Common Risks and Protective Measures
Each instance of new construction or demolition next to an existing historic structure will involve varying risks to that structure. The proximity of the historic site to the project and the scope of the project are two of the most significant variables. Construction of a high rise building with deep foundations is more likely to affect a neighboring structure than the rehabilitation of a nearby rowhouse. However, the converse may be true if the rowhouse is directly adjacent to and sharing a wall with the historic structure. Other factors influencing the degree of likely impact include the age, construction type and structural integrity of the historic building, as well as the depth and makeup of its foundation and its surrounding soil types.
Owners should also anticipate the effect increased dust, vibration and fire risk will have upon interior architectural features and furnishings. For the most sensitive objects, such as chandeliers, paintings and glassware, temporary removal to an off-site location may be the safest course. Those features that cannot be easily removed, including plaster ceiling medallions and cornices, can be cushioned and buttressed by padded wood supports. Additional information concerning the safeguarding of interior features can be found in the preceding Tech Note in this series, "Temporary Protection, Number 2. Specifying Temporary Protection of Historic Interiors During Construction and Repair."
The remainder of this section addresses some of the more common dangers to historic structures when new construction or demolition activities occur nearby. The description of each potential impact is accompanied by suggested approaches for reducing or eliminating those risks.
Vibration
Demolition and new foundation work are common sources of vibrations that can affect adjacent structures. The tools and methods used in demolition, such as impact hammers, wrecking balls, pavement breakers and implosion blasting, produce vibrations that may be transmitted to the historic structure. Similarly, techniques used to prepare new foundations (pile driving and blasting) create potentially dangerous vibrations. Vibrations may also be caused by increased truck traffic accompanying new construction or demolition work. In all cases, the strength of the vibrations reaching the adjacent historic structure depends upon the activity generating the vibrations, the distance between the source and the existing structure, and the type of soil or pavement found between the two.
Historic structures may be particularly vulnerable to the effects of vibrations generated at an adjacent site. Deferred maintenance and past alterations may have produced structural weak points that are susceptible to damage. Historic finishes, such as plaster walls and ceilings, lack the flexibility to accommodate abnormal movement, while shallow foundations (common in historic buildings) may lack the rigidity to resist vibration induced movement.
Mitigating the effects of vibrations should begin during the consultation process when acceptable levels can be set and alternative processes explored. Hand demolition is an appropriate substitute when conventional demolition activities may cause excessive vibrations. If pile driving is likely to damage adjacent structures, the contractor may be able to employ non-displacement piles that are inserted in bored holes rather than driven. Lower vibration levels can also be achieved by "jacking-in" or pressing the piles into the ground. Locating delivery entry and exit points farther from the historic site may reduce vibrations caused by increased vehicular traffic. Once construction is under way, continual crack and vibration monitoring provides an effective warning system, indicating that established safe thresholds have been crossed.
Movement
Excavation and foundation work can also cause ground displacement and movement of an adjacent historic building. New construction almost invariably calls for digging a foundation that is much deeper than the foundations of neighboring historic buildings. This is especially true for projects that include underground parking facilities. A historic structure, with a shallow masonry or stone foundation and wall footings, may experience corresponding displacement that can result in major structural damage.
A building adjacent to excavation activity is vulnerable to both vertical and horizontal movement. When underpinning placed beneath the historic structure is installed incorrectly or proves insufficient, the building may settle primarily along a vertical plane. Vertical movement can also take the form of heave, as ground and building are lifted by soils displaced by pile driving. Horizontal movement can occur when the construction site excavation wall fails to hold back adjacent soil, causing lateral ground displacement.
Efforts to control movement should begin during the consultation phase. Whether the developer's engineer selects underpinning or strengthened excavation walls with tie backs as the means to resist movement of the adjacent structure, the historic building team should retain its own engineer to review the plans. The consulting engineer should ensure that the selected approach addresses the unique characteristics and vulnerabilities of the historic structure and that even incidental movement is restricted.
Water
A well functioning water drainage system is essential to the protection of any historic structure. This system can easily be rendered ineffective by neighboring construction or demolition work. Debris originating at the construction site often finds its way to the gutters, downspouts and drains of an adjacent building. Drainage mechanisms may also become inoperable when excavation workers inadvertently seal off or collapse old pipes running from neighboring buildings. If blocked pipes cannot remove water from both above and below the surface of an historic site, excessive moisture levels or flooding may result.
Regular visual inspections (part of the monitoring program described later) are one of the best means of thwarting increased moisture levels. The inspection procedure should include checking gutters, valleys and exposed drains for any obstructions. Also, indications of dampness or water damage in the basement and where gutters and downspouts meet other building surfaces should be noted.
Construction site runoff from cement mixing and cleaning and dust suppression activities should not flow toward the historic property. Although placing screens and wire cages over exposed areas of the drainage system may provide some protection from obstructions, such installations need to be inspected just as frequently. Low-pressure water washes can occasionally be used to flush the system of dirt and debris. To reduce the possibility that drainpipes will be blocked at the adjacent construction site, all concealed pipes should be traced from their origins at the historic structure and the information passed on to the appropriate contractors. Final landscaping and grading patterns on adjacent construction sites should be examined to ensure that rainwater is not routed towards the historic building.
In some cases, the lack of water beneath an historic structure can lead to damage. Buildings located in areas with a high water table were often constructed upon timber piles. When groundwater or storm water is removed from a neighboring site during foundation excavations (a process known as "dewatering"), the groundwater level beneath the historic site may also drop. Previously submerged timber piles that are exposed to air can quickly undergo dryrot. If there is reason to suspect that the structure was built on such a foundation, the property manager should work with the neighboring construction team to maintain the existing water table. This can be done using watertight excavation support systems such as slurry walls which ensure that most of the water pumped out of the construction site does not come from adjacent properties. Dewatering of soft clay ground may also result in settlement of a neighboring building, as ground water pressure is reduced and the soil consolidates.
Fire and Security Concerns
The heightened possibility of fire accompanies many demolition and new construction activities. Temporary heating devices, torches, sparks, molten metal and undersized electrical utility panels are some of the most common sources of fire at construction sites. Additionally, the improper storage of fuels, cloth rags and brushes also presents opportunities for fire to ignite and spread. The Tech Note, "Specifying Temporary Protection of Historic Interiors during Construction and Repair," provides detailed information on reducing the likelihood of fire in situations involving work near historic structures.
The security of a historic building can be threatened when adjacent construction provides opportunities for illegal entry. Newly constructed floor levels at the building site may make the neighboring historic structure's ledges, windows and rooftops accessible to trespassers. Window openings on the historic building should be fastened and all doors from the roof to the interior should be locked. Where a historic structure is protected by an intruder alarm system, that system should be upgraded to protect rooms that are rendered accessible from the outside. In cases where the historic structure does not directly abut new construction or demolition activity, attention should still be paid to the possibility that incidents of vandalism and theft will carry over to the historic site.
Physical Impact
Construction or demolition can cause direct physical damage to neighboring historic features and materials. Cranes, hoists and workers on upper floors of a construction site can drop building supplies and tools onto an adjacent historic structure. Misdirected debris chutes and backing vehicles may also leave their mark.
Generally, to counter these occurrences, protective barriers are placed over any area of the historic structure deemed at risk. If the new construction will rise significantly above the historic building, plywood sheets should be placed over the roof to distribute the force of dropped materials. Plywood covers should also be placed over decorative roof embellishments such as finials and balustrades. Alternately, horizontal netting can be rigged to shield vulnerable rooftop features.
Facades that are directly exposed to adjacent construction sites should receive close attention. To avoid damage, windows should be covered with plywood. Layers of cushioning materials can be placed between the plywood covering and particularly fragile windows, such as stained glass.
If entire wall surfaces are vulnerable, scaffolding should be erected against the façade and debris netting placed on the outside of the scaffolding. Plastic sheeting can provide added protection in areas where acidic cleaning solutions may splash onto historic facades, windows and other surfaces.
The best means of protecting a historic structure from physical impact, however, is often to have adequate horizontal and vertical netting and barriers in place at the construction site. When adjacent buildings are adequately considered in the construction site netting and scaffolding plans, protective measures at the historic site can be less intrusive, and the likelihood of damage reduced even further.
Additional Dangers
Other byproducts of new construction and demolition, such as dirt and dust, can also pose threats to an adjacent historic structure.
Dust suppression measures including the installation of fabric enclosure systems should first be employed at the building site. Despite these efforts, historic building owners will undoubtedly have to deal with raised levels of dust infiltration. Accordingly, vulnerable interior objects and artifacts should be covered or temporarily moved to another location. Windows can be taped shut or temporarily sealed with clear polyethylene sheets. Additional mats and carpets near entrances can help reduce the amount of dirt tracked inside. An accelerated maintenance program that includes thorough and frequent cleaning and HVAC filter replacement, is an effective means of addressing the degraded environment surrounding a construction site. To lessen the chance of airborne asbestos infiltration, the exhaust from sealed work areas must be properly filtered and vented away from historic buildings.
The owner of a historic property should anticipate the increased rodent and household pest presence that accompanies major demolition activity. Newly opened holes in old foundations are easy escape routes that should be promptly sealed. The construction or demolition site rodent control plan should include provisions for protecting adjacent historic resources. Concurrently, the historic property owner should consider securing a contract with an independent extermination company. Plans should include both preventive measures to reduce conditions favorable to infestation as well as a system of eradication such as rodenticide and traps.
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