Open caisson construction method for large-scale p

2022-08-06
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Open caisson construction method for large and medium-sized pump house of Tianjin flowing plastic sludge the sewage treatment plant of Tianjin Economic and Technological Development Zone is an environmental protection infrastructure project established to improve the investment environment. Its sewage pump house is built on the sea water sedimentation tank formerly known as yanwangzi. About 2m below the surface is filled with plain soil, and then the flowing plastic sludge deposited for many years. The buried depth of the underground pump sump of the pump house is 13m, which is in a convex shape. Due to the nearby substation, dehydration machine room and mud cake stacking warehouse, it is impossible to purchase 8.1 factory products. The product certificate includes: open excavation foundation pit or deep foundation maintenance structure, which is designed as a polygonal open caisson structure. Sinking the open caisson in the flowing plastic sludge is very easy to cause sudden sinking, partial sinking and sliding, water gushing, mud gushing and excessive sinking in the well, and the speed and direction of sinking the open caisson are very difficult to control, which is a construction problem that has not been well solved so far. According to the design intention, the third division of China Railway 18th Bureau, which undertook the project, adopts pre driving powder jet grouting piles at the foot of the open caisson blade to form a row of pile type underground continuous wall. For the open caisson wall to form a drag bearing and supporting wall with certain strength, the process of sinking the open caisson in the sludge is turned into a controllable process, which successfully solves this construction problem

1 construction preparation

1.1 excavate and fill soil, reduce the initial settlement elevation

according to the geological conditions of the open caisson, in order to ensure the balanced settlement of the open caisson in the initial settlement stage, excavate the artificial fill layer, and set the prefabrication and initial settlement elevation of the open caisson to 0.48M, which can create two favorable conditions: first, because the initial settlement stratum is silt, its water content and bearing capacity are uniform, which is convenient for the stable initial settlement; Second, the total sinking amount of the open caisson is reduced by 2.5m. The 0.5m thick open caisson in the upper section III can not be used as the open caisson construction, but poured after the closed bottom of the open caisson, which not only reduces the dead weight of the open caisson (still meeting the requirements of the sinking weight), but also shortens the sinking depth, killing two birds with one stone

1.2 construction of dry jet grouting piles

driving dry jet grouting piles, strengthening the soft soil at the foot of the open caisson blade, and making the open caisson sink under the support of the cement continuous wall formed by continuous driving dry jet grouting piles. The following points shall be paid attention to in the construction of dry jet grouting pile:

① the position shall be accurate, the outer edge of the pile shall be tangent to the outer edge of the shaft wall, and shall not be exposed, so as to prevent the cement soil from squeezing out of the shaft wall and losing its support function when the open caisson sinks

② the pile bottom shall be 16m below the design elevation of the edge foot of the open caisson

③ at the foot of the outer ring pile bottom blade and within 1.0m of the pile top, the amount of sprayed mud is 10% (calculated according to the pile weight), and the rest of the pile body is 7%

④ the cement spraying amount of inner circle pile is 10%

⑤ the spacing between the inner and outer rows of piles is 10cm, so as to ensure that the outer pile will not be damaged when excavating the inner pile

2 open caisson prefabrication

2.1 plane layout of prefabrication site

as the open caisson shaft wall is the pump pool wall of the joint pump house, the pump pool location is selected as the open caisson prefabrication location. In order to facilitate the construction and consider the needs of concrete pouring for the open caisson support wall, the soft soil surface of the site is compacted with 15cm and 8% lime soil, and another 10 lime soil platform is built at the bottom of the support wall

2.2 prefabrication method

the original design of the open caisson is three sections. After the surface fill is excavated, the ground elevation is just the top elevation of the second section of the open caisson. (7) two aluminum alloy vans of Shengtong group have been listed in the list of recommended models for hanging by the Ministry of transport. Therefore, the third section of the open caisson is no longer regarded as the open caisson structure, but will be connected after the two open caissons are sunk in place and sealed. In order to reduce the weight of the first concrete pouring and avoid concrete cracking caused by excessive settlement and uneven settlement, the first section of concrete pouring is completed in two times. The blade foot (1.0m high) is poured first, and the rest is poured when the concrete strength reaches more than 70% of the design strength, and the second section is constructed in turn. After the concrete of two sections of open caisson is completed, it shall be sunk in place at one time

2.3 the bottom formwork of the blade foot and the bottom formwork of the supporting wall

according to the preliminary design, two rows of powder spraying piles are driven at the blade foot to strengthen the soft soil layer. The original ground is mucky loam with an allowable bearing capacity of 80kPa. The cement content at the top of the powder jet grouting pile is 10%. The unconfined compressive strength of the cement soil can reach 600kpa in 7 days and 800 ~ 1000kPa in 28 days. The total amount of open caisson concrete is 444m3, and the total weight is 3610t based on the capacity of 2.5t/m3. The bottom area of open caisson blade is 44.88m2. Therefore, when only the bottom surface of the blade foot is used as the supporting surface, the bearing load is 804kpa

according to the above calculation and taking into account the construction error of the powder jet grouting pile, the blade foot of the open caisson is placed on the top of the powder jet grouting pile, and the bearing capacity can not fully meet the requirements of

. During the prefabrication of the open caisson, the side of the blade foot has not been loaded, so the supporting bottom formwork is added at the bottom of the supporting wall to share the weight of some of the open caisson

3 sinking of open caisson

3.1 preparation work

the open caisson must not start sinking until the concrete strength reaches the design strength. The following preparations must be made before sinking:

① draw observation signs outside the shaft wall, and set leveling observation points at the four corners of the open caisson to observe the sinking amount and balance; A vertical line is set at the central axis to observe the displacement and balance of the open caisson

② remove the module

③ after the surface lime soil is excavated

the bottom formwork of the supporting wall is removed, the open caisson slightly sinks, but the side of the blade foot immediately bears the force, and the open caisson stops sinking

3.2 settlement coefficient calculation

settlement coefficient formula:

k = q/(f · h · L)> 1 (1)

where? Q?—— Dead weight gravity of open caisson

f—— Friction coefficient, for soft soil, 9.8 ~ 11.76kn/m2

h—— Maximum sinking depth

l—— The friction coefficient of the outer wall perimeter of the open caisson

is the maximum value of the soft soil. The self gravity sinking coefficient of the general structural open caisson can reach 3.0. Moreover, there is no stagnation problem in the silt. The existing problem is that when the sinking depth meets the requirements, it will still sink, so control measures must be taken

3.3 mechanism of controlling subsidence of continuous wall with dry jet grouting piles

① guiding and preventing sudden subsidence and soil inflow

according to the preliminary design concept, within the range of shaft wall density and under the cutting edge, pre driving two rows of dry jet grouting piles to strengthen the stratum is a comprehensive measure to prevent sudden subsidence of open caisson, too fast settlement speed indicator and soil inflow. Its working principle is as follows: first, dry jet grouting piles form a continuous wall of cement land, For the open caisson, it is a bearing wall enclosed and sandwiched in the silt. The whole sinking process of the open caisson is also the excavation process of the bearing wall. In this way, the sinking speed and stability of the open caisson can be completely controlled by manually excavating the powder jet pile. Second, the open caisson is implemented in the muddy soft soil, which is prone to soil gushing in the process of excavation and sinking, that is, the fluid plastic sludge outside the shaft wall slides due to the instability of the soil surface inside and outside the shaft due to the large height difference between the soil surface inside and outside the shaft, and the foot of the sliding arc surface is squeezed into the shaft, so that only the soil is excavated in the shaft but the soil elevation does not drop, while the soil elevation outside the shaft sinks, which is very easy to produce unbalanced earth pressure, resulting in the inclination of the open caisson. Two circles of dry jet grouting piles were driven at the foot of the blade in advance to form a 1.2m thick cement wall, preventing the sliding of soil outside the shaft wall from sliding into the shaft wall

② design depth for controlling the sinking of the open caisson

when the open caisson sinks to the design elevation, and the soil layer under the bottom surface of the blade foot is still silt, it can be seen from the sinking coefficient that the bearing capacity of the silt soil is far from enough, and it will sink continuously. Now, the underground continuous wall of the powder jet grouting pile is pre driven at the foot of the blade foot, which can make the bottom surface of the open caisson blade foot sink to the design elevation and fall on the pile top surface of the underground continuous wall of the powder jet grouting pile, which solves the problem of controlling the sinking depth of the open caisson

when the open caisson needs to sink, it only needs to chisel off the pile head. The open caisson can sink by overcoming the friction and supporting force of the soil by its own weight. Once the blade foot is solid to the pile head, it can stop sinking. According to the above inference, when the open caisson is about to reach the design elevation (the difference should be 50 ~ 70cm), chisel the powder jet grouting pile at the design elevation in advance, so as to control the open caisson in place and stop sinking accurately

3.4 excavation method

the hard shell of lime soil on the surface shall be excavated manually, and the rest of the sludge shall be washed by high-pressure water gun. Every 50cm is a layer, which shall be washed and stripped down layer by layer. In order to prevent the support wall and ground beam from bearing bending moment and the need of water collection and mud pumping, the soil layer is flushed into a pot bottom type (low in the middle and high around). For each layer of DJM piles exposed at the foot of the open caisson blade, the inner circle DJM piles shall be excavated along the same elevation, and the outer circle DJM piles shall be excavated at intervals to make the remaining DJM piles crushed by the open caisson and removed manually

3.5 deviation correction of open caisson

due to the guiding effect of the powder jet grouting pile, the open caisson generally does not have large deviation. In order to ensure that the positioning error of the open caisson is within the allowable range of the specification, the following control measures are taken:

a. monitor the horizontal and central axis with the sinking of the open caisson, and adjust the position and height of the dug down powder jet grouting pile at any time

b. when the sinking of open caisson is almost in place, stop chiseling piles, excavate soil for 24h, and observe the settlement. If there is no obvious settlement, it can be sunk in place at one time, and no other settlement prevention measures will be taken; If there is obvious settlement, the cause shall be found out, and anti settlement measures shall be added

c. when the open caisson is almost in place, if the axis displacement or inclination exceeds the allowable range, it can be corrected by means of unilateral compaction and filling, unilateral excavation and load reduction, counterweight, etc

4 bottom sealing of open caisson

after the sinking of open caisson, the deviation shall comply with the specifications:

① there are 5 axis displacements at the set input port, which are not greater than 1% of the well depth

② elevation: +40mm, -60mm

③ inclination ≯ well depth 0.7%

after the open caisson is in place for 2 ~ 3 days, the blade foot has stably fallen on the top of the powder jet grouting pile, and the open caisson can be sealed. In order to avoid large buoyancy caused by the accumulation of underground water, the top crack bottom sealing concrete can be evenly arranged on the bottom plate with 2 ~ 3 seepage wells. The seepage pipes can be buried in the wells, and radial gravel ditches are built around the seepage pipes to divert water. After the pump sump structure is completed, the wellhead can be blocked

5 conclusion

due to the characteristics of poor bearing capacity and small friction coefficient of the formation, it is very easy to have sudden sinking, soil gushing, too fast sinking speed, too large over sinking displacement and inclination during the sinking process of the open caisson, which is difficult to control. The design and construction of the open caisson made full use of the characteristics of cement soil. Two rows of powder jet piles were driven at the foot of the open caisson blade in advance to form a continuous bearing wall with appropriate strength in the soft soil layer, which is like a reliable guide rail during the sinking of the open caisson. The supporting force can be adjusted by chiseling the head of the powder jet grouting pile in sections and positions, and the attitude, sinking speed and depth of the open caisson can be accurately controlled

from the above construction process, it can be seen that in the design and construction of open caisson in similar soil layers, the bearing capacity can be adjusted by changing the blade foot area, the length, diameter and strength of the powder spraying pile (by adjusting the amount of powder spraying). The method is diverse, the process is simple, and the cost is low. It is a successful construction process

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