Mark Ruberti, PE, Principal Geotechnical Engineer, Kleinfelder, Inc.
Riverside Road in Springfield, Massachusetts has a long history of sinkholes, dating back as far as 2007. The sinkholes have been a focus of multiple past study and mitigation efforts. Recently, a sinkhole compromised a water transmission main at the intersection of Orchard Street and Riverside Road. The subsidence caused a valve to fail, leading to critical infrastructure being taken offline. The Springfield Water and Sewer Commission (SWSC), and independent regional utility, tasked Kleinfelder with replacing the impacted water main and valve, assessing any further damage, and supporting the valve to help prevent any future failures.
The location of the recent sinkhole is adjacent to an Army Corps of Engineers floodwall that protects the City of Springfield from flooding in the Connecticut River. That floodwall is under the jurisdiction of the United States Army Corps of Engineers and is owned and maintained by the City of Springfield, a separate entity from SWSC. Additionally, critical drainage infrastructure owned and operated by the City of Springfield Department of Public Works (DPW) exists in the vicinity. With three parties affected by the sinkhole, careful consideration of each stakeholder’s interests was required when coordinating field activities, weighing different design options, and coordinating on new developments as they arose in the field.
Subsurface Investigations
A key first step in any geotechnical evaluation is gathering data on the subsurface conditions to inform design. The goal of the subsurface investigation program was to evaluate the potential causes of the sinkholes and to inform the design of a foundation system capable of supporting the valve in the event of any future sinkholes. For this article, we will focus on the foundation design investigation.
We completed a boring in the vicinity of the compromised water main and the subsurface conditions were what we would expect immediately adjacent to a major waterway: granular fill over very loose to loose silty sands underlain by coarser-grained and more dense riverbed deposits. Glacial till was encountered at depth, almost 75 feet below ground surface. The looser silty sand represented about 23 feet of the formation and extended to around 30 feet below ground surface. Groundwater was observed in the loose, finer-grained river deposits between 21 and 25 feet, as shown in the snippet of the boring log below.
Foundation Design
Based on the subsurface conditions encountered in the boring and our knowledge of the persistence of sinkholes in the area, we knew we had to design support to go beyond any loose zones and anchor the valve’s foundation into a dense layer below. Glacial till is a very dense, glacially deposited soil type that is an amalgamation of the four soil types, gravel, sand, silt, and clay, which makes for an extremely tight soil matrix. This soil makes for an excellent bearing layer to set a foundation on. Now that we had our bearing layer identified, we needed to select an appropriate foundation type.
Driven and drilled piles are two deep foundation types that could be utilized to support the water valve. In our scenario, we have nearby sensitive structures, such as the floodwall and the Commission’s Connecticut River Interceptor nearby which may be susceptible to vibrations, so a driven pile type is off the table. Micropiles are a common drilled foundation type that are relatively cost-effective to install and can achieve high-capacities in tight spaces. They can be installed using conventional excavation equipment with the proper attachments and use readily available materials: steel casing, rebar, and grout. The design selected was a micropile embedded in the glacial till and connected to the water valve with a pile cap. This approach will ensure the water valve will be minimally impacted if any future sinkholes occur in the upper loose zones.
Micropile Installation
Installation of the micropile began by setting the casing and advancing the hole with a rollerbit. Advancement went smoothly until about 30-35 feet below ground surface (bgs), when the soft silty sand transitioned to a denser, gravelly deposit. Difficulty advancing the casing combined with the surrounding loose silty sands resulted in some surface subsidence, as shown in the photo below.
The contractor was attempting to push the casing down to overcome the difficulty in advancing the casing and in the process, lifted and pushed the casing multiple times, which led to local compaction of the looser surrounding soils. Kleinfelder’s resident project representative and contractor witnessed this settlement and made the decision to take a step back and notify the Engineer and Owner of the difficulty in advancing the casing, failed methods of overcoming this difficulty, and the resulting settlement. The contractor recommended switching to a hollow bar system, which allows the borehole to be advanced without the need for permanent casing. After coordination with the Commission, we alerted the other major stakeholders of the situation and presented the intended path forward. In hollow bar installation, a thin grout is used to keep the hole open and is permeated under low pressure through the center of the hollow bar, which also acts as the reinforcement for the micropile. Since grout needs to flow through the center of the bar, it results in a larger diameter reinforcement than originally designed. The low-pressure element was key in securing USACE buy-in specifically, as they were concerned with hydraulic fracturing, which would further exacerbate the sinkhole issues. Additionally, the contractor’s engineer was able to shorten the length of the micropile now due to the larger bar being used as reinforcement. The micropile was installed to a shorter length and less steel and grout was used than originally planned for, while still providing adequate support and protection of the valve from future subsidence.
Conclusion
Construction, even of relatively straightforward foundation elements, in sensitive geologies and around critical infrastructure with multiple stakeholders is a collaborative effort that involves all parties. From the owner to the designer to the contractor, everyone needs to be continuously engaged to manage adaptation to changing field conditions and deliver a successful project. Installation methods can vary across the same foundation type, and it is important to consider all variables when selecting the appropriate method for the project. Field conditions can bring about unforeseen issues during construction and engineer oversight is crucial in identifying these problems, communicating them in a timely manner, and recognizing when work should be halted to prevent damage.