Texas is home to three of the fastest growing metropolitan areas in the United States. Heavy construction in these urban areas is at an all-time high. And now that much of the prime real estate in downtown Austin located north of Lady Bird Lake has seen development, focus has shifted to more difficult construction sites along Shoal Creek and south of the Colorado River. With closer proximity to major ground water sources, new below grade project developers are turning to an old, but innovative construction technique to deal with the challenges of excavation below the water table. The use of the diaphragm wall is back on the rise in Texas.
History of use in Texas
Diaphragm walls, also known as slurry walls, have been used in the US since the early 1960’s. From the beginning, most of this work was performed in coastal metropolitan areas such as San Francisco, Boston, New York and Chicago. The method is ideal for providing a water barrier for deep excavations near large bodies of water or shallow water tables. As the design of building foundations began to require deeper excavations in other interior urban areas of the country, the need for cutoff walls to hold back water for these deep excavations further advanced the demand for the system. Cities such as Denver, Omaha, Philadelphia, Washington, DC and Dallas began to see the method as more common place. In all, there have been well over 400 diaphragm wall projects completed in more than 50 US cities since 1962.
In 1972, the foundation for the First International Building (now the Renaissance Tower) in downtown Dallas was constructed using a diaphragm wall, one of the first such applications in Texas. Through the early 80’s and 90’s, there were no fewer than 10 more projects in the Dallas area to utilize the method for deep urban excavations and public transportation projects. In just the last 2 years, the method has been applied on 3 deep excavation projects in downtown Austin, with one more currently in the design phase.
Application and Construction
Diaphragm walls can be used for large building foundation walls, cutoff walls for dams and tunnel access shafts, or any below grade enclosure structure. They are most effective when used as the permanent basement wall in combination for both vertical superstructure support and permanent earth retention for basements extending below the water table. When socketed into an underlying rock formation, they effectively cutoff water infiltration into the excavation. Typical support systems to resist lateral earth pressures include tieback anchors and/or internal steel bracing, much like those employed for temporary soldier pile and lagging walls.
Since the permanent basement wall is constructed prior to basement excavation, the method is considered a top-down approach. In the initial phase of construction, a guide wall is constructed at the surface along each side of the wall alignment. Guide walls are usually temporary but are an essential part of the construction process. They provide for the proper positioning of the wall, as well as elevation control for rebar cage placement. Once complete, guide walls serve as a working platform for personnel and support many of the ancillary items used during concrete placement such as tremie pipes and end stops.
The soil between the guide walls is excavated as a trench to depths below final subgrade needed in design for resistance to earth pressures. Historically, walls were excavated with massive devices known as clam shells. These mechanical grab tools where typically very heavy and had an upper body as tall as 20 to 25 feet requiring large crawler cranes for handling. They tended to be free swinging buckets that required manual positioning between the guide walls as they were lowered into the trench. Each pass produced a minimal amount of excavated material and the spoils had to be drained prior to haul off and disposal. The weight of the bucket as well as the height of the body provided for verticality control as the trench advanced to depth. Tolerances for verticality ranged from 1 to 2 percent of the wall height. When rock was encountered, a large chisel device replaced the bucket and broke up rock with repeated drops along the trench bottom. The bucket would then be re-attached in order to clean the bottom of the trench. Rock penetrations were generally limited to a few feet.
Excavation tooling for diaphragm wall construction has advanced significantly in the last three decades. The Hydromill trench cutter is used extensively in today’s market. Large circular cutting wheels located at the bottom of the cutter excavate soil and rock much more quickly and to much greater depths than traditional clam shell and chisels. The cutter body is also significantly taller than a clam shell with heights of up to 50 feet. This allows for tighter vertical tolerances for the excavated trench. In addition, the cutter body has pressure plates located on all sides which can be controlled by the mill operator. Sensors in the tooling indicate when the tool is starting to stray from true vertical. An operator can literally “drive” the trencher by apply pressure against the side of the trench at the top and bottom of the cutter body. This is significantly improved vertical tolerances for wall installation, typically within 0.5% over the height of the wall. Hydromills are also adept at cutting into rock strata that were impossible for clam shell buckets to excavate.
As the trench progresses, a slurry is added to provide additional lateral support for the trench walls during cutting. Slurries are generally comprised of bentonite clay and water with the occasional use of synthetic polymers added to improve slurry viscosity and bleed performance. The slurry level is kept near the top of the trench to provide positive head pressure against trench walls below the water table. As the slurry bleeds into the soil, a bentonite “cake” is formed along the sides of the trench. This adds additional stability to the trench wall and prevents excessive slurry bleed off. Spoils from the trench are mixed into the slurry suspension and pulled up to the surface through the center of the cutter body with a high-powered suction pump. As slurry is removed from the bottom through the cutter, more slurry is added to the open trench top.
Slurry with the suspended solids is pumped to a centrally located de-sanding unit and processed. As the mixture travels through a series of hydroclones (or fluid cyclones), the solids are removed from the suspension. The remaining slurry is pumped back to storage tanks for replenishing and reuse. The solids, i.e. gravels, sands and silt, are deposited on the ground, collected and hauled off site.
The Hydromill excavates the ground in rectangular panels which measure approximately 10.5 feet long by 2’-8” in width. Each primary panel consists of 3 overlapping passes by the trencher. Secondary panels, also known as closure panels, consist of one pass of the trencher and overlaps primary panels on both sides. By cutting into the adjacent primary panel concrete, the closure pour excavation creates a roughened joint between concrete pours. Once a panel has been excavated, the slurry in the trench is tested to verify proper viscosity and sand content prior to placement of concrete.
A few hours before a panel is to be poured, a prefabricated rebar cage is lifted and lowered into position within the trench. Vertical reinforcing steel for the diaphragm wall typically consists of #10 or larger reinforcing bars placed front and back face with smaller sized horizontal bands. In addition to the re-steel, steel pipe sleeves with bearing plates are tied into the cage as reservations for future tieback anchor penetrations through the wall. As the excavation advances, these sleeves are exposed to provide easy passage through the wall for anchor drilling. If internal bracing is to be used for wall support, bearing plates with headed studs are used in lieu of anchor sleeves. Elevation control is accomplished by measurements from the pick point at the top of the cage. This is the point at which the cage will be hung from the guide wall.
Concrete is placed by tremie method so that contamination of the cement and aggregate is minimal. As concrete advances to the top of the panel, the tremie pipe can be shortened to allow for better concrete flow, but never to be pulled out of the previously placed concrete. The displaced slurry is collected off the top of the panel and returned to storage tanks for replenishing and reuse similar to the processed slurry from the de-sander. By design, panels are poured to the surface as determined by the top elevation of the adjacent guide walls. Panel concrete may be held low in areas where large penetrations through the wall are required by final design. When this is done, some chipping of concrete at the top of the pour is required to remove latent concrete mixed with coagulated bentonite from the slurry.
Once placement of the diaphragm wall concrete has been completed for the entire perimeter of the basement, the interior mass excavation may commence. The soils are removed in lifts corresponding to each level of bracing required for the wall. After a level of bracing is installed, the excavation is exposed down to the next bracing elevation and continues in sequence until final subgrade is achieved. Bracing is usually considered temporary as the permanent slabs of the structure tend to serve as strut supports for resisting the long-term lateral earth and water pressures on the wall.
Many diaphragm wall structures are performed as “Design-Build” projects. Since diaphragm walls are such a specialty application, there are only a hand full of qualified constructors nationwide. Design is closely tied to constructability or means and methods. The specialty contractor typically designs the wall to handle the temporary lateral earth pressure condition with tieback anchors and/or internal bracing. Once an initial design is completed, the structural engineer-of-record verifies that the permanent loading condition requirements are accounted for in the wall design. This process involves a very coordinated effort between wall contractor and engineer.
In many applications, diaphragm walls are temporarily supported with tieback anchors placed outside of the property line. Tiebacks are de-tensioned after completion of the below grade, internal floor slab system. The internal structural slabs must be designed for axial loads due to external earth pressures on the wall. Perimeter column loads from superstructure framing can easily be carried by diaphragm walls founded on rock and installed in thicknesses ranging from 24 inches to over eight feet. Standard wall thickness for the most recent projects completed in Austin was 32 inches with perimeter column loads of over 4200 kips per column.
Design of earth retention systems can be quite complicated. The interaction of soils and structure must be thoroughly evaluated in order to create cost efficient designs. Since the equipment used in diaphragm wall construction is large and expensive to mobilize, the demands on efficient design relative to production processes are significant over other more conventional shoring techniques. For this reason, designs are often performed using finite element analyses to determine wall loading and bracing requirements. Shear and bending moments in the diaphragm wall are dependent on the deflection condition during excavation. It is only possible to determine the serviceability state of the system with an understanding of soil stiffness parameters and their relevance to FEA computations. The employment of an experienced geo-structural engineer is vital to the successful implementation of diaphragm wall design.
As the demand for commercial development in our urban centers continues to grow, the need to address poor subsurface conditions on less lucrative building sites will require engineers to seek more innovations in construction techniques. As general contractors in the city of Austin have discovered, the use of diaphragm walls can be an attractive cost-effective way to deal with large excavations below the water table. As buildings continue to rise into skylines across our state, the diaphragm wall will be there to support them.
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