Soil-Cement and Other Construction Practices in Geotechnical Engineering

Along normally dry rivers in the arid southwestern United States soil-cement has become a popular method of protecting banks and bridge abutments from erosion and collapse during floods. This paper focuses on construction of soil-cement bank protection. Construction topics discussed Include control of water, central mixing plants, and the equipment needed to, transport, spread, compact and cure soil-cement built in stair-stepped fashion on relatively steep riverbanks. Additionally the materials, mixture proportions and section dimensions of soil-cement bank protection, are discussed. Production rates and cost factors are also presented.

The uses of soil-cement in the water resources applications have broadened over the past years. Still one construction related task has remained essentially the same: quality control. Soil-cement is a highly compacted mixture of soil; usually sand, portland cement, and water. It has the properties of a low-strength concrete. Without adequate controls on the quality of the materials and construction methods, the performance of the project may not meet expectations. A successful soil-cement project begins with the development of a quality control program with the responsibilities of each party clearly defined. The quality control plan should address submittals which must receive approval from the project engineer; mixture proportioning considerations; minimum requirements for equipment such as the mixing plant, compactors, and curing equipment; types and frequencies of material testing; and acceptance criteria for construction procedures and individual material tests. Quality control practices during construction do not have to negatively impact the contractor's production. Each member of the project team, from the laborer to the project manager, can be aware and trained in their responsibilities to ensure a quality product. The benefits of having everyone on the project buy into and execute their responsibilities will minimize any rework and ensure that the performance criteria will be met or exceeded.

Bidding soil cement is somewhat unique. The nature of mix in-place soil cement does not allow a 7 a.m. to 3 p.m. work schedule. When the cement is applied the material cannot be left overnight, you must complete the operation even if overtime work is necessary. Working around utilities can be a very challenging matter. Of major concern during a soil cement operation is the amount of rock in excess of softball size. Successful completion of a project requires the use of proper equipment. It is important to determine and use the appropriate pulvermixer for the soil type.

Thousands of miles of existing flexible pavements throughout the U.S. are deteriorating. Street and highway departments continually seek economical, yet durable ways to rehabilitate their rutted and pot-holed roadways. Many have chosen to recycle their worn flexible pavements with cement. Recycled flexible pavements save money by utilizing in-situ materials, thus eliminating haul/disposal costs, and avoiding the expense of producing and importing new aggregate. Cement-treated flexible bases provide a moisture-resistant pavement base capable of supporting light to heavy traffic for many years. This paper discusses mixture and thickness design procedures, presents the basic construction sequence, details recycling costs and documents quality control/assurance issues. Projects in Texas, Washington, California and Florida are presented as examples which demonstrate the diverse methods used to produce durable pavements at minimum cost through recycling.

Relocation of the Cape Hatteras Lighthouse and associated structures across a coastal dune system was planned by the National Park Service to protect the structures from continuing coastal erosion. The lighthouse is a masonry structure about 61m tall and weighing about 4,400 metric tons. Preparation of the loose sands along the move route required a combination of proofrolling, in—place densification and placement of compacted stone layers to control settlement during the move. A geotechnical site characterization program consisting of conventional exploratory borings supplemented by dilatometer testing and a large scale area load test was performed to determine the necessary ground improvement. Elastic analyses were performed to estimate settlements under the planned move system. Measurements of the deflections of the steel mats during the move were obtained and compared to predicted deflections.

The paper summarizes the results of pile installation during construction of the $50 million P-700 Aircraft Carrier Wharf. The project was undertaken by the Navy to homeport a Nimitz-class aircraft carrier at the North Island Naval Air Station in San Diego, California. The existing berthing facilities were modified by creating a new land reclamation area, a new containment rock dike, and a new wharf. The wharf is supported by 450 concrete and steel pipe piles driven through the rock dike and into a dense bearing layer. A major concern during design and construction was the drivability of the piles through the rock dike and into the dense bearing layer. Results of the indicator and production pile programs are presented as they relate to construction issues. A discussion of the construction sequencing and construction difficulties encountered during pile installation are also discussed.

Tiebacks are frequently used to support deep excavations. Installations of tieback systems require certain favorable subsurface conditions as well as skill and experience. When the subsurface conditions are not suitable for tieback installation, an alternative bracing method is needed to achieve the same degree of effectiveness without the use of anchors. This paper presents a method of using earth-filled wide-base hollow piers to brace a deep excavation for construction of Penyun Hotel Building in Shenyang, China. The building covers 16,000 m² and had a 100-m high main tower with a 24-m high apron building. The depth of excavation varied from 12.3 m to 16.5 m. The foundation site contained a 4.1-m thick fill of construction debris followed by a 0.7-m thick clay, a 5.5-m thick sand, and a very thick gravelly sandy soil stratum. The ground water table was near the foundation base. The design, construction, and performance of the bracing system are presented and discussed. It was concluded that the earth-filled wide-base hollow piers could effectively utilize a portion of unstable backfill to become a part of the retaining structure. As a result, a considerable saving in concrete material was realized. Therefore, it is an effective and economical method for bracing deep excavations when the subsurface condition are not favorable for installation of tieback systems.

The Twin Lakes Dam Enlargement project, located in Northern Wyoming, was completed in 1998. Foundation excavation for the embankment dam required removal of deep peat bog deposits. The excavation depths and bid quantity were estimated based primarily on hand probe data, with an additional amount added to allow for uncertainty. Due to strong competition, the successful bid for construction was substantially lower than the Engineer's Estimate. The low bidder planned to complete the work in a single construction season, about half of the duration estimated by the Engineer. During construction, much deeper than expected peat bog deposits were encountered. This impacted the schedule, and the Contractor eventually took three full seasons to complete the project. Ultimately, the Contractor negotiated additional compensation for increased costs associated with the muck excavation, including delay impacts and inefficiencies. However, even with large increases from the low bid price, the final construction cost was within 10 percent of the original budget estimate.

This paper presents a deep excavation case study of a project in Fuzhou City. Fuzhou is located on the southeast coast of China. The excavation covers a plan area of about 7000-sq. meters and extends to a depth of 10.7 m. The site is underlain by soft soils composed of recent fluvio-marine sediments. The design and construction of the retaining structures are discussed in the paper. It is shown that soldier piles with internal bracing system are appropriate retaining structures for deep excavations in Fuzhou.

The South Fork Tolt Dam Intake Tower was designed and built in accordance with the standards of practice current in the early 1960's. Thirty years later, analysis based on modern structural design criteria and updated seismic hazard data concluded that the tower was highly vulnerable to earthquake damage and needed strengthening. Due to the critical importance of the water supply, the Owner mandated that the reservoir remain in service and that water quality be protected during the repair work. The innovative seismic upgrade design called for six high-capacity post-tensioned anchors, installed into holes drilled with high accuracy down through the thin walls of the tall, slender structure. Specialized construction techniques and equipment were employed to successfully complete this challenging project, which required some of the largest and longest post-tensioned anchors ever used to seismically strengthen an intake tower structure.