Excavation support in an urban environment

April 2004 » Features » PROGRESSIVE ENGINEERING
Mechanical soil anchors provide an alternative on sites with severe constraints.
Vincent A. Siefert, P.E.
The project area is within an existing courtyard surrounded by buildings and with limited access.

Most construction activities in the urban environment require support of excavation or underpinning of adjacent structures. Many of these projects have severe site constraints including accessibility; therefore, support of excavation with conventional sheeting or soldier piles and timber lagging is becoming an increasingly difficult option. Similarly, conventional underpinning with mass concrete is very labor intensive and is becoming expensive and time consuming and may not fit well with the project schedule.

In general, support of an excavation could be divided into two broad categories: external support system and internal support system. External systems such as cantilever walls, walls with rakers and walers, and soldier piles with lagging provide support exclusively to the outside face of the excavation. Internal systems such as soil nails, mechanical soil anchors, and grouted tiebacks with walers provide support to the excavated surface with all elements installed within the soil mass. A successful project in New York City utilized mechanical soil anchors with a concrete facing as an alternate to the conventional soil retaining system specified on the contract drawings.

Mechanical soil anchoring is one of the external systems which has been successfully used during the last 50 years and is becoming increasingly popular. Mechanical soil anchoring is a part of retaining wall systems with top-down construction compared with conventional retaining wall systems where construction proceeds from bottom to top. This is often an advantage for projects because of reduced construction time. Mechanical soil anchors are also well suited for urban projects where disposal/storage of material is a challenge due to logistics.

In the urban environment, the mechanical soil anchor system, which is part of an internal support system, has many advantages:

  • It is installed along the line of excavation without infringing into the building line.
  • No heavy equipment is required. Typically these anchors are installed with a 5,000-pound vehicle with 500 foot-pound pavement breakers.
  • Installation generates minimal noise and vibration, which suits well for vibration- and noise-sensitive sites.
  • The system can be installed in restricted access areas where mobilization of heavy equipment is not feasible.
  • The system can accommodate irregular shapes and difficult-to-reach areas since anchors can be installed at any location and then shotcreted.
  • It is well suited for stabilizing existing retaining walls with no excavation and backfilling.
  • An architectural facing can be provided to meet project requirements.
  • A thinner facing can be provided compared with conventional underpinning.
  • The system can be used for both temporary and permanent structures.
  • It can be designed to handle some surcharge loads.

Project challenges
Contractor EE Cruz retained Siefert Associates, LLC, to provide construction engineering services for support of excavation for a foundation project located within the courtyard of the Tisch Hospital, NYU Langone Medical Center in Midtown Manhattan, N.Y. Depth of excavation varied from 10 feet to 25 feet from the existing ground. A combination of sloped excavation, mini piles functioning as soldier piles with lateral anchor support, and mechanical soil anchors were utilized for the project.

The project area had severe site constraints with material and equipment accessibility being one of them. The only access for the site was through a double door located in the main lobby and mobilization of heavy equipment and material required for conventional soldier piles was ruled out. Mechanical soil anchors proved to be the most viable alternate solution for several areas of excavation.

Because of access restrictions, contractors used small, lightweight equipment to excavate within the courtyard of NYU Langone Center, N.Y.

The site being within the courtyard of a fully operational hospital had noise and vibration restrictions too. Mechanical soil anchors proved to be ideal for these restrictions also. Soil anchors were installed with minimal noise and vibration and required no vibration monitoring. Mobilization of small and lightweight equipment — low-height vehicles and tools for installation of anchors — enabled the contractor to maintain the project schedule.

The project site was also close to the East River and the groundwater was relatively high. The soil anchor option minimized the required dewatering elevation and expedited the project construction.

Apart from the aforementioned site constraints, the project also encountered many challenges during construction. Some of those which required adjustment/modification to the design included:

  • very loose soil conditions at localized areas
  • obstructions encountered during installation of anchors
  • change in permanent building design resulting in increased loading on anchors
  • limited subsurface soil data
  • caving in of the excavation surface

Mechanical soil anchors were installed at difficult-to-access locations with minimal effort and fully accounted for all surcharge loadings and for varying heights of soil excavation at several different locations of the project. For a typical external support system with soldier pile and lagging, modification to account for these varying site conditions would have been difficult, costly, and time consuming.

Shotcrete was used as a facing for the soil anchors.

The facing for the soil anchor system was permanently placed concrete — shotcrete — designed to meet architectural requirements including waterproofing. Relatively thin shotcrete facing proved to be sufficient compared with the thicker width of a typical retaining wall. Shotcrete was installed in areas that were difficult to access and was the ideal system for the tight spots encountered with irregular shapes.

Another advantage of mechanical soil anchors and shotcrete is the ability of the system to be used as an alternate to underpinning. At some locations, excavation was adjacent to an existing vault that required underpinning to avoid settlement. Conventional concrete cast-in-place underpinning in small sections with hand excavation close to the water elevation would have resulted in much longer project duration apart from being expensive. With the soil anchors option, sufficient lateral support was provided to the soil mass including the effects of surcharge and no underpinning of the adjacent structure was required.

The mechanical soil anchors can be tested soon after their installation and design capacities can be verified instantly. This allows for any required modification to the design. Project site monitoring data and information gathered during field testing was used to modify the design quickly with additional anchors or by altering the spacing of anchors.

Summary
Foundation construction in the urban environment is becoming increasingly challenging as these projects demand a fast schedule, provide minimal work area, and are bid extremely competitively. These foundation projects require a very site-specific design and analysis tailored to the existing field conditions. Careful planning and verification of field conditions to the extent feasible enables a construction engineer to provide appropriate recommendations. Variations in field conditions are inherently part of foundation/support of excavation design and mechanical soil anchors are a perfect fit to revise and adjust the design to adapt to the field conditions without compromising the project demands.

Vincent A. Siefert, P.E., is the owner and president of Siefert Associates, LLC, Naugatuck, Conn. Ram Kasturi, P.E., is a senior geotechnical engineer at Siefert Associates, LLC. They can be contacted at vincent.siefert@siefertassociates.com and ram.kasturi@siefertassociates.com, respectively.


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