Economic benefit analysis of low-level high vacuum compaction method from the perspective of low carbon

2.2.1. Quantities

Wuwei County Wucheng Road Soft Foundation Project is about 1100 m long, 40 m wide and 44,000 m² treated area. According to the design scheme, the design height of the top surface of the roadbed and the calculation of the road plane design plan, 43442 powder-sprayed piles are required for the motor lane, non-motor lane and sidewalk of this section. And the pile length is about 370593.2 m, the cleaning surface is 116958.697 m³, while the filling volume is 1152.592 m³, of which about 77202.564 m³ is hill-skin stone backfill, about 33284.694 m³ is graded gravel, and the plain earth backfill and two-way fiberglass geogrid is 4515.334 m³and 43560 m²respectively. Fig. 1 shows the cross-sectional plan of foundation treatment in the Eighth Road Standard Section. The boundary lines of foundation treatment are 2m out of the road red line and the foundation treatment width of the standard section is 44m. The boundary lines of foundation treatment in the non-standard section are also controlled according to the above principles.

Fig. 1Planning standard cross-sectional map of foundation treatment of eighth road sign (dimensions in meters except indicated)

2.2.2. Project direct cost

According to the above calculation criteria, it can be seen that DJM pile ($d=$50 cm, cement content 60 kg/m) is 55 yuan/m, Hill pebble backfilling is 89.7 yuan/m³, two-way glass fiber geogrid is 15.87 yuan/m², graded gravel backfilling is 134.55 yuan/m³, plain soil backfilling is 31.45 yuan/m³, surface cleaning is 26.85 yuan/m³, the distance between abandoned party and debit party is 5 km. The preliminary design budget for the treatment of soft foundation by dry jet grouting pile in Wucheng Road is shown in Table 1.

The total direct cost of soft foundation treatment with DJM piles is 35.7598 million yuan, and the unit cost is 35.7598 million yuan/(1,100 m×40 m/10,000) = 812.72 yuan/m².

2.3.1. Quantities

According to the scheme and the calculation of the design height of the top surface of the roadbed, the engineering quantities are as follows: the cleaning surface is 66107.8 m³; the backfilling volume of hilly stones is about 36059 m³; the plastic drainage board is 417347 m without portal frame bag sand rig; the geotextile is 66107.8 m²; the vacuum film is 135.6 m²; the full ramming energy of dynamic compaction soil foundation is less than 120 kN·m, ramming once, and the soft foundation area of repeated ramming hammer at each position is 60098 m²; The tamping energy of base point is less than 2000 kN·m, the number of tamping points per 100 m² is less than 12, and the soft foundation area is 18094 m² with repeated tamping times less than 4 times at each tamping point.

2.3.2. Project direct cost

The direct cost of the low-level high-vacuum compaction method consists of cleaning the road surface, strong tamp, precipitation, etc. The calculation standard is based on the above.

According to the above calculation criteria, plastic drainage plate without portal frame bag sand rig 11.48 yuan/m, geotextile 8.10 yuan/m², vacuum film 8.10 yuan/m², dynamic compaction soil base full compaction 9.26 yuan/m², dynamic compaction soil base point compaction 21.5 yuan/m², light well point construction dewatering 7.7 yuan/m²; sand bottom 63.93 yuan/m²; vacuum preloading period (2 months) 75 yuan/m². The preliminary design budget of high vacuum compaction treatment for soft foundation of Wucheng Road is shown in Table 2.

The total direct cost of soft foundation treatment by low-level and high vacuum compaction method is 24.717 million yuan, and the unit cost is 24.7147 million yuan/(1,100 m×40 m/10,000) = 561.7 yuan/m².

2.4.1. Direct cost comparison

Taking Wucheng Road soft foundation treatment project in Wuwei County of Wanjiang City Belt as an example (1100 m long, 40 m wide and 44,000 m² engineering volume), high vacuum compaction soft foundation treatment method is adopted instead of traditional dry jet pile method. The direct cost comparison analysis results are shown in Figure 2.

Fig. 2 shows that the direct cost per unit of DJM pile soft foundation treatment method is 812.72 yuan/m², while the corresponding part of low-level high vacuum compaction method is 561.70 yuan/m². Compared with the traditional construction method, the unit direct cost of the low-level high vacuum compaction method is reduced by 251.03 yuan/m², and the direct cost of the project is saved by 30.89 %. That is to say, if the case engineering uses the low-level high vacuum compaction method instead of the powder jet mixing pile to treat the soft foundation, the project will save 11.4451 million yuan in direct cost.

Fig. 2Direct cost comparison of powder-jet pile method and low-level high vacuum compaction method

The low-level and high vacuum compaction method has replaced the traditional method to deal with the bad roadbed. It has been used in the Yangxiang Road Project of Wuwei County in Anhui Province, the Eighth Road Project of Wuhu City Planning, the Second Phase Project of Jiangbei Industrial Concentration Area in Anhui Province, the Second Phase Extension Project of Jiangbei Industrial Concentration Area in Anhui Province, the Third Phase Project of Jiangbei Industrial Concentration Area in Anhui Province and Jiangnan Industrial Concentration Area in Anhui Province. It has been successfully applied in at least 6 projects. The basic situation of engineering application is shown in Table 3.

From Table 3, it can be seen that six soft foundation projects, such as Yangxiang Road Project in Wuwei County, Wanjiang City, Anhui Province, have a treatment area of 9717.11 million m². The project cost [20, 21] of powder-jet pile method and low-level high vacuum compaction method are 789 million yuan and 545 million yuan respectively. By replacing the traditional method with the low-level high vacuum tamping method, only the above six engineering applications can save 244 million yuan of direct cost, with extremely significant economic benefits. According to the statistics bulletin of the Ministry of Housing and Urban-Rural Construction in 2016, as shown in Fig. 3, the total length of roads has increased by 20.771 million kilometers and the total area of roads has increased by 39.68 billion square meters in recent years.

From the statistical chart, we can see that the road area increases linearly every year. The data are linearly fitted by Excel. The result shows that the linear relationship between the road area and the year is $y=38057x-7596.7$, that is, the average additional road area is 380570 million square meters per year. The results are shown in Fig. 4.

The difference of direct cost between the two methods is 251.03 yuan/m² (812.72 yuan/m²-561.7 yuan/m²). Assuming that 10 % of the new roads are treated by low-level and high vacuum compaction instead of traditional methods, the average annual investment will be saved by 388.1 billion m²×25.103 billion yuan / m² = 9.564 billion yuan, which is about 380.57×10 % = 0.3805.7 billion m² taking 0.38 billion m².

Fig. 3Statistical map of urban road construction by ministry of construction in 2016

Fig. 4Linear fitting map of urban road area for urban and rural construction of the ministry of construction in 2016

2.4.2. Contrast of environmental protection benefits

According to the introduction of China Academy of Environmental Sciences and China Cement Association, the cement industry is a key polluting industry, and the problems of resource consumption and ecological destruction are prominent. Carbon emission in cement production is second only to that in electric power industry. Particulate matter emission accounts for 20-30 % of the national particulate matter emission, and sulfur dioxide emission accounts for 5-6 % of the national emissions. Fluorite is added to some shaft kilns to reduce burning heat consumption, and fluorine pollution in surrounding areas will also be caused.

According to relevant information, about 110 kWh of electricity is needed for every ton of cement production. Electric power consumption in cement production can produce indirect CO2 emissions. According to the data released by the competent national authorities, the indirect CO2 emissions from electric power consumption in cement production are about 728 kg/t.

The cement consumption per square meter for soft foundation treatment by dry jet pile method is 505 kg, or 0.505 tons. The electricity price of cement industry is generally 0.8 yuan/kWh, the charge of CO2 emission is 60 yuan/ton, and the standard coal is 400 yuan/ton. Thus the energy consumption per square meter is 89.73 yuan (110×0.8+0.728×60+0.115×400)×0.505 tons/m².

It can be seen that the promotion of low-level high vacuum densification method will greatly reduce the consumption of natural ore and fuel, significantly reduce the intensity of greenhouse gas emissions per unit project, while saving a large number of comprehensive environmental protection treatment costs, resulting in considerable environmental benefits. The environmental benefits of the above six soft foundation projects are shown in Table 4.

1) Low-level high vacuum compaction method has been successfully applied in at least six projects, such as Yangxiang Road Project in Wuwei County, Wanjiang City Belt, Anhui Province, with an area of 9717.11 million m² and an additional environmental benefit of 87.191 million yuan (9717.11 million m² ×89.73 yuan/m²).

2) According to the statistical bulletin of the Ministry of Housing and Urban-Rural Construction on urban-rural construction, a linear fitting is made. It can be seen that in recent years, about 381 million m² of new road area has been added every year. Assuming that 10 % of the new roads in China need foundation treatment every year, the average annual environmental benefit will be 344563.2 billion yuan (381 million m² × 10 %×89.73 yuan/m²).