The building has 500,000 gsf (46,500 sq m)of medical office space with garage parking for 2500 vehicles that covers an area ao over two city business district blocks for a total structure area of 1.5 million sq ft (140,000 sq m). The parking garage is the largest modern garage of its kind built in Houston. The reinforced concrete tower rises 32 floors above a two level basement that had an excavation depth of 29 ft (9m) to the base of foundations with a plan geometry of 237 ft by 200 ft (72 by 65 m). Six levels of parking and one basement level extend 250 ft (76 m) south of the tower south face. The garage basement extends into the tower so that two basement levels are beneath the tower and part of the garage, Figure 1.

Finished floor levels of Basement level 2 and Basement Level 1 are 22 ft (7m) and 12 ft (4m), respectively.

An excavation retention system was needed because the project extended to the property lines in the Texas medical Center and the most heavily traveled streets bordered the property on three sides with an extensive network of utilities. To minimize the use of property for temporary excavation retention, designing the underground structure to function for both temporary and permanent condtions was the cost effective approach selected. Drilled piers, 42 in.(110 cm) dia. At 18 in. (46 cm) clear spacing , were selected for the temporary basement wall to satisfy both of the requirements of the construction cantilever wall design and support of the perimeter columns. Accordingly the all exterior columns for the tower and garage were supported on the basement wall to avoid cantilevering the structure floors and reducing the interference of perimeter columns on the north, east, and west sided of the tower, fig 2. The south side perimeter columns at the garage transition are supported by large spread footings. A mat foundation is used to support the Tower core. Intermediate columns along to ease-west axis are supported by drilled pier groups.

Drilled piers 42 in. dia. Designed as a cantilever arranged in a single line group along three sides to form the temporary excavation wall, permanent basement wall, and full foundation support for the building and garage exterior columns

Shallow spread foundations to support the interior garage loads

Mat foundation to support the interior building core

Mini mats 30 ft by 15 ft in plan that support the exterior building columns bordering the parking garage

Six-pier groups to support intermediate columns between the mat and basement wall along the east-west building axis

Although the building floor slab in Basement Level 2 is 20 ft below street level, the building mat foundation base extended 29 ft below street level and the garage foundation bases were 16 to 18 ft below street level.

The drilled pier retaining wall along with using multiple foundation systems exploded the project success because the piers allowed maximum efficient utilization of the property. Although we considered auger cast in place piles, we chose drilled piers installed by the Slurry Displacement Method because the fewer variables to control in construction made the use of piers a more steadfast foundation system and the reinforcing could be installed in advance of concreting instead of attempting reinforcing installing after grouting.

The edge of the core mat was limited so that the pier cantilever could be designed only for the lateral earth pressure between the street level and top of Basement Level 2 floor slab and the axial loads from the structures. Pier sections were 42 in. dia. by 86 ft bordering Basement Level 2 and 30 in dia by 40 ft along the Parking Garage Basement level 1.

Drilled Piers. A total of 379 piers were installed by the Slurry Displacement Method consistent with ACI 336.1-89 and 01 based on a design principally made by the Design Geotechnical Engineer and Project Specifications prepared by the Design Geotechnical Engineer. Nearly 180 piers were 42 in. diameter by 70 to 120 ft to support the exterior columns of the thirty story building, act as the temporary excavation wall, and permanent basement wall. A total of 202 piers at 30 in. diameter were installed to depths ranging from 37 to 92 ft to achive the same objectives as the large diameter piers , but for a one story basement.

Pier clear spacing was typically 18 in. Reinforcing amounted to 1.2 to 1.3% of the pier cross sectional area using an asymmetrical spacing for efficient use of pier reinforcing steel. Although the reinforcing spacing was initially designed for a minimum clear spacing of 4 in. or 4 times the size of the maximum coarse aggregate, the Geotechnical Engineer acquiesced to a smaller clear spacing of 3 in. consistent with past positive experiences with strong construction engineering on site.

Pier Concrete. The pier concrete mix, Table1, was used successfully on past projects with the supplier and was again confirmed for suitability by trial batch for maintaining a 6 in. slump for 4 hrs because cement manufacturers and mid-range retarder product manufacturers had changed since the last use of the mix with these design properties.

The general subsurface conditions are variable between soil borings, consistent for Houston Area deep excavations, Table 2. Beyond the surface fill, the underlying soils are medium to strong clay with sand and silt inclusions at varying thicknesses and positions. From the base of the fill to about 30 ft below street level, the soil is clay with a distinct but discontinuous silt zone from 70 to 80 ft below street level. The clay grades to dense silty fine sand below 30 ft to about 60 ft. Strong clay are below the sand.

The Undrained shear strength of the clay soils below about 10 ft is more than 0.7 tsf and becomes more than 1.0 tsf below the upper sand. The sand and silt is medium dense to dense in condition provide the groundwater is controlled properly during site excavation. The sand and deep clay have siltstone inclusions at varying positions but generally smaller than 2 ft. The depth to ground water was measured to be 24 ft below the ground surface in the upper sand and slightly deeper in the zone from 70 to 80 ft depth.

The shallow foundations in the structure interior consisted of a core mat for the tower, mini mats 15 by 30 and spread footings 12 ft by 12 ft. The tower mat was design using the DAM procedure initially reported by Ulrich (1995) for the design of mat foundations and adjusted for local experience with tall buildings. The assessment of building foundation movement was analyzed by extending the DAM procedure to the perimeter walls considering Busssinesq and Wetergarrd Models. The expected differential movement between the basement wall and tower core of less than 1 in. was achieved by top out in June 2006.

Proper control of the groundwater is needed for both construction and long term conditions. Inadequate groundwater control would cause the excavation to be unstable and the mat subgrade would become quick resulting in catastrophic building performance. A system of deep, 8 fully penetrating wells with submersible pumps was installed outside of the tower to depressure and dewater the building excavation. The temporary dewatering was not designed to allow drilled pier installation by the dry methods because local experience has shown that the dewatering system is seldom effective in preventing premature collapse of the shafts below the original groundwater level. Combined flow to the discharge location was 70 to 80 gpm.

The temporary system was replaced by an underslab, gravity system using a double stage sand and gravel filter to PVC collector pipes designed and detailed by the Geotechnical Engineer.

Drilled pier construction followed the project specification prepared by the Design Geotechnical Engineer in general accordance with ACI 336.1- (01) and first used in Texas to define the pier construction method on major Texas projects (Ulrich, 1990, 1995). Before the emergence of the new ACI pier construction standard, piers installed by the slurry displacement method were popular primarily with the Texas Department of Transportation and in Florida.

The principal parameters measured on this assignment were mud weight, mud viscosity, tremie tip position, and concrete flow characteristics. Concrete delivered to the pumping location not containing at least a 7 in . slump was rejected. Except for 6 piers wherein the concrete was not expunged beyond the top of concrete, all piers were clean smooth and free of significant defects to the depth exposed in the excavation.

Example of an occasional hole only extending to the rebar.

The drilled pier installation process and results were an awesome improvement over the results which occurred without a responsible pier installation standard and needed construction engineering, Fig. 8.

Conclusions

Drilled piers are a highly productive and economical foundation system that can be used independently or with other foundation types to safely support structural loads and accommodate architectural innovation in both temporary and long term conditions. All foundation systems have the potential for defects. Flawless construction is not possible, a distinct advantage of drilled piers over Augercast piles is that pier construction using the Slurry Displacement Method has less construction variables to control; hence, the risk for significant defects should be less. Success in underground construction occurs with a team commitment and the underground specialist contributes appropriately as a design team member.  The Memorial Hermann Medical Plaza now stands as a welcoming beacon to those in search of innovative medical care.

            The author gratefully acknowledges the Memorial Hermann Hospital System for their innovation and focus in design and construction. The Mischer Development Corporation developed the project and provided guidance where light was hard to find under the leadership of Robin Harrison. Harvey Construction Company as the contractor, led by Kelly Hall,  was well organized and committed as the best of the best. Kirksey Architects, led by Jim Deitzman,  are appreciated for both their architecture and sensitivity to construction. Some may say such attributes are opposites. Not enough can be said about Haynes Whaley Associates, Structural Engineers who always left the closet door open and the key in the hands of the Design Geotechnical Engineer. Thankyou to Robert Tyler and Ronal Chen. From top to bottom, a dedicated design-build team conquered the unthinkable and the result was significant cost savings for ownership. Bell Bottom Foundation Company was superb as usual and TXI was exemplary in providing the pier concrete.  

Ulrich, E. J. (1995). Subgrade Reaction in Mat Foundation Design, ACI SP-152 Design and performance of Mat Foundations, A State of the Art Review, Farmington Hills, MI, pp 95- 116.

Ulrich, E. J. and Ehlers, C.J. (1995). Tallest Building in the Texas Medical Center: St. Luke’s medical Tower, ACI SP-152 Design and Performance of Mat Foundations, A State of the Art Review, Farmington Hills, MI, pp 219- 244.

Ulrich, E.j. & Ehlers (1990). Arrest of a Texas Landslide, Concrete International, ACI, Farmington Hills, MI.,pp