The permeability coefficient of soil profile is one of the problems concerned by engineers, and the determination of permeability coefficient method mainly relies on the laboratory permeability test and field pumping test, but these tests are time-consuming and inefficient, and especially the permeability coefficient of soil under the condition of partial drainage was difficult to determine; in this paper, the modern digital CPTU technology was used. Dimensional permeability
With the development of foundation pit engineering to larger and deeper and the surrounding environment is increasingly complex, especially in the city downtown area, some excavations close to the high-rise buildings, some excavations close to the subway tunnel. At the same time, the groundwater treatment around the excavation cannot be avoided, and the environmental problems around the excavation caused by dewatering are becoming more and more serious. How to arrange dewatering to reduce the impact as much as possible is the key issue for many scholars and technicians. In order to reasonably arrange the dewatering well, we must grasp permeability characteristics of each soil accurately. The key parameter of permeability characteristics is the permeability coefficient. The permeability coefficient is the relative strength index of soil. It is a basic parameter and it must be used for seepage calculation; therefore, the accurate determination of soil permeability coefficient is a very important work, and it plays an important role in the success of the excavation. The current methods for determining the permeability coefficient are routine laboratory permeability test, field pumping test, water pressure test, and so forth. Routine laboratory permeability test is divided into constant head permeability test and variable head permeability test. The laboratory permeability test is widely used, mainly because of its easy operation and simple equipment; however, it has some disadvantages: (1) soil disturbance, (2) inaccurate permeability test, especially sand inclusion or interbed, (3) not simulating field boundary conditions, and (4) the heavy workload and high cost. These drawbacks make much difference for the permeability coefficient between field test and laboratory test.
Piezocone penetration test (CPTU) is a time-saving, little disturbance, convenient and economic method for permeability coefficient of soil. The permeability coefficient of the soil was able to relatively accurately obtain, especially mixed-layer or thin interbed experts and scholars at home and abroad dedicated to the study method that can accurately determine the permeability coefficient.
Since the advent of multifunction, a lot of the relationships between permeability coefficient and multifunctional CPTU parameters have been proposed. The main methods of permeability coefficient of cohesive soil are divided into two categories: (1) to estimate permeability coefficient based on the soil classification and (2) to estimate permeability coefficient based on the pore pressure dissipation test. The correct tip resistance
View of the calculation of consolidation coefficient is less than 7.1 × 10−5 m/s2, the consolidation coefficient of the cohesive soil can be determined by the pore pressure dissipation in the completely undrained condition, and then the permeability coefficient can be determined by the consolidation coefficient according to the relationship between permeability coefficient and consolidation coefficient. However, the consolidation coefficient between 7.1 × 10−5 m/s2 and 1.4 × 10−2 m/s2 was taken for the part of drainage condition, so the pressure dissipation was not used. The partial drainage state boundary range
For the saturated clay under undrained conditions, cylindrical cavity expansion stress was proposed by using [
Under undrained conditions, the excess pore pressure caused by cavity expansion can be calculated using the average total stress increment
In the CPTU penetration process, the horizontal stress is assumed to be equal to the cavity expansion stress. For the cohesionless soil
Tip local conditions. Cone expansion stress is
CPTU sounding yields profiles of the correct tip cone resistance,
If the adhesion of the cone sleeve is assumed to be equal to the undrained shear strength,
It is assumed that the penetration of a standard cone has a stable speed (i.e., 20 mm/s). According to the fluid continuity theorem and the Darcy law, the water volume is equal to the cone penetration volume in unit time, as shown in (
Geometry of process zone surrounding advancing penetrometer.
Take into consideration
and assumption that
Obtain
Substitute
Equation (
South and North Anchorage of the 4th Nanjing Yangtze River Bridge are located in the Yangtze River levee on both sides of south and North, belonging to the lower reaches of the Yangtze River alluvial floodplain. The site of north anchor of the 4th Nanjing Yangtze River Bridg is located in Nanjing fine mag Technology Co. Ltd. factory area, the ground elevation is about 5.5~6.1 m. The northern boundary of south anchor is apart from dyke and river more than 60 meters and 200 meters respectively. The north anchorage ground elevation is about 4.5~5.2 m, the site and the surrounding for forest, the southern boundary of north anchor is apart from the dyke and river about 120 m and 120 m respectively. The Quaternary loose sediment thickness is more than 60 m in north anchor.
From June 17, 2007, to June 23, 2007, the CPTU tests (six holes) were carried out in north anchor of Nanjing 4th Yangtze River Bridge. The soil profile of north anchorage of the Yangtze river 4th bridge is shown in Figure
The soil profile of north anchorage of the Yangtze river 4th bridge
Layout diagram of CPTu holes in Yangtze River four bridge north anchorage.
Results of CPTu.
Contoured plots of
Based on laboratory tests, the internal friction angle of slit is 30.9°, and the internal friction angle of fine sand is 32.5° and the internal friction angle of silty soil is 31°. The permeability coefficient related to the depth can be calculated by the equation (
Estimated soil permeability (
SBTn zone | SBTn | Range of |
|
---|---|---|---|
1 | Sensitive fine grained | 3 × 10−10 to 3 × 10−8 | NA |
2 | Organic soils clay | 1 × 10−10 to 1 × 10−8 |
|
3 | Clay | 1 × 10−9 to 1 × 10−9 |
|
4 | Silt mixture | 3 × 10−9 to 1 × 10−7 |
|
5 | Sand mixture | 1 × 10−7 to 1 × 10−5 |
|
6 | Sand | 1 × 10−5 to 1 × 10−3 |
|
7 | Dense sand gravelly sand | 1 × 10−3 to 1 |
|
8 | Very dense/stiff soil* | 1 × 10−8 to 3 × 10−5 | NA |
9 | Very stiff fine-grained soil* | 1 × 10−9 to 3 × 10−7 | NA |
Curve of permeability coefficient changing with depth.
The dimensionless penetration index
The engineering application shows that the permeability coefficients obtained from the pumping test and CPTU test are identical. This method can provide the reference for practical foundation pit dewatering.
The authors declare that there is no conflict of interests regarding the publication of this paper.
The work described in this paper was supported by the Outstanding Youth Fund of the Education Department of Anhui Province (Project no. 2011SQRL045), Master/Doctor Fund of the Anhui University of Science and Technology, Young Scholar Fund of the Anhui University of Science and Technology, and the National Natural Science Foundation of China (Project no. 51208005).