A method for determining the thickness of a foundation treatment for replacing an underground water supply and drainage pipeline

Abstract

The application discloses a kind of underground water supply and drainage pipeline replacement cushion layer thickness determination ground treatment methods, S1, determine the pipeline foundation type is concrete foundation or sandstone foundation;S2, calculate the ground bearing capacity required;S3, check whether the uncorrected pipe bottom soil layer ground bearing capacity meets the design requirement, if meet, no need to carry out replacement cushion layer ground treatment;If not satisfied, replacement cushion layer ground treatment is carried out, and the replacement cushion layer thickness is calculated according to the following formula;The application improves the accuracy and speed of replacement thickness calculation, improves work efficiency, reduces unnecessary waste, and the application proposes the determination of ground bearing capacity required and the ground treatment method of replacement cushion layer thickness for different pipe materials, pipe diameters and soil thicknesses, determines the ground bearing capacity required according to different pipe materials, pipe diameters and soil thicknesses, and when there is soft soil layer at the bottom of the pipe, the ground treatment method of replacement cushion layer is proposed, which can meet the requirement of ground bearing capacity and ensure the safe operation of the pipeline.

Description

Technical Field

[0001] This invention belongs to the field of foundation treatment technology for pipelines in municipal water supply and drainage, water conservancy and water diversion projects, and particularly relates to a foundation treatment method for determining the thickness of the replacement layer for underground water supply and drainage pipelines. Background Technology

[0002] Underground water supply and drainage pipelines are affected by geological conditions and surrounding external environmental loads, causing additional pressure and horizontal thrust, resulting in deformation and displacement. This leads to pipeline defects such as cracking, misalignment, and leaks, which not only interrupt water supply and drainage functions but also affect surface traffic, surrounding buildings, and the safety of other underground pipelines. Therefore, studying the surrounding external loads of urban underground water supply and drainage pipelines, determining their foundation bearing capacity requirements, and conducting reasonable foundation design is a very necessary and important task.

[0003] When designing underground water supply and drainage pipelines, different requirements are typically set for the bearing capacity of the soil layer beneath the pipe material. For example, the bearing capacity of the soil layer beneath concrete pipes is not less than 100 kPa, ductile iron pipes not less than 80 kPa, and plastic pipes not less than 60 kPa. Some requirements also specify a bearing capacity of not less than 100 kPa for pipes with a diameter less than 1000 mm and not less than 120 kPa for pipes with a diameter greater than 1000 mm. When the bearing capacity of the soil layer beneath the pipe does not meet these requirements, foundation treatment is necessary. A common method is the replacement layer method. Whether the bearing capacity requirement varies depending on the pipe diameter and material is unclear. A review of relevant patents and literature does not provide sufficient evidence or discussion regarding different minimum bearing capacity requirements for different pipe materials, nor does it provide sufficient calculation basis for whether the bearing capacity requirement differs for different pipe diameters. Although patent CN110904759A proposes a method for calculating the thickness of the pipe bottom replacement layer, it takes the bearing capacity of the pipe bottom as the research object and does not consider the influence of pipe material, pipe diameter, thickness of soil cover on the pipe top, and ground vehicle load on the foundation bearing capacity required by the pipeline, and ignores the difference between water unit weight and soil unit weight.

[0004] It is evident that there is currently a lack of sufficient research on the differences in the required foundation bearing capacity for different pipe materials and diameters, as well as the determination of the replacement thickness when using replacement cushion layers for foundation treatment. Summary of the Invention

[0005] The purpose of this invention is to provide a foundation treatment method for determining the thickness of the replacement layer for underground water supply and drainage pipelines, which can improve the accuracy and speed of calculating the thickness of the replacement layer and increase work efficiency.

[0006] The objective of this invention is achieved through the following technical solution: a foundation treatment method for determining the thickness of the replacement layer for underground water supply and drainage pipelines, characterized by comprising the following steps:

[0007] S1. Determine the type of pipe foundation, whether it is a concrete foundation or a gravel foundation, based on the pipe material and interface type.

[0008] S2. Calculate the required foundation bearing capacity f: If the pipe foundation type is a gravel foundation, the required foundation bearing capacity is the sum of the vertical pressure of the ground live load acting on the pipe, the weight of the soil covering the pipe, the weight of the water inside the pipe, and the self-weight of the pipe; If the pipe foundation type is a concrete foundation, the required foundation bearing capacity is the sum of the vertical pressure of the ground live load acting on the pipe, the weight of the soil covering the pipe, the weight of the water inside the pipe, the self-weight of the pipe, and the self-weight of the concrete foundation.

[0009] S3. Verify whether the bearing capacity of the uncorrected bottom soil layer meets the design requirements. If it does, no foundation treatment with a replacement layer is required. If it does not meet the requirements, foundation treatment with a replacement layer is required. Calculate the thickness z of the replacement layer according to the following formula.

[0010] (1) When the required foundation bearing capacity f is less than or equal to 80 kPa,

[0011] The pipeline foundation is a concrete foundation, and the thickness z of the replacement layer is:

[0012]

[0013] The pipeline foundation type is a sand and gravel foundation, and the thickness z of the replacement layer is:

[0014]

[0015] (2) When the required foundation bearing capacity f is greater than 80 kPa, the pipeline foundation type is a concrete foundation.

[0016] When the soil cover thickness H above the pipe is 0.7m ≤ H < 3m, the thickness z of the replacement layer is:

[0017]

[0018] When the soil cover thickness H above the pipe is H≥3m, the thickness z of the replacement layer is:

[0019]

[0020] The pipe foundation type is a gravel foundation.

[0021] When the soil cover thickness H above the pipe is 0.7m ≤ H < 3m, the thickness z of the replacement layer is:

[0022]

[0023] When the soil cover thickness H above the pipe is H≥3m, the thickness z of the replacement layer is:

[0024]

[0025] In the formula: D is the outer diameter of the pipe, in meters; f ak The characteristic value of the uncorrected bearing capacity of the subgrade soil layer; when the ground vehicle load is a Class A urban vehicle load, q v 36.46H -1.16 When the ground vehicle load is a Class B urban vehicle load, q v 28.68H -0.93 .

[0026] This invention can calculate the required foundation bearing capacity of pipelines under different conditions such as overburden, pipe diameter, and ground live load. It can directly obtain the thickness of the replacement layer according to different foundation types, improving the accuracy and speed of the replacement thickness calculation, increasing work efficiency, and reducing unnecessary waste. Moreover, this invention proposes a method and structure for determining the required foundation bearing capacity of pipelines with different pipe materials, pipe diameters, and overburden thicknesses, as well as for foundation treatment based on the thickness of the replacement layer. This method can determine the required foundation bearing capacity based on different pipe materials, pipe diameters, and overburden thicknesses. Furthermore, when there is a weak soil layer at the bottom of the pipe, a foundation treatment method and structure using a replacement layer are proposed to ensure that the foundation bearing capacity requirements are met and to guarantee the safe operation of the pipeline.

[0027] The vertical pressure exerted on the pipeline by the ground live load in this invention is taken as the larger of the accumulated load of 10 kPa and the vertical pressure at the top of the pipe caused by the wheel pressure of ground vehicles. The vertical pressure at the top of the pipe caused by the wheel pressure of ground vehicles is:

[0028]

[0029] In step S3 of this invention, the uncorrected bearing capacity of the soil layer at the bottom of the pipe not only needs to be greater than the calculated required bearing capacity, but also needs to meet the bearing capacity requirements of the soft soil layer, that is, the sum of the self-weight stress and additional stress of the soil layer at the bottom of the cushion layer is not greater than the corrected characteristic value of the bearing capacity of the soft soil layer at that location.

[0030] The pipeline foundation type of this invention is a gravel foundation, and the soil cover weight on top of the pipe is: Water weight: Pipeline self-weight:

[0031] In the formula: H is the thickness of the soil cover over the pipe (m); γ s The unit weight of soil (kN / m³) 3 For multi-layered soils, the weighted average value based on the soil layer thickness can be used; γ w water specific weight (kN / m³)3 );γ g The density of the pipe (kN / m³) 3 );γ c Unit weight of concrete foundation (kN / m³) 3 D0 is the inner diameter of the pipe (m); D is the outer diameter of the pipe (m).

[0032] The pipeline foundation type of this invention is a concrete foundation, and the soil cover weight on top of the pipe is: Water weight: Pipeline self-weight: Self-weight of concrete foundation:

[0033] For reinforced concrete pipes, the foundation type is a gravel foundation. The sum of the weight of the soil covering the pipe, the weight of the water inside the pipe, and the weight of the pipe itself is:

[0034] G = γ s H+(0.215γ s +0.545γ w +0.24γ g D;

[0035] The pipeline foundation is a concrete foundation. The sum of the weight of the soil covering the pipe, the weight of the water inside the pipe, the weight of the pipe itself, and the weight of the concrete foundation is:

[0036] G = γ s (H+0.24D)+(0.44γ c +0.363γ w +0.16γ g )D.

[0037] For ductile iron pipes, the foundation type is a sand and gravel foundation. The sum of the weight of the soil covering the pipe, the weight of the water inside the pipe, and the weight of the pipe itself is:

[0038]

[0039] For plastic pipes, the foundation type is a gravel foundation. The sum of the weight of the soil covering the pipe, the weight of the water inside the pipe, and the weight of the pipe itself is: G = γ s H+(0.215γ s +0.785γ w )D.

[0040] When the calculated required foundation bearing capacity is greater than 80 kPa, the calculated required foundation bearing capacity is taken; when the calculated required foundation bearing capacity is less than or equal to 80 kPa, 80 kPa is taken.

[0041] In step S3 of this invention, the bearing capacity of the soil layer at the bottom of the pipe without depth correction is not less than 80 kPa and is greater than the calculated required bearing capacity. At the same time, it meets the bearing capacity requirement of the soil layer at the bottom of the cushion layer, that is, the sum of the self-weight pressure and the additional pressure of the soil layer at the bottom of the cushion layer is not greater than the characteristic value of the foundation bearing capacity of the soil layer at the bottom of the cushion layer after correction.

[0042] p z +p cz ≤f ak +η b γ(b-3)+η d γ m (d-0.5)

[0043] p z =b(pp) c ) / (b+2ztanθ)

[0044] In the formula: p z The additional pressure on the subgrade soil, kPa; p cz f is the self-weight pressure of the subgrade soil, in kPa; ak η represents the characteristic value of the uncorrected bearing capacity of the subgrade soil layer; b η d These are the width and depth correction factors for the bearing capacity of the foundation subgrade soil layer, respectively; γ is the unit weight of the subgrade soil layer, kN / m³. 3 ;γ m The weighted average unit weight of the soil layer from the bottom of the foundation to the ground surface is expressed in kN / m³. 3 d is the foundation depth, in meters; b is the width of the strip foundation, in meters; p is the pressure on the foundation bottom surface, in kPa; p c The self-weight pressure of the foundation soil, kPa, p c =γ m (H+D); z is the thickness of the cushion layer, m; θ is the pressure diffusion angle, °.

[0045] When the bearing capacity calculation of the subgrade soil layer in this invention shows that the thickness of the replacement subgrade layer is less than or equal to 0, the replacement subgrade layer shall be a sand and gravel subgrade layer with a thickness of 0.1m-0.3m.

[0046] Compared with the prior art, the present invention has the following significant effects:

[0047] (1) This invention can calculate the required foundation bearing capacity of pipelines under different conditions such as overburden, pipe diameter, and ground live load. It can directly obtain the thickness of the replacement layer according to different foundation types, thereby improving the accuracy and speed of the replacement thickness calculation, increasing work efficiency, and reducing unnecessary waste.

[0048] (2) The implementation process of this invention is clear and explicit, the calculation process is accurate, and the calculation results can effectively guide the design and construction.

[0049] (3) This invention can quickly determine the thickness and structure of the replacement layer for underground water supply and drainage pipelines of different foundation types. It has wide applicability and is suitable not only for urban water supply and drainage pipeline projects, but also for water conservancy projects such as water diversion and regulation.

[0050] (4) This invention can quickly and effectively solve the problem of soft soil foundation treatment at the bottom of the pipeline under different conditions, and ensure the safe operation of the pipeline. Attached Figure Description

[0051] The present invention will now be described in further detail with reference to the accompanying drawings and specific embodiments.

[0052] Figure 1 It is a schematic diagram showing the relationship between the standard value of the vertical earth pressure on the top of the pipe generated by the wheel pressure of ground vehicles and the value of the soil cover thickness H on the top of the pipe;

[0053] Figure 2 This is a flowchart of the present invention;

[0054] Figure 3 This is a schematic diagram of the pipeline gravel foundation replacement bedding layer structure of the present invention;

[0055] Figure 4 This is a schematic diagram illustrating the calculation of the thickness of the replacement cushion layer for the pipeline foundation according to the present invention;

[0056] Figure 5 This is a schematic diagram of the concrete foundation replacement pad layer structure for the pipeline of the present invention;

[0057] Figure 6 This is a schematic diagram illustrating the calculation of the thickness of the concrete foundation replacement layer for the pipeline in this invention. Detailed Implementation

[0058] The technical solution of the present invention will be clearly and completely described below with reference to the embodiments. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0059] There are two main types of foundations for pipelines: gravel foundations and concrete foundations. Metal pipes (ductile iron pipes, steel pipes), plastic pipes, etc., are flexible pipelines and generally use gravel foundations; concrete pipes, reinforced concrete pipes, prestressed steel cylinder concrete pipes (collectively referred to as concrete pipes), etc., are rigid pipelines, and either gravel foundations or concrete foundations can be selected depending on the joint type. This invention is specifically a foundation treatment method for determining the thickness of the replacement bedding layer for these two types of foundations: gravel foundations and concrete foundations.

[0060] Based on the specific conditions of the project, initial parameters such as ground load type, soil cover over the pipe, pipe material, pipe diameter, bearing capacity of the soil layer under the pipe, and groundwater level can be obtained.

[0061] like Figure 3 As shown, a replacement layer 2 and a gravel foundation 3-1 (compaction coefficient of 0.85-0.90) are sequentially installed on the soil base 1 in the trench. A concrete pipe, metal pipe, or plastic pipe 4 is installed on the gravel foundation 3-1. Around the pipe, from bottom to top, a gravel foundation 3-2 (compaction coefficient not less than 0.93), backfill soil 5, and medium-coarse sand or gravel 6 backfilled on top of the pipe are sequentially installed. Plain soil 7 (compaction coefficient of backfill soil according to ground requirements) is then backfilled on the backfill soil 5 and medium-coarse sand or gravel 6, and the road surface structure 8 is laid. Support structures 9 are installed on both sides of the trench.

[0062] like Figure 4 The diagram shows the calculation of the thickness of the replacement cushion layer for the pipeline foundation. In the trench, from bottom to top, there are soft soil layer 11, replacement cushion layer 12 under the foundation, and soil layer 14 above the bottom of the foundation. The foundation width is 13.

[0063] like Figure 5 As shown, a replacement layer 2 and a concrete foundation 10 are sequentially installed on the soil base 1 in the trench. The pipeline is installed on the concrete foundation 10. Backfill soil 5 is filled on both sides of the concrete foundation 10 and the pipeline. Medium-coarse sand or gravel 6 is backfilled on top of the pipe. Plain soil 7 (compaction coefficient of backfill soil according to ground requirements) is then backfilled on the backfill soil 5 and medium-coarse sand or gravel 6. Road surface structure 8 is then laid. Support structures 9 are installed on both sides of the trench.

[0064] like Figure 6 The diagram shows the calculation of the thickness of the replacement cushion layer for the concrete foundation of the pipeline. From bottom to top in the trench, there are soft soil layer 11, replacement cushion layer 12 under the foundation, and soil layer 14 above the bottom of the foundation. The foundation width is 13.

[0065] like Figure 2 As shown, the present invention provides a foundation treatment method for determining the thickness of the replacement layer for underground water supply and drainage pipelines, comprising the following steps:

[0066] S1. Determine the type of pipe foundation, whether it is a concrete foundation or a gravel foundation, based on the pipe material and interface type.

[0067] The width of the gravel foundation is based on the pipe diameter. For the concrete foundation, according to the national standard drawing set, a 180° concrete foundation is taken, the concrete thickness at the bottom of the pipe is 2t, and the distance from the edge of the foundation in the pipe to the outer edge of the pipe is 2.5t. Therefore, the width of the concrete foundation is D + 2.5t * 2 = D + 5t, where t is the thickness of the concrete pipe wall.

[0068] S2. Calculate the required foundation bearing capacity:

[0069] Underground water supply and drainage pipelines refer to buried pipelines laid underground for transporting drinking water, rainwater, sewage, and water from rivers and lakes. They are generally subjected to multiple loads, including the pipeline's own weight, the weight of the water inside the pipe, the water pressure outside the pipe, the load from the overlying soil, and the ground load. Because the longitudinal length of the pipeline is much greater than its transverse dimension, the ground load diffuses through the soil layer to the top of the pipe, while other external loads act evenly on the soil layer at the bottom of the pipeline. Therefore, the pipeline structure can be regarded as a strip foundation.

[0070] The required bearing capacity of the foundation is the sum of all loads acting on the bottom of the pipe (concrete foundation bottom). When the pipe foundation type is a gravel foundation, the required bearing capacity is the sum of the vertical pressure of the ground live load acting on the pipe, the weight of the soil covering the pipe, the weight of the water inside the pipe, and the self-weight of the pipe. When the pipe foundation type is a concrete foundation, the required bearing capacity is the sum of the vertical pressure of the ground live load acting on the pipe, the weight of the soil covering the pipe, the weight of the water inside the pipe, the self-weight of the pipe, and the self-weight of the concrete foundation. The ground live load is taken as the larger of the accumulated load (10 kPa) and the vertical pressure at the top of the pipe generated by the wheel pressure of ground vehicles. According to the specification, the standard value of the pressure exerted on the top of the pipe by two or more single-row wheel pressures can be calculated using the formula... Calculations yield a scatter plot showing the vertical pressure exerted on the top of the pipe by urban-A and urban-B class vehicle loads under the most unfavorable vehicle wheel pressure, in relation to the soil cover thickness H above the pipe. Figure 1 As shown, it can be seen that as the thickness H of the soil cover over the pipe increases, the vertical pressure on the pipe top generated by the vehicle wheel decreases. Regardless of the type of vehicle load, when the soil cover thickness H ≥ 3m, the vertical pressure on the pipe top generated by the vehicle wheel is ≤ 10kPa. Therefore, when the soil cover thickness H ≥ 3m, the ground live load is taken as the accumulated load of 10kPa. To facilitate the calculation of the required foundation bearing capacity, the vertical pressure on the pipe top generated by the most unfavorable vehicle wheel (q) is used. vA q vB The result is expressed as a function of the soil cover thickness H above the pipe using curve fitting, i.e.:

[0071]

[0072] Similarly, the weight of the soil covering the pipe, the weight of the pipe itself, and the weight of the water inside the pipe can all be expressed as functions of the soil covering thickness H or the outer diameter D of the pipe. Thus, the relationship between the required foundation bearing capacity under different conditions and the soil covering thickness H and the outer diameter D of the pipe can be obtained.

[0073] The calculated required bearing capacity indicates that when the pipeline is small and the overburden thickness H is small, the required bearing capacity may be less than 80 kPa. However, the standard still requires that the uncorrected bearing capacity of the bottom soil layer be no less than 80 kPa. This requirement is not wasteful, but rather because when the bottom soil layer acts as the bearing stratum, it must not only meet the bearing capacity requirement but also the foundation deformation requirement. Large foundation deformation can lead to uneven settlement, causing damage such as cracking or misalignment at the pipeline joints. The requirement of 80 kPa for the uncorrected bottom soil layer is actually a requirement for the soil properties, implicitly including a requirement for the deformation of the bottom soil layer. Therefore, when the calculated required bearing capacity is greater than 80 kPa, the calculated required bearing capacity is used; when the calculated required bearing capacity is less than or equal to 80 kPa, 80 kPa is used.

[0074] The required foundation bearing capacity is calculated, and the load list is as follows:

[0075]

[0076]

[0077] (Table 1)

[0078] In the formula: H is the thickness of the soil cover over the pipe (m); γ s The unit weight of soil (kN / m³) 3 For multi-layered soils, the weighted average value based on the soil layer thickness can be used; γ w water specific weight (kN / m³) 3 );γ g The density of the pipe (kN / m³) 3 );γ c Unit weight of concrete foundation (kN / m³) 3 D0 is the inner diameter of the pipe (m); D is the outer diameter of the pipe (m).

[0079] The wall thickness of reinforced concrete pipes is generally 10% of the inner diameter of the pipe. Therefore, t = 0.1D0, D = D0 + 0.2D0 = 1.2D0. Substituting these values ​​into equations (1) to (4) above, the required foundation bearing capacity (kPa) can be obtained as follows:

[0080]

[0081] The nominal wall thickness of ductile iron pipe is a function of the nominal diameter (DN). Taking the K10 grade pipe with wall thickness classification as an example, the nominal wall thickness of the ductile iron pipe is t = 10(0.5 + 0.001DN) = 5 + 0.01DN (mm). The required foundation bearing capacity is:

[0082]

[0083] Plastic pipes, such as PE pipes, have a density of 950 kg / m³.3 The density of polyethylene pipe is 1400 kg / m. 3 Since its density is close to that of water, it can be approximated as the density of water for calculation. The required foundation bearing capacity is:

[0084] G = γ s H+(0.215γ s +0.785γ w D (7)

[0085] As can be seen from equations (5) to (7), under the same pipe diameter and soil cover load, concrete pipes require the greatest foundation bearing capacity, mainly due to the thicker pipe wall and greater self-weight.

[0086] Therefore, taking concrete pipes as an example, after considering the ground vehicle load, the required foundation bearing capacity (0.7≤H<3) is shown in equation (8) (when H≥3, the ground live load is taken as 10kPa ground surcharge):

[0087]

[0088] As shown in the above equation, the soil cover load on the pipe top has a linear relationship with the required foundation bearing capacity. The required foundation bearing capacity increases with the increase of the soil cover thickness. The vertical pressure generated by the vehicle load decreases with the increase of the soil cover thickness. Therefore, after the two are combined, the required foundation bearing capacity of the pipeline reaches an extreme point. It first decreases with increasing depth, and then increases with increasing depth. The rate of decrease before the extreme point is greater than the rate of increase after the extreme point. Differentiating equation (8-1) (City-B Class Vehicle Load) yields that the required foundation bearing capacity is minimum when H = 1.226. Substituting into equation (8), the minimum required foundation bearing capacity corresponding to different pipe diameters can be obtained.

[0089] S3. Calculation of replacement layer thickness:

[0090] ① Calculation of bearing capacity of the replacement layer foundation:

[0091] The calculation of the replacement layer thickness determines the thickness of the cushion layer when replacement layer treatment is required. If the bearing capacity of the uncorrected soil layer at the bottom of the pipe meets the design requirements, then foundation treatment with a replacement layer is not required; if the bearing capacity of the uncorrected soil layer at the bottom of the pipe does not meet the design requirements, then foundation treatment with a replacement layer is required.

[0092] The uncorrected bearing capacity of the soil layer at the bottom of the pipe must not only exceed the calculated required bearing capacity, but also meet the bearing capacity requirements of the soil layer at the bottom of the cushion layer. Specifically, the sum of the self-weight pressure and additional pressure of the soil layer at the bottom of the cushion layer must not exceed the corrected characteristic value of the bearing capacity of the foundation at the bottom of the cushion layer. When the bearing capacity verification calculation of the soil layer at the bottom of the cushion layer shows that the required replacement cushion layer thickness is less than or equal to 0, it indicates that only a very thin cushion layer is needed to replace the soil layer at the top of the pipe, and the corrected bearing capacity of the foundation at the bottom of the cushion layer will meet the requirements. In other words, even if the calculated replacement thickness is less than 0, a thin cushion layer still needs to be laid at the bottom of the pipe. This serves two purposes: firstly, to meet the requirement that the uncorrected bearing capacity of the soil layer at the bottom of the pipe is not less than 80 kPa; and secondly, because manual or mechanical excavation causes varying degrees of disturbance to the soil layer at the bottom of the pipe, a cushion layer with controllable quality can meet the bearing capacity requirements of the foundation and reduce uneven settlement. This thin cushion layer can generally be a sand and gravel cushion layer of 0.1-0.3 m in thickness.

[0093] Based on the required foundation bearing capacity calculated in step S2, verify whether the foundation bearing capacity of the soil layer at the bottom of the pipe meets the requirements. If it does not meet the requirements, for cases where the thickness of the weak soil layer at the bottom of the pipe is small, the foundation treatment can be carried out by the replacement cushion layer method. According to the specifications, the thickness of the replacement cushion layer should be 0.5m to 3m. The replacement cushion layer method diffuses the load on the top of the cushion layer to the bottom of the cushion layer through the diffusion effect of the cushion layer, so that the sum of the self-weight pressure and the additional pressure of the soil at the bottom of the cushion layer is not greater than the corrected characteristic value of the foundation bearing capacity at the bottom of the cushion layer. Its expression is shown in equation (9).

[0094] p z +p cz ≤f ak +η b γ(b-3)+η d γ m (d-0.5) (9)

[0095] In the formula, p z Additional pressure (kPa) on the subgrade soil; p cz The self-weight pressure of the subgrade soil (kPa); f ak η represents the characteristic value of the uncorrected bearing capacity of the subgrade soil layer; b η d These are the width and depth correction factors for the bearing capacity of the foundation subgrade soil layer, respectively. When b is less than 3m, the value is taken as 3m; when b is greater than 6m, the value is taken as 6m. γ is the unit weight of the subgrade soil layer (kN / m³). 3 When the groundwater level is below the groundwater level, the buoyant weight is taken; γ m The weighted average unit weight (kN / m³) of the soil layer from the bottom of the foundation to the ground surface. 3 For soil layers below the groundwater level, the buoyant unit weight is taken, and d is the foundation (pipeline) burial depth (m), where d = H + D. For soft soil and composite foundations, only depth correction is required.

[0096] Treating the pipeline as a strip foundation, the additional pressure of the soil at the bottom of the cushion layer is shown in equation (10):

[0097] p z =b(pp) c ) / (b+2ztanθ) (10)

[0098] In the formula, b is the width of the strip foundation (m); p is the pressure on the bottom surface of the foundation (kPa); p c denoted as σ, where σ is the self-weight pressure of the foundation soil (kPa), z is the thickness of the cushion layer (m), and θ is the pressure diffusion angle.

[0099] The pressure on the foundation bottom surface is the bearing capacity of the foundation required by the pipeline, and the self-weight pressure p of the foundation bottom soil. c =γ m (H+D).

[0100] ② Calculation of minimum replacement layer thickness:

[0101] The required foundation bearing capacity f of the pipeline should not be greater than the foundation bearing capacity of the soil layer at the bottom of the pipe. Substituting into equation (9), we can obtain:

[0102]

[0103] In the formula, γ m1 Weighted average unit weight of soil layers from the foundation to the ground surface (kN / m³) 3 );γ m2 The weighted average unit weight (kN / m³) of the soil layer from the bottom of the foundation to the ground surface. 3 ), γ 垫 The specific weight (kN / m) of the subbase material 3 ), where z is the thickness of the cushion layer (m). Assume γ m1 =γ m2 =γ 垫 =18, substituting into the above formula, the thickness of the cushion layer can be calculated as shown in formula (12).

[0104]

[0105] As can be seen from equation (12), the thickness of the cushion layer is related to the required foundation bearing capacity f of the pipeline, the pipe diameter D, the thickness of the soil cover H above the pipe, and the characteristic value of the uncorrected foundation bearing capacity f of the soil layer under the cushion layer. ak And it is related to the pressure diffusion angle θ.

[0106] According to the specifications, when the calculated required foundation bearing capacity f is less than or equal to 80 kPa, f is taken as 80 kPa, and the minimum thickness of the replacement cushion layer can be obtained as shown in equation (13):

[0107]

[0108] It is evident that, under the same pipeline burial depth and uncorrected characteristic value of the foundation bearing capacity of the subgrade soil, the replacement thickness has a linear relationship with the pipe diameter; as the pipe diameter increases, the replacement thickness increases. For the same pipe diameter, the thickness decreases with increasing pipeline burial depth and also decreases with increasing characteristic value of the bearing capacity of the subgrade soil. Furthermore, let z... min =0, from equation (13) we can get:

[0109]

[0110] For example, when f ak When the pressure is 60 kPa, we can obtain d = 1.61 m, and the minimum replacement thickness z. min The value is 0, regardless of the pipe diameter. Its practical significance is that when the foundation depth is 1.61m, a soft soil layer with a bearing capacity characteristic value of 60kPa can reach 80kPa after depth correction. Therefore, when d is greater than 1.61m, the replacement thickness is negative, and replacement is not required. However, according to the specifications, the uncorrected bearing capacity characteristic value of the soil layer at the bottom of the pipe should not be less than 80kPa, meaning the bearing capacity needs to reach 80kPa before depth correction. Clearly, when the cushion layer thickness is 0, the uncorrected bearing capacity of this soil layer is 60kPa, which does not meet the specifications. Therefore, when the uncorrected bearing capacity characteristic value of the soil layer at the bottom of the pipe is less than 80kPa, a very small cushion layer thickness is taken. After depth correction, the bearing capacity characteristic value of the soil layer at the bottom of the cushion layer can meet the requirements. Therefore, when the calculated replacement thickness is less than 0, the replacement thickness of 0.1m to 0.3m can be used according to the standard design drawings for sand and gravel foundations.

[0111] Table 2 shows the embedment depth d corresponding to the minimum replacement thickness of the subgrade soil layer with different foundation bearing capacity characteristic values ​​(corresponding to a bearing capacity requirement of 80 kPa).

[0112]

[0113] (Table 2)

[0114] ③ Determining the thickness of the replacement layer:

[0115] The thickness of the replacement layer is determined based on the following three situations:

[0116] ① When the calculated required foundation bearing capacity is ≤80kPa, the cushion layer shall be determined based on the required foundation bearing capacity of 80kPa;

[0117] ② When the calculated required foundation bearing capacity is >80kPa and the thickness of the soil cover H on the top of the pipe is <3m, the cushion layer shall be determined according to the calculated required foundation bearing capacity.

[0118] ③ When the required foundation bearing capacity is >80kPa and the thickness of the soil cover H on the top of the pipe is ≥3m, since the foundation bearing capacity of the general uncorrected soil layer is not less than 19kPa, the calculated replacement thickness is less than 0.

[0119] When the required foundation bearing capacity f for the pipeline is less than 80 kPa, the replacement thickness is calculated according to formula (15).

[0120]

[0121] When the required bearing capacity of the pipeline is greater than 80 kPa, take γ. c 22kN / m 3 γ g 25kN / m 3 Substituting equation (8) into equation (12) yields:

[0122]

[0123]

[0124] Equation (16) represents the urban-B class vehicle load. When the ground surface is an urban-A class vehicle load, the load is 36.46H. -1.16 Replace 28.68H -0.93 In equations (16) and (17), the second term... The result is small and can be ignored. From equations (15) to (17), it can be seen that the thickness of the replacement layer is mainly related to the pipe diameter D, the overburden thickness H, and the characteristic value f of the bearing capacity of the underlying soft soil layer. ak (The following explanation uses a gravel foundation under a Class B vehicle load in a city as an example).

[0125] When the required foundation bearing capacity is less than 80 kPa, the thickness of the replacement layer decreases with the increase of the overburden depth. When the thickness of the cushion layer z ≤ 0; when the uncorrected bearing capacity f of the foundation soil layer at the bottom of the cushion layer... ak When the backfill thickness H is constant, the relationship between the backfill thickness and the pipe diameter is a parabolic curve with the opening facing downwards from the origin (f ak (Generally greater than 9 kPa). When When the equation of the axis of symmetry is positive, but because When the thickness of the replacement layer z is less than or equal to 0, it is necessary to calculate the thickness of the replacement layer. When the pipe diameter is 0.028(89-f), the replacement thickness is greater than 0. As the pipe diameter increases, the replacement layer thickness first increases and then decreases. ak -18H), the maximum value for achieving the minimum replacement thickness is (Assume the pressure diffusion angle θ is 20°). When 89-f akWhen -18H<0, the equation of the axis of symmetry is negative; when D>0, the calculated replacement thickness z<0, which is consistent with the above.

[0126] If the pipeline requires a foundation bearing capacity greater than 80 kPa, when the soil cover thickness is 0.7 ≤ H < 3, the thickness of the cushion layer decreases as the soil cover thickness increases. When the backfill thickness z ≤ 0, and the backfill thickness is constant, the relationship between the backfill thickness and the pipe diameter is a parabolic curve with the opening facing downwards from the origin. The equation of the axis of symmetry is positive, therefore... At this time, the thickness of the replacement layer first increases and then decreases with the increase of pipe diameter. When pipe diameter D = 0.187 (28.68H) -0.93 -f ak +9), the maximum value for achieving the minimum replacement thickness is... (Assume the pressure diffusion angle θ is 20°). If 28.68H -0.93 +9-f ak <0, the equation of the axis of symmetry is negative. Since the calculated replacement thickness z<0 when the pipe diameter D>0, the thickness of the replacement cushion layer can be taken as 0, and only the cushion layer of the sand and gravel foundation needs to be set.

[0127] When the backfill thickness H≥3, the thickness of the replacement layer is independent of the backfill thickness. The relationship between the replacement thickness and the pipe diameter is a parabolic curve passing through the origin and opening downwards. Since the characteristic value of the soil bearing capacity is generally greater than 19kPa, the replacement thickness z<0 is calculated according to equation (17). Combined with the aforementioned backfill thickness when the foundation bearing capacity is less than 80kPa... When the replacement layer thickness is zero, and the characteristic value of the foundation bearing capacity is greater than 30 kPa, the calculated foundation bearing capacity is irrelevant, and the replacement layer thickness can be taken as 0, requiring no replacement treatment. Its engineering significance is that when the pipeline cover depth is large, a very small layer thickness can meet the bearing capacity requirements through depth correction. The process for determining the replacement layer thickness is as follows: Figure 2 As shown.

[0128] It is worth mentioning that when the replacement thickness determined by this method is greater than 3m, the rationality of adopting the replacement cushion layer method should be determined based on the soil properties, thickness, groundwater level, and surrounding environment of the bottom soil layer. Generally, a replacement thickness greater than 3m will lead to an increase in the depth of the foundation pit, while also increasing the difficulty of groundwater dewatering and affecting the construction quality of the cushion layer. Therefore, its economy and reliability need to be carefully considered.

Claims

1. A foundation treatment method for determining the thickness of the replacement layer for underground water supply and drainage pipelines, characterized in that... Includes the following steps: S1. Determine the type of pipe foundation, whether it is a concrete foundation or a gravel foundation, based on the pipe material and interface type. S2. Calculate the required foundation bearing capacity f: If the pipe foundation type is a gravel foundation, the required foundation bearing capacity is the sum of the vertical pressure of the ground live load acting on the pipe, the weight of the soil covering the pipe, the weight of the water inside the pipe, and the self-weight of the pipe; if the pipe foundation type is a concrete foundation, the required foundation bearing capacity is the sum of the vertical pressure of the ground live load acting on the pipe, the weight of the soil covering the pipe, the weight of the water inside the pipe, the self-weight of the pipe, and the self-weight of the concrete foundation. S3. Verify whether the bearing capacity of the uncorrected soil layer at the bottom of the pipe meets the design requirements. If it does, there is no need to carry out foundation treatment with replacement layer. If the requirements are not met, foundation treatment with a replacement layer shall be carried out, and the thickness of the replacement layer z shall be calculated according to the following formula. When the required foundation bearing capacity f is less than or equal to 80 kPa, The pipeline foundation is a concrete foundation, and the thickness z of the replacement layer is: ； The pipeline foundation type is a sand and gravel foundation, and the thickness z of the replacement layer is: ； When the required foundation bearing capacity f is greater than 80 kPa, the pipeline foundation type is a concrete foundation. When the soil cover thickness H above the pipe is 0.7m ≤ H < 3m, the thickness z of the replacement layer is: ； When the soil cover thickness H above the pipe is H≥3m, the thickness z of the replacement layer is: ； The pipe foundation type is a gravel foundation. When the soil cover thickness H above the pipe is 0.7m ≤ H < 3m, the thickness z of the replacement layer is: ； When the soil cover thickness H above the pipe is H≥3m, the thickness z of the replacement layer is: ； Where: D is the outer diameter of the pipe, m; H is the thickness of the soil cover over the pipe, m; θ is the pressure diffusion angle, °; f ak This represents the characteristic value of the uncorrected bearing capacity of the subgrade soil layer; when the surface vehicle load is a Class A urban vehicle load, for When the ground vehicle load is a Class B urban vehicle load, .

2. The foundation treatment method for determining the thickness of the replacement layer for underground water supply and drainage pipelines according to claim 1, characterized in that: The vertical pressure exerted on the pipeline by the ground live load is taken as the larger of the accumulated load of 10 kPa and the vertical pressure at the top of the pipe generated by the wheel pressure of ground vehicles.

3. The foundation treatment method for determining the thickness of the replacement layer for underground water supply and drainage pipelines according to claim 1, characterized in that: When the ground vehicle load is a Class A city vehicle load, the vertical pressure at the top of the pipe generated by the wheel pressure of the ground vehicles , ; When the ground vehicle load is a Class B urban vehicle load, the vertical pressure at the top of the pipe generated by the wheel pressure of the ground vehicles... , .

4. The foundation treatment method for determining the thickness of the replacement layer for underground water supply and drainage pipelines according to claim 3, characterized in that: The pipe foundation type is gravel foundation, and the soil cover on top of the pipe is: ; Weight of water in the pipe: Pipeline self-weight: ; In the formula: H is the thickness of the soil cover over the pipe, in meters; The density of soil is kN / m 3 For multi-layer soil, the weighted average value is taken based on the thickness of each soil layer; The specific weight of water is kN / m³. 3 ; The density of the pipe is kN / m 3 ; The unit weight of the concrete foundation is kN / m³. 3 ; D is the inner diameter of the pipe, in meters; D is the outer diameter of the pipe, in meters.

5. The foundation treatment method for determining the thickness of the replacement layer for underground water supply and drainage pipelines according to claim 3, characterized in that: The pipe foundation type is a concrete foundation, and the soil cover weight on top of the pipe is: ; Weight of water in the pipe: Pipeline self-weight: Self-weight of concrete foundation: ; In the formula: H is the thickness of the soil cover over the pipe, in meters; The density of soil is kN / m 3 For multi-layer soil, the weighted average value is taken based on the thickness of each soil layer; The specific weight of water is kN / m³. 3 ; t is the pipe's inner diameter, in meters; t is the pipe's nominal wall thickness, in meters. The density of the pipe is kN / m 3 ; The unit weight of the concrete foundation is kN / m³. 3 .

6. The foundation treatment method for determining the thickness of the replacement layer for underground water supply and drainage pipelines according to claim 4, characterized in that: For reinforced concrete pipes, the sum of the weight of the soil covering the pipe, the weight of the water inside the pipe, and the weight of the pipe itself is: ； For ductile iron pipes, the required foundation bearing capacity is: ； For plastic pipes, the required foundation bearing capacity is: ； In the formula: The density of soil is kN / m 3 ; The specific weight of water is kN / m³. 3 ; The density of the pipe is kN / m 3 D is the outer diameter of the pipe, in meters; DN is the nominal diameter of the pipe, in meters.

7. The foundation treatment method for determining the thickness of the replacement layer for underground water supply and drainage pipelines according to claim 5, characterized in that: For reinforced concrete pipes, the sum of the weight of the soil covering the pipe, the weight of the water inside the pipe, the weight of the pipe itself, and the weight of the concrete foundation is: ； In the formula: The unit weight of the concrete foundation is kN / m³. 3 ; The density of soil is kN / m 3 For multi-layer soil, the weighted average value is taken based on the thickness of each soil layer; The specific weight of water is kN / m³. 3 ; The density of the pipe is kN / m 3 .

8. The foundation treatment method for determining the thickness of the replacement layer for underground water supply and drainage pipelines according to claim 6 or 7, characterized in that: When the calculated required foundation bearing capacity is greater than 80 kPa, the calculated required foundation bearing capacity shall be used; when the calculated required foundation bearing capacity is less than or equal to 80 kPa, 80 kPa shall be used.

9. The foundation treatment method for determining the thickness of the replacement layer for underground water supply and drainage pipelines according to claim 8, characterized in that: In step S3, the characteristic value of the bearing capacity of the soil layer at the bottom of the pipe without depth correction is not less than 80 kPa and is greater than the calculated required bearing capacity. It also meets the bearing capacity requirements of the soil layer at the bottom of the cushion layer, meaning the sum of the self-weight pressure and the additional pressure of the soil layer at the bottom of the cushion layer is not greater than the corrected characteristic value of the bearing capacity of the soil layer at the bottom of the cushion layer. The following formula is used for calculation: ； ； In the formula: The additional pressure on the subgrade soil is expressed in kPa. The pressure is the self-weight pressure of the subgrade soil, in kPa; This represents the characteristic value of the uncorrected bearing capacity of the subgrade soil layer. These are the correction factors for the width and depth of the foundation bearing capacity of the subgrade soil layer, respectively. The unit weight of the subgrade soil is kN / m³. 3 ; The weighted average unit weight of the soil layer from the bottom of the foundation to the ground surface is expressed in kN / m³. 3 d is the foundation depth, in meters; b is the width of the strip foundation, in meters; p is the pressure on the foundation bottom surface, in kPa. The pressure is the self-weight pressure of the foundation soil, in kPa. z is the thickness of the cushion layer, in meters; θ is the pressure diffusion angle, in degrees.

10. The foundation treatment method for determining the thickness of the replacement layer for underground water supply and drainage pipelines according to claim 9, characterized in that: When the bearing capacity calculation of the subgrade soil layer shows that the thickness of the replacement subgrade is less than or equal to 0, a sand and gravel subgrade with a thickness of 0.1m-0.3m shall be used for the replacement subgrade.

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