Evaluation of a global safety factor that includes the nailed soil and the surrounding ground requires determination of the critical sliding surface. This surface may be located totally inside, totally outside, or part inside and part outside the nailed zone. Limit equilibrium methods are usually used, and the Davis method is recommended because of its simplicity and availability in the public domain (C. K. Shen et al., “Field Measurements of an Earth Support System,” Journal of the Geotechnical Division, American Society of Civil Engineers, vol. 107, no. 12, 1981). The Davis method has been modified (V. Elias and I. Juran, “Soil Nailing,” Report for FHWA, DTFH 61-85-C, 1988) to permit input of interface limit lateral shear forces obtained from pullout tests, separate geometric and strength data for each nail, facing inclination, and a ground slope at the top of the wall. The concrete facing elements (shotcrete, cast-in-place concrete, or prefabricated panels) are considered for design to be analogous to a beam or raft of a unit width equal to the nail spacing supported by the nails.
Although procurement and contracting practices vary among the European countries, there are some common elements that tend to distinguish European practices from those in the United States. These include (1) strong industry, academic, and government
cooperation in research and development and the introduction of new technologies; (2) a partnering approach among all parties involved in a particular project; (3) less litigation; and (4) a high level of contractor involvement in the conceptualization and design phases, as well as during construction.
In France, the contractor design-build approach appears to be dominant. For public agency work, a prequalified group of contractors are typically asked to prepare a final design and bid, based on a preliminary design prepared by the owner or the owner’s consultant. Alternative designs may also be prepared by the contractor at this time, and may be selected if they are technically and financially viable and meet the overall performance and scheduling requirements of the project. French contractors tend to be much larger and stronger than their U. S. counterparts, and the major groups tend to support significant research and development efforts. Contractor-consultant-academic — government cooperation in areas requiring major research and development is particularly well developed in France.
In Germany, public agency work is again usually bid on a conceptual or preliminary design prepared by or for the agency, with the contractor required to submit a bid on the original design and also encouraged to submit any alternative design that will provide an equivalent wall at a reduced price. Ultimately, award is made for the lowest — cost responsive bid. Soil nailing in Germany requires the involvement of one of a small group of prequalified or “licensed” contractor organizations. As in France, these contractors tend to be technically and financially very strong. Private work, like public work, tends to be awarded on the basis of low bid.
Based on the European, and particularly the French, experience, two main recommendations are offered for encouraging the development of innovative construction methods and improving the construction performance for such methods. First, stronger and more formal government-academic-industry cooperation should be established to develop new technologies and disseminate the information in a nonproprietary manner. This should also include participating in corresponding European programs when the opportunity arises and when the information will be of mutual benefit. Second, alternative bidding, including contractor design-build alternatives, performance-oriented specifications, and the use of carefully prequalified specialty contractors, should be encouraged.
