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\chapter{\uppercase {Introduction}}
\label{chap:Intro}
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\section{Motivation }
On Sunday morning, April 5, 1987, during a fifty year flood event a pier on the New York Thruway (I-90) bridge over Schoharie Creek collapsed.  The collapse caused the third and fourth spans (of five total) to fall approximately 20 m into the creek along with a tractor-trailer and one car.  Three more cars that were unable to stop also drove into gap, falling into the creek before the bridge was closed.   Ten people were killed.  The aftermath is shown in Fig.~\ref{fig:schoharie}.
\begin{figure}[!ht]
\centering 
\begin{subfigure}[Police searching the river]
{\includegraphics[width=70mm, height = 60 mm]{figures/schoharie}\label{fig:popo}}
\end{subfigure}
~~
\begin{subfigure}[An exposed section of one of the fallen vehicles]
{\includegraphics[width=70mm, height = 60 mm]{figures/car}\label{fig:ruroh}}\end{subfigure}
\caption{On April 9, 1987 police search for bodies, four days after the collapse of the Schoharie Creek bridge \citep{McK}} 
\label{fig:schoharie}
\end{figure}
\noindent Upon later inspection, investigators determined that the cause of the pier to collapse was excessive scour.  In response to the failure, the Federal Highway Administration (FHWA) published the ``Interim Procedures for Evaluating Scour at Bridges" \citep{FHWA88} that included, for the first time, scour assessment at bridges.  The 1998 FHWA publication marks the beginning of the national bridge scour evaluation program.

Despite the new interim guidelines for nationwide scour assessment, on April 1, 1989 bents 70 and 71 of the northbound lanes of US-51 over the Hatchie River in Tennessee collapsed.  Three spans of the roadway and five cars also plunged into the river as a result of the collapse, killing eight.  Like the Schoharie Creek bridge, the Hatchie River bridge failed due to localized scour.  Several other bridge collapses due to scour have occurred since; some were fatal.  

As the scour and bridge collapse problem became more apparent across the nation, the FHWA took several steps towards mitigation.  The ``Procedures for Evaluating Scour at Bridges"  has been updated five times \citep{FHWA12} and as of 2008 the FHWA addressed the issue of unknown foundations.  The ``Technical Guidance for Bridges over Waterways with Unknown Foundations" \citep{FHWA08} is directed specifically towards the process ``to reduce or eliminate the population of bridges over waterways identified as having unknown foundations, which in turn would allow bridge owners to evaluate these bridges for their scour vulnerability."  This report states that in 2008 each state was required to identify the bridges in their database coded as '113' or as missing information related to foundations details in scour vulnerable waterways.  If a state was unable to identify all of it's unknown foundations by 2010, it was required to have a plan of action (POA).  This research 
is the findings for the State of Texas POA.

\section{Background}
\noindent A full review of the existing methods for determining unknown bridge foundations and a review of the two proposed methods are included in Chapter 2.
\subsection{Unknown Foundations}
\noindent A bridge with an unknown foundations is a bridge where details pertaining to the type and/or depth of the foundation are missing in the National Bridge Inventory (NBI).  As-built details of a foundation, particularly the depth of embedment of a foundation, are critical when bridges are evaluated for scour vulnerability.  Fig.~\ref{fig:bridge} is an example of different elements of a bridge.
\begin{figure}[!ht]
\centering
\includegraphics[width=\textwidth]{figures/Bridge}
\caption{Variables of an unknown bridge foundation \citep{Ols98}}
\label{fig:bridge}
\end{figure}
\noindent The elements related to the substructure as shown on Fig.~\ref{fig:bridge} are considered the pertinent information for scour evaluation; in particular, those elements below the surface.  Fig.~\ref{fig:bridge} shows piles beneath a pile cap.  Piles can be driven and made of reinforced concrete, steel, or timber or reinforced concrete drilled shafts.  Piles may also exist without a pile cap, with a column connecting the superstructure to the pile or foundations can be shallow footings that look like pile caps without piles beneath.
\subsection{Bridge Scour}
\noindent Bridge scour is a significant issue in the United States and globally.  Scour is the localized removal of soil due to water flow, which is especially dangerous when it occurs around a bridge foundation.  Approximately 60\% of all bridge collapses are due to scour \citep{Parola97}.  Fig.~\ref{fig:scour} shows a schematic of a bridge as it was originally built and after scour has occurred. 
\begin{figure}[!ht]
\centering
\includegraphics[width=\textwidth]{figures/scour}
\caption{Potential scour around bridge foundations \citep{mel}}
\label{fig:scour}
\end{figure}
\noindent In Fig.~\ref{fig:scour}, the original riverbed is shown with the abutments and piers embedded to safely support the bridge for the traveling public.  As shown on the figure, significant scour can occur around both the abutments and piers as well as between the piers.  General scour, local scour, and contraction scour are the three types of scour that may occur.  These types can occur individually or simultaneously as shown on Fig.~\ref{fig:scour}.  General scour refers to scour that naturally occurs over time regardless of the presence of a bridge.  Local scour and contraction scour, on the other hand, are due to the bridge elements that have been constructed through the river.  Local scour is caused by the pier or abutment being in the riverbed, forcing water to flow around it versus an unobstructed path as it did before the bridge.  Contraction scour is due to the accelerated flow of a river between piers, such as shown on Fig.~\ref{fig:scour}.  

Scour can be very deep.  The Schoharie Creek bridge failed because the third pier was undermined by scour.  Approximately 7.5 to 9 m along the length of the third pier fell into a scour hole that was approximately 9 m deep \citep{NT}.

\section{Technical Need}
\noindent Ultimately, one universal method is not feasible for determining the depth of unknown bridge foundations.  Each bridge is a unique combination of the structure and its surrounding environment.  Different bridges require different levels of confidence in the foundation estimation and certain approaches to determine the foundation depth may or may not be feasible.  For this reason, there is a technical need for a variety of methods for determining the depth of unknown foundations.  

Induced polarization (IP) and electrical resistivity imaging (ERI) are electrical methods in the field of near surface geophysics that image the subsurface using spatial distributions of the response of the ground.  The two methods are very similar, requiring an extra electrode line to collect IP data to ensure minimal noise.  ERI has already been used for bridge foundations \citep{{rung}, {Hoss}} but collecting ERI data does not increase IP survey time.  It is shown that IP is better at imaging long slender piles; however, because IP is fundamentally based on the measurements collected in an ERI study, the ERI data is a good representative of the quality of the IP data.  Using ERI and IP in conjunction with each other is a unique tool to add to the unknown bridge foundation testing method database.

\section{Objectives}
\noindent The objective of this research is to establish that foundations as well as other infrastructure elements possess measurable IP responses in the field. The use of IP for unknown bridge foundations along with an established protocol for determining the depths of the foundations are in themselves a contribution to the scientific world.  Determining the reliability of IP for bridge foundations and extending the method to other problems strengthens experimental contribution. The objectives are accomplished in several phases, beginning with deterministic forward modeling to identify influential parameters before the simplified field testing begins. Successful foundation measurements are achieved by using IP and ERI for determining known infrastructure elements near Texas A\&M University. The complexity of the research is then increased by moving to actual bridge foundations. After the method is used on assorted bridges, the data are used to assess the reliability of the method.  The objectives of this research are to 

\begin{enumerate} 
\item Establish that bridge foundations possess a measurable IP response on known foundations at the control site. 
\item Establish the range of applications for IP and ERI in the field to detect known foundations.
\item Develop a post processing algorithm for predicting the depth of bridge foundations.
\item Develop a probability of exceedance curve for estimating unknown foundation depths with collected data from known foundations.
\item Evaluate the method for applications to other subsurface civil engineering infrastructure. \end{enumerate}

\section{Organization of the Dissertation}
\noindent This dissertation is organized into nine chapters.  Following this introduction is a literature review of existing methods for determining unknown bridge foundation depths as well as an overview of the ERI and IP methods.  Next several forward models are created and discussed serving as a preliminary study before field work is conducted to identify influential parameters on the use of ERI and IP for reinforced concrete structures.  The bulk of the information is in Chapter~\ref{chap:Experiment} which includes tests at the control site, representing a laboratory study, and tests at four bridge sites.  The post processing method for estimating the foundation depth and a probability of non-exceedance curve are developed and described in the next chapter, Reliability.  Next, the developed non-exceedance curve is used for a prediction of an unknown foundation in Chapter~\ref{chap:NavasotaRiver} where a blinded study is conducted.  The following chapter discusses the use of ERI and IP for another subsurface infrastructure, a buried gas line.  The final two chapters summarize the scientific contributions of this research ending with the conclusions.

