Eurocode groud type S1 seismic action

Are you asking for ground types A-E or ground types S1 and S2 (special ground types)?
If you're asking about the actual procedure, you're probably better off asking in geotechnical engineering general discussion forum.
If you're asking what the code says, it's the following.

For S1 this is the case (copied from the code):
Deposits consisting, or containing a layer at least 1O m thick, of soft clays/silts with a high plasticity index (PI> 40) and high water content.
v_s30 < 100 m/s (indicative)
c_u 10 - 20 kPa

For sites with ground conditions matching either one of the two special ground types S1 or S2, special studies for the definition of the seismic action are required. For these types, and particularly for S2, the possibility of soil failure under the seismic action shall be taken into account.
NOTE Special attention should be paid if the deposit is of ground type S1. Such soils typically have very low values of v_s, low internal damping and an abnormally extended range of linear behaviour and can therefore produce anomalous seismic site amplification and soil-structure interaction effects (see EN 1998-5:2004, Section 6). In this case, a special study to define the seismic action should be carried out, in order to establish the dependence of the response spectrum spectrum on the thickness and v_s value of the soft clay/silt layer and on the stiffness contrast between this layer and the underlying materials.