A　series　of　small-scale　1g　X-section　cast-in-place　concrete　(XCC)　pile-penetration　model　tests　were　conducted　to　study　the　effects　of　soil　density　and　pile　geometry　on　the　lateral　responses　of　an　existing　pile　and　the　variations　in　surrounding　soil　stress.　The　results　showed　that　the　bending　patterns　of　existing　XCC　piles　varied　with　penetration　depth.　The　lateral　response　of　the　existing　pile　was　sensitive　to　the　change　in　relative　density　and　pile　geometry.　For　example,　the　bending　moment　of　the　existing　pile　increased　along　with　these　parameters.　The　development　of　the　radial　stress　sigma　＇(r)/sigma　＇(v0)　of　the　soil　around　an　existing　pile　showed　different　trends　at　various　depths　during　the　penetration　of　the　adjacent　pile.　Moreover,　the　change　in　radial　stress　during　the　penetration　of　the　XCC　pile　did　not　exhibit　the　＂h/R　effect＂　that　was　observed　in　the　free-field　soil,　due　to　the　shielding　effect　of　the　existing　piles.　The　peak　value　of　radial　stress　sigma　＇(r_max)/sigma　＇(v0)　decreased　exponentially　as　the　radial　distance　r/R　increased.　The　attenuation　of　sigma　＇(r_max)/sigma　＇(v0)　with　r/R　in　the　loose　sand　was　faster　than　in　the　medium-dense　or　dense　sands.　The　sigma　＇(r_max)/sigma　＇(v0)　at　the　same　soil　location　increased　with　the　cross-section　geometry　parameter.