Design analysis of underground water supply socket cast iron pipe

Design and Analysis of Underground Water Supply Pipe Cast Iron Pipeline Design Li Yongkang Zhuang Wenguang (China Textile Industry Design Institute, Beijing 100037) (Qingdao Huajin Group Co., Ltd. Qingdao 266021) The water supply socket cast iron pipe laid underground in the Syrian Jiebile Cotton Mill factory area is 6720m About 6000m of them are laid under the road in the factory area. After the road is fully excavated and installed, the soil is backfilled for compaction. The construction is progressing smoothly and the quality is guaranteed. It has been running for a year and a half, but due to the operation, the operator accidentally increased the water pressure by 2kg/cm2, causing the pipe to burst in three places. Everything else is normal, and the leakage of the whole plant pipeline is within the allowable value.

The design of the underground water supply cast iron pipe is quite complicated. It is related to the foundation condition, the buried depth of the pipe, the load on the upper part of the pipe, the water pressure inside the pipe, the pipe and so on. The following is mainly the relationship between the ground and the soil.

1 Before the design of the soil condition of the foundation, it is first necessary to know the underground soil condition along the direction of pipeline laying. For this reason, soil samples of different depths can be taken at a certain distance, and then the triaxial test of the soil sample can be done to determine the cohesion of the soil ( c) and the internal friction angle (,), which can have a more comprehensive grasp of the soil condition, so as to classify the foundation soil at different points along the pipeline laying direction, and provide a basis for the buried depth design of the pipeline.

0 ring normal stress; T0, T cylindrical surface and radial plane position change; 2 establish a model to analyze the pipeline stress situation buried in the ground, while being affected by external foundation soil pressure, load and internal water pressure, now available Simplified as a model, the force condition of the pipeline is analyzed approximately in accordance with the condition that the cylinder is subjected to a uniform distributed pressure, q1 represents water pressure, and q2 represents external pressure. Here, regardless of the rigid displacement, an angle from the r-ring to the r-direction (radial) is cut from the pipe; A and C depend on the boundary conditions and the constraints.

The boundary condition of the problem is that a unit body consisting of two cylindrical surfaces spaced apart from each other and two radial planes forming a d0 angle establish polar coordinates, so that the stress can be derived as follows: (2) into (1) we can obtain: = 2C ten first author: Li Yongkang male December 1967 born and then her master engineer 80 AC values ​​into (1) can be derived Lrn answer: Bian b2 according to formula ( 3) The radial normal stress and the circumferential normal stress of the pipeline can be judged to determine whether the selected pipeline strength can meet the requirements.

3 Pipe Support Pier Design Buried water supply cast iron pipe fixed support pier is an important auxiliary structure for removing unbalanced thrust, supporting pipes and their accessories to ensure the normal operation of the pipe network. Abutments are required at the pipe elbows (horizontal, vertical), tees, etc. Therefore, the design of the fixed pier of the pipeline is a very important part. If the design of this section is unreasonable, during the operation, the pipeline will be displaced or even disjointed, causing water leakage at the socket. However, the design of the pipe horizontal elbow and the butt at the tee is the most complicated. As shown, the pipe elbow or tee is partially poured into the concrete to make it integral with the concrete buttress. The water pressure in the pipe acts on the concrete buttress, and the force on the pier is applied to the soil. In this way, it is necessary to analyze the force and deformation between the concrete buttress and the undisturbed soil and backfill, that is, to analyze the active earth pressure and passive earth pressure of the soil. The earth pressure is generated when the soil is displaced (or strain is generated). It is difficult to produce plastic equilibrium in the soil at the same time. In most cases, it is a gradually developing phenomenon. The two circles (CA and AD) which can pass through the common point A and are tangent to the rupture line represent a plane strain plastic equilibrium state, and other circles such as EA, AF, etc., all represent elastic states.

The active earth pressure is related to the plastic equilibrium state determined by the rupture circle AC. The stresses OA and OE are first applied to obtain a K (stable) state.

Then gradually reduce the OE to OC, and as a result, the resulting slip line forms an angle as shown, and the stress OA (maximum) and the stress OC (minimum) can be used to draw a moiré circle. The active earth pressure (\) is as follows: the stress is always accompanied by deformation, so in b and 3c, when the lateral pressure is reduced from OE to OC, the soil has lateral expansion, when the lateral constraint OE is provided by the concrete buttress If the concrete buttress is absolutely stationary, the buttress will move laterally away from the vertical plane under stress. If this happens, the soil expands laterally and the lateral pressure decreases simultaneously. If the pressure behind the buttress is reduced from some high value to the active (minimum) state, the buttress must be laterally translated.

Elastic balance and plastic balance () stress state before (destruction) and failure (plasticity); (b) initial condition under constant ambient stress OA; (c) shear for active earth pressure theory Destructive surface; (d) - shear failure surface for passive earth pressure theory 1 - plasticity; 2 - elasticity; 3 - fracture line passive earth pressure is formed by b, 3c increasing lateral pressure from OE to OD This is similar to the buttress pushing laterally toward the soil, and the soil is subject to deformation, but the deformation is resistant to the surrounding soil, so the pressure caused by the deformation must be and must be much larger, which is the support pier at a specific active pressure The reason for the forward displacement under large pressure and the fact that the lateral pressure is reduced cannot be displaced back to the original position, of course, the back-translational translation will be performed along a sliding line different from the forward translation.

Only by analyzing the main and passive earth pressures can we correctly design the strength, orientation and size of the butt joints, so as to ensure that the piers and soils do not shift under the action of water pressure.

4 Summary Through the design of the project, the following conclusions can be drawn: (1) The design work of the underground water supply socket cast iron pipe should comprehensively and meticulously understand the underground soil quality. In the previous design, the designer ignored this work, resulting in poor design quality. After the construction was completed, the pipeline was broken and misaligned. (2) At the same time of designing, the stress state of the pipeline should be analyzed and calculated in detail, and the force between the concrete buttress and the soil should be calculated to ensure that the pipeline will not be broken under the action of load and earth pressure; The pier is strong and does not shift. In the previous design, because the designer did not calculate the pipe, soil and concrete buttress, the pipe burst, the socket joint disjoint, the concrete pier piercing, displacement and other phenomena caused by the underground pipe filling operation caused the project Great economic loss. (3) The design of the underground socket pipe can not be carried out only according to the general requirements in the design manual. Because the engineering geological conditions are complex and varied, the design manual cannot be all-encompassing, and specific problems must be analyzed.

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