Monitoring study on vertical bearing capacity of pile foundation in soft rock of lhasa human settlements

Abstract

In view of the weak vertical bearing capacity and low safety performance of the traditional soft rock pile foundation for human settlements, this paper proposes to monitor the vertical bearing capacity of the soft rock pile foundation for human settlements in Lhasa. The vertical mechanism of soft rock is analyzed, including the uniaxial compressive strength of rock around the pile, the relative displacement of pile rock and the interface condition of pile rock. The vertical bearing capacity of the pile foundation in soft rock is calculated by load transfer method. The side friction model and the pile end of the pile are established to determine the pile foundation. The calculation formula of the load settlement curve is obtained by using the side friction model and the pile end resistance model. The vertical bearing capacity of the pile foundation in soft rock is analyzed when the pile side rock is in the elastic stage and the surrounding rock is in the plastic stage. The test results show that friction pile plays an important role in the test pile under the working load. This method can effectively monitor the vertical bearing capacity of pile in the soft rock geological environment of human settlements, and can be applied to practical projects.

Keywords

Lhasa human settlements soft rock geological pile foundation vertical bearing capacity

1 Introduction

Human settlement environment is the place where human beings live together, the surface space closely related to human survival activities, the base on which human beings live in nature, and the main place where human beings use and transform nature. Human settlement environment focuses on the relationship between human and environment. It emphasizes the study of human settlement as a whole [3]. Its purpose is to understand and master the objective laws of the occurrence and development of human settlement, so as to better build a settlement environment in line with human ideal.

Lhasa is located on the North Bank of Lhasa, with a total area of nearly 30000 square kilometers and a urban area of 59 square kilometers. It is located in the river valley impact plain, with the terrain inclined from east to west. It is one of the cities with the highest elevation in the world. The climate belongs to the plateau temperate semi-arid monsoon climate. The annual sunshine time is more than 3000 hours, so it is called “sunshine city”. The annual precipitation is 200–510 mm, mainly from June to September. The frost free period of the whole year is 100–120 days [16].

The architectural form of Lhasa residential area is constantly changing, not only the construction technology and building materials, but also the traditional residential form has been greatly affected. Lhasa began to pay attention to the combination of greening, landscape and residential areas. Lhasa as a tourist city, in order to maintain the existing city style, the overall planning of the old urban area has made clear provisions [6]. The height of buildings shall be strictly controlled in the historical urban area, the traditional flat and open space form of the historical urban area shall be protected, and the visibility between important cultural relics and historic sites shall be maintained. In principle, the height above the ground of new buildings in the historical urban area shall not exceed 15 meters, and the height above the ground of new buildings within 30 meters on both sides of the red line of traditional streets and lanes shall not exceed 12 meters. With the continuous development and economic development of the new area, high-rise buildings begin to appear in the new area.

Pile foundation is one of the oldest building forms, which transfers the upper load of the building to the foundation soil or rock layer, and has certain rigidity and bending capacity. Pile foundation can bear more vertical and horizontal loads, adapt to the requirements of high, heavy and large buildings, and effectively resist earthquake disasters and wave impact [23]. Pile foundation can be used as the foundation of various engineering structures such as abutment, retaining wall, wharf and mechanical engineering by adopting different materials, interfaces and pile forming methods and supporting in different soil layers.

Pile foundation not only has a long history, but also is durable. In ancient China, many heavy and high-rise buildings built on soft foundation and historic bridges have successfully used pile foundation. In the long process of human using piles, the types and techniques of piles have been greatly developed and changed [19]. Since the mass production of cement and steel, concrete piles, reinforced concrete piles, prestressed concrete piles, steel piles and soon have gradually disappeared the dominant position of wood piles because of their good compressive performance, easy forming, low cost and other huge advantages. In particular, the bored pile is used to pour concrete into the pre formed borehole, avoiding the adverse factors such as noise, vibration and ground arching caused by pile driving. When the concrete is poured into the rock stratum, it combines with the surrounding rock mass to form a whole structure bearing the load, and then the rock socketed cast-in-place pile is formed. This pile type makes full use of the foundation resources and the material strength of the pile body, and saves the resources to the greatest extent. With the development of effective equipment [7] that can obtain large direct and ultra-long drilling holes in soil and rock, rock socketed cast-in-place pile has become the most commonly used foundation form in the field of foundation construction, and has achieved good economic and social benefits. The vertical bearing capacity of pile foundation refers to the maximum load that the pile foundation can bear without losing stability and excessive deformation under the external load. The vertical bearing capacity of pile foundation is an important index to evaluate its quality [11].

Many scholars have studied the load transfer mechanism of rock socketed cast-in-place pile. Liu Songyu of Southeast University, aiming at the vertical bearing test of the large-diameter muddy soft rock socketed pile adopted in the eastern part of our country, has given out the basic law of load transfer of the muddy soft rock socketed pile. The rock socketed pile in the area of argillaceous soft rock has the characteristics of general rock socketed pile, which mainly shows the characteristics of friction pile, and the resistance at the end of pile is generally small. Although it has effectively analyzed the characteristics of rock socketed piles in the muddy soft rock area, it has not been applied to the overall practice and the application effect is not clear. Tang Shengchuan et al. discussed the determination method of rock socketed pile bearing capacity based on the measured data of single pile bearing capacity in soft rock area. Based on the comprehensive consideration of the influence of the mutual coordination between the side resistance and the end resistance of the rock socketed section on the vertical bearing capacity provided by the rock socketed section, a new calculation method was proposed and verified. This method has complex steps in practical operation, easy to produce errors and bad application effect. Based on the design theory of rock socketed pile, Ding Cuihong and QianShikai studied the load transfer behavior of large-diameter cast-in-place rock socketed pile on the argillaceous soft rock in the east of China and reached the same conclusion as Liu Songyu. The total resistance of rock socketed section is mainly borne by side resistance. It is reasonable to deepen the rock socketed depth of rock socketed pile on soft rock [20]. The strength of pile body and foundation can be designed to be equal, so as to maximize the strength of pile body material and obtain the maximum economic benefit. This method is suitable for the residential buildings in the east of China, but it has some limitations in the western plateau area.

Taking the soft rock geological pile foundation of human settlements in Lhasa city of Tibet Autonomous Region as the main research object, this paper studies the monitoring of vertical bearing capacity of the soft rock geological pile foundation of human settlements in Lhasa, in order to provide basis for improving the construction quality of human settlements in Tibet. The main research route is as follows:

First of all, the vertical bearing mechanism of pile in soft rock is affected by three factors: uniaxial compressive strength of rock around pile, relative displacement of pile rock and interface condition of pile rock.

Secondly, the calculation method of the vertical bearing capacity of the soft rock pile foundation is obtained by constructing the friction model and the resistance model of the pile end.

Then, the characteristics of the rock mass on the pile side in the elastic stage and the rock around the pile in the plastic stage are analyzed, and the load settlement curve is calculated.

Then, experimental analysis is carried out to verify the effectiveness of the proposed method.

Finally, the conclusion and the future work direction are obtained through the experiment.

2 Materials and methods

2.1 Vertical bearing mechanism of pile foundation in soft rock

The bearing capacity of single pile under vertical load is composed of pile side resistance and pile end resistance, or it is borne by pile side friction and pile end resistance separately. The characteristics of the pile side friction and the pile end resistance of the soft rock geological pile foundation not only show the generality of the rock socketed pile, but also have its own properties. Therefore, the study of the vertical bearing mechanism of the soft rock geological pile bottom plays an important role in monitoring its vertical bearing capacity [12].

The total resistance at the end of the pile is smaller when the pile is embedded in the strongly weathered rock or in the fresh bedrock. According to the load test data collected from the rock socketed pile, the ratio of the side resistance of all the piles to the side friction of the pile to the bearing capacity of the pile is more than 60%, most of which is more than 80%. The soft weak rock socketed pile has the common characteristics of the rock socketed pile.

In fact, the exertion of the side friction of rock socketed pile is the downward transfer of the load along the pile body, and the process of the diffusion of the friction to the rock mass is constantly overcome. For the side friction of rock socketed pile, the measured and finite element analysis results show that the side resistance is not uniformly released, and the near surface (soft rock surface) is small, and increases rapidly with the depth [4]. When the depth ratio is 0.5 left to right, it increases to the maximum, and then decreases with the depth.

The exertion degree and distribution mode of pile side friction force of rock socketed pile change with the change of bedrock properties and other factors, which is a complex problem affected by many factors, mainly including:

2.1.1 Influence of uniaxial compressive strength of rock around pile

The strength of the rock around the pile is the most important factor affecting the side friction of the pile.

Based on nearly 66 representative rock socketed pile test data in China and abroad, the relationship between the measured data and the current calculation method is analyzed [10]. It can be seen that the control factors and influence degree that affect the exertion of the ultimate side friction resistance of rock socketed section. If the pile side friction is regarded as a function of the rock uniaxial compressive strength σ_c, then the magic resistance α coefficient is defined as α = τ_max/σ_c (τ_max is the limit value of pile side friction). Table 1 is the comparison table of the relationship between the side friction coefficient α and the rock uniaxial compressive strength σ_c of 66 test piles, as shown in Table 1.

Table 1
Statistics on pile side friction coefficient

Rock uniaxial compressive strength σ_c Rock hardness Measured average value of ultimate frictional resistance of pile side

σ_c ≤5 Mpa Extremely soft rock 0.18

5 Mpa< σ_c ≤15 MPa Soft rock 0.13

15 Mpa< σ_c ≤30 MPa Slightly soft rock 0.073

30 Mpa< σ_c ≤60 MPa Slightly hard rock 0.0321

Rock uniaxial compressive strength σ_c	Rock hardness	Measured average value of ultimate frictional resistance of pile side
σ_c ≤5 Mpa	Extremely soft rock	0.18
5 Mpa< σ_c ≤15 MPa	Soft rock	0.13
15 Mpa< σ_c ≤30 MPa	Slightly soft rock	0.073
30 Mpa< σ_c ≤60 MPa	Slightly hard rock	0.0321

It can be seen from Table 1 that with the increase of the silhouette degree of soft rock, the pile side friction coefficient is decreasing, that is to say, the exertion degree of the ultimate friction resistance of the rock socketed pile in soft rock is higher than that of the rock socketed pile in hard rock, which is also one of the unique statistical characteristics of the rock socketed pile in soft rock.

2.1.2 The influence of pile rock relative displacement

The trend of relative displacement between pile and rock is the premise of the exertion of pile side friction. At the beginning of loading, the relative displacement of pile rock is small or no displacement [8], which only shows the elastic compression of pile, and its stress-strain relationship is linear. The relative displacement required to reach the limit value of friction resistance is small, but it is not a fixed value and has no regularity to follow [13]. Table 2 shows the empirical value of relative displacement corresponding to the ultimate side friction of rock socketed section in different rock layers.

Table 2
Relative displacement of the rock-socketed section exerting the limit side resistance

Rock name Relative displacement/mm

Broken sandy clay rock 4

Broken fine sandstone 4

Complete fine sandstone 3

Complete limestone 2

Complete granite 2

Rock name	Relative displacement/mm
Broken sandy clay rock	4
Broken fine sandstone	4
Complete fine sandstone	3
Complete limestone	2
Complete granite	2

2.1.3 Influence of pile rock interface conditions

The characteristics of pile rock interface depend on the roughness of pile surface, the roughness of hole wall and the thickness of mud skin. In the case of relatively smooth interface between pile and rock, the friction resistance of pile rock interface is obviously brittle failure [1], that is to say, it is processing softening phenomenon, and the residual resistance is smaller than the peak resistance; while in the case of rough interface, the unit side resistance of rock socketed section increases with the increase of displacement until reaching the limit value, which is processing hardening phenomenon, and the yield process is relatively gentle.

In a word, the side friction of the pile foundation in soft rock geology has not only the general characteristics of the rock socketed pile in soft rock geology, but also its own unique statistical characteristics. Its exertion degree is related to the variables such as pile length, rock socketed depth, rock strength, construction method, mud property, etc.

2.2 Calculation method of vertical bearing capacity of pile foundation in soft rock based on load transfer method

Based on the vertical bearing mechanism of soft rock geological pile foundation, load transfer method is selected to calculate the vertical bearing capacity of soft rock geological pile foundation.

2.2.1 Pile side friction model

There are many pile side friction models. Elastic-plastic model with clear concept and convenient parameter selection is selected to represent the pile side friction model. Its mathematical expression is as follows: ${\begin{matrix} τ (z) = ks s < s_{f} \\ τ (z) = τ_{f} s ⩾ s_{f} \end{matrix}$ (1)

In the formula, τ (z) is the side friction of pile, k is the side resistance transfer coefficient of pile rock contact surface, s is the displacement of pile rock, s_f is the relative displacement of pile rock required for the full play of the side resistance of pile rock contact surface, and the relative ultimate side resistance is τ_f.

It can be determined according to the measured shear displacement curve of pile rock contact surface $k = τ (z) / s (z) = τ_{f} / s_{f}$ (2)

It can be seen from the above formula that if the ultimate friction resistance τ_f and the ultimate relative displacement s_f are not obtained in the test, the s_f can be obtained from the linear elastic part curve of the model, the ultimate friction resistance τ_f can be obtained from the theoretical calculation, and then the ultimate relative displacement s_f can be obtained. Therefore, the incomplete shear displacement curve can be supplemented completely.

2.2.2 Pile end resistance model

The model q_p - s_b of pile end resistance and pile bottom displacement can be used as the following three fold function, and its mathematical expression is as follows: ${\begin{matrix} q_{p} = k_{b 1} s_{b} s_{b} < ξ_{0} \\ q_{p} = k_{b 1} ξ_{0} + k_{b 2} (s_{b} - ξ_{0}) ξ_{0} ⩽ s_{b} < ξ_{f} \\ q_{p} = q_{pf} s_{b} ⩾ ξ_{0} \end{matrix}$ (3)

In the above model, q_p represents the resistance at the end of pile, s_b represents the displacement at the bottom of pile, k_b1 represents the influence of sediment at the bottom of pile, and k_b2 represents the influence of sediment at the bottom of pile is not considered. When the pile end displacement is less than ξ₀, the pile end resistance is all provided by sediment [2]. If the pile bottom is well emptied, ξ₀ = 0 can be made, then: ${\begin{matrix} q_{p} = k_{b 2} s_{b} s_{b} < ξ_{f} \\ q_{p} = q_{pf} s_{b} ⩾ ξ_{f} \end{matrix}$ (4)

In the above model, there are two important parameters, q_pf, ξ_f and k_b2, among which there are two independent parameters. The value of q_pf for soft rock has always been a research hotspot, which is the same as the pile side friction resistance. If q_pf is not obtained in the test, the ultimate relative displacement ξ_f can still be obtained through the elastic part curve of the model line, the ultimate friction resistance q_pf can be obtained according to the theory, and the pole can be obtained [14]. The incomplete model curve of pile end resistance can be completed by limiting the relative displacement ξ_f.

2.3 Calculation formula of load settlement curve

Taking the load transfer function of pile side into the differential control equation $\frac{d^{2} s (z)}{{dz}^{2}} - \frac{U}{E_{p} A_{p}} τ (z) = 0$ , we can get: $\begin{matrix} \frac{d^{2} s (z)}{{dz}^{2}} - \frac{U}{E_{p} A_{p}} ks = 0 s < s_{f} \\ \frac{d^{2} s (z)}{{dz}^{2}} - \frac{U}{E_{p} A_{p}} τ_{r} = 0 s ⩾ s_{f} \end{matrix}$ (5)

In Equation (5), s (z) is the displacement of pile body, U is the perimeter of pile body, A_p is the section area of pile body, E_p is the elastic modulus of soft rock, RR is the resistance of pile side.

In the formula, the characteristic equation of the first equation is: $λ^{2} - \frac{Uk}{E_{p} A_{p}} = 0$ (6)

In Equation (6), λ is the characteristic coefficient, k is the shear rigidity between piles and soil, also known as the side friction transfer coefficient.

It has two different characteristic roots: $λ = \pm \sqrt{\frac{Uk}{E_{p} A_{p}}}$ (7)

So the general solution of the first equation is: $s (z) = c_{1} e^{λ z} + c_{2} e^{- λ z}$ (8)

Due to the existence of hyperbolic functions: $sinh (z) = \frac{e^{z} - e^{- z}}{2}$ (9) $cosh (z) = \frac{e^{z} + e^{- z}}{2}$ (10)

Therefore, Equation (8) can be further converted into: $\begin{matrix} s (z) = (c_{1} + c_{2}) sinh (λ z) \\ + (c_{1} - c_{2}) cosh (λ z) \end{matrix}$ (11)

The undetermined coefficients c₁ and c₂ in the formula can pass through the boundary conditions, so they can be expressed as follows: $s (z) = c_{1} sinh (λ z) + c_{2} cosh (λ z)$ (12)

Directly integrate the above formula twice to get: $s (z) = \frac{1}{2} \frac{U τ_{r}}{E_{p} A} z^{2} + c_{3} z + c_{4}$ (13)

According to the different load conditions of each construction stage, the settlement value corresponding to each load can be derived [5], so as to draw the vertical load displacement curve of pile foundation, and realize the calculation of the vertical bearing capacity of rock socketed pile controlled by the settlement of pile top.

2.3.1 Rock mass at pile side is in elastic stage

Set the settlement of pile top as s₀, the number of soil or rock layers around the pile as n, and the length of pile body as l. The boundary and displacement continuity conditions are as follows: ${\begin{matrix} S_{1} |_{z = 0} = S_{0} \\ S_{i} |_{z = l_{i}} = S_{i + 1} |_{z = l_{i}} i = 1, 2, ..., n - 1 \\ \frac{d S_{i}}{d z} |_{z = l_{i}} = \frac{d S_{i + 1}}{d z} |_{z = l_{i}} i = 1, 2, ..., n - 1 \\ E_{p} A_{p} \frac{d S_{n}}{d z} = - k_{b 1 S_{n}} |_{z = l_{n}} \end{matrix}$ (14)

According to the boundary and displacement continuity conditions, the layered solution of the above formula is as follows: $s_{i} = c_{1 i} sinh (λ_{i} z) + c_{2 i} cosh (λ_{i} z) i = 1, 2, \dots, n$ (15)

In the above formula: ${\begin{matrix} c_{21} = s_{0} \\ \begin{matrix} c_{1 i} sinh (λ_{i} l_{i}) + c_{2 i} cosh (λ_{i} l_{i}) = \\ c_{1 (i + 1)} sinh (λ_{i + 1} l_{i}) + c_{2 (i + 1)} cosh (λ_{i + 1} l_{i}) i = 1, 2, \dots, n - 1 \end{matrix} \\ \begin{matrix} λ_{1} [c_{1 i} cosh (λ_{i} l_{i}) + c_{2 i} sinh (b_{i} l_{i})] = \\ λ_{i + 1} [c_{1 (i + 1)} cosh (λ_{i + 1} l_{i}) + c_{2 (i + 1)} sinh (λ_{i + 1} l_{i})] i = 1, 2, \dots, n - 1 \end{matrix} \\ \begin{matrix} c_{1 n} cosh (λ_{n} l_{n}) + c_{2 n} sinh (λ_{n} l_{n}) = \\ - β_{1} [c_{1 n} sinh (λ_{n} l_{n}) + c_{2 n} cosh (λ_{n} l_{n})] \end{matrix} \end{matrix}$ (16)

Where. $λ_{i} = \sqrt{\frac{{Uk}_{si}}{EA}}$ , $β_{i} = \frac{k_{bi}}{EA λ_{n}}$ , i = 1, 2.

There are 2n unknowns and 2n simultaneous linear equations in the above formulas. The coefficients c_1i and c_2i can be obtained by solving them. The displacement s_i (z) of each weak stratum can be obtained by introducing this into formula (15). The load Q on the top of the pile is as follows: $Q = Q (0) = - EA \frac{{ds}_{1}}{dz} | z = 0 = - EA λ_{1} c_{1 i}$ (17)

2.3.2 All the rocks around the pile are in the plastic stage

When all around the pile enters the plastic stage, it shows that the pile side friction reaches the limit value. According to the pile end resistance model, the pile end is in the stage of sediment bearing, bedrock bearing linearity and bedrock bearing plasticity. When the resistance at the pile end is in the plastic bearing stage of bedrock, the displacement at the pile top is already very large [21], and there will be obvious inflection point on the load displacement curve, that is, the pile foundation has reached the ultimate bearing capacity, the load corresponding to the inflection point is the ultimate bearing capacity, and its formula is as follows: $Q_{u} = \sum_{i = 1}^{n} τ_{Γ i} l_{i} + [k_{b 1} ξ_{0} + k_{b 2} (ξ_{f} - ξ_{0})]$ (18)

In Equation (18), k_b1 and k_b2 respectively represent the end resistance transfer coefficient of sediment compression and rock elastic compression stage, and ξ₀ and ξ_f respectively represent the boundary displacement between the initial and end points of elastic compression of pile end.

Through the above, the load transfer function of pile end and pile circumference in various stress states can be obtained [15, 17]. The load Q of pile top can be calculated directly according to the settlement ofpile top. Through a series of corresponding points of Q and s, the Q - s curve can be drawn to monitor the vertical bearing capacity of pile foundation in soft rock geology.

3 Results

In order to effectively monitor the monitoring effectiveness of the vertical bearing capacity of the pile foundation in weak rock geology, this paper selects the staff residence of the labor and Social Security Department of Tibet Autonomous Region as the research object. The staff residence of the department of labor and social security of Tibet Autonomous Region is located in the courtyard of the department of labor and social security on the south side of Beijing West Road and the west side of Luding South Road in Lhasa city. The construction area of the project is 3810 m², the building height is 28.95 m, the main structure on the ground is 9 floors, the indoor and outdoor height difference of the building is 0.7 m, the structure adopts frame structure, the equivalent absolute elevation of±0.0 is 3650.9 m, and the foundation buried depth is 3.0 m. The safety level of the engineering building structure is level II, the design service life is 50 years, the seismic fortification category of the building is level C, the design level of the foundation is level C, and the seismic level of the frame is level II. The seismic fortification intensity of the site is 8 degrees.

The survey data show that the geomorphic unit of the site belongs to the first terrace of Lhasa River, and the site is located in the temperate semi-arid climate area of the South Tibet Plateau, with the maximum frozen soil depth of 26 cm. The soil layer of the site is mainly quaternary alluvium, and the distribution of each main soil layer is shown in Table 3.

Table 3
Basic conditions of the construction site soil layer

Soil name Soil thickness/m Soil description Bearing capacity characteristic value/kPa Modulus of deformation/Mpa Compression modulus/Mpa

Miscellaneous fill 0.7–1.1 Composed of medium sand, gravel, sand, etc.

Sandy soil 0.2–1.0 Can be transformed into fine sand or silt 105 5

Sand 0.5 Loose, saturated, can change into sandy silt due to changes in sand content 95 3

Boulder 0.4–3.0 Physical and mechanical properties vary widely, being a weak interlayer 65 6

Pebble Can be divided into two layers: precision pebble, medium and dense pebble 295 17–31

Soil name	Soil thickness/m	Soil description	Bearing capacity characteristic value/kPa	Modulus of deformation/Mpa	Compression modulus/Mpa
Miscellaneous fill	0.7–1.1	Composed of medium sand, gravel, sand, etc.
Sandy soil	0.2–1.0	Can be transformed into fine sand or silt	105		5
Sand	0.5	Loose, saturated, can change into sandy silt due to changes in sand content	95		3
Boulder	0.4–3.0	Physical and mechanical properties vary widely, being a weak interlayer	65	6
Pebble		Can be divided into two layers: precision pebble, medium and dense pebble	295	17–31

The groundwater of the site is buried in the pore phreatic water in the Quaternary pebble layer, which is mainly supplied by atmospheric precipitation, snowmelt and groundwater runoff in the upstream, and flows from northeast to southwest. During the survey, the static groundwater level elevation of the site is 3647.83–3647.97 m. January to April is the dry season, and June to September is the wet season. The Quaternary pebble layer has strong water permeability and is the main aquifer of the site. The ground water of the site is not corrosive to the concrete. From the depth of the foundation to the compression layer, the foundation soil is mainly composed of pebble layer. Pebble layer is a good bearing layer for natural foundation because of its large thickness, high bearing capacity and low compressibility, meeting the load requirements. However, there are pebble layers in the site. Some survey points have large thickness of pebble layer, shallow burial at the top, extremely loose and low bearing capacity, which has a great impact on the integrity of pebble layer.

After the main soil samples are collected, the indoor test shall be carried out. In the natural state, the vertical bearing capacity of the uniaxial ultimate compressive strength test monitor shall be carried out for the soft rock type, and the cohesionless soil (such as fine sand, sand pebble and sand gravel) in the overburden shall be tested, combined with the actual situation of the test site.

The design of the test pile adopts the scheme of 18.00 m long pile and 1.0 m diameter. The upper part of the test pile adopts the full length configuration of 16φ23 longitudinal reinforcement, and the stirrup of the spiral reinforcement is φ12@15. Along the pile body, a φ20 stiffening hoop is added at a spacing of about 2 m. After the reinforcement cage is made, the diameter of the reinforcement cage is measured to be about 80 cm. The reinforcement drawing of the pile body section is shown in Fig. 1.

Fig. 1

Pile section reinforcement diagram.

The pile end resistance can better reflect the resistance of pile end rock to deformation. In order to verify the effectiveness of the proposed method, the relationship between the displacement of pile end and the settlement of pile end is compared. The measured relationship between pile end settlement and pile end load is shown in Fig. 2.

Fig. 2

Relationship between pile end resistance and displacement.

From the analysis of Fig. 2, it can be seen that the displacement of pile end in this method is directly proportional to the settlement of pile end. With the increasing of the displacement at the end of pile, the settlement at the end of pile increases gradually. However, there is no complete rule between the displacement at the end of pile and the settlement at the end of pile in the traditional method, which has certain randomness. The experimental results show that the method in this paper has strong resistance to deformation.

In order to verify the effectiveness of this method, the experiment calculates the pile end load and the pile shaft friction through this method and the traditional method, and obtains the different proportion of the pile top load borne by the rock socketed section friction and the pile side soil resistance. The results of the load sharing ratio are shown in Fig. 3.

It can be seen from the test results in Fig. 3 that the soil resistance on the side of pile and the friction resistance of rock socketed section are the key parts of rock socketed pile. When the load level increases, the soil resistance of pile side will change correspondingly. In this paper, the pile side soil resistance of the method is lower than that of the traditional method, the highest is about 69%, and that of the traditional method is up to 82%. However, the friction of rock socketed segment increases slowly and presents a downward trend. The test results further prove that the pile side friction bears most of the working load, while the end resistance only accounts for a small proportion. It can be seen from the test results that the performance of this method is better under the working load.

Fig. 3

Load sharing ratio results.

When the rock socketed ratio calculated by this method is 6, the limit value of pile side friction and the ratio of pile end resistance to rock mass strength are shown in Table 4.

Table 4

Side resistance and end resistance coefficient calculation table

Project	Measured friction resistance limit	Ultimate side friction resistance coefficient	Measured end resistance limit	Limit end resistance coefficient	Experimental result
Natural state	436	0.08	2489	0.33	14500
Secondary water saturation	369	0.12	1509	0.44	10300
Four times full of water	233	0.15	1439	0.98	9100
Average Value		0.12		0.55
Standard deviation		0.024
Standard Value		0.06

It can be seen from the experimental results that the difference between the ultimate friction of single pile obtained from the uniaxial compressive strength of rock mass and the ultimate friction of the prototype pile calculated from the model test results is about 2–5 times. The bearing capacity of the pile is equivalent to the standard value of the ultimate bearing capacity of the pile calculated by the rock mass parameters under the natural conditions.

The monitoring results of the vertical bearing capacity of the pile foundation in soft rock geology of the residential environment obtained by this method are shown in Table 5.

Table 5

Calculation results of vertical bearing capacity of single pile under different rock parameters

Category	Pile end load/Mpa	Calculate the side friction/KN	Measured side friction/KN	Calculating end resistance/KN	Calculate the standard value of vertical bearing capacity of single pile/KN	Standard value of pile concrete bearing capacity/KN
Rock block	19.9	26154.13		17165.73	44331.1	21610.03
	19.9	26154.13		17165.73	44331.1	21610.03
	17.3	22501.65		14624.05	38226.8	21610.03
	2.1	2873.25		1878.45	4862.65	21610.03
Rock mass	5.02	9167.21	10700	5687.85	14066.2	14500
	2.81	5152.03	9500	3178.35	8441.58	10300
	1.43	3036.03	5500	1855.89	5002.81	9100
	0.21	386.89	1100	247.57	635.48	3100
Geological survey recommended value	Strong weathering	5314.81		3289.81	8706.01	Physical measured ultimate bearing capacity
	Stroke	8932.1		8932.15	17075.4
Natural exploration	15.55	21487.85		14058.76	36553.8
Geological survey saturation	10.73	13693.13		9789.72	23406.1

It can be seen from the calculation results that the method in this paper can effectively monitor the vertical bearing capacity of pile foundation in soft rock geology of human settlements. Different rock uniaxial compressive strength values lead to different results. The ultimate bearing capacity of single pile obtained by saturated uniaxial compressive strength of rock block under different working conditions is far greater than the standard value of concrete compressive strength of pile body. The calculation results are in accordance with the code for design of building foundation and foundation. The results are the same and the uniaxial compressive strength of rock has a significant influence on the calculation results.

4 Discussions

Pile foundation is the foundation form of bearing superstructure with pile. In China, pile foundation monitoring has a history of more than 30 years from introduction and exploration to in-depth research and promotion. In addition to reinforced concrete piles, a series of pile systems have been developed, such as steel pile series, cement pile series, special pile series (ultra-high strength, ultra-large diameter, variable section, etc.), sand pile, lime soil pile and lime pile of natural materials. With the development of high-rise buildings, the upper main building is getting higher and higher [9], and the buried depth of underground foundation is getting larger and larger. Pile foundation has the advantages of high bearing capacity, good integrity, seismic performance and small deformation, which is widely used. The bearing capacity of pile foundation directly affects the stability and safety of building structure. Pile foundation monitoring is an important means to verify the construction quality of pile foundation. Objective and accurate data of pile foundation is an important basis for engineering quality evaluation.

The main factors affecting the monitoring of vertical bearing capacity of pile foundation in soft rock are as follows:

4.1 Personnel factor

The professional level, practical experience, sense of responsibility, understanding ability and mastery degree of monitoring standards of monitoring personnel directly affect the scientificalness and representativeness of monitoring data. Therefore, the monitoring personnel must be trained to understand and master the monitoring technology and monitoring standards, be familiar with the use conditions and connection methods of instruments and equipment, and understand the impact of the setting of the number on the whole monitoring process and monitoring results. We should not only understand the work to be done in each step, but also understand the problems that should be paid attention to in doing a lot of work, realize the importance of monitoring work from the ideological point of view, improve the responsibility and monitoring technology level of the monitoring personnel, and provide reliable guarantee for improving the monitoring quality.

4.2 Selection of equipment and instruments

The instrument and equipment must be used within the valid verification period.

Although modern technology makes the measuring instruments of various monitoring instruments more precise and the monitoring process can be controlled automatically, the damage of measuring instruments or the change of measuring parameters caused by environmental factors and human factors lead to inaccurate test results. Therefore, the measuring instruments used for monitoring must be sent to the legal measurement unit for regular verification, and must be within the validity period of measurement verification when used.

The jack and load sensor or oil pressure sensor of corresponding range shall be configured according to the bearing capacity of the site monitoring. The measurement error of the sensor shall not be greater than 1%, and the accuracy of the pressure gauge shall be better than or equal to 0.4 level. Displacement sensor or large range dial indicator should be used for settlement measurement.

The number of jacks and displacement test instruments shall be configured. One or more jacks of the same model and specification shall be configured as required. When the diameter or side width of the inspected pile is greater than 500 mm, four displacement measuring instruments shall be symmetrically arranged in two directions. When the diameter or side width is less than or equal to 500 mm, two displacement measuring instruments can be symmetrically arranged. The settlement measuring plane should be below 200 mm of the pile top, and the measuring point shall be firmly fixed on the pile body.

4.3 Determination of monitoring time

The bearing capacity monitoring shall meet the double requirements of the rest time of foundation soil and the age or design strength of pile body concrete. The start time of monitoring the vertical compressive bearing capacity of single pile shall not be less than 7 days for sandy soil foundation, 10 days for silty soil foundation, and 10 days for unsaturated cohesive soil, except when the concrete age of the tested pile reaches 28 days or the strength of the reserved curing test block reaches the design strength [18]. When there is no experience in mature areas, the rest time shall not be less than 15 days, 25 days for saturated cohesive soil, and the rest time shall be appropriately extended for slurry wall protection cast-in-place pile.

4.4 Selection and requirements of reaction device

According to the comprehensive consideration of site conditions, safety and investment benefits, the loading reaction device can be selected as an economic and applicable reaction device. The common reaction devices include the reaction device of the cross beam of the tracing pile, the reaction device of the weight platform, the combined reaction device of the anchor pile and the weight, and the reaction device of the ground anchor. No matter what reaction device is adopted, the following requirements shall be met: the reaction forces provided by reaction device shall not be less than 1.2 times of the maximum load. before loading, check the strength and deformation of all components of the loading reaction device. when the anchor pile is used to provide the reaction force, the anti-pulling force of the anchor pile (foundation soil, anti-pulling reinforcement, pile joint) shall be checked; when the engineering pile is used as the anchor pile, the number of anchor piles shall not be less than 4, and the pull-up amount of the anchor pile shall be monitored; when the counterweight platform is used to provide reaction force, the weight should be fully added before monitoring and placed on the platform evenly and firmly; the pressure exerted on the foundation by the platform should not be greater than 1.5 times of the characteristic value of the foundation bearing capacity. During the test, the bottom area of the platform can be calculated according to the size of the foundation bearing capacity and the amount of the pressure weight, so as to meet the test needs; when conditions permit, it is best to use the engineering pile as the fulcrum of the pile load.

In view of the above factors affecting the monitoring results of the vertical bearing capacity of the pile foundation in soft rock, the following measures should be taken to improve the vertical bearing capacity of the pile foundation in soft rock:

4.4.1 In order to ensure the goal of the total construction period, it is necessary to implement subsection control and dynamic control

During the implementation of the project, the schedule shall be revised and adjusted in time according to the actual situation after the change without affecting the overall schedule. Material supply and payment of progress payment shall be timely [22] to ensure project quality.

4.4.2 Strictly control the quality of materials.

The materials shall meet the national standards (including environmental protection standards) and design requirements, and the material acceptance system shall be strictly implemented. To ensure the quality of the main structure: the quality of the main structure is related to the quality and safety of the whole project and the life safety of each employee. Therefore, it is necessary to ensure the quality of the main structure. Pay attention to decoration quality: in the stage of construction decoration, in fact, we should overcome common quality problems, do a good job in detail treatment, be superior in decoration level, and have new innovation and new technology. Project completion acceptance is the last procedure of construction project and the final inspection summary of project design and quality. After the completion of the project, seven working days before the acceptance, the construction unit shall notify the project supervision engineer to carry out quality inspection. During the acceptance, the project acceptance standard issued by the State shall be strictly implemented, and the acceptance evaluation shall be carried out item by item. Specifically, the methods of engineering technical documents inspection and physical engineering quality inspection can be adopted. For the supervision of project completion acceptance, the construction unit must submit the time, place and list of personnel organized by the acceptance institution to the project quality supervision institution 7 working days before the acceptance. After the project is accepted, the construction unit shall go through the filing formalities with the filing department with the completion acceptance report.

5 Conclusions

The vertical bearing capacity of building pile foundation is an important index to evaluate its safety performance. This paper studies the monitoring of the vertical bearing capacity of the weak rock geological pile foundation in Lhasa residential environment. Taking Lhasa in Tibet Autonomous Region as the research object, after analyzing the vertical bearing mechanism of the weak rock geological pile foundation in the residential environment, the vertical bearing capacity of the weak rock geological pile foundation is calculated by the load transfer method to realize the weak human settlements environment, and it can effectively monitor the vertical bearing capacity of pile foundation in weak rock of Lhasa human settlements. The method can be applied to the monitoring of the vertical bearing capacity of the pile foundation in the weak rock of the actual living environment.

Although the method has achieved some results at this stage, there are still many deficiencies. The future research work will focus on the following work:

In the experiment, the environment should consider the change of natural environment and experimental data caused by the change of different seasons.

In the vertical direction of the monitoring, the angle should choose a variety of, and then analyze the accuracy of detection.

Footnotes

Acknowledgments

The research is supported by Study on the Vicissitudes of Human Settlements and Environment in Tibetan Cities since the Middle of the 20th Century (National Social Science Fund major bidding projects, Code: 16ZDA135).

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Monitoring study on vertical bearing capacity of pile foundation in soft rock of lhasa human settlements

Abstract

Keywords

1 Introduction

2 Materials and methods

2.1 Vertical bearing mechanism of pile foundation in soft rock

2.1.1 Influence of uniaxial compressive strength of rock around pile

Table 2 Relative displacement of the rock-socketed section exerting the limit side resistance Rock name Relative displacement/mm Broken sandy clay rock 4 Broken fine sandstone 4 Complete fine sandstone 3 Complete limestone 2 Complete granite 2

2.2 Calculation method of vertical bearing capacity of pile foundation in soft rock based on load transfer method

2.2.1 Pile side friction model

4.1 Personnel factor

4.2 Selection of equipment and instruments

4.3 Determination of monitoring time

4.4 Selection and requirements of reaction device

4.4.1 In order to ensure the goal of the total construction period, it is necessary to implement subsection control and dynamic control

4.4.2 Strictly control the quality of materials.

5 Conclusions

Footnotes

Acknowledgments

References

Table 2
Relative displacement of the rock-socketed section exerting the limit side resistance

Rock name Relative displacement/mm

Broken sandy clay rock 4

Broken fine sandstone 4

Complete fine sandstone 3

Complete limestone 2

Complete granite 2