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 (settlement or heave) and horizontal displacements of the soil occur during the construction of excavation retaining walls (usually steel sheet piles, piles, or barrettes), during excavation of foundation pits, during dewatering of excavation pits, or during the recovery of steel sheet piles.
– When prefabricated sheet pile walls are vibrated or pressed, the ground surface tends to rise and the soil is pushed away. Conversely, when constructing bored piles or barrette piles, the surrounding ground surface subsides and the soil shifts horizontally towards the drilling location.
– During excavation of foundation pits, the surrounding soil settles and shifts horizontally towards the excavation. The degree of settlement and horizontal displacement depends on the excavation depth, soil characteristics, support structures, and excavation procedures. Large displacements often occur when excavating deep pits in weak soil.
– When pumping water for excavation, the groundwater level is lowered, increasing the settlement of the surrounding soil. The degree of settlement depends on the extent of groundwater level lowering, the characteristics of the soil, and the construction time.
– When steel sheet piles are removed, soil shifts into the voids left by the piles, causing subsidence in the area surrounding the sheet pile wall.
– Instability 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.
Landslides are a type of landslide.Local 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

– Excavation work for underground levels or foundations of buildings is a complex task that requires strict management, supervision, and execution at all stages, from surveying and designing construction methods to construction and handling unforeseen situations during construction.

– Buildings with large crowds, or those of significant cultural or political importance located within the excavation area's impact zone, need to be proactively supported and reinforced before excavation begins.

– Survey for the design of construction methods

– The volume and depth of geotechnical surveys for designing excavation methods must comply with the requirements of the following standards: TCVN 4419:1987 – Surveying for Construction. Basic Principles; TCVN 160:1987 – Geotechnical Surveying for Design and Construction of Pile Foundations; TCXD 194:1997 High-rise Buildings – Geotechnical Surveying; 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.

– The survey of existing structures on the surface adjacent to the excavation pit must ensure that these structures are classified according to their importance and sensitivity to soil displacement. The survey scope includes structures located at a position equal to twice the excavation depth from the edge of the pit. The 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).

– Underground pipelines, cables, and compression tunnels need to be surveyed within a surface area equal to one time the excavation depth from these technical devices to the edge of the excavation pit. The information to be collected includes: their characteristics, depth, dimensions, and distance from the excavation pit.

– Design of construction methods.

– When designing construction methods, avoid using excavation wall supports made of non-waterproof piles such as bored piles, driven piles, or pressed piles. Prioritize the use of barrette piles in weak soil conditions with groundwater, especially when constructing two or more basement levels. Steel sheet piles can be used for constructing two basement levels in good soil conditions or one basement level in weak soil conditions with groundwater.

– Calculating the stability of the excavation wall support system for the underground level must take into account soil pressure, the load of structures in the vicinity, and other loads that may arise during construction. The soil pressure acting on the excavation wall support is taken as the soil pressure at rest; for weak soil layers, a lateral pressure coefficient K=1.0 is applied.

– The depth of pile driving must ensure the stability of the excavation walls, with a focus on sliding stability. If the sandy soil is below the groundwater level, piles should be driven to the most water-impermeable soil layer at the greatest excavation depth. Creating a water-impermeable layer at the bottom of the excavation or applying water infill outside the excavation should be considered in the construction design when maintaining the groundwater level is necessary to protect adjacent structures.

– When designing construction methods, it is necessary to assess their impact on neighboring structures and propose measures to mitigate 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 sheet pile walls with high bending stiffness, preferably those placed in the ground;

– Use bracing and crossbars of sufficient rigidity;

– The connections between bracing and struts and the wall, or between them, must have good contact to eliminate initial deformation, prevent local instability, and ensure even load distribution; ;

– Limit axial deformation of the strut 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:

– Construction is carried out quickly in each stage;

– Create a waterproof layer at the bottom of the excavation pit using methods such as cement grout injection, cement/bentonite grout, or silicate injection;

– Maintain a stable groundwater level outside the excavation pit by injecting water.

– The design documents must specify the quantity, type, location, and timing of geotechnical monitoring. The quantity 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 the displacement values corresponding to each construction phase, thereby establishing corresponding warning thresholds to prevent incidents during subsequent monitoring in the construction process.

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

– Excavation work 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.

– When excavating pits supported by retaining walls in the ground, the following points should be noted:

– When constructing wall panels, if the width of the excavation trench collapses (5-10%), it is necessary to change the type or density of the wall-stabilizing fluid, raise the guide wall to increase the pressure for the fluid, or change the excavation technology.

– The construction site must be level, free from flooding, and the foundation must be compacted or reinforced to withstand the weight of the construction equipment.

– The slope of the guide wall must not exceed 1/100. The guide wall must guide the excavator bucket to ensure straight excavation of the wall sections, maintain the necessary stable fluid level, and support the main wall reinforcement. The guide wall should be constructed to a depth at least equal to the depth of the shallow foundations of adjacent structures, or to the depth of the backfill layer, usually about 0.7 – 1.5m. If the foundations of adjacent structures are shallower, or are likely to slide when excavating the guide wall, foundation reinforcement measures must be taken before excavation.

– The number of concrete pouring tubes (tremie tubes) for a wall section (panel) must ensure that the distance from the tube to the furthest edge of the panel does not exceed 1 m, and that the concrete rise is not less than 3 m/hour. The tube must be submerged in the concrete to a depth of at least 2 m. If delays in concrete delivery are anticipated, a slow-setting admixture should be used.

– The quality and homogeneity of the concrete are checked using ultrasonic testing according to standard TCXDVN 358:2005 “Bored piles – Method for testing the homogeneity of concrete by ultrasonic testing”. The ultrasonic tubes are arranged at a distance of no more than 1.5 m and no less than 2 tubes for each wall panel. Ultrasonic testing is required to check the quality of the panels and the joints between the panels.

– For horizontal bracing, choose structural steel with dimensions one size larger than the design calculations. Reserve structural steel must be available to immediately reinforce the bracing system in case of damage or incidents in adjacent structures. For structures with more than two underground levels, the top-down construction method should be prioritized.

– Excavation should be done in stages, with each stage no deeper than 1 meter. Begin digging from the shorter side of the foundation pit and from the center, then gradually work outwards.

– Monitoring must be carried out before and during construction. Specifically, 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 supervisory consultant immediately after each on-site monitoring. When the on-site monitoring value reaches 70%, the value calculated in the design, monitoring should be intensified and preventive measures should be prepared.

– Excavation work must be stopped to assess the level of danger to adjacent structures when the monitoring values reach one of the following limits:

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

– When the observed value has not yet reached the 70% threshold (the calculated value in the design), but signs of danger have been detected in a neighboring structure.

– The assessment of the danger level of adjacent structures is carried out according to TCXDVN 373:2006 “Guidelines for assessing the danger level of building structures”.

– When monitoring groundwater levels, if a localized drop in groundwater level is detected at several monitoring points, the measuring equipment, the results of the geotechnical surveys, the pile driving depth, and the quality of the pile walls should be re-checked to take necessary corrective measures.

4. Handling damages and incidents

– Repairing damageWhen 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 driving process, if the cause of damage is determined to be due to inappropriate pile driving technology, depending on the specific conditions, one of the following measures may be applied:

– Utilize construction technologies that minimize vibration;

– Apply auxiliary measures for pile driving (drilling, water jetting);

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

– During excavation, if the cause of damage is determined to be settlement and lateral displacement exceeding the values specified in the design, it is necessary to reinforce the excavation walls or backfill part or all of the excavation with soil.

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

– Create a reverse filter layer using materials with appropriate gradation or geotextile fabric;

– Pump water into the foundation pit until it reaches the initial groundwater level;

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

– Troubleshooting

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 reinforce nearby structures that are at risk of collapse;

– Reinforce the support structure of the excavation pit that has been partially damaged;

– Backfill the entire excavation pit if the cause of the incident is due to sliding or excessive displacement beyond the calculated limits; ;

– Pump water into the excavation pit or backfill 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.