almost any text book has the chart. However, more likely your usable bearing capacity is less than computed since settlement probably governs.

### RE: Finding the phi angle for layer

I presume this is for a pile since you have referenced toe capacity.

You can use your SPT data and Meyerhof (1956,1976) from page 895 of Bowels 5th edition. This will be a rough guide and should be confirmed against your local code.

Ult capacity = Ap(40N)(Lb/B).......pick the lesser of that and Ap(380N)

Ap - area of pile base
N - N70, you will need to convert your N60 to N70 by using the SPT hammer efficiency
Lb - penetration to bearing stratum
B - width of pile diamater

Note - N is the average Nvalue of 8B above pile toe and 3B below plie toe. Use a conservative average.

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