The foundation constitutes the lower structure that transfers loads from buildings to the subsoil, serving as a critical link between superstructures and subsoil. It bears internal forces generated by the interaction of superstructure loads and subgrade reaction forces. Conversely, the reaction forces at the foundation's base act as loads on the subsoil, inducing stress and deformation. The subsoil refers to the limited area supporting the building foundation. When the subsoil's bearing capacity fails to meet the loads transferred from superstructures, foundation treatment or pile foundation solutions become necessary to satisfy load-bearing requirements. The effectiveness of subsoil treatment directly impacts the project's quality, cost, and schedule. Common subsoil treatment methods include replacement bedding, preloading, compacted subsoil, composite subsoil, and grouting reinforcement. This discussion focuses on CFA pile composite subsoil, a prevalent technique in composite subsoil applications.

CFA pile composite foundations are widely adopted in construction and foundation treatment projects due to their simple construction, cost-effectiveness, and strong adaptability, particularly in multi-story residential buildings, public structures, and industrial plants. During the design phase, comprehensive adjustments and optimizations are typically required for key parameters such as pile length, diameter, and spacing to meet multiple requirements including foundation bearing capacity, deformation control, and cost-effectiveness. While measures like increasing pile length, enlarging pile diameter, or reducing spacing can effectively enhance bearing capacity, practical engineering experience shows that different parameter adjustment strategies often have significantly varying impacts on project costs, construction timelines, and feasibility. Therefore, detailed analysis and judgment based on actual conditions are essential.

Ⅰ. Pile length

The primary consideration for determining pile length in is bearing capacity. Typically, pile length should be determined by integrating design requirements, bearing capacity, and the natural bearing capacity of the underlying stratum. Using a pile diameter of 400mm and spacing of 3-5 times the pile diameter, combined with geotechnical column charts, a relatively favorable bearing stratum can be identified. Once the bearing stratum at the pile end is confirmed, the pile length is essentially finalized.

·  The pile length should also be determined by settlement control, especially for strata with hard upper layer and soft lower layer or thick soft interlayer. Since the bearing layer is of good quality, the contribution of soil between piles is large, and the shorter pile length may meet the bearing capacity requirement. However, because the soil quality of the underlying stratum is poor, the pile length should not be determined only by bearing capacity, but should penetrate the soft soil layer into the better soil layer or the pile length should meet the control requirement.

·  When determining pile length, the maximum borehole depth of construction machinery and operational conditions must be considered. Currently, domestically produced long spiral drills can achieve a maximum borehole depth of 40m, but most on the market are models with depths under 30m. For long spiral drills exceeding 30m in borehole depth, market research is essential. Generally, larger equipment results in higher per-square-meter construction costs.

Ⅱ. Pile diameter

The pile diameter selection should consider construction techniques, spacing, length-to-diameter ratio, and material utilization efficiency. Generally, 400mm and 500mm pile diameters are the most economical. If the length-to-diameter ratio remains excessively high or the spacing is too dense with 400mm diameter piles, increasing the diameter should be considered.

III. Pile Spacing

The spacing between piles should be determined by considering pile length, diameter, bearing capacity requirements, and pile layout methods. Within the recommended range of 3 to 5 times the pile diameter, a larger spacing is preferable. Increasing the pile length ratio to reduce spacing is more beneficial for settlement control.

From a construction standpoint, increasing pile length reduces the required number of piles, thereby decreasing the frequency of equipment repositioning and enhancing overall efficiency. The wider spacing between piles also minimizes soil displacement effects and the likelihood of hole migration. When extended pile lengths allow the pile ends to reach superior soil layers—where higher bearing capacity and compressive modulus exist, and where CFA piles can better exert their end resistance—this approach yields significantly greater technical and economic benefits compared to reducing pile spacing. In such cases, the maximum allowable pile length within the equipment's drilling depth should be utilized, while proportionally increasing the spacing between piles.

IV. Case Analysis

This commercial building features six above-ground floors and one basement level, constructed with a frame structure. The design proposes independent foundations beneath the frame columns, supplemented by CFA pile composite foundations for localized treatment. The total vertical load standard value is 6000 kN. The foundation's bearing layer consists of medium-density fine sand, with a natural bearing capacity of 160 kPa. The pile end bearing layer, a 13-meter-thick coarse sand layer, demonstrates high bearing capacity and compressive modulus, making it an ideal choice for pile end support.

The CFA pile diameter is set at 400mm. Given the favorable and relatively thick bearing stratum at the pile end, which allows for greater pile length flexibility, a design approach combining fixed spacing with reverse calculation of pile length was adopted. Two spacing schemes were implemented: 4d and 3d spacing, with comprehensive technical-economic comparisons conducted alongside independent foundation analysis. Following the principle of uniform pile replacement rate under the foundation, the pile edge spacing was uniformly set at half the pile spacing. Consequently, the foundation dimensions for the 4d scheme are 4.8m × 4.8m, while those for the 3d scheme are 3.6m × 3.6m, as illustrated in the figure below.

Option 1 (left figure): 4d pile spacing, 13.5m pile length

Option 2 (right): 3D pile spacing with 16.0m pile length

The calculated bearing capacity characteristic values based on the resultant vertical force and foundation plan dimensions should be no less than 265 kPa and 460 kPa, respectively. Based on the required bearing capacity characteristic values and pile spacing, the single pile bearing capacity characteristic values should be no less than 685 kN and 835 kN, respectively. All concrete strength grades must meet C30 requirements. The pile lengths derived from the single pile bearing capacity characteristic values are 13.5 m and 16.0 m, respectively. Both pile lengths penetrate the coarse sand layer, with penetration lengths of 1.65 m and 4.15 m, respectively.

Both schemes can meet the requirement of bearing capacity, but there is a big difference in the comprehensive technical and economic effect.

From a technical standpoint, the 16-meter pile penetrates the coarse sand layer over a longer distance, ensuring greater load-bearing capacity. In terms of foundation deformation control, longer piles outperform shorter ones. Economically, while the total concrete consumption for 16-meter piles increases, the concrete usage for independent foundations is significantly reduced. The combined concrete consumption for piles and foundations can be decreased by 11.3m³, representing a 23.7% reduction. Additionally, due to the smaller foundation dimensions and shorter lever arms, the reinforcement is substantially reduced despite increased base pressure. With the same foundation height, the number of steel bars can be reduced from 32 bars of 22mm (825kg) to 21 bars of 22mm (406.1kg), achieving a reduction of over 50%.

Therefore, under the condition of constant column bottom load, the scheme of increasing pile length and reducing foundation plane size has obvious economic advantage than that of reducing pile length and increasing foundation plane size.

V. Summary

The pile placement for CFA pile composite foundations is typically confined to the foundation projection area. The selection of pile length, diameter, and spacing not only directly impacts the foundation's overall load-bearing capacity and deformation characteristics but also significantly affects project costs, construction duration, and organizational efficiency. Therefore, prior to finalizing the pile layout plan and foundation type, a comprehensive multi-parameter and multi-scenario technical-economic analysis should be conducted. This analysis must consider the specific geological conditions, structural requirements, and construction resources of the project to ultimately select the optimal solution that offers the best economic benefits, technical rationality, and suitability for the project's implementation conditions.