Settlement, which is a downward motion of foundations due to applied loads exceeding design limits, can occur and may cause movement in structures. However, settlement can be tolerated as long as the loads do not exceed the allowable bearing pressures.
When investigating foundation defects, a thorough examination is essential to obtain an accurate diagnosis for a report. It is crucial to gather and carefully examine all available evidence before deciding on a method for repair. While the inspection may take time, it is important to investigate extensive defects properly. It is also important to consider that multiple causes may be responsible for a defect, even though the primary cause should be investigated.
For instance, foundation movements may cause a fractured external wall, but rainwater could penetrate the fracture and result in dampness on the internal face of the wall. Therefore, it is important to consider all possible factors when diagnosing foundation defects.
Collapse of a building in New York City
Thorough investigation is crucial when it comes to foundation repairs for existing buildings, as they are often challenging and expensive to carry out. The main objective of the investigation should be to assess the characteristics and stability of the subsoil under the existing building’s load. This is particularly important in New York, where the construction of a new structure can have significant effects on an existing structure, leading to defects that require remedial solutions.
1. Suburb Sewers Line Project
During the development of a sewer line project in Brooklyn, it became necessary to excavate deeply near some old residential structures. The plan was to have the sewer lines run close to the existing footings, which posed a challenge as the excavation was within the influence zone of the footings. To complete the project, piles were installed using a vibratory pile driver, and unfortunately, there was no way to monitor the vibration levels during the pile driving process.
As a result of the construction activity, numerous structures close to the project site experienced settling, tilting, cracking, and movement. The footings of the affected buildings were compromised, causing considerable damage. To mitigate the damage, inclining supports were added as a brace for lateral support. Unfortunately, despite these efforts, one of the buildings collapsed, and one of its residents lost their life.
Further investigation into the cause of the incident revealed that the affected building had been constructed using timber footings, a method commonly used at the turn of the century. Although the footings were in acceptable condition, they had been adversely affected by the construction activity associated with the sewer line project.
1.1 Restoration Measures
The collapse of a structure occurred due to excavation work carried out in the influence zone of a timber footing, which resulted in the loss of strength of the soil supporting the vertical support. As a consequence, the building collapsed due to settlement despite the presence of a high-grade steel bracing framework system that was unable to save the structure since the soil support was lost below the foundation level.
To restore the integrity of the vertical support, the initial plan was to underpin the structure. However, the bid prices for this option were exorbitant. As an alternative, it was decided to purchase the damaged buildings and demolish them since this approach was more cost-effective.
1.2 Lessons Learned
Rewrite only given context in paragrahs
Paragraph 1: To ensure the safety of adjacent structures during construction, a thorough preconstruction study must be conducted. This study should involve reviewing existing plans, conducting soil borings, and assessing the strength and condition of nearby buildings and structures. The study must determine whether protection and underpinning are necessary.
Paragraph 2: When excavating for sewers or other utilities, it is crucial to avoid being too close to existing buildings. The excavation cut should be located outside of the footing influence lines to prevent damage to adjacent structures.
Paragraph 3: In loose soils like sands, it is best to avoid using vibratory equipment for pile driving. The use of such equipment could be potentially dangerous and damaging to nearby structures. Therefore, alternative methods should be employed.
Paragraph 4: To ensure the suitability and proper energy of the driving hammers, vibration readings should be taken regularly using seismographs. The frequency of readings should be determined by the requirements of the construction project. This measure will help to maintain the safety and integrity of the surrounding structures.
2. Electronics Plant
An electronic plant was being constructed in Upstate New York when a building designed to house some of the electronics experienced settlement, causing it to move vertically and laterally towards a nearby valley. The building’s primary framing elements, including columns and beams, were made of steel, while the floors were reinforced concrete. As a result of the building’s settlement, cracks appeared in the concrete slab. However, it was found that the bolted connections at the column-beam and beam-to-beam junctions were not deformed beyond their capacity. Through the stress-relief method, the strength of the bolted connection was measured, and it was determined that the main reason behind the building’s cracks was the significant movement, which measured approximately 102 millimeters.
2.1 Restoration Measures
The structural corrective measure that was carried out involved enlarging the existing footings. The aim of this was to ensure that the structure could support the additional live loads. The reason for this was to prevent considerable settlements from occurring.
By enlarging the footings, the hope was that the structure would be able to withstand the increased weight and pressure that would be placed upon it. This was seen as the best solution to the problem, as it would allow the structure to continue functioning without the need for major changes or alterations.
It was hoped that this approach would prove effective in addressing the concerns that had been raised about the stability of the structure. By taking this step, the hope was that the structural integrity of the building would be preserved, and that it would be able to continue serving its intended purpose for many years to come.
2.2 Lessons Learned
When designing high footings adjacent to severe soil slopes, it is important to consider the potential for settlements. To mitigate this risk, it is recommended to use low allowable bearing pressures in the design process. Additionally, in more severe cases, the use of retaining walls should be considered to provide additional support and stability.
When evaluating the adequacy of a distressed structure, it is essential to assess the actual stress level within the structure. This provides a more accurate measure of the structure’s condition and helps to determine the appropriate course of action for repairs or remediation.
The stress-relief method has proven to be a useful tool in the strength evaluation of distressed structures. By using this method, engineers can better understand the level of stress within a structure and identify potential areas of weakness or failure. This information can then be used to develop effective repair and strengthening strategies to improve the overall structural integrity of the building or infrastructure.
3. East Side Hospital
During the construction of East Side Hospital, an urgent alarm was sounded for all those involved in the high-rise hospital project. It was discovered that during the excavation for the hospital’s foundation, it was revealed that the adjacent high-rise residential building was precariously resting on a narrow strip of rock. As a result, all construction activities were immediately halted, and the fully occupied residential building was evacuated for safety reasons.
High-rise residential building unsteadily sitting on a sliver of rock
During the investigation, it was discovered that the width of the rock sliver, which was supporting the high-rise building and of unknown strength, measured approximately 355 mm. Surprisingly, this was the only structural element supporting the entire building, which was quite remarkable.
Furthermore, it was revealed that the footings as delineated in the original design for the existing apartment building were not built at all. This oversight was unexpected and raised concerns about the structural integrity of the building.
Another surprising finding was that the easterly wall of the building, which was originally thought to be a standard concrete wall, was actually a rock face with a stucco finish. This unexpected construction method added to the complexity of the investigation and raised questions about the building’s compliance with building codes and regulations.
Additionally, it was discovered that the plans that had been filed with the Department of Buildings did not accurately reflect the as-built conditions of the building. This discrepancy further complicated the investigation and pointed to potential violations in the construction process and documentation.
Overall, the investigation revealed multiple unexpected conditions, including the sole reliance on a rock sliver for structural support, the absence of built footings, the unconventional construction of the easterly wall, and inaccurate plans filed with the Department of Buildings. These findings raised serious concerns about the building’s structural integrity and compliance with building codes, requiring further investigation and corrective measures.
3.1 Restoration Measures
Following the investigation results, it was found necessary to implement an emergency plan for underpinning. The plan involved the construction of new laterally braced concrete piers into the rock sliver. The purpose of this plan was to provide additional support to the existing structure in order to prevent any possible structural damage or collapse. The plan was executed flawlessly, and as a result, the alarm was deactivated.
According to the New York City Building Code, it is the responsibility of the neighbor who is carrying out new construction to underpin the existing structure if their footings go below the bottom of an existing footing and are more than 3.0 meters below curb level. This means that if the neighboring construction goes below the existing footing, it is their responsibility to provide additional support to the existing structure to ensure its stability and safety. The underpinning process involves the installation of additional piers or support structures to reinforce the foundation and prevent any potential damage or collapse. Adherence to the regulations outlined in the New York City Building Code is crucial in ensuring the safety and integrity of existing structures during construction activities.
3.2 Lessons Learned
It is strongly advised that individuals refrain from using the rock located outside their own property to provide support for their structures, even if it seems like a cost-effective solution in the short-term. The only rock that should be utilized for foundation support is that which is entirely within one’s own property line. It is not recommended to rely on thin rock slivers for the purpose of supporting a structure’s foundation.
4. Manhattan Hospital Complex
During the deep excavation for the Manhattan Hospital project, adjacent buildings experienced cracks and sinking as the excavation progressed towards a depth of 14 m below grade. Although the initial expectation was that the excavation would encounter sound rock, gneiss, the adjacent structures had single-level basements and were constrained by a sump siphon for ground-water control under an existing basement slab. It was determined during the design stage that underpinning of the adjacent structures would be required, as per the requirement of New York City Construction regulations. Accordingly, an expert specialist was engaged to develop a plan for underpinning, which involved the use of post-tensioning cables to resist earth pressures in supporting pits.
Cracks in adjacent buildings to Manhattan Hospital during foundation excavation
During the excavation of the pits, movement was detected in the building located directly east of the site. Water was found several feet below the pits, and attempts were made to dewater the soil around the excavation. However, this proved to be futile as the soil was impermeable. Well points were then used for dewatering, but they also proved ineffective due to silts in the soil clogging the well point screens. A third alternative using jack-piles was considered. This method involved pushing pipe segments into the ground, utilizing the building’s weight as a reaction for the jacking forces. However, it became apparent that the footings of the existing building, which consisted of decomposed rubble, did not offer sufficient resistance to the jacking loads.
Lateral bracing of adjacent structures
As construction efforts were underway, another unfavorable shift was observed in the nearby building. This raised concerns about the safety of the structure, and temporary braces were promptly installed. As a result, construction operations came to a halt, as ensuring the safety of the workers and the structure took precedence.
4.1 Restoration Measures
The two buildings on the east are showing serious cracking throughout, with the one closest to the excavation site settling and causing the entire building to pivot around the foundation walls. As a result, the party wall between the two buildings has moved westward, leaving a 152 mm gap at the top. This movement has caused significant damage to the interior of the building, including settled floors, cracked ceilings and walls, and even a stuck elevator due to deformation of its shaft. On the fourth floor, there is a separation between the wood joists and the party wall, causing the floor to settle by 28 mm.
Cracking of partition wall
When all attempts to support the building failed, the final option was to use an expensive method of soil solidification through freezing. Fortunately, the saturated soil actually facilitated the freezing process. The installation of this process, which involved pipes and equipment, took approximately three weeks. However, it was ultimately successful in stopping almost all movements within the adjacent structure.
To further stabilize the building during subsequent construction operations, a cross-lot bracing system was installed just before freezing the soil. This system, consisting of large braces that spanned the entire lot, prevented any further movement in the building and allowed construction to proceed uninterrupted. Once the soil under the building was consolidated, the original underpinning design was implemented. Finally, after the completion of the underpinning, the adjacent buildings were restored.
4.2 Lessons Learned
Accurate preconstruction subsoil study is crucial to ensure the success of foundation work. Without adequate test data, there is a risk of serious delays, cost overruns, damage, and even collapses. If precise information about the subsoil had been obtained beforehand, it would have been possible to plan for design changes and soil solidification before starting the construction process. This could have prevented unforeseen complications and difficulties during the project.
During underpinning operations, it is essential to carefully monitor any adjacent structures. If any movements are detected, changes in methods or procedures can be implemented promptly to prevent any potential damage or accidents. It is critical to remain vigilant and address any signs of instability or shifting during the underpinning process to ensure the safety of both the workers and any neighboring structures.
In summary, obtaining accurate test data before construction is necessary to prevent any unwanted delays, costs, and damage. During underpinning operations, monitoring adjacent structures is vital to detect any movements promptly and implement necessary changes to ensure the safety of all involved.
5. Queens Apartments
The construction of Queens apartments took place on marshy land, and after the project was completed, cracks were observed in the masonry walls at various levels of the buildings, both on the exterior and interior walls. Further investigation revealed that some of the piles in a three-pile group that supported the distressed corner of the building had broken. The cause of pile breakage was determined to be the consolidation of the organic clay stratum when loads were applied to the pile. As the stratum consolidated under the load, it resulted in breakage of the piles.
Cracks developed in the masonry walls
Tests were conducted in a research laboratory and field program to evaluate the potential for additional settlement and determine its possible extent. In the laboratory, the rate of settlement resulting from secondary compression was estimated. In the field, a sensitive settlement plate was installed and monitored for approximately six months using a depth caliper with a precision of 0.025 mm. The chosen location for the settlement plate was the inside corner of the pantry room, selected due to its proximity to settlement cracks.
5.1 Restoration Measures
Laboratory data analysis has revealed that the organic clay is expected to settle by approximately 25 mm for every ten years due to the weight of the fill above it. However, the settlement plate measurements taken from the field indicated a higher level of settlement than what was anticipated from the laboratory data. According to the field data, settlements were about twice as much as what was suggested by the laboratory data.
To prevent any further settlement, repair work was carried out by adding new reinforced-concrete beams. The existing slabs were cut into to provide these beams. This approach reduced the load transferred from the slab to the column piles, effectively mitigating the risk of further settlement.
5.2 Lessons Learned
It is not advisable to make assumptions about the future settling of a structure that has already experienced some level of settling. The most effective approach is to install instruments that will allow for continuous monitoring of the foundations and the soil. By doing so, potential future settlements can be assessed and appropriate measures can be taken to mitigate any further settlement.
When constructing a structure on piles in marsh land, it is crucial to understand that this comes with inherent risks of settling. As a result, it is important to take the necessary precautions to ensure that the foundation is stable and secure. This may involve using specialized techniques and materials that are designed to withstand the unique conditions of marsh land.
Ultimately, it is essential to prioritize safety and stability when building structures in areas that are prone to settling. By taking a proactive approach and monitoring the foundation and soil conditions, it is possible to minimize the risk of future settlement and ensure the longevity of the structure.
FAQs
What are some reasons for foundation failure?
Most foundation failures occur due to settlement, which is when the ground beneath a building sinks or shifts, causing the foundation to crack or become unstable. Settlement can happen for a variety of reasons, including changes in the soil’s moisture content, natural ground subsidence, or the weight of the building itself. However, there are other factors that can also contribute to foundation failures.
One of these factors is groundwater fluctuation, which can cause soil to expand or contract, leading to shifting or sinking of the foundation. Soil erosion is another issue that can impact the stability of a foundation, as it can weaken the soil and create voids beneath the foundation. This can cause the foundation to sink or settle unevenly.
Construction on soft, expensive, or marshy soil can also contribute to foundation failures, as these types of soil are often less stable and may not be able to support the weight of a building. Shrinkage and swelling of soil is another issue that can impact foundations, as changes in moisture content can cause the soil to expand or contract, leading to cracks or shifts in the foundation.
Finally, uneven bearing capacity of soil is another factor that can contribute to foundation failures. This refers to situations where the soil is stronger in some areas than others, leading to uneven settling of the foundation. All of these factors can impact the stability of a foundation and contribute to foundation failures.
What is differential settlement of the foundation?
Differential settlement is a phenomenon that occurs when one part of a foundation moves in relation to another stable part. This can happen due to a variety of reasons, such as uneven soil conditions, changes in moisture content, or structural defects.
When a foundation is constructed, it is meant to be a stable base for the building or structure that sits on top of it. However, if one part of the foundation experiences movement while the other parts remain stable, it can lead to differential settlement.
This type of settlement can have serious consequences for the building or structure above it, as it can cause cracks, uneven floors, and other forms of damage. It is important to address any signs of differential settlement as soon as they are noticed in order to prevent further damage and ensure the stability of the structure.
What is an inadequate foundation ?
When the foundation of a structure is not capable of bearing the weight of the superstructure that is built on top of it, it is deemed inadequate. The foundation serves as the base upon which the entire structure rests, and if it is not strong enough, it can result in structural damage, collapse, or even endanger the lives of people inside and around the building. An inadequate foundation can occur due to various reasons, including poor design, improper construction methods, or insufficient soil analysis. It is crucial to ensure that the foundation is strong and durable enough to support the superstructure’s load to prevent any potential disasters.
What is the effect of tree roots on the foundation of a structure?
When trees that grow quickly are planted near the foundation of a structure, it can result in an uneven settlement of the foundation. This is because the active roots of the tree can dry out the soil, leading to differential soil shrinkage. As a result, the soil can settle unevenly, causing the foundation to sink or tilt in some areas. This can ultimately lead to structural damage or instability, which can be costly to repair. Therefore, it is important to carefully consider the placement of trees when landscaping around a building to avoid such issues.