1. The document outlines preventive and contingency measures for diaphragm wall and bored pile production near underground structures. It identifies causes of potential damage and collapse during construction and recommends measures to address them.
2. Key risks include erroneous surveying damaging underground utilities, trench collapse from low slurry levels or loose soils, and tunnel movement from nearby construction activities.
3. Recommended measures involve careful surveying, utility detection, instrumentation, maintaining safe slurry levels, stopping work if movement or collapse occurs, and backfilling trenches if necessary. Salvage plans are also outlined to address potential equipment losses.
1. The document outlines preventive and contingency measures for diaphragm wall and bored pile production near underground structures. It identifies causes of potential damage and collapse during construction and recommends measures to address them.
2. Key risks include erroneous surveying damaging underground utilities, trench collapse from low slurry levels or loose soils, and tunnel movement from nearby construction activities.
3. Recommended measures involve careful surveying, utility detection, instrumentation, maintaining safe slurry levels, stopping work if movement or collapse occurs, and backfilling trenches if necessary. Salvage plans are also outlined to address potential equipment losses.
1. The document outlines preventive and contingency measures for diaphragm wall and bored pile production near underground structures. It identifies causes of potential damage and collapse during construction and recommends measures to address them.
2. Key risks include erroneous surveying damaging underground utilities, trench collapse from low slurry levels or loose soils, and tunnel movement from nearby construction activities.
3. Recommended measures involve careful surveying, utility detection, instrumentation, maintaining safe slurry levels, stopping work if movement or collapse occurs, and backfilling trenches if necessary. Salvage plans are also outlined to address potential equipment losses.
1. The document outlines preventive and contingency measures for diaphragm wall and bored pile production near underground structures. It identifies causes of potential damage and collapse during construction and recommends measures to address them.
2. Key risks include erroneous surveying damaging underground utilities, trench collapse from low slurry levels or loose soils, and tunnel movement from nearby construction activities.
3. Recommended measures involve careful surveying, utility detection, instrumentation, maintaining safe slurry levels, stopping work if movement or collapse occurs, and backfilling trenches if necessary. Salvage plans are also outlined to address potential equipment losses.
Applicable Contingency Measures for Diaphragm Wall & Bored Pile Production in Vicinity of Underground Structures
S/No. Cause Identification
Applicable Preventive / Contingency Measures for Diaphragm Wall & Bored Pile Production in Vicinity of Underground Structures. page 1 1 Erroneous setting-out and damage cause to the numerous vital underground structures and utilities underlying propose development Establish Project Setting-Out Control, Existing Underground Structures and Surrounding Development Survey, and incorporate the same with construction tolerances provided in Section 9 of Particular Specifications into Propose Development Setting-Out Plans to eradicate erroneous setting-out and prevent damage causes to the numerous vital underground structures and utilities underlying propose development. Setting out shall be carried out by qualified surveyor. 2 Underground utilities damages, underground obstruction, slurry outflow and sudden loss of slurry in trench, resulting in trench collapse and potentially endangering platform stability. Carryout utilities detection by Licensed Cable Detection Worker along propose work layout to ascertain physical layout and function [live, abandon] status of known utilities. Trial trench and survey to ascertain position or absence of utilities example: sewer lines, cable ducts or ground cavities and implement appropriate measures [protection, diversion, termination, weep holes plugging, etc] prior to guide walls construction or casing installation to ensure against underground utilities damages and slurry outflow preventing trench collapse and endangering platform stability. Verify historic land use, study soil log to ascertain presence or absence of disturb ground, obstructions, and ground cavities. Implement relevant measures where applicable. 3 Underground structures damages All works within the Railway Protection Zone and within 6m of Common Service Tunnel shall adopt restricted activities guidelines stipulated in Code of Practice for Railway Protection and Standard Operating Procedure for Works within 6m of CST Structure. Setout 3.0m high Flag Marker Stumps, demarcate Reserve Lines layout on ground and hoarding and inculcate awareness in workmen on prohibited work processes to avert damages to Underground structures.
Notwithstanding the minimal influence concluded by ERRs on Existing North-South Line Tunnel section due to Silo Loadings, buoyancy from Bentonite Pond Construction and Heavy Machinery Movements across underlying structures, the following contingency measures shall be adopted under the following events: 4 Excessive Tunnel Movement during Bentonite Slurry Silo Loading Redistribute slurry to more distant silos and limit silo storage capacity to minimize tunnel movement. 5 Excessive Tunnel Movement during Slurry Pond Excavation Stop further excavation, backfill to higher elevation where applicable to minimize tunnel movement. 6 Excessive Tunnel Movement during Heavy machineries traverses or loading over tunnel.
Relocate position of Heavy machineries to minimize influence over tunnel.
Applicable Contingency Measures for Diaphragm Wall & Bored Pile Production in Vicinity of Underground Structures
S/No. Cause Identification
Applicable Preventive / Contingency Measures for Diaphragm Wall & Bored Pile Production in Vicinity of Underground Structures. page 2 7
Minor Collapse during excavation, due to lack of cohesion in sandy stratum; trench slurry level too low due to seepage, lack of topping up or excessive pumping; under circumstance where loose material stop raveling and platform stability is maintained. Analyze lowest slurry level where trench stability could be achieved and maintain slurry level during ongoing trenching process. Provide wider open trench within guide walls to increase reservoir of slurry to minimize slurry head drop. Banksman monitor rate of supply and operator could monitor rate of suction. Instrument trial panel to ascertain stability of soil stratum susceptible to loosening, at presence of very plastic marine clay to verify open trench or borehole stability. Conduct ultrasonic trench profile with Koden machine to ascertain extent of collapse do not undermine platform stability. Proceed with diaphragm wall or boring activities when open trench or borehole raveling stop and safe platform stability is ascertained. Trench Stability Analysis. Operating procedure of trench cutter and boring rig. Instrumented Trial Panel. Readings from Koden machine. 8 Major Collapse during excavation, due to lack of cohesion in sandy stratum; trench slurry level too low due to seepage, lack of topping up or excessive pumping; under circumstance where loose material keeps raveling and platform stability is in question. In event of major collapse, shift heavy machinery away from open trench. Check for signs of cracks and depression on platform. Ascertain extent of trench collapse and platform stability ultrasonic trench profile with Koden machine. If equipment is in trench, where feasible, devise the salvage plan after reviewing the extent of collapse, otherwise backfill the open trench with excavated spoil from discharge pit, or other cohesive material available on site, particularly suitable would be excavated materials.
9 Minor Collapse during cage installation, due to lack of cohesion in sandy stratum, trench slurry level too low due to seepage, lack of topping up or excessive pumping; trench squeeze in very plastic marine clay, or caused by heavy machineries surcharge during cage installation; especially after desanding where slurry density are generally lighter than excavation slurry; under circumstance where platform stability is maintained and service crane could salvage or extract cage in trench. Analysis on lowest slurry level where trench stability could be achieved and trial panel to ascertain stability of stratum susceptible to loosening. Prevent excessive lowering of slurry level by limiting suction level of submersible pump. During cage installation observe for presence of bubbles from slurry in trench, which can be deduced the cause of air pockets release through collapse medium. Note differences from crane hoist load indicators to signify load reduction or increase caused by squeezing or collapse trench or borehole. Check depth soundings, where toe level difference is within 100mm, remove loose material through airlift or redesanding after trench raveling stop; otherwise withdraw cage, repair cages where necessary, remove excess loose material, redesand and proceed with diaphragm wall production processes when trench and platform stability is verified stable through Koden readings.
Applicable Contingency Measures for Diaphragm Wall & Bored Pile Production in Vicinity of Underground Structures
S/No. Cause Identification
Applicable Preventive / Contingency Measures for Diaphragm Wall & Bored Pile Production in Vicinity of Underground Structures. page 3 10 Major Collapse during cage installation, due to lack of cohesion in sandy stratum, trench slurry level too low due to seepage, lack of topping up or excessive pumping; trench squeeze in very plastic marine clay, or caused by heavy machineries surcharge during cage installation; especially after desanding where slurry density are generally lighter than excavation slurry; under circumstance where platform stability may be in question and service crane could not salvage or extract cage in trench.
In event of major collapse, shift heavy machinery away from open trench. Check for signs of cracks and depression on platform. Ascertain extent of trench collapse and platform stability through readings from Koden machine ultrasonic trench profile or other feasible probing means. If steel cage is in trench, where feasible, devise the salvage plan after reviewing the extent of collapse. Backfill the collapse trench with excavated materials or other non-high strength cementious mixes. Review information from collapse event; devise and implement applicable ground improvement measures prior to subsequent diaphragm wall construction. Precaution Salvage plan devised shall preferably increase the number of lifting points [load distribution] at structurally suitable layout, but otherwise strengthening of lifting points, which currently does not cater to withstand unlimited or unknown loadings.
11 Trench Collapse during Casting Where Salvage and Extraction is Not Feasible owing to Cage Embedment in Concrete
Shift heavy machinery away from vicinity of open trench when stability of platform is in doubt. Under dire situation where cage extraction is not feasible owing to cage embedment in concrete where inactivity due to lack of other practical solution will aggravate the untoward circumstances, it is inevitable but prudent to adopt the following procedure to alleviate overall risk exposure. Measure level of concrete and compare with anticipated rise, too much a rise signifies necking or collapse. Check for signs of cracks and depression on platform. Note the level and magnitude where anomaly occurs and continue with casting operation. Do not cut tremie pipes shorter than theoretical concrete embedded depth. Where possible maintain tremie pipes as low as possible to enable increased flushing. Overcast panel till fresh concrete overflows from top of guide wall or assurance attained by the extent of overcast. Carryout PIT test, coring test, etc. to verify soundness of constructed element and provide remedial proposal where applicable.