đź•‘ Reading time: 1 minute
Foundations are susceptible to movements if the applied loads are not in proportion with the design, causing a downward motion known as settlement. Settlement can be tolerated by the structure provided the loads do not exceed the allowable bearing pressures.
When foundation defects are to be investigated, a thorough examination is imperative. The primary objective of the examination is to obtain an accurate diagnosis as a basis for a report. It is, therefore, extremely important that all available evidence is gathered and carefully examined before adopting a method for repair.
The inspection may take considerable time but it is essential that extensive defects are properly investigated. It is always advisable to bear in mind, when making a diagnosis, that more than one cause may be responsible for a defect, although it is necessary to investigate the primary cause.
For example, foundation movements may have been responsible for a fractured external wall, but rainwater could penetrate the fracture causing dampness on the internal face of the wall.
Foundation repairs to existing buildings are generally the most difficult and costly to effect, which is again a good reason why a thorough investigation should be carried out. The objective of the investigation should be to determine the nature and strength of the subsoil under load.
This article describes the effects of construction of a new structure on an existing structure in New York, the defects that occurred in the existing structure, and the recommended remedial solutions.
Contents:
1. Suburb Sewers Line Project
Construction of a sewer line development in Brooklyn required deep excavation nearby some existing old residential structures. The sewer lines were intended to run near to existing footings and the excavation was within the influence zone of the existing footing.
Also, the piles were installed using a vibratory pile driver. Moreover, there was no means to check the vibration levels during pile driving. Therefore, numerous close by structures settled, tilted, cracked, and moved along the side.
After the damage occurred, inclining supports as a brace were added for lateral support. One structure imploded and led to the death of one of the residents.
The investigation revealed that the building was constructed using timber footings, a construction method common at the turn of the century. The footings were discovered to be in acceptable condition yet had been upset by the construction of the sewer line project.
1.1 Restoration Measures
The structure collapsed because the excavation for the sewer line was carried out within the influence zone of timber footing. Thus, the strength of soil supporting the vertical support was lost and the building collapsed due to settlement. The high-grade steel bracing framework system was not able to save this structure as the soil support was lost beneath the foundation level.
To re-establish the integrity of the vertical support, it was initially decided to underpin the structure. The bid prices obtained for underpinning were very high. However, it was more cost-effective to buy the damaged buildings and demolish them.
1.2 Lessons Learned
- Careful preconstruction study to determine the need for protection and underpinning of adjacent structures is required. This study should review existing plans, taking sufficient soil borings, and evaluating the condition and strength of existing buildings and other structures.
- Excavations for sewers or other utilities should be moved away from existing buildings and should be located so that the excavation cut is outside footing influence lines.
- Use of vibratory equipment for pile driving should be avoided in loose sands and similar soils.
- Vibration readings, using seismographs, should be taken as often as needed to establish the suitability and proper energy of the driving hammers.
2. Electronics Plant
During the construction of an electronic plant at Upstate New York, an entire building settled vertically and also moved laterally towards the adjacent valley. The main framing elements, columns, and beams of the building were made from steel and the floors were constructed of reinforced concrete.
During the settlement of the building, the cracks occurred in the concrete slab. However, the bolted connection at column-beam and beam to beam junction was not deformed beyond their capacity.
Thus, it was concluded that the main reason behind the occurrence of cracks in the building was due to the effect of a large movement of the order of 102 mm. The strength of the bolted connection was measured using the stress-relief method.
2.1 Restoration Measures
The only structural corrective measure that was executed was to enlarge the existing footings so as to enable the structure to support the additional live loads without considerable settlements.
2.2 Lessons Learned
- High footings adjacent to severe soil slopes should be designed using low allowable bearing pressures to reduce possible settlements.
- In extreme cases, the use of retaining walls should be considered.
- The actual stress level in a deformed structure is the true yardstick for evaluating the adequacy of a distressed structure.
- The stress-relief method proved to be an effective tool in the strength evaluation of distressed structures.
3. East Side Hospital
During the construction of East Side Hospital, an alarm went out for everyone associated with the high-rise hospital project. It was found that while excavating the foundation for the hospital, the adjacent high-rise residential building was unsteadily sitting on a sliver of rock. After that, the construction was stopped, and the fully occupied residential building was evacuated.
Investigation revealed the following conditions:
- The width of the rock sliver, of unknown strength, was approximately 355 mm. Incredibly, it was the only structural element supporting the high-rise building.
- Even more remarkably, the footings dileanated in the original design for the existing apartment building were not built at all.
- The easterly wall of the building was not a standard concrete wall but was actually a rock face with stucco finish.
- The plans that had been filed with the Department of Buildings did not show the as-built conditions.
3.1 Restoration Measures
After the investigation results, an emergency plan was immediately launched for underpinning. The plan included the construction of new laterally braced concrete piers into the rock sliver. The plan was executed perfectly, and the alarm was turned off.
According to the New York City Building Code, when neighboring construction goes below the bottom of an existing footing, it is the responsibility of the neighbor doing the new construction to underpin the existing structure when their footings are more than 3.0 m below curb level.
3.2 Lessons Learned
- Do not use the rock outside your property for support of your structure, regardless of the apparent immediate savings.
- You may support your structure only on the rock totally within your own side of the property line.
- Rock slivers should not be relied upon for foundation support.
4. Manhattan Hospital Complex
During deep excavation for the foundation of the Manhattan Hospital project, the adjacent buildings started showing cracks, and the buildings slowly started to sink in. An excavation extending across an entire site was progressing toward a total depth of 14 m below grade.
However, the existing adjacent structures had only single-level basements. Initially, it was anticipated that the excavation would be through gneiss rock, which is sound rock.
The ground-water level inside the structures was constrained by a sump siphon situated under an existing basement slab. It was recommended clearly in the design stage that the adjacent structures would have to be underpinned.
The New York City Construction regulation requires that underpinning will be planned by an expert specialist. This necessity was met, and a plan for supporting pits with post-tensioning cables to resist the earth pressures was formulated.
As excavation for the pits was proceeding, some movement was detected in the building directly east of the site. As water was found several feet below the pits, attempts were made for dewatering the soil surrounding the pit excavation. But this proved futile because of the impermeable nature of the soil existing at this location.
This was followed by dewatering by well points. This also proved ineffective, because the silts in the soil clogged the well point screens. A third alternative using jack-piles was then considered.
This method of pushing pipe segments into the ground requires the use of the building weight to act as a reaction for the jacking forces. Unfortunately, it readily became apparent that the footings of the existing building, which consisted of decomposed rubble, did not offer sufficient resistance to the jacking loads.
While these efforts were going on, a further adverse movement was detected in the adjacent building. At this time, the safety of the structure was questioned, and temporary braces were installed. For all practical purposes, construction operations came to a halt.
4.1 Restoration Measures
The two buildings on the east exhibited serious cracking throughout. The one nearest the excavation settled, causing the entire building to pivot about the foundation walls. This pivoting effect, in turn, caused the entire party wall to move westward, leaving a 152 mm gap at the top. This movement caused enormous distress on the interior of the building.
The floors settled, and the ceilings and walls cracked, and even the elevator got stuck as its shaft got deformed and would not permit free movement of the cab. On the fourth floor, there was actually a separation between the wood joists and the party wall, causing the floor to settle by 28 mm.
When all efforts to support the building failed, the last resort was used, which involved very costly soil solidification by freezing the soil. The saturated soil proved to be an advantage for the freezing process.
This process, with all its pipes and equipment, took about three weeks to be installed. It proved to be a total success as practically all movements within the adjacent structure stopped.
Once the soil under the building was consolidated, the original conventional underpinning design was implemented. Just prior to freezing the soil, a cross-lot bracing system was installed. The purpose of this system was to eliminate any further movement in the building and to stabilize it during the subsequent construction operations.
The giant cross-lot braces that spanned the entire lot permitted construction to proceed without interruption. After completion of the underpinning, the adjacent buildings were restored.
4.2 Lessons Learned
- Accurate preconstruction subsoil study is essential to the success of foundation work.
- Inadequate test data can cause serious delays, cost overruns, damage, and even collapses. In this case, had accurate information been obtained, design changes and soil solidification could have been planned prior to the start of construction.
- Adjacent structures must be carefully monitored during underpinning operations, so if and when movements are detected, changes in methods or procedures can be implemented.
5. Queens Apartments
Queens apartments were constructed over a marshy land. After the completion of the project, many cracks developed in the masonry walls at many levels of the buildings due to the settlement. Cracks were observed in both exterior and interior walls. Timber piles were provided for the foundation of this project.
Investigation confirmed that some of the piles in the three-pile group supporting the distressed corner of the building had broken. The piles broke because of the consolidation of the organic clay.
When the loads were applied to the pile, the stratum started consolidating. Thus, breakage occurred due to the load resulting from the consolidation.
To assess the potential for extra settlement and their possible extent, tests were executed in a research laboratory and field program. In the lab, the rate at which settlement could have occurred due to the secondary compression was estimated.
In the field, a sensitive settlement plate was monitored for around half a year utilizing a depth caliper precise to 0.025 mm. The zone chosen for installing the plate was the inside corner of the pantry room. This zone was picked because of its closeness to settlement cracks.
5.1 Restoration Measures
Analysis of laboratory data has suggested that settlement of the organic clay, as a result of the fill above it, can be expected to be about 25 mm in every ten years. The field measurements from the settlement plate indicated an even greater settlement. The field data suggested settlements about twice those indicated by the laboratory data.
To reduce further settlement, the repair work was carried out by adding new reinforced-concrete beams. The beams were provided by cutting into existing slabs. Thus, the load transferred by slab into the column piles was reduced.
5.2 Lessons Learned
- Do not guess whether a structure that has settled will continue to do so. Instrument the foundations and the soil to assess potential future settlements.
- Placing a structure on piles bearing in marsh land always involves risks of settlements.
FAQs
Settlement is the main reason for most of the foundation failures. The other reasons for foundation failure are discussed below:
1. Groundwater fluctuation
2. Soil erosion
3. Construction on soft, expensive, and marshy soil
4. Shrinkage and swelling of soil
5. Uneven bearing capacity of soil
When one part of the foundation moves relative to the other stable part then a differential settlement occurs.
A foundation is called inadequate if it is not able to support the superstructure load.
Fast-growing trees close to the foundation of a structure can cause unequal settlement when active tree roots dry out the soil causing differential soil shrinkage.
Read More
Taj Mahal: Construction of its Invincible Foundation
Venice: Foundation Details of the Biggest Floating City in the World
