ZBrushCentral

STL file messed up when divide option is being used

Hi all,
I am new to ZBrush and I have a question regarding STL file importing and editing.
I build dioramas and I want to create a stone bridge in ZBrush. I created the basic form with a cad program and I converted into STL file. Then I opened it in ZBrush and I tried to place a stone alpha on it to create the stone pattern on the bridge but the stone pattern did not show up as I expected. Instead aof that , all I could see was bulged driangles coming out of the bridge. Then I pressed the divide option to multiply the polygons but the bridge was greatly deformed (edges, faces etc). Also the stone/brick alpha now was visible when I tried to draw it on the deformed bridge. So how can I keep the initial form of the bridge and also create the stone/brick effect I want with the alpha? Any suggestions ?