Tangent auger cast walls are constructed by drilling low-vibration auger cast piles at spaced intervals followed by installation of additional auger cast piles between those spaces. This forms a continuous wall, sealing off the excavation. While these walls may be tied back, internally braced, or cantilever, tangent auger cast walls are often a good solution when seeking a vibrationless earth retention system adjacent to existing structures or critical utilities.

Featured Project

Project Details

Hardman Construction was fortunate enough to work on the Michigan State Capitol Building’s utility upgrade project. The project consisted of combining new utilities with a new central utility vault at the south end of the building.  In order to complete the extensive renovations, the site required an earth retention system. The excavation depth for the new central utility vault was 15 – 25’ below grade, with an overall earth retention wall length just under 500 lineal feet.

Due to the historic nature of the capitol building, vibration and movement sensors were monitored on a 24-hour basis. The sensors notify all construction parties if certain thresholds have been exceeded. Hardman construction utilized a vibrationless earth retention system and a tangential augercast earth retention system. Driving sheeting or installing driven H-pile was simply not an option.

The tangential augercast wall first needed a template poured. Doing this keeps the piles in the correct alignment. Piles are then drilled with the auger and grouted to the top. Every third augercast pile was reinforced with a vertical H-pile. The H-piles holds the majority of the load from the retained earth. The reinforced piles are installed in front of the non-reinforced or filler piles. Filler piles are installed behind the beam piles and are not reinforced with steel. The arched alignment of the piles helps with load transfer to the reinforced H-piles and makes it difficult for gaps to develop between the piles. It’s important to maintain no gaps between piles. Once all piles are installed, the excavation can take place. In areas where the cut is deeper, the wall is reinforced with steel walers. The walers span from one side of the pit to the other creating a box like structure. This gives the perimeter earth retention walls much more rigidity.

Hardman Construction worked on the University of Michigan Ross School of Business demolition and expansion project. The old School of Business was demolished to make room for the new one, which features a new facade as well as additional rooms below grade. In order to complete the extensive renovations, the site required an earth retention system. The excavation depth for the new building was 20’ to 27’ below grade, with the overall wall length just under 1,000 lineal feet. Hardman Construction installed a tangential augercast wall. Every third augercast pile was reinforced with an H-pile. The reinforced piles are installed in front of the non-reinforced or filler piles. Filler piles are installed behind the beam piles and do not have steel reinforcement. By doing this, the soil load is transferred into the beams. To resist lateral loads, earth anchors were used. The earth anchors were installed through anchor pockets which are fabricated in the H-pile reinforcement. Depending on the depth of excavation, up to two rows of ties were used. Another interesting aspect of the project was that approximately 150 lineal feet of the earth retention system had to be installed with limited headroom of 15′, which was required to protect a 250-year old oak tree. We utilized a low headroom drill rig and had to splice the H-piles while in place. An ongoing challenge faced was where the new Ross building tied into the two existing buildings. The new foundations were 15’ deeper than the two existing buildings. If the foundations for the new building were excavated, it would undermine the foundations of the existing buildings. Conventional earth retention systems could not be used due to space constraints. The only option was to stabilize the soil using micro fine cement and conventionally underpinning the existing foundations. We installed 40,000 gallons of micro fine cement to help stabilize the soil while the foundations were then segmentally underpinned. Ross School of Business opened for the 2016 school year.

The Mosher Jordan Hall at the University of Michigan was renovated using Hardman Construction to reinforce the foundation of the existing building. The renovation included the addition of a new dining area which was 18’ below the elevation of the existing building footings and remodeling of the existing dorm rooms. The excavation next to the existing hall was constructed using a permanent earth retention system which was a tangential augercast wall with permanent earth anchors and a structural shotcrete facing. In addition to the permanent ERS wall, micropiles were installed in both the outside and inside areas. The micropiles that were installed inside of the existing building were placed in 8’ of headroom and were utilized to support the existing columns during the time they were removed and the new footing were placed at a lower elevation. The micropiles that were placed outside the building were installed to support the existing area ways during excavation. In these areas the permanent ERS system was not utilized as underpinning was required.

The new University of Michigan Law School required the installation of augercast tangential walls, chemical grouting, and underpinning for the construction of the new buildings. The ERS was a tangential pile wall with earth anchors. Our contract was held by Walbridge and our scope of services was a prime contract. Our work included the tangential walls, grouting and permanent underpinning of adjacent structures. The North Addition was in the existing courtyard and presented some very difficult challenges for the project team. The existing foundation required grouting and underpinning as the proposed footings were to be 8’-10’ below the existing foundations. In addition, a tangential earth retention wall was required for access into the work area. This work had to be performed during the university’s summer break and an extensive amount of overtime and coordination was required to complete this work on time. The inside work demanded limited access equipment and signifcant scheduling. The South Addition was an augercast tangential pile wall with earth anchors. The wall was over 1,000 LF long and enclosed the perimeter of the site. The South Addition was started in the fall and completed throughout the winter. There were areas of the tangential wall where a permanent shotcrete facing was required. This area of shotcrete was performed in the winter and required heating and housing to protect the work. The interesting part of this project is that the south addition ERS walls connected to the previously installed tangential walls that U of M first used on the Gerald Ford Public Policy Building. The previous project was a trial ERS wall that U of M, selected after having diffculty achieving successful ERS walls on campus. The installation, construction, pricing and settlement results of the Gerald Ford project, have continued to be used on future work at U of M. The two building areas were extremely limited, and delivery of materials had to coordinated daily. The settlement of the existing structures was monitored by settlement points. These points were checked throughout the project’s duration and no settlement was recorded in the adjacent structures or utilities. The settlement of the existing building was a concern for all parties, and it was successfully completed by using good construction techniques and engineering. The anchors were installed and tested per the specifcations. The permanent shotcrete facing was placed using wet mix methods.

The $50 million dollar 5th Avenue parking structure required the installation of a temporary Earth Retention System consisting of a tangential auger cast pile wall & soldier pile & lagging system. Augercast tangential walls were selected over secant walls for the tangential pile areas, and the soldier pile & lagging system was per the original scope of services. The primary reason for the augercast wall was the economics and schedule constraints that the secant wall presented. The project was set up as a secant wall design, however when the final bids were received, the project team decided to use the tangential auger cast pile wall as it was a substantial saving to the project. The scope of services was the construction of the new parking garage adjacent to the existing downtown buildings. The site was extremely congested, and deliveries had to be scheduled days in advance because of this. The existing buildings were still in service and our work could not affect these operations. The project required an ERS system to support the existing residential, & commercial building that were on site, numerous utilities that serviced the existing buildings, and tower cranes. The ERS system had to be vibration free, due to the sensitive nature of the existing operations, which were directly adjacent to our work. Due to the site conditions, weekly meeting with project managers were held throughout the project. Daily activity meeting with the site superintendent and look-ahead schedules were required. The settlement of the existing structures were monitored by settlement points. These points were checked throughout the project’s duration and to date there has been no settlement of any adjacent structures or utilities. The project included 1,510 lineal foot of tangential auger cast pile wall (855’) & soldier piles & lagging (656’). The excavation depths were up to 71’ deep. There were over 40,000 lineal foot of augercast pile drilling installed, 1,130,000 lbs. of reinforcing steel installed, and over 6,000 cubic yards of grout used for the project. To our knowledge, this is one of the deepest excavation depths which utilized this type of system. The tangential auger cast pile wall utilized earth anchors which were installed using duplex drilling and self-drilling/grouted anchors. Over 700 earth anchors were installed. All of the anchors were tested and locked at their required loads. The duplex drilled earth anchors were tested to maximum of 235 Kips and the self-drilling anchors were tested to 144 Kip. The duplex-drilled anchors were installed using water, grout, and air as the drilling mediums.