## 5 Deformation Control Measures
### Comparative Analysis of Deformation Results
#### 5.2.4 Engineering Cost Comparison Analysis  
In subway foundation pit projects, the cut-and-cover method is widely adopted due to its ability to reduce the long-term occupation of ground traffic. This paper demonstrates that in engineering practice, by adding steel supports, the deformation of the retaining structure can be effectively controlled. However, the increase in the number of supports will significantly increase the project cost. Therefore, a quantitative correlation between \"deformation control requirements\" and \"economic costs\" needs to be established to provide a scientific basis for engineering decision-making. Based on the actual project overview, the width of the calculation model is 8.2 m, and the longitudinal length is set at 50 m; all the steel supports are made of \(\varphi 609\times 16\) ordinary pre-stressed steel supports, with a horizontal spacing of 3 m. The density of the steel is 7850 kg/m\({}^{3}\). Thus, the weight of a single steel support is approximately 1.915 tons. According to the current market price, the comprehensive unit price of the steel supports is 5000 CNY/ton (including material procurement and on-site installation costs), where the material cost accounts for 70% (3500 CNY/ton), and the installation cost accounts for 30% (1500 CNY/ton). The steel supports are reusable materials. After removal, the steel can be recycled, and the recycling price is calculated based on 40% of the material cost (installation costs cannot be recycled). It can be calculated that the net cost (total cycle cost) of a single steel support, the total purchase cost minus the recycling value, is approximately CNY 6894. The longitudinal length of the foundation pit is 50 m, and the longitudinal support spacing is 3 m per row (that is, one horizontal steel support is set every 3 m). Since the two ends of the foundation pit need to be equipped with sealing supports, the actual number of support rows needs to be calculated by rounding up, which gives approximately 17 steel supports. For Plan 2, 1 steel support is set on each floor, and a total of 3 horizontal supports are needed for the 3-storey main structure; the total number of supports is 51; the total cost is approximately 351,594 yuan (about 352,000 yuan). For Scheme Three, 22 steel supports are set on each floor, and a total of 6 horizontal supports are needed for the 3-storey main structure; the total number of supports is 102; the total cost is approximately CNY 703,188 yuan (about CNY 703,000). Regarding the construction period of the foundation pit, in the original scheme, each floor is poured at once, and the curing time for each floor is approximately 5 days, with a total curing time of 15 days; for Plan 2, each floor is poured in two stages, with a total curing time of 30 days; for Plan 3, each floor is poured in two stages, with a total curing time of 45 days. Regarding the cost of the curing period, since it is relatively small, it is temporarily not included, but Plan 3 has the longest working hours, followed by Plan 2, and Scheme 1 has the shortest. The economic performance of the scheme is evaluated by the \"unit deformation control cost\" indicator, that is, \"total cost increment/deformation reduction amount\", as shown in Equation (1):  
\[C_{\text{uc}}=\frac{C_{\text{tc}}}{\Delta\delta} \tag{1}\]  
In the formula, \(C_{\text{uc}}\) represents the unit cost for deformation control; \(C_{\text{tc}}\) represents the total cost increment; and \(\Delta\delta\) represents the reduction in deformation. According to Equation (1), it can be calculated that the unit deformation control cost of Plan 2 is approximately 33,400 CNY/mm; the unit deformation control cost of Plan 3 is approximately 46,300 CNY/mm. Plan 3 increases the number of steel supports by a factor of 2 (102 vs. 51), but only reduces the deformation by 4.64 mm (15.18 mm vs. 10.54 mm). However, the cost increases by a factor of 2 (703,000 yuan vs. 352,000 yuan), and the unit deformation control cost is 38.6% higher than that of Plan 2 (46,300 CNY/mm vs. 33,400 CNY/mm).