Shotcrete in the Urban Jungle
Jason Myers
In today’s complex urban jobsite the easy to build on sites are long gone and harder to reach sites and sites that require creative solutions are the ones that are left to develop. 1395 22nd St is a complex of five apartment buildings that were built into one of the many hillsides of San Francisco, Ca. and in an area of the city that is surrounded by existing buildings and adjacent properties. Several of the tallest buildings for the project have one level below grade, one level at grade, and ten stories above grade. As part of the foundation system of the project horizontal and vertical grade beams were installed along the hillside for the first five stories of the buildings and then anchored into the hillside with highly loaded tiebacks. The excavated slope was up to a 1.5:1 slope which made for very difficult access to get to each of the grade beams. Each of these first five floors of the project were anchored into the hillside by the floor slab sitting on the grade beam and each of the grade beams were over 550 lf (167 m) long and as much as 75 ft (22.8 m) up the slope. A typical grade beam was around 3 ft (0.9 m) square, but some of the grade beams included larger footings that were 3 ft (0.9 m) deep and up to 5 ft (1.5 m) across. The volume of the grade beams was over 1,350 cy (1030 m3).
The General Contractor was looking at a very difficult formwork process to try to cast in place the grade beams with very little access to each of the grade beam locations. The General Contractor also realized that once they started to develop the detailed schedule for the project they realized that the formwork sequencing was not going to fit into the original timeline. The General Contractor contacted Dees Hennessey, Inc (DHI) to figure out if there was a way we could help with the budget, construction limitations, and timeline issues that they were facing on the project. After some initial brainstorming we figured out this was a perfect shotcrete application. Some of the constraints facing the General Contractor that we looked at were the access issue, how to do the formwork (and how to brace it on the side of a hillside when there is nothing to brace to), timeline to install the formwork, and getting the concrete to cure so that the tiebacks could be locked off and the next level started. Shotcrete was able to solve all of these issues.
The plan that was developed was for the General Contractor to develop a work platform that could sit on top of a previously poured grade beam and since all of the grade beams were uniform in distance in height and setback the deck would provide a work platform that could be easily lifted to each level by sitting on the previous grade beam with little modification. Once the platform was installed the rebar cage could be installed and then shotcrete was installed on each of the grade beams. Since shotcrete was used it created a situation from the General Contractor having to figure out impossible formwork to not requiring any formwork at all. Also shotcrete was able to work around and provide the proper finishes for the anchor bolts for the work platform, the tieback anchor plates, or a nozzle finish for a grade beam that did not need it and was going to be buried. The design strength of the grade beams were 5,000 psi (34.5 MPa) but in order to decrease the timeline for the stressing of the tiebacks an accelerator was used to increase the set time of the shotcrete and an 6,000 psi (41.3 MPa)mix was used in case of any strength loss due to the rapid set. The plan was for the grade beams to reach design strength and to turn the tiebacks over to be stressed five days after shotcrete was installed but instead the shotcrete normally reached design strength and the grade beams were typically turned over in three days. The overall schedule was originally for each grade beam to take three to four weeks per level to get access, formwork installed, poured, stripped, and stressed but with the shotcrete process we were able to get a grade beam level every one and half weeks.
Once the grade beam contract was wrapping up we negotiated a change order to install shotcrete for all of shear walls, parapets walls, and retaining walls for all of the vertical concrete walls on the project. This lead to additional schedule and economic benefits to the project. An additional benefit was the flexibility with our daily routine and shotcrete some of the first structural walls in the same day as the grade beams, and also install some of the retaining walls at the end of the shift. The scope of the shear walls, parapet walls, and retaining walls included shotcrete up to 8,000 psi (55.2 MPa), pumping distance up to 1,000 lf (305 m), and a scope of over 3,400 cy (2,600 m3) and 90,000 sf (8360 m2).
Project Name 1395 22nd St
Project Location 1395 22nd St, San Francisco, Ca
Shotcrete Contractor Dees Hennessey, Inc.
General Contractor Pacific Structures
Architect Perry Architects
Structural Engineer Nishkian Menninger
Material Supplier Cemex
Project Owner RP Pennsylvania, LLC
Author
Jason Myers graduated from California Polytechnic University at San Luis Obispo in 1995 with a Bachelor’s Degree in Civil Engineering and from Golden Gate University in 2015 with a Master’s In Business Administration with an emphasis in Project Management. Jason started out his professional career working for an earth retention subcontractor where he learned the importance of budgeting, scheduling, and client relationships. Also during this time he was introduced to the use of shotcrete and its applications. After working for a General Contractor for a couple of years he realized that he enjoyed the tighter knit of working for a subcontractor and the ability to construct projects on a tighter time frame with several going at once. Jason also enjoys the process of handling most of the procedures that go into constructing a project rather then seeing only a small portion of the process. Jason joined Dees Hennessey in 2004 and has been a part owner of the company since 2007. Jason currently serves as the Vice President of Operations as well as the safety director.
Shotcrete Magazine Article
Outstanding Shotcrete Project – 2015
Architecture/New Construction Project
Sufism Reoriented
Sufism Reoriented is an American spiritual order that focuses on the principles of divine love as the central focus of their lives. They are in the process of constructing a permanent home in Walnut Creek, CA that is nearing completion. The new sanctuary will be set among the serene gardens on three acres of land surrounding by a suburban neighborhood. The design of the structure was designed under the guidance of Murshida Carol Weyland Conner and the highly distinguished architectural firm Philip Johnson/Alan Ritchie with Soga and Associates being the Architect of Record. The project had its ground breaking ceremony on May 24, 2012 and the sanctuary is expected to be completed in the middle of 2016.
An important symbol to the order is the circle. The circle is expressed in the gently sloping saucer domes vaulting the Prayer Hall and the adjacent rooms. The outline of the domes is to reflect the soft, rolling hills that border the surrounding valley. Within the sanctuary, the domes create tranquil and uplifting interior spaces for prayer, meditation, and communion with God. The roof of the sanctuary consists of eight small domes, four medium domes, and in the center one large dome.
The eight small domes have a diameter of 22’ 8” (6.91 m) with the design of the project having the small domes constructed out of fiberglass. The four medium domes have a diameter of 37’ 6” (11.43 m) and a height of 13’ 2” (4.01 m) with a concrete thickness of 7 1/2” (0.19 m) with an approximate area of 1,500 sf (140 m2) each. The single large dome has a diameter of 76’ (23.16 m) and a height of 21’ 6” (6.55 m) with a concrete thickness of 7 1/2” (0.19 m) with an approximate area of 5,800 sf (540 m2). The Concrete Contractor, Overaa Construction out of Richmond, Ca was awarded the concrete portion of the project and had originally planned on doing the domes with a cast in place placement but once the project got out of the estimating department into the construction phase they realized that the cast in place was not the best solution. At this point Overaa Construction started some conversations with Dees Hennessey about the possibilities of using shotcrete. Shotcrete proved an ideal solution for the domes because the complexity of the formwork was eliminated and the architectural features of the domes could immediately be seen and evaluated by the Owner, Contractor, and Dees Hennessey to make sure the domes were geometrically correct before the shotcrete had set up and make correction if needed during placement. A full height section of the dome was constructed as a preconstruction test panel to verify the nozzlemen as well as to make sure that the architectural finish was acceptable for the General Contractor, Architect, and Owner.
Each of the domes had an opening in the top of the dome (oculus) which required the concrete contractor starting out each of the domes with pouring a compression ring of concrete around the base and top of each of the domes to lock in the rebar and the structural frame of the structure. Each of the medium domes was then shot monolithically during a single mobilization. Due to the size and weight of the large dome the dome was split into six sections and two nonadjacent sections were shot during the three mobilizations to complete the large dome. The wiring of these domes was extremely problematic because of the geometry, surveyors checking the geometry, and as each section was complete there was fewer areas to walk on which only increased the chances of the wires being damaged. There were numerous evenings which the Contractor’s Representative or Owner’s Representative had to be chased off of the domes because they wanted to visualize what the domes were going to look like.
As previously discussed one of the challenges for the shotcrete, and ones of its advantages, was shotcreting the dome shape. After the shotcrete was complete the waterproofing consisted of a fluid waterproof membrane that was applied directly over the concrete. This meant that all of the shotcrete surfaced had to be finished exactly on the curved surface within construction tolerances because any deviation would have been very noticeable as well as any patchwork would have shown through the waterproof membrane. The advantage with shotcrete was that the final layout could be continuously checked and adjusted as needed to make sure the proper shapes were achieved. This was extremely problematic in the large dome with such a large structure and installing it in six different sections and could not be visualized until all six sections were complete. This was accomplished by Dees Hennessey with no issues or patching required. In the end the Owner was very satisfied with the shotcrete surface and geometrics and was able to use a thinner and simpler waterproofing membrane because of the quality of the shotcrete finish. Another challenge was that each of the domes has a drainage channel and ladder along the curved surface that required additional detail work.
Dees Hennessey relied on its over thirty years of shotcrete experience on difficult projects to ensure that the quality control, safety standards, and correctness along with the industry standards were used on this project. Even though this was a very difficult project it showed that with the proper preplanning and thinking through how the project is going to be constructed it can greatly simplify a project and eliminate a lot of the problems that may occur down the road. The success of this project is due to the great teamwork between all parties and individuals.
Project Name Sanctuary for Sufism Reoriented
Project Location Walnut Creek, CA
Shotcrete Contractor Dees Hennessey, Inc.
Concrete Contractor Overaa Construction
Design Architect Philip Johnson Alan Ritchie Architects
Architect of Record Soga and Associates
Material Supplier Central Concrete
Project Owner Sufism Reoriented
Author
Jason Myers graduated from California Polytechnic University at San Luis Obispo in 1995 with a Bachelor’s Degree in Civil Engineering and from Golden Gate University in 2015 with a Master’s In Business Administration with an emphasis in Project Management. Jason started out his professional career working for an earth retention subcontractor where he learned the importance of budgeting, scheduling, and client relationships. Also during this time he was introduced to the use of shotcrete and its applications. After working for a General Contractor for a couple of years he realized that he enjoyed the tighter knit of working for a subcontractor and the ability to construct projects on a tighter time frame with many going at once. Jason also enjoys the process of handling most of the procedures that go into constructing a project rather then seeing only a small portion of the process. Jason joined Dees Hennessey in 2004 and has been a part owner of the company since 2007. Jason currently serves as the Vice President of Operations as well as the safety director.
