【Excerpts】What are the factors that cause the foundation pit failure?

Typical engineering cases and analysis of foundation pit failure

 

 

Factors leading to foundation pit failure:

 

1.The design safety degree is not enough, not well-considered

2.Wrong concept of the design

3.The designers are not familiar with the local soil nature

4.Imperfect survey data and poor reliability

5.Local mutation of geology

6.The construction does not follow the requirements of the design drawings

7.Construction quality of retaining pile (wall)

8.Construction quality of mixing pile (rotary spray pile)

9.Anchor rod (soil nail) construction quality

10.Construction quality of the supporting rod

11.Internal damage of the support rod (internal injury)

12.The excavation of the envelope structure does not reach the design strength

13.Excavator and vehicle damage to the envelope structure

14.Excavator and vehicle hovering support operation

15.The construction load outside the foundation pit exceeds the design permit, especially the dynamic load exceeds the standard;

16.Insufficient slope unloading of the foundation pit top

17.Foundation pit excavation is too deep, and there is no design requirement for layered excavation

18.The foundation pit is exposed for a long time, and the cushion and floor did not timely follow up the influence of bad climate for a long time

19.The influence of the surrounding excavation and piling construction on the foundation pit

20.Water stop curtain fails

  

 

 

Problem description: the unqualified construction quality leads to the support failure.

Cause analysis: the center line of the ring beam, the reinforced concrete support and the tie are not in the same plane, and the ring beam produces local torsion effect, resulting in concrete cracking, which leads to the serious decrease of the lock stiffness.

Preventive measures: strictly follow the design drawings to ensure the construction quality of the support system.

 

 

Problem description: soil is piled outside the pit, causing support failure.

Reason analysis: Soil piling outside the pit leads to excessive force on the support system.

Preventive measures: When piling soil outside the foundation pit, the height of piling soil at 1m beyond the edge of the foundation pit shall not exceed 1.5m.

 

 

Problem description: Insufficient resistance of the support pile.

Cause analysis: the embedded length of the retaining pile is insufficient.

Preventive measures: the setting depth of the retaining pile should be calculated, that is, whether the horizontal resistance in the passive soil area is satisfied.

 

 

Problem description: retaining pile kick.

Reason analysis: the soil pressure outside the pit is too large.

Preventive measures: to find feasible ways to find signs of damage as soon as possible to ensure safety, such as setting up inclined tube and axial force meter.

  

 

Problem description: Column failure.

Cause analysis: it is related to many factors such as pit bottom uplift, vertical excavation unloading, excavation method, formation characteristics, bearing head, support type and number of support channels caused by the excavation of foundation pit excavation.

Prevention measures: When the foundation pit size is large, the calculated length of the support can be reduced.

  

 

Problem description: the overall unstable foundation pit.

Cause analysis: at the top of the slope, the slope is too steep; the excavation depth is too large; the soil increases the soil weight with water; the seepage of groundwater produces a certain dynamic water pressure; the water in the vertical cracks of the soil produces lateral hydrostatic pressure.

Preventive measures: Considering the various factors affecting the stability of the slope, determine the earthwork slope according to the experience, ensure the size of the slope, make the slope top load meet the specification requirements, or set up the necessary support.

 

 

Problem description: pit bottom bulge.

Cause analysis: Due to the rebound volume caused by unloading after excavation, the soil around the foundation pit raises the subsoil of the pit upward under the action of dead weight.

Preventive measures: strengthen the monitoring of pit bottom displacement. Once it is found that the displacement of the bottom of the pit reaches the warning value, immediately backfill the soil here until the bottom of the pit is no longer displaced, and then use the rotary jet and equipped cement to reinforce the soil.

 

 

Problem description: the pile body defect.

Cause analysis: the protective cushion of steel cage is not provided.

Prevention: Reduce the number of catheter lifts. The more the lifting of the pipe, the more likely it is to bring the ballast into the concrete. When the height of the top surface of the concrete from the hole of the pipe is long enough, it is forbidden to pull the pipe randomly, and the concrete in the pipe will produce super pressure or impact force to pour automatically down. When pouring to the end of the concrete is really difficult to irrigate, the number of lifting should be controlled to ensure the quality of the top concrete. Set up the protective layer pads according to the design requirements.

  

 

Problem description: the pile body defect.

Cause analysis: the protective cushion of steel cage is not provided.

Prevention: Reduce the number of catheter lifts. The more the lifting of the pipe, the more likely it is to bring the ballast into the concrete. When the height of the top surface of the concrete from the hole of the pipe is long enough, it is forbidden to pull the pipe randomly, and the concrete in the pipe will produce super pressure or impact force to pour automatically down. When pouring to the end of the concrete is really difficult to irrigate, the number of lifting should be controlled to ensure the quality of the top concrete. Set up the protective layer pads according to the design requirements.

  

 

Problem description: pipe pile inverted.

Cause analysis: improper earthwork stacking, foundation pit excavation in violation of relevant norms, pipe pile foundation is too much shear, resulting in the collapse.

Preventive measures: the construction party shall take special protective measures for the excavation of the foundation pit and the disposal of the earthwork. The supervisor shall not effectively deal with the illegal and illegal behaviors of the construction party and the construction party, and shall report the hidden dangers of the construction phenomena in time.

  

 

 

The basic principle and methods of emergency rescue

 

Basic principle: to maintain the existing force balance system

The most concise emergency rescue method: external unloading and filling inside

Common methods:

1.Remove the soil on the outside side of the foundation pit

2.Unloading on the outside of the foundation pit (various building materials, temporary rooms, vehicles, etc.)

3.Fill the inside of the foundation pit with earthwork or sandbags

4.inside the foundation pit reinforced concrete

5.Back-pressure of building materials

6.Water filling balance in the foundation pit

7.Add temporary support (concrete or steel support)

8.Set up temporary anchor bolts

9.Add retaining walls

10.Add water stop curtain (high spray or mixing pile)

11.Galla anchor

12.Add the temporary column (borrow the engineering pile)

13.sticky steel or carbon fiber repair support

14.Change the digging line

15.Stop the relevant construction around the foundation pit

16.Change the basement drawings conditionally

 

Remove soil outside the pit

 

Support repair

 

Support repair

 

Inverse pressure in the pit

 

Inverse pressure in the pit

 

Add temporary support

 

 

  

  

 ——Source: Chikulong Geotechnical

                                                                                                                                                                

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