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Application of soil slope stability analysis in the safe construction of deep foundation pit

with the rapid development of social economy and engineering technology, high-rise buildings are increasing day by day, and the application of deep foundation is also widely used. The safety management in construction, especially the safety work in deep foundation pit construction, is also becoming more and more important. Several safety construction schemes with strong professionalism, such as scaffold, temporary power supply for construction, formwork, etc., can be quantified skillfully, such as scaffold stress calculation, distribution line load calculation, formwork bearing capacity calculation, etc. these quantitative treatments play an important role in safe construction. In the construction of deep foundation, such as the depth of foundation pit, the mechanical properties of foundation soil, the effect of external load on the slope top, etc., have a decisive impact on the safety of foundation pit. Quantitative technical treatment in foundation pit construction is also a key link for construction safety. The purpose of this paper, which ensures the high accuracy of the experimental force, is to combine the practical problems in ordinary projects, use the soil slope stability analysis theory, and introduce a simple foundation pit safety checking method, so that the foundation pit safety can be quantitatively analyzed safely and quickly, and the foundation pit safety construction scheme can be formulated and implemented reasonably

I. Analysis of common conditions of foundation pit in engineering

deep foundation pit sites generally carry out grading treatment of different sizes. Sliding of soil slope is a common failure form of foundation pit. Soil slope sliding means that the soil slope moves downward and outward along a sliding surface (generally a curved surface similar to an arc) within a certain range and loses its stability. Typical common forms of clay slope failure are shown in Figure 1. The following is a discussion on the safety of foundation pit according to several common situations in engineering practice

Figure 1 the sliding surface of cohesive soil slope

(I) the stress analysis of cohesionless soil slope

cohesionless soil slope is shown in Figure 2. Cohesiveness (c=0) does not exist between non cohesive (such as sandy soil and silty soil) soil particles, only friction T. as long as the slope does not slide, the soil slope will maintain its stability without damage. Set the slope angle as β， The weight of soil particles on it is mg, and the internal friction angle of sand is φ， Then the normal reaction force and tangential sliding force of soil particles on the slope surface are:

Fig. 2 failure stability analysis of cohesionless soil

n=mg cos β T=mg sin β

tangential friction along slope: t '=ntan φ= mg cos β tan φ

stability safety factor:

k=t ′/t=mg cos β tan φ/（mg sin β）

for cohesionless soil slope, when the slope angle β Equal to the internal friction angle of soil, i.e β=φ When t '=t, the stability safety factor k=1, and the soil slope is in the limit equilibrium state; When β＜φ (at this time, k> 1), it can be considered that the soil slope is safe. In order to ensure that the soil slope has sufficient safety reserves, k=1.1~1.5

(II) there are two methods for the safety check calculation of cohesive soil slope

cohesive soil slope: one is the slice method, and the other is the Taylor stable number table method. Due to the large workload of finding the center of the most dangerous sliding circle, the slice method is more cumbersome compared with the number table method. Here, the method of Taylor stabilizing the number table is only introduced as an example. Taylor's stable number table is shown in Table 1 and table 2

Table 1 taylor stability number table

Table 2 taylor stability number table (saturated soft clay)

1. For unsaturated soft clay, table 1: its abscissa represents the slope angle of soil slope β， The ordinate represents the stability number ns according to the internal friction angle of soil φ Values are different, and different [ns] curve values are used as the safety design value of the stability number. The actual stability number can be calculated by the following formula:

ns=rh/c

r- is the unit weight of the soil C - is the cohesion of the soil

h - is the height of the soil slope

when NS <[ns], it is considered that the calculated soil slope is safe; When ns=[ns], the checked soil slope is considered to be in a critical state; When ns ＞ [ns], it is considered that the checked soil slope will produce sliding failure. In other words, only when ns ＜ [ns], the checked soil slope is in a safe state. Similarly, in order to ensure that the soil slope has sufficient safety reserves, k=[ns]/ns=1.1~1.5 can be taken

the usage of taylor stability number table in soil slope stability checking calculation is diverse, and the above description is the known slope angle β、 Cohesion c and internal friction angle of soil layer φ、 The unit weight R, slope height h and rescue stability ns can also be applied to the following two situations according to the actual needs of the project: ① the slope angle is known β And soil layer C φ、 R and the corresponding [ns], find the stable slope height h; ② Known h, C φ、 R and the corresponding [ns], find the stable slope angle β。

2. for saturated soft clay soil, use table 2: its abscissa to represent the slope angle β， The ordinate represents the stable number ns. When slope angle β＞ At 53 °, the failure surface passes through the slope angle, and the [ns] value of the left half of the curve in the table is adopted; When slope angle β＜ At 53 °, the failure depends on the slope angle β And the depth coefficient Nd (the ratio of the distance from the hard soil surface to the top of the soil slope to the height of the soil slope). When the thickness of the soft soil layer is large (nd> 4), nd= ∞ can be taken, and it is known from table 2 that [ns]=5.52. At this time, the critical height of the soil slope:

hc=5.52cu/r

cu - the undrained (rapid shear) strength of the soil

(III) soil stratification of the soil slope

soil stratification of the soil slope is a natural phenomenon in the process of soil formation. When the depth of the foundation pit is less than the thickness of the soil layer, the impact of stratification is generally not considered (except for saturated soft clay). When the soil slope is composed of different types of soil layers within the height range, the safety and stability checking calculation of the soil slope must be carried out according to the depth of each layer and the mechanical property index of the layer

(IV) loading on the upper part of the soil slope

loading on the edge of the foundation pit is a common phenomenon in practical engineering, such as placing tower cranes on the top of the slope, stacking building materials, opening material transportation roads, etc. In such cases, it should be converted into equivalent soil weight according to the action of actual load, that is, the hypothetical soil weight should be used to replace the uniformly distributed load, and the thickness of equivalent soil layer is h=q/r (q is the concentration of uniformly distributed load, and R is the unit weight of soil layer constituting soil slope)

II. Project example

(I) project overview

Shandong Pingyin County People's Hospital Comprehensive outpatient building, the main building is a reinforced concrete frame structure, reinforced concrete raft foundation; One floor underground and eleven floors above the ground, with a building scale of 15214.9m2 and a height of 43.2m. The natural depth of the foundation pit of the outpatient building is 6.8m, and the water level height is 4.5m below the natural terrace. The rock and soil in the proposed site can be divided into four layers (including sub layers locally), and the overview of each layer is as follows: ① (qml4) miscellaneous fill; Variegated, slightly wet, loose ~ slightly dense, mostly silty soil, ranging from ash and broken bricks, unevenly distributed, with a thickness of 0.2~1.5m. ② (q4al+pl) loess like silty clay; Grayish brown, yellowish brown, hard plastic, mixed with a small amount of ginger stone, gravel and plant roots, with a small amount of pores, local ginger stone content of 10%~15%, particle size of 0.5~3cm. This layer is stably distributed, with a thickness of 1.05~3.6m. ③ (q3al+pl) clay: brownish red, hard plastic, locally plastic, containing a small amount of ferromanganese nodules, locally containing dolomitic limestone gravel, the content varies from 10% to 40%, individual content exceeds 50%, and the particle size is 0.5~3cm. This layer of soil is stably distributed, with a thickness of 0.1~4.5m. ④ (olz) dolomitic limestone: grayish white, grayish yellow, crystalline structure, medium thick layered structure, serious karst phenomenon in this layer, and local karst caves and fissures are developed. See Table 3 for the mechanical performance indexes of each layer related to the stability of soil slope

mechanical property index of foundation pit soil layer table 3

see table

(II) the problem is put forward that

one set is placed at the edge of the foundation pit by Shandong ×× QTZ63 tower crane produced by the group company has a weight of 30548kg, a maximum lifting weight of 6000kg, a counterweight of 12000kg, and its lower part is a reinforced concrete foundation (weight 5m × 5m × 1.3m × 2500kg/m=81250kg）。 The test calculates the safety and stability of the foundation pit

(III) safety check calculation

when installing the tower crane, the ① layer of soil has been occupied by the tower crane foundation, and the ④ layer of soil is rock, so there is no need to check calculation. Only the ② and ③ layers of soil can be checked for stability, as shown in Figure 3

slope angle β= Arctan (5.5/0.6) =83.78

total mass of tower crane including all attachments:

w=30548kg+6000kg+12000kg+81250kg=129798kg

equivalent soil layer thickness:

h=129798kg/[(5m × 5m） × 2710kg/m3]=1.93m

1. Checking calculation of soil stability of the second layer

equivalent warning of soil slope h=h+h1=1.92m+1.90m=3.82m

ns=rh/c= (2.71 × one thousand × 9.8N/m3 × 3.82M) ÷ 25000n/m2=4.06

according to the taylor stability table: [ns]=6.5

k=[ns]/n2=6.5/4.06=1.60

therefore, the soil of layer ② meets the requirements of safety and stability

2. Stability check calculation of layer ③ soil

the 1.3m thickness of the lower part of layer ③ soil is below the water level. This layer of soil should be considered as saturated soft clay, and the data in Table 2 should be used

equivalent height of soil slope h=6. The tensile testing machine must be worn and operated before use to ensure that there is no fault before loading and use h+h2=1.92m+1.9m+2.3m=6.12m

β= 83.78> 53

according to Taylor stable number table 2: [ns]=4.3

ns=rh/c=[e-commerce platform will form big data analysis in spot trading, supply chain finance, warehousing and logistics 2.74 × one thousand × 9.8N/m3 × (2.3+1.9+1.92) m] ÷ 45000n/m2=3.65

k=[ns]/ns=4.3/3.65=1.18

therefore, the ③ layer of soil also meets the requirements of safety and stability

3. conclusion: the foundation pit where the tower crane is installed meets the requirements of safety and stability; Furthermore, there is no tower crane at the top of the foundation pit or the additional load Q ＜ 129798kg ÷ (5m × 5m) =5191.92kg/m2, it also meets the safety and stability requirements of soil slope

III. several points that should be paid attention to in maintaining the safety and stability of soil slope

(1) the shear strength of the soil mass constituting the soil slope decreases. If the water content in the soil increases and the excess hydrostatic pressure increases, the soil will develop in the opposite direction of drainage consolidation, and the shear performance of the soil in line with the development trend of circular economy will decline, thereby reducing the stability of the soil slope. In practical engineering, pay attention not to be eroded by water within a certain range of the slope top, especially pay attention to the formation of water diversion conditions such as joints, holes and pits of the soil on the slope top. In addition, pay attention to the leakage of underground fire fighting, living and heating pipelines near the foundation pit. Therefore, in order to prevent surface water from eroding the soil slope, it is necessary to protect the slope surface and slope top, such as spraying or pouring fine aggregate concrete with a certain thickness on the surface as the closed protective layer of the soil slope

(2) soil slope bears external action. ① Static action. For example, stacking bricks, sand, stones, steel bars and other building materials or building (structures) (temporary facilities such as cement silo) on the top of the slope, so that the top of the slope can bear or increase the load, or the top of the slope is a road for vehicles, which is equivalent to increasing the thickness of the equivalent soil layer, so that [ns]/

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