The Colosseum at Caesars Palace: A Case Study of Design Build Foundations
CFA Piles, Drilled Shafts, Restricted Access Drilling & Secant Walls
The construction of the Colosseum at Caesars Palace provided challenging deep foundation design and construction. Limited site access and working room, the presence of a structurally sensitive masonry box culvert cutting across the site, high column loads and a 15 foot cut in site grade required unique solutions. In addition, tight budget and schedule limitations had to be met.
Drawing on extensive background experience, Morris-Shea Bridge Company produced a design build option, meeting all engineering requirements and site restrictions while providing project cost and schedule savings.
The Colosseum at Caesars Palace Hotel and Casino was designed as a circular structure, approximately 12550 square feet in area and 120 feet high. The steel-frame building, which has column loads of up to 1400 tons, was to be built over and around an existing masonry box culvert. Where columns were located over the culvert, heavy section girders were used to pick up the columns and transfer loads to foundations adjacent to the culvert. Finished floor elevations are approximately 15 feet below existing site grade.
Inspection of the masonry culvert disclosed a very sensitive structure, which was occasionally flooded. Site specific soil borings disclosed fill soils overlying a highly variable sequence of alluvial soils, with caliche layers.
Original Foundation Plan:
The initial foundation plan comprised a total of 123 3-foot diameter shafts, in addition to a number of shallow foundations where light column loads allowed. Due to the presence of the 51 foot wide culvert, which could not be crossed with conventional drilled shaft equipment, the use of a large base crane with an extension for the power auger was proposed. This, however, was prohibitively expensive, and would occupy the entire site area, severely impacting schedule.
Earth retention, to construct the basement, required the construction of a temporary lagging wall, and permanent reinforced concrete retaining wall.
Design Build Option:
Due to cost, schedule and sequencing concerns, Morris-Shea was invited by Perini Building Co., the project general contractor, to look for alternative solutions.
Three main issues were identified by Morris-Shea:
Optimization of the deep foundation system
Installation of deep foundations in areas of restricted access
Morris-Shea worked closely with the Martin and Pelton, the project structural engineer to incorporate the changes into the building, and ensure no conflicts in structural issues occurred.
The cost of temporary shoring to a depth of 15 feet around the basement cut and schedule impact was a major concern. After construction of temporary support, a permanent wall would be required. To reduce this cost and reduce schedule impact, Morris-Shea proposed a secant piled retaining wall.
A secant wall is formed by a series of interlocking piles, and can be included as part of the permanent basement wall. One meter diameter Continuous Flight Auger (CFA) piles with a 175mm overlap were proposed for the wall. Piles were installed using a Hitachi KH 180 base crane with a high torque mast and electric turntable.
A total of 165 1-meter diameter CFA piles were installed in a total of 12 single shift working days to provide earth retention and form the permanent basement wall.
Unreinforced “female” piles, which comprise piles 1,3,5 etc – all odd number piles, were installed prior to a second pass where reinforced male piles (even number piles) were installed, cutting into the female piles, and thereby forming a water tight retaining wall.
Column loads along the secant wall were taken directly on the piles, thereby eliminating the need for some column foundations. All 9 girders picking up columns over the 51 foot wide culvert were supported directly onto the secant wall.
The finished basement wall can be a finished concrete pour against the piled wall, or as used in the Colosseum project, the piles can be left as the finished wall after pressure washing and application of a painted sealant.
Deep Foundation Optimization:
A review of the project geotechnical report, performed by Western Technologies, Inc. indicated a very variable subsurface profile, with discontinuous layers of very hard caliche. Drilled Shafts were recommended as a deep foundation system, with an available load versus depth plot included within the report to determine shaft length at each column location. Due to the groundwater conditions and County approval requirements for drilled shafts, it was considered that a large diameter CFA system would prevent loss of shaft friction from water induced softening, and optimize production.
The CFA piling system places concrete as the auger is withdrawn, thereby eliminating softening of soils due to decompression and water softening. This system also increases production from 1 to 5 drilled shafts per day to 10 to 20 CFA piles per day, resulting in significant schedule savings.
To better assess potential variations in sub-soil conditions, Morris-Shea performed supplemental borings. In addition, an instrumentation system was installed on the CFA drill rig to measure drill torque, drill rate, and auger rotation, to allow determination of the presence or absence of caliche layers. This system, which was calibrated against several borings, allowed verification of a minimum thickness of caliche in load bearing piles.
A record of drilling and concreting parameters, was recorded for each pile, printed and submitted to the project geotechnical engineer to provide additional quality assurance.
To allow determination of unit shaft friction values for CFA piles, and therefore allow optimization of pile length, a strain gauge instrumented full scale load test was performed.
As anticipated, extremely high friction values were measured with the CFA pile system, allowing high pile capacities to be taken by single piles, and significant saving in pile length over those indicated in the original soils report.
This process prevents soil decompression, softening of soils and siltation, thereby optimizing pile capacity. Concrete pressure and pumped volume are continuously monitored and displayed in-cab, to ensure a monolithic concrete column is formed. Monitoring of torque, rotation speed, and drill rate allows verification of soil stratigraphy, and therefore allows immediate determination of required pile length.
Upon concreting of the CFA piles, the reinforcing steel is installed, and the concrete dipped out to the required cut-off elevation.
The conventional construction approach for drilled shafts involves the use of a short auger to drill into the ground. The auger is continually inserted and withdrawn until the tip elevation is reached. In soils below the water table, this can result in softening of soils, mud caking of shaft sides, and accumulation of silts at the bottom of the shaft. These processes result in loss of shaft and end bearing capacity.
Image 7: Installation of CFA piles, involves the drilling to required tip elevation with a continuous auger in one pass. On reaching the required depth, concrete is pumped through the hollow stem auger, filling the void as the auger is withdrawn.
The use of high capacity large diameter CFA piles enabled column loads up to 1000 tons to be taken on single piles, enabling a reduction in pile numbers and cap sizes. In addition, where loads allowed, anchor bolts were installed directly into the top of the piles, thereby eliminating pile caps, which resulted in significant cost and schedule savings.
Installed pile lengths were shorter than those required for shafts of a similar diameter. While installation of conventional drilled shafts averages 1 to 5 per day, due to hard drilling, inspection requirements, and the need to clean out and tremie concrete, CFA pile production averaged approximately 15 piles per day. Custom made tooling was made to allow piles to be drilled without the need to drill pilot holes through hard layers, as is often required with lower torque drilled shaft equipment.
In the main basement area, shallow foundations originally proposed for lightly loaded columns were replaced with short CFA piles to avoid excavation of the main work area, and achieve schedule acceleration.
Ultimately, a total of fifty-five 1-meter CFA piles were installed in the basement area.
Restricted Access Drilling:
As previously indicated, the access restriction caused by the masonry culvert, and need to install piles in tight areas adjacent to the existing building precluded the use of conventional equipment. High axial, lateral and moment load conditions made micro piles an expensive and time consuming option.
Morris-Shea therefore mobilized compact Tescar and Delmag RH2813 drilled shaft rigs. These rigs could cross the culvert, with suitable protection, access the shaft locations and work within the confined conditions. The use of the Tescar rig, allowed installation of 1-meter drilled shafts within existing buildings.
A total of forty-eight 24-inch and eleven 1- meter drilled shafts were installed with the Tescar and Delmag rigs, at an average production of 4 shafts per day.
The introduction of new concepts and systems by Morris-Shea, together with a close working relationship with Perini, Martin and Pelton, and Western Technologies resulted in a successful foundation solution. Completion of all piling work relating to the foundation piles and secant wall took approximately 6 weeks, including a week delay while the basement mass excavation took place. A schedule saving of at least one month was realized as a direct result of Morris-Shea’s alternative construction methods and specialized equipment.
The elimination of the need for temporary shoring for the basement excavation,together with a reduction of foundation shaft and cap quantities generated project cost savings in excess of $1million.
The successful design and construction of design-build deep foundations requires extensive experience, and the ability to offer systems that suit the structural loads and soil profile. Cooperation of project structural and geotechnical engineers also helps optimize foundations and provide cost and schedule savings for the client.
Teamwork between Morris-Shea, Martin and Pelton, and Western Technologies was key to the success of this project.
Contact Morris-Shea for more information on design build options, and our state-of-the-art deep foundation systems.