Incident Prevention Guide for Nearby Works

DDigging into soil and building pile walls in excavation pits when constructing foundations or underground floors has recently caused many incidents to works adjacent to excavation pits, especially in the city. Ho Chi Minh, Hanoi and some other cities. The incident occurred during both the construction of the pile wall and during excavation. The main incidents that have occurred are: structural cracks, broken pipes, tilted houses, ground subsidence, collapsed fences, and house collapses. These phenomena often occur in areas with weak clay or flowing sand when the dug pile wall is not stiff enough or lacks water insulation (pressure piles, intermittent bored piles, melaleuca piles or some types of piles). other). At some construction projects, incidents have occurred even when the ground is not too weak but the pile wall is not hard enough or when the pile wall is a wall in the soil that is hard enough but is defective, unable to prevent underground erosion of water and sand.

 The construction of excavations in principle needs to ensure safety for both the items inside the excavation and the structures adjacent to the excavation. In 2006, after a number of incidents occurred for structures located in excavated holes, the Ministry of Construction issued the document "Technical guidelines for preventing construction incidents when digging deep foundation holes in soft soil areas". to prevent such incidents.

Vibrations arising when lowering piles or lowering casings to drill bored piles can cause settlement of the foundations of neighboring structures resting on some types of loose, poorly compacted soil or cause structural damage by direct impacts. onto them;
– Soil displacement
– Vertical displacements (subsidence or heaving) and horizontal displacements of soil occur when constructing pit pile walls (usually steel sheet piles, piles or barrettes), when excavating foundation pits, and when draining water from the pit excavation or when recovering steel sheet piles.
– When vibrating or pressing pre-made pile walls, the soil surface tends to lift and the soil is pushed away. On the contrary, when constructing bored piles or barrettes, the surrounding soil surface sinks and the soil moves horizontally towards the hole drilling location.
– When excavating the foundation pit, the ground in the surrounding area sinks and moves horizontally towards the excavation. The degree of settlement and horizontal displacement depends on the excavation depth, characteristics of the ground, supporting structure and excavation process. Large movements often arise when constructing deep excavations in soft soil.
– When pumping water to construct dug holes, the underground water level is lowered, increasing the settlement of the ground in the surrounding area. The degree of settlement depends on the level of groundwater level lowering, soil characteristics and construction time.
– When recovering steel sheet piles, soil moves into the voids left by the piles, causing subsidence in the area around the pile wall.
- Unstable HThe excavation can become unstable due to the support system not being able to bear the force or due to deep sliding. In this case, the structures adjacent to the excavation will experience large displacements and may collapse immediately.
– Landslide is hLocal collapse of dug trenches and boreholes when constructing pile walls and piles using the cast-in-place method can leave small cavities in the soil. Larger-scale cavities are formed when soil is washed away by the flow of water into the foundation pit through gaps between sheet piles or through defects in the pile structure. When the earth dome above these cavities collapses, it will cause ground collapse or the failure of structures above it. This phenomenon is likely to occur when dewatering excavation pits for foundation and basement construction in water-saturated sand.

3. Incident prevention measures

- General requirements

– Construction of excavated pits for underground floors or foundations of works is a complex job that needs to be managed, supervised and implemented strictly at all steps from survey, design of construction methods, and execution. to handle situations that arise during construction.

– Crowded works, culturally or politically important works within the scope of influence of the excavation need to be proactively supported and reinforced before constructing the excavation.

– Survey to serve the design of construction methods

– The volume and depth of geotechnical surveys serving the design of excavation construction methods must comply with the requirements of standards TCVN 4419:1987 – Survey for Construction. Basic rules; TCVN 160:1987 – Geotechnical Survey for pile foundation design and construction; TCXD 194: 1997 High-rise buildings – Geotechnical survey work; TCXD 205:1998 Pile foundations – Design standards.

When preparing a geotechnical survey outline, it is necessary to refer to available data in the vicinity of the project. If there is not enough data, a number of survey points must be arranged along the edge of the excavation with an initial distance of no more than 30 m/point. Survey density should be no greater than 20m/point when one of the following conditions exists:

a) Large excavation depth (over 2 underground floors or over 6 m);

b) Ground conditions have many fluctuations in properties and thickness of soil layers;

c) Within the excavation area, there are sand layers saturated with water and a high groundwater level;

d) Constructions in the surrounding area have sunk and cracked.

* Survey results, in addition to serving normal foundation design, need to provide additional data on:

a) Soil water, including surface water and fluctuations in groundwater levels according to the seasons of the year;

b) Criteria for permeability of soil layers, in which the permeability of loose soil layers needs to be determined by field testing;

c) Other specific criteria (if any) determined according to the requirements of the method of calculation and design of construction measures.

* After completing the survey, the drill holes must be filled with waterproof material.

– Surveying the current status of structures on the ground adjacent to the excavation pit needs to ensure that those structures are classified according to their importance and sensitivity to ground displacement. The survey scope is works at a location equal to 2 times the excavation depth from the edge of the excavation. Information collected includes:

a) Type of construction, location and distance to the excavation pit;

b) Elevation and characteristics of the foundation structure;

c) Scale and structural characteristics: plan, number of floors, type of structure (masonry, steel, reinforced concrete), tilting, subsidence of the building, structural cracking (shown on the location drawing , width if any).

– Pipelines, cable routes, and underground technical compression pipes need to be surveyed within a plan size equal to 1 time the excavation depth from these technical equipment to the edge of the excavation. Information that needs to be collected includes: Characteristics, depth, size and distance to the excavation hole.

– Design construction methods.

– When designing construction methods, do not use structures to support the excavation wall with piles that do not have a water-proof connection such as conventional bored piles, driven piles or pressed piles. Prioritize the use of barrette piles in weak soil conditions with underground water, especially when building 2 or more basement floors. Steel sheet piles can be used to construct 2 basements in good soil conditions or 1 basement in weak soil conditions with groundwater.

– Calculating the stability of the excavation support system for the underground floor must take into account earth pressure, load of structures in the surrounding area and other loads that may arise during construction. The earth pressure acting on the retaining wall of the dug hole is equal to the earth pressure at rest. For soft soil layers, apply the horizontal pressure coefficient K=1.0.

– The depth of pile placement must ensure the stability of the excavation wall, with the focus being on sliding stability. In case the sandy foundation is below the water table, it is necessary to lower the piles to a layer of soil capable of insulating water below the maximum excavation depth. Creating a water-proof layer at the bottom of the excavation or forcing water outside the excavation should be considered in construction design when the water table level must be maintained to protect adjacent structures.

– When designing construction measures, it is necessary to evaluate its impact on neighboring works and propose measures to limit adverse effects, including:

a) Vibration in the surrounding area when constructing pre-fabricated sheet pile walls, barrette sheet pile walls or building piles (if any). When lowering prefabricated piles, static pressing methods should be chosen to limit vibrations;

b) Displacement (settlement or heaving and horizontal displacement) when constructing prefabricated pile walls. Priority should be given to using steel piles to minimize ground displacement when lowering and withdrawing piles. In case the piles are arranged too close to neighboring structures, the piles should not be recovered after completing the underground construction;

c) Displacement (subsidence and horizontal displacement) of the surrounding area corresponding to each stage of excavation construction. To limit displacement, measures can be taken to increase the stiffness of the excavation wall support system such as:

– Use pile walls with high bending resistance, giving priority to using walls in soil;

– Use bracing systems and horizontal struts with sufficient stiffness;

– The connection between braces and struts with the wall or between them must have good contact to eliminate initial deformation, not cause local instability and ensure uniform load distribution;

– Limit axial deformation of struts by preloading.

d) Settlement in the surrounding area due to the impact of lowering the water table in the foundation pit. Measures to prevent and limit the effects of lowering the groundwater level are:

– Quick construction of each stage;

– Create a water-proof layer at the bottom of the excavation pit by drilling and injecting cement mortar, cement mortar/bentonite, and silicate;

– Stabilize the underground water level outside the excavation pit by pressing rehydration measures.

– Design documents need to specify the volume, type, location and time of conducting geotechnical observations. The amount of monitoring depends on the scale and complexity of the project but must not be less than 3 points for each type of monitoring.

Commonly monitored parameters are: Settlement, horizontal displacement of soil according to depth and water level in the soil. Monitoring the water table must be done when digging in sand below the water table. Equipment installation and monitoring must be done before construction begins.

– The design needs to calculate displacement values for each construction stage, thereby providing corresponding warning thresholds to prevent incidents when monitoring during future construction.

When calculating and checking, you can refer to the following displacement data:

a) The settlement of the ground when constructing a wall in the soil is about 0.1%-0.15% of the wall depth;

b) The limits of settlement and horizontal displacement of neighboring structures are:

+ If small cracks are accepted on reinforced concrete structures and load-bearing walls of neighboring structures: Limited cumulative differential settlement of surrounding structures due to underground construction ( ).

+ Calculated deformation due to tensile stress in adjacent building structures

– Construction of excavation pits carried out in accordance with designed construction methods. It is recommended to start constructing piles to hold dugouts from areas far from existing structures to check construction technology and evaluate its impact on the surrounding area such as the impact of vibrations and soil displacement. , quality of concrete pouring, joints and other impacts. It is necessary to change technology or adjust design when technology does not meet requirements through trial construction.

– Excavated holes supported by walls in the ground should note the following points:

– When constructing wall panels, if the width of the trench is eroded by 5-10%, it is necessary to change the type or density of the wall retaining solution, raise the guide wall to increase the pressure head of the solution or change the excavation technology.

– The construction site must be flat, not flooded, and the foundation must be compacted or stabilized enough to bear the load of construction equipment.

– It is necessary to ensure that the slope of the wall is not more than 1/100. The guide wall needs to guide the digging bucket to ensure straight excavation of wall sections, maintain the solution level at the necessary stable level and hang the main wall reinforcement. The guide wall is constructed to a depth at least equal to the depth of the shallow foundation bottom of neighboring structures, or to the depth of the backfill layer, usually about 0.7 - 1.5m. If the bottom of the foundation of neighboring structures is shallower, or there is a possibility of slipping when digging the leading wall, measures need to be taken to strengthen the foundation before digging.

– The number of concrete pipes (tremie pipes) for a section of wall (panel) must ensure that the distance from the pipe to the farthest edge of the slab is not more than 1 m, and at the same time ensure that the concrete rise is not less than 3m/hour. . The pipe is submerged in concrete at least 2m deep. If delays in concrete delivery are expected, slow-setting admixtures should be used.

– The quality and uniformity of concrete are checked by ultrasonic method according to standard TCXDVN 358:2005 "Drilled piles - Method of checking concrete uniformity by ultrasonic method". Ultrasonic tubes are arranged at a distance of no more than 1.5 m and no less than 2 tubes per wall panel. Ultrasonic testing is needed to check the quality of the panels and the joints between the panels.

– The horizontal support system should choose shaped steel one level larger than the design calculation. There must be reserve shaped steel to immediately strengthen the supporting system when there are signs of damage or incidents in neighboring structures. For projects with more than 2 underground floors, priority should be given to applying the top-down construction method.

– It is necessary to dig the soil in batches, the depth of each batch is not more than 1m. Start digging from the short side of the foundation hole and from the middle of the hole and gradually work your way around.

– It is necessary to carry out monitoring before starting construction and during the construction process. As follows:

a) Monitor settlement and tilt of neighboring structures. Settlement markers should be mounted at the corners of the building and on the main load-bearing structures. For pipelines, cable routes, and technical tunnels, arrange monitoring markers 15 to 25 m apart along the route. Constructions that are sinking and deformed close to the warning threshold as stated in section 3.3.6, need to be continuously monitored.

b) Monitor the horizontal displacement of the ground. Use a depth-based horizontal displacement monitoring device (inclinometer) with an inclined measuring tube arranged outside the pile wall. Priority is given to arranging monitoring points on the side of structures that are expected to sink and deform up to the warning threshold as stated in section 3.3.6 and at the space between the edges of the excavation. The bottom depth of the monitoring pipe must be embedded in hard soil at least 2 m or 3 m deeper than the tip of the pile, whichever is the larger of the above two values.

c) Monitoring underground water level. It is necessary to monitor the groundwater level in the non-cohesive soil layers (sand, sandy loam) located above and immediately below the excavation depth. Monitoring points are arranged outside the pile wall, no more than 25 m apart along the perimeter of the wall and no less than 1 point at each edge of the excavation. Monitoring points should be added on the side of projects that are expected to sink and deform to the warning threshold as stated above.

d) Monitor axial force at struts or anchors. Monitoring is performed using a strain gauge or a load cell. Measuring equipment should be arranged at all levels with struts or anchors and mounted on not less than 15% total number of bars.

– Monitoring results are provided to the investor and supervision consultant immediately after each on-site monitoring. When the on-site monitoring value reaches 70% the value calculated in the design, it is necessary to strengthen monitoring and prepare to take measures to prevent incidents.

– Construction of excavation pits must be stopped to assess the danger level of neighboring structures when the monitoring value reaches one of the following limits:

– When the field monitoring value reaches 100% the value calculated in the design; 

– When the monitoring value has not reached the threshold of 70% calculated value in the design but adjacent structures have been detected with signs of danger.

– The assessment of the level of danger of neighboring structures is carried out according to TCXDVN 373: 2006 "Instructions for assessing the level of danger of house structures".

– When monitoring the groundwater level, if a local decrease in the groundwater level is detected at a few monitoring points, it is necessary to re-check the measuring equipment, the results of the geotechnical survey performed, the pile depth, pile wall quality to take necessary remedial measures.

4. Handling damages and incidents

– Damage handling kWhen constructing the foundation and underground floor according to the established methods but the neighboring works still suffer damage as stated in section 2 of this document, it is necessary to temporarily stop construction, find the cause and take appropriate measures.

– During the pile lowering process, if the cause of damage is determined to be due to inappropriate pile lowering technology, depending on the specific conditions, one of the following measures can be applied:

– Use construction technology that causes less vibration;

– Applying auxiliary measures for lowering piles (drilling, water erosion);

– Change the pile type (switch to a pile type that causes less soil movement).

– During the excavation process, if the cause of damage is determined to be settlement and horizontal displacement exceeding the expected value in the design, it is necessary to strengthen the support of the excavation wall or backfill part or all of the excavation. .

– During the excavation process, if the cause of ground cracking or structural damage is determined to be due to underground soil erosion, construction must be stopped and one of the following measures must be applied:

– Create a reverse filter layer with suitable graded materials or use geotextile;

– Pump water into the foundation pit to the initial underground water level;

– Survey the pile wall, identify defects (if any), create piles on the side of the defect or use appropriate measures to ensure that water does not continue to erode sand through the defect location;

- Resovle problem

When constructing the foundation and underground floors according to the established methods but the neighboring works still suffer from problems as stated in section 2 of this document, it is necessary to stop construction and urgently apply the following handling measures at the same time:

– Immediately support neighboring structures at risk of collapse;

– Reinforce the partially damaged excavation support;

– Fill the entire excavated hole with soil if the cause of the incident is due to sliding or displacement greater than the expected calculation;

– Fill the hole with water or fill it with soil if the cause is underground erosion;

Construction will continue only after determining the cause of the incident and redesigning the construction method.