As a general rule you do not need to apply p-multipliers for lateral loading in the transverse dirrection of a single row of drilled shafts if the center-to-center spacing is at least 3 shaft diameters.  If the spacing is less than 3 diameters, the reduction factor can be estimated using the formula:

p-mult = 0.64*(s)^0.34, where s is spacing expressed as number of shaft diamters

David,

Thanks for the response.  Do you have a reference for this formula?

The formula is presented in figure 6.4 of the GROUP 8.0 (Ensoft 2010) Technical Manual.  It is based on a fit of data from studies by (Cox, Dixon, an Murphy 1984), (Prakash 1962), (Wang 1986), and (Leing 1988).  If you can get a copy of the GROUP Technical Manual, section 6.1.1 is what you are looking for.

I don't have GROUP 8.0 Technical Manual.  Does the Manual have a formula for load along the row of pile also?