Richard J. Driscoll, Consulting Engineer (RJDCE) provides consulting structural and foundation engineering services to owners, design professionals, contractors and other construction project stakeholders in the northeast and mid-Atlantic regions. The practice specializes in managing the technical challenges and risks associated with foundation systems, below-ground structures, existing structures and construction in the urban environment. RJDCE utilizes a broad base of experience and knowledge to assist stakeholders balance performance, cost and risk on projects subject to uncertainty.
Have you ever noticed how much the construction industry seems to love silos? Not the kind of silos that hold grain and other bulk materials, but metaphorical silos within organizations in which subgroups have different asymmetric information and interests and limited points of communication with each other. The resulting “silo mentality” is a result of poor information sharing and hierarchical communication, specialization and conflicting incentives. This may lead to subgroups working at cross-purposes and counter to the goals of the organization as a whole.
In the construction industry, the organizations are typically ad hoc, assembled for a particular project. The subgroups include the owner, the architect, engineers, the contractor and subcontractors. On simple projects, the hierarchy might be rather flat. An architect and contractor report to the owner, each of who has one tier of subs. However, larger and more complex project may have a few more tiers. For example, the excavation contractor might have a specialty foundation contractor, who might, in turn, have an engineer. A project organization of this complexity is bound to develop a few silos.
Last week, I made a hastily planned out-of-town trip for a consulting engagement at a client’s office. The client had gathered several engineering and construction practitioners from around the country, along with several of the client’s staff to help them plan a new project. A few of us made brief presentations about our past work as it applied to the problem at hand and the remainder of the work day comprised a series of interesting and wide-ranging discussions of various aspects of the project.
Nate’s Silver’s book, The Signal and the Noise (The Penguin Press, New York, 2012) was released in the run-up to the 2012 election. Silver and FiveThirtyEight.com project were well on their way to accurately predicting most of the electoral college and senate results and there was a good deal of interest and controversy in the political media regarding his work. The book is fundamentally about making predictions in an environment of uncertainty. Silver uses examples from a variety of disciplines including weather, baseball, economics and gambling to illustrate why some predictions fare better than others. However, the lessons can be applied to anyone whose field puts them in the prediction business. And engineers, whether they realize it or not, are in the prediction business. After all, engineering analysis and design of engineered systems require making predictions about the performance of a system, typically with uncertain loads and initial conditions and with the stakes being the health, welfare and safety of the public.
A few days ago, I saw a brief case study from a foundation contractor’s who had installed a proprietary pile system for a building in Northwest Washington, DC. Being somewhat familiar with the subsurface conditions and typical building heights in the area, I immediately doubted whether a deep foundation system was necessary. You will have to forgive my skepticism about this sort of thing; I have seen it a lot, and it seems to be getting more prevalent.
I recently looked at a small support of excavation project in a major city that illustrated some of the issues that can arise when support of excavation or underpinning is not considered during the design process of an urban project, especially a small urban project.
The project involved a horizontal addition to an attached single family dwelling: a very typical project type in cities, and about as simple as new construction can get in the urban environment. However, the existing basement was to be extended below the addition, requiring several feet of excavation on a small site with impacted abutting properties on two sides. Consequently, support of excavation would be required, unless the neighbors would be willing to lose the use of their backyards during construction.
Most significant new constructions projects begin with a subsurface exploration. Part of the purpose of the subsurface exploration is to provide a basis for geotechnical engineering services necessary to design the foundations, earthwork, pavement and utilities for the project. In addition, the subsurface exploration and geotechnical engineer’s interpretation of the conditions observed inform the construction contractor’s pricing and planning of below-grade work. Given the importance of geotechnical considerations to the cost and risk of a construction project, it should be apparent that adequate budget and scope need to be authorized to address these considerations. However, often the scope of services agreed to by the geotechnical firm and their client, be that the owner, another design professional or some other stakeholder, does not take advantage of opportunities to reduce construction cost and risk of below-ground work. So who should be responsible for determining the right geotechnical scope of services?
The bearing capacity is the primary design parameter for proportioning shallow foundations (i.e. footings and mats). If the average pressure applied by a footing is less than the allowable bearing capacity, then the footing area is adequate. This method has been used for over a century and actually predates soil mechanics. Empirical “safe” bearing capacities were provided in many codes and reference handbooks in the late nineteenth century.
The foundation is one of the most critical components of any structure. Poor foundation design and construction can impair the serviceability of the entire structure and put adjacent structures at risk for movement and damage. The concepts, design parameters and construction considerations for a structure’s foundation system are based on a geotechnical subsurface investigation and memorialized in the geotechnical report.
On a lot of projects, the geotechnical services are treated as a commodity. To be cost-competitive, some firms budget very little project-specific engineering and instead provide minimal interpretation of the subsurface conditions, excessively conservative design parameters and generic recommendations.