Keywords

1 Introduction

A large amount of groundwater is stored in the soil voids and rock fissures, and the groundwater will produce uplift pressure on the underground structures buried in or above the rock and soil bodies. The problem of excessive uplift pressure of underground structures is often not given enough attention to cause accidents. The reason for this is that the design does not consider the most unfavorable situation in the construction conditions, and the second is that the groundwater level exceeds the design water level (such as during heavy rainfall). Insufficient decompression will eventually lead to pit instability, pit surge, basement floor cracking, basement overall uplift and other problems, and may also affect the surrounding pipelines or neighboring buildings [1].

Especially for mega buildings on slopes in the southern rainy areas, the groundwater level is high, and the original slope seepage field forms a new seepage condition under the interference of mega buildings, and the influence on the basement becomes very complicated. If the value of basement water uplift pressure calculation is small, it will cause low safety reserve or even accidents, and if the value is conservative, it will lead to high construction cost [2].

Since the damage occurring in the uplift pressure problem is mainly infiltration damage of the soil, the terminology used in the literature of previous studies on infiltration damage is more varied and has changed considerably over time. Terzaghi in doing sheet pile weir impermeable sand trough model experiments, the downstream sand surface is floated by the top of the water flow phenomenon called pipe surge, indicating the general term for the infiltration deformation of the foundation soil [3, 4]. USACE used internal stability to define the ability of the filter layer to resist separation of coarse and fine particles and the formation of tubular channels [5]. Subsequently, Kenney and Lau defined the internal stability of the soil as the ability of the granular material to resist the loss of fine particles due to a disturbance, which can be seepage or vibration [6]. Kezdi first used the term suffusion to define this phenomenon: when seepage flows through the pores of a medium, the water carries away the fine particles of the medium but does not destroy the structure of the medium [7]. Mansur let piping is defined as: in the dike base permeable layer, due to water pressure and local channel concentration seepage resulting in the bottom of the dike or the overlying soil layer of sand particles or other soil particles are gradually eroded phenomenon [8]. Moffat and Fannin gradually unified the understanding that when fine particles in cohesionless soils are lost in the pores of coarse particles without changing the total volume of the soil, the phenomenon is called “suffusion”; and as the number of lost fine particles increases, it causes a reduction in the soil skeleton, which is called “suffosion” [9, 10].

For the calculation of depressurized wells, the steps for solving the multi-well system given by S.Y. Wu are: superimposing the solution of a single well and making it conform to the boundary conditions, listing the equations for the coefficients to be determined, and then solving for the coefficients to be determined to obtain the solution of the multi-well system. Based on the above method, for wells under general double-layered foundations, S.Y. Wu gave approximate and rigorous solutions for single-row wells [11]. Mao Changxi, on the other hand, gave the solution for a cluster of pressurized wells with arbitrary arrangement [12]. If the well cluster is arranged along a closed boundary, the precipitation funnel in the area of the well cluster is nearly circular, the radius of influence of each well is nearly equal, and the flow rate of each individual well is approximately the same, a simplified calculation of the well cluster equivalent to a circular distribution can be obtained [13]. In the design of the pit precipitation, the water barrier of the water stop curtain was considered by Lingao Wu, who treated the water stop curtain as a complete water barrier boundary and solved the unsteady seepage process from the beginning of pumping to stabilization in the pit [14].

In this paper, for a sloping basement residential district basement construction project in Guangzhou, the drainage decompression method based on the combination of water stop curtain and pressure reducing well is adopted, and the regional multi-layer horizontal surface seepage finite element calculation is used to simulate the seepage field, the monitoring of basement floor uplift pressure is carried out during the construction period, the drainage decompression scheme is developed, The applicability of this method to the problem of excessive uplift pressure is analyzed, which will provide useful experience for solving engineering problems.

2 Project Overview

The project is located in the eastern part of Luogang District, Guangzhou, and the site is surrounded by mountains on three sides, with the elevation of the top of the mountains being about 80 ~ 100 m, and located in a valley, the elevation of its low point is about 34.5 m, which is basically in the south west - north east direction. Due to the mountains on three sides, abundant groundwater converges to the site, causing surface overflow and forming fish ponds and ditches. There are roads around the site, the elevation of the southeast road is 39 ~ 45 m, the elevation of the northeast road is about 37 m, and the elevation of the west road is about 38 ~ 45 m. Figure 1 shows a typical geological profile.

Fig. 1
An illustration of 3 vertical columns with different layers of soil in between. The layers of soil from the top are fill soil, silty soil, medium sand, sandy clayey soil, and fully weathered granite. The length of each layer varies between the columns.

Typical geological profile

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2.1 Basement Burial Depth

Several buildings are proposed to be built in the site area, mostly 12-storey high, with single-storey integral basement in the lower part, the area of basement is about 56,000 m2. the ± 0.00 elevation of the building is + 42.15 m. The elevation of basement floor is not uniform and there are leap levels, the main elevations are as follows: + 36.25 m (Zone A), + 33.65 (Zone B) and + 37.75 (Zone C), as shown in Fig. 2.

Fig. 2
An illustration of a closed irregular shape that has 3 zones, A to C. The elevations of the bottom of the subgrade are z equals plus 36.25, plus 33.65, and plus 37.75 meters respectively. The upper constant load is 33, 28, and 33 kilopascals. The load and tower weight are not accounted for.

Basement floor bottom elevation and upper constant load

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2.2 Hydrological Conditions

The groundwater types include upper layer stagnant water, pore water in the sand layer and fracture water in the rock layer: upper layer stagnant water is mainly stored in the fill, and the source of recharge is mainly atmospheric precipitation, which is obviously influenced by the season; diving is mainly stored in the sand layer, and the source of recharge is mainly atmospheric precipitation and fish pond recharge, which is obviously influenced by the season and locally has micro-pressure; fracture water in the rock layer is related to the fracture development of the bedrock and its connectivity. The fracture water in the rock layer is related to the fracture development of bedrock and its connectivity, and the main source of recharge is fish pond recharge, and the amount of recharge is obviously influenced by the degree of rock fragmentation and the degree of topographic relief.

2.3 Problems

The ground elevation, groundwater level, top elevation of the sand layer and bottom elevation of the sand layer in the analysis area were interpolated by the natural proximity method based on 140 geological drilling data holes in the site. From the interpolation results of the ground and water levels, the analysis of several typical sections shows that the permeable layer presents a pressurized nature and the basement is in the overburden layer, which does not penetrate deeply into the permeable layer and does not interfere with the hydraulic conductivity of the permeable layer. Comparing the burial depth of the basement and the water table, it can be seen that the basement is subjected to an average uplift pressure of 30 ~ 40 kPa, and the local foot of the hill is as high as 50 kPa. The water table was measured under the condition that it did not rain during the exploration period, and the problem of floating stability of the basement will be more prominent in the rainy season. Especially near the foot of the hill (south, southwest and northwest of the site) or at the lower basement floor, the basement is more threatened by the uplift pressure.

The preliminary analysis knows that the following two main problems should be solved: first, under unfavorable (heavy rainfall) conditions, seepage analysis of the site to derive the uplift pressure on the groundwater chamber; second, for the characteristics of the project, to propose effective, safe and economic drainage decompression measures.

3 Seepage Analysis

The seepage analysis is calculated by the regional multilayer horizontal surface seepage finite element method[15], and the calculation program is developed by Professor Cao Hong of South China University of Technology, which has been applied to many important water conservation projects, foundation excavation projects and urban groundwater environment research projects.

3.1 Calculation Area and Boundary Conditions

The boundary of the calculation area should be theoretically divided to the watershed. Since the road is opened at the side of the cell, the seepage water of the mountain ridge will overflow at the roadside during heavy rain, so the road is taken as the calculation boundary. The northeastern direction extends along the rill towards Nangang River, taking about 180 m, and the area of the calculation area is 130,000m2.

The sand layer is only exposed in the middle of the site, and the sand layer is missing at both sides of the site, i.e. at the foot of the hill. There is no obvious demarcation line between the area with sand layer disclosure and the area with sand layer disappearance, so a transition area is set between the two areas in the calculation.

Considering that the overflow will occur at the foot of the mountain when the rainfall is high, the calculation boundary of various working conditions is taken as the head boundary under the most unfavorable conditions when calculating with the finite element model, i.e. the head value is equal to the boundary elevation.

3.2 Calculate the Model Soil Layer

In order to properly reflect the interrelationship of seepage characteristics of each soil layer, the site soil layers are simplified when establishing the calculation model. In the missing sand layer, the sandy clayey soil is treated as a strong permeable layer, and the silt, clay and fill soil are treated as a water barrier. In the transition zone, the sand layer and sandy clayey soil are treated as strongly permeable layers, and the silt, clay and fill soil are treated as water barrier layers. In the area with sand layer exposure, because the permeability coefficient of sand layer is much larger than that of sandy clay, the sand layer is regarded as a strong permeable layer, and the sandy clay, silt, clay and fill are regarded as a water barrier. The bedding layer is made of gravel and sand with better permeability, which is a strong permeable layer.

From the finite element calculation model, it can be seen that the distribution of the strongly permeable layer is in the shape of an obvious washout, and the strongly permeable layer at the foot of the hill is thin and located in the shallow part of the stratum, while the sand layer in the middle of the site is unevenly distributed with a large variation in thickness. The maximum thickness of the strongly permeable layer is 12.4 m, the minimum value is 0.5 m, and the average thickness is 5.5 m.

3.3 Selection of Calculation Parameters

The permeability coefficient of the sand layer in the finite element model calculation was selected based on the results of the field pumping test; the permeability coefficients of other soil layers were selected based on engineering experience; the fine sand was exposed in only four holes in the site and was not considered in the calculation. The values of permeability coefficient of each soil layer are shown in Table 1.

Table 1 Calculated parameters
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3.4 Original Working Condition Seepage Field Analysis

After finite element calculation, the seepage field of the original landform of the site during the storm period and after the basement was completed were obtained. For the original geomorphology, the results of the distribution of equal head lines show that the groundwater seepage in the site is the seepage field of the pressurized aquifer, and the head is higher than the ground, and the head value is higher at the foot of the hill. The maximum head value of the proposed basement site is 40.5 m; there is a sand layer in the middle of the site to expose the area with a small hydraulic slope; the groundwater of the site flows from the foot of the hill in the southwest along the gully to the Nangang River in the northeast.

After the basement was completed, the basement played a role in suppressing the overflow, and the head congestion was about 1 m high; the head value of the basement floor bottom and the strong permeable layer was basically the same. The whole basement floor is subject to high head pressure, and the maximum head pressure is about 50 kPa in area B (see Fig. 2 for partition). In order to reflect the head distribution in the basement more visually. As shown in Fig. 3, four typical profiles were selected to analyze the results. the head values of all four profiles were higher than that of the basement floor. The profiles AB and CD are subjected to a maximum uplift pressure of about 50 kPa, and the head is higher at the foot of the hill as shown by the profile GH.

Fig. 3
An outline illustration of the basement floor area with 3 zones. 3 parallel diagonal lines A B, C D, and E F are over zone B from the right. Another diagonal line G H from the left, between zone A and intersects lines A B, C D, and E F.

Typical profile

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The comprehensive seepage field calculation results show that the bottom of the basement is subjected to a large uplift pressure, and the uplift pressure in most areas of the basement is greater than the upper load of 30 kPa, so drainage decompression measures must be taken. In the area without sand layer exposure, due to the small permeability coefficient, it is suitable to use anti-drawing piles. In the area of basement B, sand layer is exposed and the uplift pressure is up to 50 kPa, so it is not economical to use anti-drawing piles. It is recommended to use the combination of pressure reducing well and anti-drawing pile for anti-floating measures.

3.5 Recommended Scheme and its Seepage Field Analysis

In this project, the groundwater is mainly stored in the sand layer, and the head is controlled by the topography around the area, so it is not easy to drain by simple precipitation; the basement floor is above the sand layer, and the uplift pressure on the floor mainly depends on the head of the sand layer; the sand layer contains a lot of clay and powder particles, and the permeability is low, and it is missing at the foot of the hill, which is slightly unfavorable to the drainage and pressure reduction; the stratum at the floor is silt, silty soil or clayey soil, and the permeability is small. Based on the above characteristics, the drainage decompression scheme of "pressure reduction well + sand bedding" is recommended, as shown in Fig. 4.

Fig. 4
A layout of the basement area with 3 zones. Zone B a the center has 6 decompression wells along the edges, with a bedding layer thickness of 0.5 meters. The thickness of the bedding layer around the zone B is 0.15 meters. A table for decompression well coordinates has values for X and Y.

Layout of bedding and depressurization wells

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The decompression wells were arranged in the B area where the elevation of the basement floor is low. The radius of the well is 0.6 m, and the bottom of the well is 1 m into the residual soil, which is 1 m lower than the bottom elevation of the basement floor in this area. to enhance the decompression effect of the decompression well, a 0.5 m thick bedding layer is arranged at the bottom of the basement floor in area B. Area B extends 50 m to area A and C, and a 0.15 m thick sand bedding layer is arranged. the elevation of the floor in areas A, B and C are different, and the sand should be backfilled outside the intersection side wall of each area to make it connected, so that the water can flow to the decompression well in area B.

The "decompression well + sand bedding" scheme was calculated by finite element to obtain the iso-head line as Fig. 5, and the pumping flow rate of the decompression well was 60 m3 /d. The head values in the figure are equal to the bottom elevation of the base plate in each area, and the contour lines are shown by thick dashed lines, from this it is clear that: 1)The bottom elevation of the bottom plate in area B is + 33.65 m, the calculated head value is 34.5 ~ 33.8 m, the uplift pressure is 8.5 ~ 1.5 kPa, the average uplift pressure is 5 kPa; 2)The bottom elevation of A area is 36.25 m, the calculated head value is 41 ~ 35 m, the uplift pressure is 48 k ~ 0 kPa, except for the local uplift pressure at the foot of the west side of the hill is big, most of the area uplift pressure is less than 20 kPa, and the uplift pressure of a small half of the area is 0 kPa. 3)The elevation of the bottom slab of area C is 37.75 m, the calculated head value is between 41 ~ 35 m, the uplift pressure is 33 ~ 0 kPa, except for the uplift pressure is big at a very small part at the foot of the southwest side of the mountain, the uplift pressure is less than 13 kPa in most of the area, and the uplift pressure is 0 kPa in a large part of the area.

Fig. 5
A contour map of the area around the basement that has higher elevations around the basement.

Isohydraulic head line of strongly permeable layer after deployment of depressurization wells

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It can be obtained that "pressure reducing well + sand layer" can effectively solve the anti-floating problem of basement slab, so that the uplift pressure of basement slab is mostly 0 or much smaller than the overlying load, and there is no need to set additional anti-drawing piles or anti-drawing anchors, which can effectively solve the problem and also save cost and improve the construction efficiency.

4 Basement Floor Uplift Pressure Monitoring

For special reasons, the project could not implement drainage decompression measures in accordance with the recommended scheme in the previous section, and only raised the elevation of the basement slab to avoid uplift pressure damage. In order to ensure the safety of the project, the construction period to monitor the project basement floor uplift pressure.

Figure 6 shows the layout of 13 measurement points, half black circle marked as deep observation wells, full black circle for general observation wells. The water level is measured by the water level meter, which is converted into the head value of the basement floor, and then the uplift pressure of the basement floor is obtained.

Fig. 6
An outline illustration of the basement area with 3 zones. 3 parallel diagonal lines, L 1, L 2, and L 3, pass over the 3 zones connecting the dots for pressure measurement wells.

Plan layout of pressure measurement wells

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From the completion of the basement floor pouring observation, a total of 25 times for a year. The frequency is about once every half month, and the observation frequency will be increased in case of heavy rain.

From the monitoring results of the pressure change law of the orifice of the pressure measurement well with time, we know that the groundwater head pressure is obviously affected by the season, and the head value of the orifice is not high from February to June, while it is higher from August to November. In addition, according to the method of the above section, the finite element analysis of the actual construction conditions of the anti-floating seepage, the head calculation value of the observation points and the actual monitoring results are compared and analyzed as shown in Fig. 7, it can be seen that the predicted head values of L1 and L2 sections are accurate. L3 section part of the measurement points exceeded the predicted value, from the site situation, it may be due to the southeast of the site near the application of mixing pile caused by the water level rise.

Fig. 7
A dot plot of water level elevation at measurement point versus measurement point number. 3 clusters of dots for measured values increase vertically with 3 decreasing lines that connect calculated values in each cluster. The lines connect calculated points numbered 1 to 4, 8 to 12, and 16 to 20.

Comparison of calculated head values and measured values

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As a result, the results obtained from the simulations are more accurate and can correctly predict the head and uplift pressure values at the basement floor, but the actual situation will also be affected by the surrounding environment and construction conditions, so the real construction should be analyzed in specific problems.

5 Conclusion

Based on the actual project, the head and uplift pressure values at the basement floor were obtained through numerical simulation, compared with the actual monitoring data, and the site seepage conditions and seepage field were analyzed, and the following conclusions were obtained.

  1. (1)

    The uplift pressure checking calculation of the basement on the slope should analyze the seepage characteristics in detail according to the geological conditions of the site, and highlight its seepage characteristics through stratigraphic simplification. Using multi-layer horizontal surface finite element method for seepage field analysis, the results are effective and reliable.

  2. (2)

    "Pressure reduction well + sand layer" measures have been shown by finite element calculations to be effective in uplift pressure reduction, which can make the basement floor uplift pressure mostly zero or much smaller than the overlying load.

  3. (3)

    Monitoring results show that the calculation and analysis method to predict the head value results are more accurate, the specific analysis also need to consider the actual construction situation and the impact of the surrounding environment.