In the collapsible loess area, the main purpose of foundation treatment is to eliminate the collapsibility of loess and improve the bearing capacity of foundation. In-hole Deep Overcompaction (SDDC) is a foundation treatment method gradually developed in recent ten years, especially suitable for deep collapsible loess and foundation treatment with high bearing capacity requirements. However, as a new foundation treatment method, it has not been included in the national standard at present, and there are still many problems worth studying in design and construction. 1 general situation of the project and geological conditions 1 floor, 4 floors above ground, with a construction area of about 7450.568m2 The local frame-supported shear wall structure belongs to Class C building, and the foundation adopts SDDC pile foundation. The site is located in the southern suburb of Xi City, and the landform unit belongs to the loess beam zone in the loess beam depression. The terrain is high in the southeast and low in the northwest, and the terrain fluctuates greatly, with a relative height difference of11.46 m. It is a self-weight collapsible loess site, and the collapsibility grade is Grade II (moderate) ~ Grade III (severe). According to the engineering survey data, the site stratum can be divided into four layers from top to bottom. See table 1 and table 1 for the stratum structure and geotechnical characteristics. List of stratum structure and geotechnical engineering features. Layer number, stratum structure and geotechnical characteristics. Layer thickness (m~m)① Plain fill Q4ml is brown, with uneven soil, containing a small amount of brick and tile fragments, ashes and plant roots.