N.Y.C. Admin. Code § BC 1817: Section BC 1817: Underpinning and Alternate Methods of Support of Buildings and Adjacent Property

1817.1 General.

Where the proposed work may disturb, displace or otherwise affect the lateral or vertical support of property or buildings, an engineer shall evaluate the need for and methods to maintain the stability and integrity of the building(s), utilities or soil adjacent to such activity.

1817.2 Minimum requirements for undeveloped adjacent property.

The minimum requirements for construction documents for support of adjacent property which does not contain a building, such as, but not limited to, empty lots, court yards, front yards, or rear yards, shall include the following information:

1. Existing grade of the adjacent property.

2. Plans, cross-sections, and elevations as necessary, to illustrate all unique conditions of the support of excavation, including the depth of the proposed excavation, the subsurface conditions, surcharge loading, the proposed method of support, sequence of construction, required material properties, and additional details as required by Section 3304.

3. Details and criteria for monitoring, including but not limited to criteria and thresholds for movements, and dewatering as specified in Chapters 17, 18 and 33.

4. The elevation of the water table, need for dewatering as noted in the Geotechnical Report, and the maximum permissible drawdown outside of the project site.

1817.3 Evaluation of adjacent buildings for suitable method of support.

At the time of foundation plan approval, an engineer shall submit an evaluation report to the department assessing the condition of the existing building and the subsurface conditions of the construction site and adjacent property. The report shall also identify acceptable method(s) of support, including underpinning or alternate methods of support, for the building. This evaluation shall be performed in accordance with the requirements of Section 1817.3.1 through 1817.3.6.

1817.3.1 Assessment of the building and the subsurface conditions.

The engineer shall assess the condition of the existing building, and the subsurface conditions of the construction site and adjacent property, to an extent sufficient for determining acceptable method(s) of support, including underpinning or alternate methods of support. As necessary, the assessment shall be based on visual observations, calculations, review of the geotechnical report prepared for the project in accordance with Section 1803.6 and review of other available documentation. The investigation shall include, as necessary, but need not be limited to, the following items:

1. An evaluation of the vertical load path of the building as it relates to the location of the proposed underpinning or alternate method of support.

2. An evaluation of the lateral load path of the building as it relates to the location of the proposed underpinning or alternate method of support.

3. Calculations of the vertical and lateral loads at the foundations to be underpinned or supported by an alternate method of support.

4. A determination of the type and condition of the above grade elements to be supported or potentially affected by the work.

5. A survey of deviations from plumb or horizontal position of the building.

6. Identification of conspicuous structural defects, including but not limited to: bowing, significant cracking, structural degradation or unusual slenderness. A detailed description of such items shall be provided, with photographs and mapping if possible.

7. A determination of acceptable thresholds for maximum vertical and lateral movement, maximum permissible vibrations, the required monitoring and the protocols for exceedances.

8. A determination of the type and condition of the foundation elements to be supported or potentially affected by the work.

9. A test pit at each substantial change in foundation type or building geometry. Records of the test pits shall include the following:

a. A description of the construction materials and condition of the footing, foundation wall and/or foundation system.

b. The bottom elevation of the wall(s) and/or footing(s).

c. The classification of the soil or rock the foundation bears upon.

d. Photographs and sketches of the test pit.

10. An assessment of the allowable bearing pressure of the soils supporting the existing foundation(s) per Section 1806.

11. An assessment of potential reductions to the allowable bearing pressure due to the proposed excavation.

12. The lateral earth, surcharge, and water pressures that will be present on the elements of the proposed underpinning or alternate method of support.

13. An analysis of the subsurface conditions and their potential impacts on the underpinning or alternate method of support work such as, but not limited to: high water table and need for dewatering, loose soils, potentially running soils, presence of boulders, or other factors that could impact the design or construction of the underpinning or alternate method of support.

14. An assessment of the allowable bearing pressure of the soils supporting the underpinning or alternate method of support during the installation sequence and in the permanent condition.

15. An assessment of the anticipated settlement during the underpinning or alternate method of support, and soil and foundation work.

16. Any additional information requested by the commissioner.

1817.3.2 Additional requirements for buildings with rubble foundation elements.

For buildings with rubble foundation elements, an assessment of the condition of the rubble element shall be performed. If the rubble foundation elements are in poor condition, recommendations for addressing the poor conditions shall be provided.

1817.3.3 Additional requirements for unreinforced masonry buildings.

Where the building being supported is of unreinforced masonry construction, the lateral stability of the masonry walls and their ability to resist the loads imposed shall be verified.

Exception: If it is not possible to verify the lateral stability of the masonry walls, lateral support shall be provided at the floor levels of the adjacent building prior to installation of the underpinning or alternate method of support. Such lateral support shall be shown on the construction documents.

1817.3.4 Additional requirements for use of rock support.

Where in situ Class 1a, 1b, or 1c rock is proposed for use in place of constructed methods of support, a subsurface investigation shall be conducted to assess the properties of the rock. Site-specific data shall be obtained to determine the predominant strike and dip of the joints, foliation, and/or other discontinuities in the rock. Recommendations for any rock stabilization necessary for the proposed work shall be provided.

1817.3.5 Evaluation report.

The evaluation report shall include the following items:

1. The address of the property.

2. A description of the acceptable method(s) of support, including underpinning or alternate methods of support, that were evaluated and found to be feasible.

3. A description of any required modifications to reinforce the existing building prior to the start of the underpinning or alternate method of support work.

4. A summary of the assessment required by Sections 1817.3.1 through 1817.3.4, including all test pit records, where performed, and any additional items deemed necessary by the engineer.

5. A list of all assumptions made in preparation of the evaluation report and a detailed plan for the verification of such assumptions.

6. If the engineer determined that specific items listed in Sections 1817.3.1 through 1817.3.4 were not required, a statement shall be included explaining the basis for that determination for each item that was not included.

1817.3.6 Responsibility for the report.

If more than one engineer participates in performing the required assessments of the building and subsurface conditions, and the author of the evaluation report is relying on the work of these additional engineer(s) in preparing the evaluation report, the findings of the additional engineer(s) shall be included as signed and sealed appendices to the evaluation report.

1817.4 Suitable methods of support.

Where a method of support for an existing building is required, such underpinning or alternate method of support shall be designed as a permanent structural element specific to the building using the methods described in Sections 1817.4.1 through 1817.4.3. The underpinning or alternate method of support shall be installed in accordance with provisions of this chapter and Chapter 33 and shall be inspected in accordance with the provisions of Chapter 17.

1817.4.1 Underpinning.

If the foundation loads are to be transferred to a lower bearing level, underpinning consisting of concrete piers, drilled piles, jacked piles, bracket piles, or similar engineered systems to transfer the existing building loads below the bottom of the proposed excavation shall be installed.

The bearing pressure at the base of the underpinning shall be computed accounting for all vertical loads and any overturning moments present due to lateral earth and water pressures, and shall be less than or equal to the allowable bearing pressure of the bearing stratum.

1817.4.2 Alternate methods of support.

In lieu of underpinning, an excavation support system and/or lateral bracing designed to restrict the movements of the adjacent buildings within the established threshold levels may be installed. Such excavation support systems may consist of secant piles, slurry walls, tangent pile walls, jet grouting, pressed-in continuous sheeting, or similar engineered system.

1817.4.3 Use of rock support.

Existing structures founded at a level above the level of adjacent new construction may be supported on Class 1a, 1b, and 1c rock in lieu of underpinning, provided the requirements of Section 1817.3.4 are satisfied.

1817.5 Design requirements.

The underpinning or alternate method of support for adjacent buildings shall be designed by an engineer and plans filed with the department for approval prior to the commencement of the work.

1817.5.1 New construction.

Materials and design shall be in accordance with the requirements of this code for new construction.

1817.5.2 Incorporation of the evaluation report.

The design shall incorporate the findings of the evaluation report and any additional evaluation and investigation deemed necessary by the engineer performing the design. If the evaluation report did not conclusively demonstrate the suitability of an underpinning or alternate method of support, such method of support shall not be used.

1817.5.3 Deviations from the evaluation report.

The engineer designing the method of support may be an engineer other than the engineer who submitted the evaluation report. If the engineer designing the method of support does not accept the evaluation report or finds it insufficient, a new evaluation report shall be submitted to the department. Similarly, if a method of support other than one(s) determined to be suitable in the evaluation report is proposed for use, an additional evaluation report shall be submitted to the department justifying the suitability of the method of support now proposed. The revised report may be submitted along with the construction documents for the design of the method of support. Alternatively, minor exceptions to the report may be incorporated into the design documents if explicitly indicated on the design documents along with the engineer's reason for override.

1817.5.4 Design loads and load combinations.

Underpinning, alternate methods of support and any associated lateral support system, shall be designed for the loads and load combinations listed in Sections 1817.5.4.1 and 1817.5.4.2.

1817.5.4.1 Loads from the existing building.

A computation shall be performed to establish the vertical and lateral loads on the elements of the method of support from the existing building. Loads and load combinations shall be computed in accordance with the requirements of Chapter 16 or where permitted for loads of prior codes for prior code buildings.

1817.5.4.1.1 Unconfirmed load path.

Where the evaluation of Section 1817.3 is unable to visually confirm the load path from the existing building, pit-pier underpinning where all horizontal loads are transferred directly to a raker or tension anchor bracing system that braces every pit-pier is permissible.

Exception: Raker bracing or tension anchors need not be installed where it is demonstrated by calculation, according to data provided by the investigation per Section 1817.3.1, that the underpinning system, analyzed independently as a retaining wall that supports the soil and water behind it, calculated both (i) with and (ii) without accounting for vertical loads from the structure above, has satisfactory bearing pressures and is stable with a 1.5 factor of safety against overturning and sliding. This exception shall not be utilized in unreinforced masonry buildings in which access to verify the lateral stability of masonry walls is not possible as per Section 1817.3.3.

1817.5.4.2 Soil and water pressures.

The design shall include at rest soil pressures, water pressures, and any surcharge pressures that arise from the adjacent portions of the building being underpinned or supported by an alternate method of support.

1817.5.5 Anticipated deflection.

A calculation shall be performed for the anticipated deflection of the method of support system and its effect on the supported building.

1817.5.6 Factor of safety.

Methods of support shall be designed to provide a minimum factor of safety of 1.5 for sliding and overturning for all loads and all anticipated interim conditions.

1817.5.7 Sequence.

The design of the method of support shall account for the means and methods of installation, sequence of operations, and all the load transfers and associated support conditions for all phases of the work.

1817.6 Minimum requirements for construction documents.

Construction documents for methods of support of adjacent buildings shall include the following information:

1. Indicate the type of adjacent foundation, including type of material.

2. The bearing elevation(s) of the adjacent building foundation and its soil classification.

3. For deep foundations, type and location including top and bottom elevations of deep foundation elements.

4. The elevations of all floor levels at grade and below in the adjacent building.

5. Plans, cross-sections, and elevations views as necessary, to illustrate all conditions of the adjacent building relevant to the operation, including the below grade portions and the depth of the proposed excavation.

6. Details and criteria for monitoring the building or wall being supported, including but not limited to criteria and thresholds for movements as specified in Section 1817.9 and Chapters 17 and 33.

7. A fully detailed design of the method of support including any required bracing.

7.1. For pit-pier underpinning, a detailed design for the timber shoring of the pit excavations shall also be provided.

8. A step by step procedure describing the installation of the elements of the method of support.

8.1. For pit-pier underpinning, the procedure shall include a diagram indicating the sequence of the pit installation and a detailed procedure of the installation of the shoring for the pit excavations.

9. The elevation of the water table, need for dewatering as noted in the evaluation report, and the maximum permissible drawdown.

10. References alerting the contractor and the special inspector to review the evaluation report of the adjacent building prior to the start of work.

11. Plans, sections, and elevation views of all methods of support.

12. Soil classification of the required bearing stratum for the elements of the method of support.

12.1. For pit-pier underpinning, also include the required allowable bearing pressure below the pier.

13. A load table or diagram shall be provided to indicate the following:

13.1. Total gravity load in underpinning pier or alternate method of support.

13.2. Total lateral load in underpinning piers or alternate method of support.

1817.7 Additional requirements for pit-pier underpinning.

When the method of support selected is pit-pier underpinning, the design shall meet the following minimum criteria:

1. After installation of each pit-pier, the approach pit shall be back filled.

2. In no case shall the general site excavation expose more than one third of the total height of a pit-pier, unless:

2.1. A pit-pier bracing system designed by the engineer is installed or;

2.2. The calculated capacity of the individual pit-pier to resist lateral loading at a greater depth is identified on the drawings.

3. Pit-piers shall be preloaded by wedging, use of permanent jacks, or by other means designed by the engineer.

4. Any voids between the bottom of the foundation and the top of the pit-pier shall be filled with dry-pack or equivalent. Dry-pack shall be composed of nonshrink material and be no more than 3 inches (76 mm) in thickness.

5. The need for jacking and all associated jacking requirements shall be determined by the engineer responsible for the underpinning design.

6. The width of pit-piers shall not exceed 4 feet (1219 mm) unless the calculations determine that a wider pit-pier is acceptable.

7. Shear transfer shall be designed and installed between adjacent pit-piers.

8. The bottom of pit-pier elevation shall be a minimum of one foot below the bottom of the future adjacent excavation.

1817.7.1 Pit-pier excavation.

Pit-pier excavation shall be subject to the requirements of Section 1817.7.1.1 through 1817.7.1.8.

1817.7.1.1 Excavation methods.

Pit-pier excavations shall be performed using hand held tools. Excavation using backhoes or other mechanical means to excavate the pit-piers is prohibited.

1817.7.1.2 Clear distance between open pits.

Clear distance between open pits shall be determined by the evaluation report and shall not be less than 12 feet (3657.6 mm).

1817.7.1.3 Pit excavation below the lowest installed lagging board.

Lagging boards shall be installed as the excavation proceeds to limit soil loss. In no case shall excavation proceed more than 2 feet (609.6 mm) below the lowest installed lagging board. At completion of pit excavation, the pit shall be fully lagged.

1817.7.1.4 Backpacking.

Backpacking of any voids shall be performed at each excavation lift.

1817.7.1.5 Excavation below the water table.

Pit excavation shall not proceed below the water table. If the depth of the underpinning requires excavation below the existing water table, a dewatering system shall be installed prior to the start of the excavation. The dewatering system shall be tested via test pit or piezometer to confirm that the water table has been sufficiently drawn down prior to the start of the pit underpinning work.

1817.7.1.6 Variable depth pits.

Where construction requires adjacent pits to be excavated to differing depths, the deeper pit-pier shall be constructed first.

1817.7.1.7 Multi-tier pit-pier underpinning.

Where multi-tier pit-pier underpinning is utilized, the upper piers shall be braced prior to the excavation of the lower pier and the installation of upper and lower piers staggered, unless the design per Section 1817.5 and drawings include explicit methods that will ensure that the installation of lower tiers shall not displace the tiers above.

1817.7.1.8 Use of tension anchors.

When tension anchors are utilized, the effects of vertical and horizontal force components shall be accounted for including any potential additional displacements.

1817.8 Additional requirements for deep foundation elements used in underpinning.

Where the method of support includes deep foundation systems such as pile supported underpinning or tie anchors, the requirements of Chapter 18 and Sections 1817.8.1 through 1817.8.5 shall be met.

1817.8.1 Pile design.

Piles used for underpinning shall be designed in accordance with Sections 1810.3 and 1810.4.

1817.8.2 Load testing of underpinning piles.

Piles used for underpinning are subject to the load testing criteria of Section 1810.4.

1817.8.3 Eccentric pile loads.

Piles used for underpinning shall be designed to sustain all moments that are generated due to any eccentricities of the load on the pile.

1817.8.4 Spanning between piles.

Unless new grade beams are provided, calculations shall be performed to confirm that the existing foundation can support the required loads spanning between the pile supports.

1817.8.5 Piles used as excavation support elements.

If underpinning piles are to be used as excavation support elements, a calculation of the anticipated pile deflection shall be performed and its impacts on the supported building shall be assessed by the engineer.

1817.9 Monitoring.

When excavation, foundation construction, underpinning, or an alternate method of support is required, adjacent structures and properties shall be monitored in accordance with a plan prepared by the engineer. The engineer shall develop the scope of the monitoring program, including location and type of instruments, frequency and duration of readings, frequency of reporting, maximum allowable time to report readings (timely reporting), reporting requirements, and permissible movement and vibration criteria. This scope shall take into account the buildings or property to be monitored and the conditions thereof. The monitoring program shall include necessary actions to address exceedances. These actions shall include notification of the commissioner. Monitoring of historic and landmarked structures shall be subject to special requirements as determined by the department. Where the building to be monitored is subject to underpinning or alternate methods of support, the monitoring plan shall be determined by the engineer designing such work.

1817.10 Special inspection.

Special inspection shall be conducted in accordance with Chapter 17.