## 4 Results and Discussion  
### Subsurface Stratigraphy  
A total of six categories (artificial ground, clayy silt, clay, silty clay, silt-fine sand, medium-coarse sand/gravel) was generalized to draw-up cross-sections. In total, 476 boreholes and 44 cross-sections were integrated into the 3D structural model (layer model), which contained 10 stratigraphic surfaces (markers) up to 50 m from the ground (Figures 6-9). The continuity of the depositional units was interrupted locally by active faults or erosional boundaries.  
Figure 6: Three-dimensional geological structures of Tongzhou District.  
Subsoil deposits up to 50 m beneath the Tongzhou area were divided into 10 major geological layers in general. Main layers were then grouped into assemblages representing three cyclothems of alluvial deposits (for example, cyclothems on Figure 4). The 3D model showed spatial stratigraphic structures comprising cyclic sequences of clay, interbedded silt, sand, and gravel with lenses (Figures 7-9).  
The upper \(\sim\)17 m of alluvial deposits (i.e., the first cyclothem) comprised layers of artificial ground, interbedded units of silt and clay, sand, and gravel. The thickness of the  
Figure 8: Schematic diagram showing the first cyclothem in the study area.  
Figure 7: Distribution of the upper fine-grained sand and the inside lenses.  
Figure 9: Distribution and thickness of geological units in the study area: (**a**) the second cyclothem; (**b**) the third cyclothem.  
gravel was around 3 m, locally identified in the north and absent from the central and southern parts of the area. Following gravel, a layer of sand with a thickness of around 9 m, described as silt or fine-grained sand, comprising feldspar, quartz, mica, iron oxides, and a few organic materials, was observed throughout the entire area. A thin layer of silty clay was intercalated in the southeastern area (Figure 7). Overlying the sand, a succession of clay could be found, identified as silty clay with heavy silty clay locally developing. The upper silt layer overlaid the silty clay, with thicknesses up to 7 m (3 m on average), and it was interpreted to be clayey silt containing mica, iron oxides, and turquoise, and silty clay lenses were locally intercalated. Following the upper claye silt, 1-2 m of artificial ground was identified throughout the whole area (Figure 8).  
The second cyclothem comprised units of clay, silt, and sand (Figure 9a). A thick layer of sand (i.e., the second sand layer) lay on the bottom of the cycle and was identified as fine-grained and medium-coarse grained sand, containing few organic materials and gravel lenses. Following the sand, an about 4.5 m thick layer of clay occurred throughout the entire area and was identified as silty clay. A unit of silt overlaid the clay, with an average thickness of approximately 4 m, described as clayy silt, which was found in the southern half of the district.  
The third cyclothem was characterized by widely-distributed units of silty clay and fine-grained sand (Figure 9b). The former mainly comprised brownish yellow or greyish-yellow silty clay, medium to stiff, with thicknesses typically ranging from 1 to 10 m (4.7 m on average), commonly containing minerals such as mica, turquoise, as well as organic or iron oxide materials. The latter consisted of brown-yellow or grey fine-grained sand, dense and saturated, with a mean thickness of around 9.6 m. Minerals included feldspar, quartz, mica, as well as organic materials. Lenses of gravel, coarse-grained sand, or clayey silt were also identified occasionally within the thick unit. Figure 10 shows the distribution of subsurface layers along the NS12 cross-section.