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\chapter{\uppercase{Experimentation}}
\label{chap:Experiment}

\section{Introduction}
\noindent This Chapter is broken into three sections, an introduction to the equipment and the testing procedure, testing at a National Geotechnical Experimentation Site (NGES), and at bridges in the Bryan TxDOT District.  The NGES testing is analogous to laboratory testing, it is used for planning full scale surveys and for determining the proper testing procedures.  The bridges in the Bryan TxDOT District are the full scale tests that are also used for finalizing the full scale survey protocol and for calculating the probability of non-exeedance curve.

\section{Equipment}
\label{Equipment}
\noindent The AGI SuperSting\texttrademark R8/IP multi-channel imaging system is used exclusively for data collection.  The SuperSting R8/IP\texttrademark is capable of simultaneously measuring in eight channels for potential readings with each current injection which decreases survey time.  Four sets of cable are available at Texas A\&M University, each with 14 electrode take-outs at 2 m spacing.  The field setup for IP is slightly different than normal resistivity surveys, requiring a line of non-polarizable (NP) electrodes parallel to each stainless steel (SS) electrode.  This means that two cables are used for the SS electrodes and two for the NP electrodes with a maximum line length of 54 m (28 electrodes, spaced the maximum 2 m).  While this limited length affects the depth of investigation, the system is capable of handling larger arrays if more cables and electrodes are available.  When set up in the field the array appears as shown in Fig.~\ref{fig:array}.  

\begin{figure}[!ht]
\centering 
\begin{subfigure}[Field setup]
{\includegraphics[width=70mm, height = 90 mm]{figures/arry}\label{fig:array}}
\end{subfigure}
~~
\begin{subfigure}[NP electrode]
{\includegraphics[width=70mm, height = 90 mm]{figures/NP}\label{fig:np}}\end{subfigure}
\caption{SuperSting R8/IP\texttrademark equipment} 
\label{fig:equipment}
\end{figure}

The current injecting SS electrodes are 45 cm long, 1 cm diameter stainless steel rods that connect directly to the cable takeouts.  The NP electrodes have a porous tip with a plastic casing that contains a copper rod surrounded by a copper sulfate solution in contact with the porous stone which is buried in the ground.  The NP electrodes (Fig.~\ref{fig:np}) are used because polarization voltages at the contact between the SS electrodes and the ground can cause debilitating noise to the survey.  As previously mentioned, IP is a phenomenon captured during resistivity surveys, so the NP electrodes are not required but the more sensitive NP electrodes collect higher quality data so they are used for bridge foundation surveys.

The SuperSting\texttrademark is powered by a 12V deep cycle marine battery.  Command files can be written in the office and then uploaded to the SuperSting\texttrademark in the field using a laptop.  An advantage of the SuperSting\texttrademark is that once data is collected it is instantly saved to the equipment so if power is lost or if the system freezes the entire survey does not have to be redone.  Once the survey is complete, the data can be extracted from the SuperSting\texttrademark and then uploaded into a data processing system like AGI's EarthImager\texttrademark.   A full data acquisition protocol manual for ERI and IP surveys of bridge foundations is included in the Appendix. 

\section{Testing Procedure}
\noindent To collect quality ERI and IP data simultaneously two sets of electrodes are necessary.  The SS electrodes should be pushed or driven straight into the ground until the cross bar is approximately 2 cm above the ground surface in a straight line at set intervals.  The NP electrodes should be buried approximately 0.3 m from each stainless steel electrode so that all of the porous stone tip is in the ground.  The stone must also have good contact with the ground.  If the ground is soft, the soil can be slightly loosened and the NP electrode pushed into the soil.  If the ground is very stiff, a hole is dug deep enough so the stone can be buried and the soil is mixed with water and back filled so the electrode can be pushed in with out damaging it.  If the porous stone does not have good contact with the soil (i.e. there are voids around the tip caused by soil clumping after the hole is dug) the IP decay curves will appear jagged and will be removed from the data set.

The electrode cables are numbered for each electrode take-out where an electrode is connected.  Each NP electrode is connected to the cable take out using alligator clips so that the copper rod exposed above the chamber is in contact with the takeout or using alligator to alligator cables (Fig.~\ref{fig:npconnec}).  The SS electrode crossbars must be in contact with the cable take-out.  Each electrode has a spring attached to it so that the cable is held in place on the crossbar during the survey.  The spring over the cable holding it in place is seen in Fig.~\ref{fig:electrodest} along with the installed electrodes.
\begin{figure}[!ht]
\centering 
\begin{subfigure}[Non-polarizable electrode connection]
{\includegraphics[width=70mm, height = 60 mm]{figures/npconnect}\label{fig:npconnec}}\end{subfigure}
~~
\begin{subfigure}[Electrode installation]
{\includegraphics[width=70mm, height = 60 mm]{figures/electrodeset}\label{fig:electrodest}}
\end{subfigure}
\caption{Electrode connections} 
\label{fig:connects}
\end{figure}

Two of the four cables are numbered 1--14 and two are numbered 15--28.  For a typical ERI/IP data set, one of each cable section is connected to another so that two long cable sections are made numbered 1--28.  For bridge foundations, the electrodes should be spaced equal to or less than the width of the foundation tested.  The electrodes should be installed so that the SS electrodes are on one side of the foundation and the NP electrodes are on the other side of the foundation for a pile, with the pile in the center of the array as shown in Fig.~\ref{setu}.  If an abutment is tested, the line of SS electrodes should be as close to the abutment as possible and straight along the side of the abutment.  The line of NP electrodes should be offset from the SS line with approximately 0.3 m space between the two lines.  

\begin{figure}[!ht]
\setlength{\unitlength}{0.8cm}
\centering 
\begin{picture}(18.89,5)
\put(0,0){\includegraphics[width=\textwidth]{figures/electrodelineshort}}
\put(2.75,3.1){1}
\put(4.15,3.1){2}
\put(5.6,3.1){3}
\put(7,3.1){4}
\put(8.35,3.1){13}
\put(9.75,3.1){14}
\put(11.25,3.1){15}
\put(12.65,3.1){16}
\put(14.1,3.1){25}
\put(15.55,3.1){26}
\put(16.95,3.1){27}
\put(18.4,3.1){28}
\put(0.6,1.2){\rotatebox{90}{SuperSting}}
\put(2.6,1.5){28}
\put(4,1.5){27}
\put(5.45,1.5){26}
\put(6.9,1.5){25}
\put(8.35,1.5){16}
\put(9.75,1.5){15}
\put(11.25,1.5){14}
\put(12.65,1.5){13}
\put(14.25,1.5){4}
\put(15.7,1.5){3}
\put(17.1,1.5){2}
\put(18.55,1.5){1}
\put(2.75,3.85){High Address}
\put(2.75,0.75){Low Address}
\end{picture}
\caption{Field setup of SuperSting R8/IP\texttrademark with 28 take-outs}
\label{setu}
\end{figure}

A schematic of the field set up is in Fig.~\ref{setu} including the cable layout.  Note that sections of the cables are removed for space in the schematic but the continuous string of electrodes are necessary in the field.   The 28th takeout with the NP electrodes must be connected to the low address port and the first takeout with the SS electrodes must be connected to the high address port on the SuperSting R8/IP\texttrademark.  The free ends of the cables away from the test box should \emph{not} be connected.  The cable sections must be laid out this as shown to use the command file and settings on the SuperSting R8/IP\texttrademark described in the Appendix.

\section{National Geotechnical Experimentation Site}
\noindent The NGES is located on the Texas A\&M Riverside Campus approximately 16 km west of College Station, Texas.  There are two soil sites on Riverside Campus available for testing; a sand site on the East side of the campus and a clay site on the west side of the campus.  The sand site is used exclusively in this research.  The stratigraphy at the sand site consists of a medium dense tan silty fine sand to 3.5 m below the surface, medium dense silty sand with clay and gravel from 3.5 to 7 m, medium dense silty sand to sandy clay with gravel from 7 to 11 m and very hard dark gray clay from 11 to 33 m.  The water table at the sand site is located 4.9 m below the surface \citep{Gib}.  The overall soil characterization at the sand site is a medium dense, silty, fine, siliceous sand \citep{Bal}.

The sand site has both shallow and deep foundations.  The deep foundations were drilled at the sand site in November of 1990 for a FHWA project.  Five drilled shafts were drilled "wet" at the sand site with planned defects, shown in Fig.~\ref{fig:shafts}.  The five shafts on the right are at the sand site.  As shown in Fig.~\ref{fig:shafts}, the planned defects were necking, a bulb, cave-in, and a tremie defect.  The shaft with necking labeled RS \#1 also has a traffic signal mounted on it.  

\begin{sidewaysfigure}[!ht]
\centering 
{\includegraphics[width=\textwidth]{figures/shafts}}
\caption{Construction plan for the drilled shafts \citep{Bal}}
\label{fig:shafts}
\end{sidewaysfigure}
\clearpage

 Five of the shallow foundations are founded at 0.76 m in the sand.  Two of these footings are 3 x 3 x 1.2 m$^3$, one is 2.5 x 2.5 x 1.2 m$^3$, one is 1.5 x. 1.5 x 1.2 m$^3$, and one is 1 x 1 x 1.2 m$^3$.  These footings were built in 1995 \citep{Gib}. There is also a 3 x 3 x 1 m$^3$ fully embedded footing that was constructed in 2010 \citep{Dunn} as well as other undocumented foundations surrounding the footings.  The overall topography of the sand site is shown in Fig.~\ref{fig:topography}.
\begin{figure}[!ht]
\centering
\includegraphics[width=\textwidth]{figures/topography}
\caption{Topography and foundation layout at the sand site \citep{rung}}
\label{fig:topography}
\end{figure}
\noindent The survey lines shown on Fig.~\ref{fig:topography} are ERI surveys \citep{rung}.
\newpage
\subsection{Shallow Foundations}
\noindent Two shallow foundations at the NGES are tested and discussed in this section.  The first is the 1 x 1 x 1.2 m$^3$ foundation.  This foundation is used for investigating three field parameters for optimal results: the effect of electrode location relative to the foundation, the effect of the IP cycle time, and the effect of the array type used.

\subsubsection{Electrode Locations}
\noindent As described in Chapter~\ref{chap:Model}, the first NGES test is the 1 x 1 x 1.2 m$^3$ foundation that is embedded 0.76 m.  This particular foundation is tested several times before larger foundations are investigated.  Using this foundation, different electrode configurations are investigated, specifically regarding the location of the foundation with respect to the electrode line.  

Fig.~\ref{fig:control} has the foundation directly in the center of the electrode line, with the 14th NP and SS electrodes on one side of the foundation and the 15th NP and SS electrodes on the other side.  The electrodes are shifted to the side of the foundation so the foundation no longer directly bisects the line but instead is side scanned.  Fig.~\ref{fig:sidescan} shows the electrode line side scanning the 1 x 1 x 1.2 m$^3$ foundation with the SS electrodes next to the foundation.

\begin{figure}[!ht]
\centering 
\begin{subfigure}[Test site]
{\includegraphics[width=70mm, height = 70 mm]{figures/negin}\label{fig:negin}}
\end{subfigure}
~~
\begin{subfigure}[Experimental setup]
{\includegraphics[width=70mm, height = 70 mm]{figures/sidescan}\label{fig:sscan}}\end{subfigure}
\caption{Side scan with SS electrodes next to the foundation.} 
\label{fig:sidescan}
\end{figure}

The inversion of the test in Fig.~\ref{fig:sidescan} is shown in Fig.~\ref{fig:ssnext}.  The resistivity section converges in eight iterations with an RMS of 2.64\%.  Despite the low RMS of the model, no clear resistivity response is shown in the top profile, similar to Fig.~\ref{fig:control}.  The large highly resistive anomalies at the bottom of Fig.~\ref{fig:ssnext} are also seen in Fig.~\ref{fig:control} as well as in other tests on this foundation.

\begin{figure}[!ht]
\setlength{\unitlength}{0.8cm}
\centering 
\begin{picture}(18.89,11)
\put(0,0){\includegraphics[width=\textwidth]{figures/SSnextshallowb}} 
\put(0,10){(a)}
\put(0,4.6){(b)}
\end{picture}
\caption{1 x 1 x 1.2 m$^3$ foundation side scan with SS electrodes next to foundation:  (a) inverted resistivity section; (b) inverted IP section}
\label{fig:ssnext}
\end{figure}
\newpage
The IP inversion shows an IP response at the bottom of the foundation, but it is a large anomaly deeper and wider than the foundation.  Comparing this to Fig.~\ref{fig:control} the interpretation of the foundation depth is less definitive.  Again, the RMS shows that the model is a good fit to the data, 1.39 mV/V.

Similarly, in Fig.~\ref{fig:npnext} the foundation is side scanned, but with the NP electrodes next to the foundation and the SS electrodes offset.  Again, there is a significant loss in the ability to identify the foundation compared to Fig.~\ref{fig:control}.  Comparing the results from Fig.~\ref{fig:control}, Fig.~\ref{fig:ssnext}, and Fig.~\ref{fig:npnext} the advantage of aligning the array so that the foundation bisects the array is clear.  

\begin{figure}[!ht]
\setlength{\unitlength}{0.8cm}
\centering 
\begin{picture}(18.89,11)
\put(0,0){\includegraphics[width=\textwidth]{figures/NPnextb}} 
\put(0,10){(a)}
\put(0,4.6){(b)}
\end{picture}
\caption{1 x 1 x 1.2 m$^3$ foundation side scan with NP electrodes next to foundation:  (a) inverted resistivity section; (b) inverted IP section}
\label{fig:npnext}
\end{figure}

The same foundation is easily discernible when the foundation bisects the array as compared to a side scan.  If a side scan array must be used because a foundation is too large to bisect the array, the SS electrodes should be placed next to the foundation with the NP electrodes parallel.
\clearpage 

\subsubsection{IP Cycle Time}
\noindent Once the optimal electrode setup is established, the effect of the IP delay time is investigated.  Previous experiments have been run with a two second cycle time.  This means that a positive current transmission is turned on for two seconds, off for two seconds, a negative current transmission is turned on for two seconds, and again off for two seconds.  The SuperSting\texttrademark has five different cycle times: 0.5 s, 1 s, 2 s, 4 s, and 8 s.  A dipole-dipole array is used with 28 NP and 28 SS electrodes spaced every 0.5 m (Fig.~\ref{fig:delay}) using one second and four seconds to see if the results vary from the control test shown in Fig.~\ref{fig:control}.  A smaller electrode spacing yields higher resolution so 28 electrodes are used instead of 14 to provide more detailed inversions with approximately the same array length (13.5 m instead of 13 m).

\begin{figure}[!ht]
\centering 
\begin{subfigure}[Test site]
{\includegraphics[width=70mm, height = 90 mm]{figures/IMG_1845}\label{fig:delaysite}}
\end{subfigure}
~~
\begin{subfigure}[Experimental setup]
{\includegraphics[width=70mm, height = 90 mm]{figures/IMG1847}\label{fig:delaysetup}}\end{subfigure}
\caption{IP delay test and array type test} 
\label{fig:delay}
\end{figure}

Beginning with one second cycle time, no discernible foundation is seen in the resistivity section in Fig.~\ref{fig:DD1secb}, as expected.  Additionally, the ERI section in Fig.~\ref{fig:DD1secb} does not converge to an RMS of less than 5\% within eight iterations as the two second cycle inversion shown in Figure~\ref{fig:control}.
\begin{figure}[!ht]
\setlength{\unitlength}{0.8cm}
\centering 
\begin{picture}(18.89,11)
\put(0,0){\includegraphics[width=\textwidth]{figures/DD1secb}} 
\put(0,10){(a)}
\put(0,4.6){(b)}
\end{picture}
\caption{1 x 1 x 1.2 m$^3$ foundation with 0.5 m electrode spacing and one second IP cycle:  (a) inverted resistivity section; (b) inverted IP section}
\label{fig:DD1secb}
\end{figure}
The IP section in Fig.~\ref{fig:DD1secb} shows a slight chargeable response, but the high response does not cover most of the foundation or penetrate to the full foundation depth as the two seconds cycle.  Because these results are not as sharp as the two seconds cycle time, the one--half second cycle is not tested.  

Next, the four second cycle time is tested without moving the electrodes.  While nothing is changed as far as installation from the one second test to the four seconds test, the number of negative apparent resistivity data greatly increases.  There are seven negative apparent resistivity readings from the one second test and 31 in the four seconds cycle time test.  Often, apparent resistivity are measured as negative when the surface voltage is too small to be measured accurately.  These negative apparent resistivity values lower the number of data that are used in the numerical model and lead to a survey with a higher RMS than in the two seconds survey.

\begin{figure}[!ht]
\setlength{\unitlength}{0.8cm}
\centering 
\begin{picture}(18.89,11)
\put(0,0){\includegraphics[width=\textwidth]{figures/DD4sb}} 
\put(0,10){(a)}
\put(0,4.6){(b)}
\end{picture}
\caption{1 x 1 x 1.2 m$^3$ foundation with 0.5 m electrode spacing and four seconds IP cycle:  (a) inverted resistivity section; (b) inverted IP section}
\label{fig:DD4b}
\end{figure}

The ERI profile in Fig.~\ref{fig:DD4b} shows little information about the foundation, but the IP section provides some details.  Two zones of high chargeability ($>$360 mV/V) are noted on the surface of the foundation as well as a zone of over 180 mV/V within the boundaries of the foundation.  Recall that the chargeability of the foundation is dependent on the amount of applied current as well as the cycle time so proceed with caution when comparing the four seconds cycle time to the two seconds cycle.  Upon visual inspection there is no advantage of a four seconds cycle time over the two seconds, and the survey time is doubled.  For this reason, the eight seconds cycle time is not investigated. All surveys herein use two seconds cycle time unless specifically noted as in this section.

\subsubsection{Array Type}
\noindent In addition to investigating the optimal electrode location and length of the IP cycle time,  the optimal array type is examined.  The dipole-dipole array is noted for its high resolution in imaging vertical structures \citep{dece} and it is used exclusively for all bridge surveys.  Because IP had not been used for investigating bridge foundations prior to this research, the Wenner and Schumberger arrays are given a trial run on the shallow footing and compared to the dipole-dipole results from Fig.~\ref{fig:control}. 

The Wenner array is used first.  As shown in Fig.~\ref{fig:wenner}, in the Wenner array the distance between the electrodes is constant and the voltage potential is measured between the current source.  
\begin{figure}[!ht]
\setlength{\unitlength}{0.8cm}
\centering
\begin{picture}(18.89,5)
\put(2.5797,0){\includegraphics[width=0.75\textwidth]{figures/wenner} }
\put(5.4,0.1){a}
\put(9.55,0.1){a}
\put(13.7,0.1){a}
\put(9.5,1.5){V}
\put(9.6,3.2){I}
\end{picture}
\caption{Wenner array}
\label{fig:wenner}
\end{figure}
\noindent The apparent resistivity using the geometric factor for the Wenner array is
\begin{equation}
\rho_\textrm{a}=2\pi a \frac{\Delta V}{I}
\end{equation}
\noindent where $\rho_\textrm{a}$ is the apparent resistivity, $a$ is the spacing as shown in Fig.~\ref{fig:wenner}, $\Delta V$ is the voltage potential, and $I$ is the electric current.  The Wenner array has the highest signal to noise ratio and has good vertical resolution.  The disadvantages of the Wenner array are its poor lateral resolution and that only a single channel can be used for each measurement so surveys take longer.

\begin{figure}[!ht]
\setlength{\unitlength}{0.8cm}
\centering 
\begin{picture}(18.89,11)
\put(0,0){\includegraphics[width=\textwidth]{figures/wenner28}} 
\put(0,10){(a)}
\put(0,4.6){(b)}
\end{picture}
\caption{1 x 1 x 1.2 m$^3$ foundation with 0.5 m electrode spacing and the Wenner array:  (a) inverted resistivity section; (b) inverted IP section}
\label{fig:wenner28}
\end{figure}
Similar results are noted between Fig.~\ref{fig:control} and Fig.~\ref{fig:wenner28} in terms of the inverted IP sections.  Both show a strong IP response penetrating over half the depth of the foundation.  The significant difference between the two tests is the RMS of the ERI data.  The RMS of the control test is 4.38\%, which is accepted as a publishable RMS.  The RMS for the Wenner test, however, is 10.70\% so the error in the fit between the gathered data and the model has over doubled.  


Similar to the Wenner array, the Schlumberger array is also only capable of using a single channel.  The Schlumberger array changes the distance between electrodes. 
\begin{figure}[!ht]
\setlength{\unitlength}{0.8cm}
\centering
\begin{picture}(18.89,5)
\put(2.5797,0){\includegraphics[width=0.75\textwidth]{figures/schlum}}
\put(6.5,0.25){s}
\put(12.7,0.25){s}
\put(9.51,1.2){a}
\put(9.5,2.65){V}
\put(9.55,4.35){I}
\end{picture}
\caption{Schlumberger array}
\label{fig:schlum}
\end{figure}
\noindent The apparent resistivity for the Schlumberger array is found by 
\begin{equation}
\rho_\textrm{a}=\frac{\pi (\frac{s^2-a^2}{4})}{a}\frac{\Delta V}{I}
\end{equation}
\noindent where $s$ is the spacing shown on Fig.~\ref{fig:schlum} and all other variables have been previously defined.  
\begin{figure}[!ht]
\setlength{\unitlength}{0.8cm}
\centering 
\begin{picture}(18.89,11)
\put(0,0){\includegraphics[width=\textwidth]{figures/schlum28b}} 
\put(0,10){(a)}
\put(0,4.6){(b)}
\end{picture}
\caption{1 x 1 x 1.2 m$^3$ foundation with 0.5 m electrode spacing and the Schlumberger array:  (a) inverted resistivity section; (b) inverted IP section}
\label{fig:schlum28}
\end{figure}
The Schlumberger array not only has an even higher RMS (13.53 \%)  than the Wenner and dipole-dipole arrays, but the IP inversion does not show the strong IP response and the two prior.  A strong chargeability anomaly is noted at the bottom of the foundation, but it appears deeper than the foundation and does not have the sharp resolution images of the two prior arrays.  Based on these experimental results of the same foundation, the dipole-dipole array is used exclusively in this research.  

The final experimental setup that is studied is whether the NP electrodes are necessary for this research.  NP electrodes increase the installation time and are not necessary for collecting IP data.  For reinforced concrete foundations, however, they are necessary in order to capture a measurable IP response.  A 28 electrode dipole-dipole array with 0.5 m electrode spacing is used with only SS electrodes for collecting the IP data.  Unfortunately, the IP decay curves showed no resemblance to the calculated IP curves and 93 \% of the data is removed based on the model criteria.  If the model is forced to keep all the data, the ERI model has an RMS of 96.20\% and the RMS of the IP section is 4005.03 mV/V.  These inversions are, therefore, pure mathematical anomalies.  In summary, all collected data unless otherwise noted utilize the dipole-dipole array with a two second IP cycle time and both SS and NP electrodes are used.

\subsubsection{3 x 3 m$^2$ Foundation with Unknown Depth}
\noindent Adjacent to the five large spread footings \citep{Gib} is a 3 x 3 m$^2$ foundation that is flush with the ground surface.  The report for this foundation is not read until after the foundation is tested for a blinded study \citep{Dunn}.  Because the foundation is 3 x 3 m$^2$ at the surface and the electrode takeouts on the SuperSting\texttrademark cables are spaced every 2 m, the array is set up to cross a corner of the foundation.
 \begin{figure}[!ht]
 \setlength{\unitlength}{0.8cm}
 \centering 
 \begin{picture}(18.89,11)
 \put(0,0){\includegraphics[width=\textwidth]{figures/matfoundationb}} 
 \put(0,10){(a)}
 \put(0,4.6){(b)}
 \end{picture}
 \caption{1 x 1m$^2$ foundation with 2 m electrode spacing and unknown depth:  (a) inverted resistivity section; (b) inverted IP section}
 \label{fig:matfoundation}
 \end{figure}
Fig.~\ref{fig:matfoundation} shows the 14 NP and 14 SS electrode line spaced every 2 m totaling 26 m.  A 1.22 m thick surface layer is noted in the ERI section which converges in eight iterations to an RMS of 4.92\%.  No foundation is detected in the inverted resistivity section.  The inverted IP section converges to an RMS of 0.59 mV/V and an IP response of over 90 mV/V is observed in the location of the foundation.  The highest response extends to approximately 1 m with a zone of over 70 mV/V extending to approximately 1.3 m.  After the inversion process is complete the report for this foundation is read.  The actual foundation is embedded 1 m and fully reinforced with \# 6 reinforcing steel every 12.7 cm \citep{Dunn}.

\begin{figure}[!ht]
\centering
\includegraphics[width=70mm, height = 60 mm]{figures/Dunnfooting}
\caption{3 x 3 m$^2$ mat foundation \citep{Dunn}}
\label{fig:mat}
\end{figure}

Thus if only the zone of the highest chargeability is accounted for, the foundation is predicted to the actual depth.  If, instead, the zone that extends to 1.3 m is considered the foundation depth is over predicted.  The decision criteria for the predicted depth of each foundations is discussed in Chapter~\ref{chap:Stats}.  Using this criteria, the predicted depth is 0.43 m.

\subsection{Deep Foundations}
\noindent The NGES also contains five reinforced drilled shafts (Fig.~\ref{fig:shafts}), additional unknown foundations, and 41 steel H-piles piles.  The 10.4 m embedded test shaft \#4 (TS \#4) from Fig.~\ref{fig:shafts} is tested several times.  The H-piles are used as a control test for testing methods on a different foundation material. 
\subsubsection{Unreinforced Foundation}
\noindent Near the shallow foundations is a foundation that is believed to be 4 m deep.  This assumed depth is the result of a resistivity survey \citep{rung}, however it has not been confirmed beyond this study.  This foundation is unique because it does not appear to be reinforced and is encased in a plastic tarp, presumably to the depth of the foundation.  Fig.~\ref{fig:unknown} shows the results of this survey.

 \begin{figure}[!ht]
 \setlength{\unitlength}{0.8cm}
 \centering 
 \begin{picture}(18.89,11)
 \put(0,0){\includegraphics[width=\textwidth]{figures/unknown}} 
 \put(0,10){(a)}
 \put(0,4.6){(b)}
 \end{picture}
 \caption{Unknown foundation without reinforcement:  (a) inverted resistivity section; (b) inverted IP section}
 \label{fig:unknown}
 \end{figure}
 
The RMS of the resistivity section converges to 1.92\% in only four iterations and the IP to an RMS of 3.02 mV/V.  Despite the low RMS, giving confidence in the numerical model, no clear foundation is seen in either section.  The large highly resistivity zone does not correlate with the foundation and no unique zone of chargeability is seen in the IP section.  The importance of rebar in a foundation in order to obtain a chargeable response is proved from this test.  Also, while rebar in concrete is necessary to estimate the depth of the foundation, this shows that IP can also be used to verify that a foundation in fact contains rebar.  

\subsubsection{Test Shaft \#4}
\noindent The deep foundation that is tested is Test Shaft (TS) \#4 from Fig.~\ref{fig:shafts} or the second closest foundation in Fig.~\ref{fig:deeps}.  This shaft extends  10.4 m into the subsurface and was built without defects.  As an initial test, 14 NP and 14 SS electrodes are spaced every 1 m (Fig.~\ref{fig:deep1}).  Only 14 electrodes are used due to limited space.
\begin{figure}[!ht]
\centering 
\begin{subfigure}[Test site]
{\includegraphics[width=70mm, height = 60 mm]{figures/deeps}\label{fig:deeps}}
\end{subfigure}
~~
\begin{subfigure}[Experimental setup]
{\includegraphics[width=70mm, height = 60 mm]{figures/deep1}\label{fig:deep1}}\end{subfigure}
\caption{Test Shaft \#4} \label{fig:five}
\end{figure}

The quality of this data is very low and it is unable to converge to an acceptable RMS.  This may be because the foundation is over three times deeper than the collected data.
\begin{figure}[!ht]
 \setlength{\unitlength}{0.8cm}
 \centering 
 \begin{picture}(18.89,11)
 \put(0,0){\includegraphics[width=\textwidth]{figures/deeppil}} 
 \put(0,10){(a)}
 \put(0,4.6){(b)}
 \end{picture}
 \caption{Test shaft \#4 at the sand site with 13 m long array:  (a) inverted resistivity section; (b) inverted IP section}
 \label{fig:deeppil}
 \end{figure}
The inverted resistivity section in Fig.~\ref{fig:deeppil} stops at ten iterations with an RMS of 40.61\% and from this model the IP section converges to an RMS of 8.07 mV/V.  A zone of very high chargeability (over 600 mV/V) is seen at the bottom of the inversion.  This may be a 'buildup' of chargeability due to the foundation that is deeper than the collected data, however the high RMS values make these interpretations unreliable.  
\newpage
Addressing the issue of collecting deeper data, TS \#4 is next tested with 1 m spacing but using 28 electrodes.  As previously mentioned, space near TS \#4 is limited; there is an excavation beginning where the small trees are in the background of Fig.~\ref{fig:deep1}, 15 m from the foundation and very tall thick brush 14 m from the foundation on the other side.   For this reason, the test with 1 m spacing is angled so the foundation is side scanned (Fig.~\ref{fig:deepscan}).

 \begin{figure}[!ht]
 \centering 
 \begin{subfigure}[Test site]
 {\includegraphics[width=70mm, height = 90 mm]{figures/deeplong}\label{fig:deeplong}}\end{subfigure}
 ~~
  \begin{subfigure}[Experimental setup]
  {\includegraphics[width=70mm, height = 90 mm]{figures/sidescandeep}\label{fig:deepscan}}
  \end{subfigure}
   \caption{27 and 54 m long array for TS \#4} \label{fig:deepsecondtest}
 \end{figure}

 Again, the data collected is not as deep as the actual foundation (10.3 m), so the inversion shows very little in terms of the foundation depth.   The inverted resistivity section has a zone of low resistivity along the middle of the pile, but it does not extend to the depth of the foundation.  The IP inversion in Fig.~\ref{fig:deepone} shows nothing relevant to the foundation depth and the RMS is very high (40.72 mV/V).  The results may have been improved if it was possible to place the electrodes across the foundation as established with the shallow foundation experiment.
 
 \begin{figure}[!ht]
  \setlength{\unitlength}{0.8cm}
  \centering 
  \begin{picture}(18.89,11)
  \put(0,0){\includegraphics[width=\textwidth]{figures/deeponemeter}} 
  \put(0,10){(a)}
  \put(0,4.6){(b)}
  \end{picture}
  \caption{Test shaft \#4 at the sand site with 27 m long array side scan:  (a) inverted resistivity section; (b) inverted IP section}
     \label{fig:deepone}
   \end{figure}


Similar results are seen in Fig.~\ref{fig:deepscan2} when the electrode spacing is increased to 2 m.  Increasing the spacing yields deeper data, but the RMS for both tests are still too high to validate the results.  Despite this, a very high zone of chargeability (700 mV/V) is located at the bottom of the foundation.

   \begin{figure}[!ht]
    \setlength{\unitlength}{0.8cm}
    \centering 
    \begin{picture}(18.89,11)
    \put(0,0){\includegraphics[width=\textwidth]{figures/deepscan2}} 
    \put(0,10){(a)}
    \put(0,4.6){(b)}
    \end{picture}
    \caption{Test shaft \#4 at the sand site with 54 m long array side scan:  (a) inverted resistivity section; (b) inverted IP section}
            \label{fig:deepscan2}
    \end{figure}

\newpage
The deep foundation is also side scanned using only SS electrodes and the Wenner-Schlumberger array in Fig.~\ref{fig:ws}.  Using this array, the IP decay curves are comparable to curves that are gathered with the NP electrodes unlike the results of the shallow foundation survey with only SS electrodes.  The inversions in Fig~\ref{fig:ws} converge to acceptable RMS values, unlike previous deep foundation tests.  Also, this test shows that it is possible to gather IP data without the more cumbersome use of the NP electrodes.  Despite the high quality IP and ERI data, still no foundation is observed from either inverted section.  By comparison with later tests using the dipole-dipole array and NP electrodes, the Wenner-Schlumberger test is not useful for identifying bridge foundations. 

   \begin{figure}[!ht]
    \setlength{\unitlength}{0.8cm}
    \centering 
    \begin{picture}(18.89,11)
    \put(0,0){\includegraphics[width=\textwidth]{figures/ws}} 
    \put(0,10){(a)}
    \put(0,4.6){(b)}
    \end{picture}
    \caption{Test shaft \#4 with a 55 m array using the Wenner-Schlumberger configuration:  (a) inverted resistivity section; (b) inverted IP section}
            \label{fig:ws}
    \end{figure}

The final experimental setup considered for deep foundations is gathering two arrays of different electrode spacing and combining them.  In this way, detailed data around the foundation can be collected with smaller spacing (0.5 m) while still obtaining deeper data so the bottom of the foundation can be estimated using 2 m electrode spacing.  In Fig.~\ref{fig:mixed}, 56 SS electrodes are used for two different arrays, with the same center point.  NP electrodes are not used because only 28 NP electrodes are available.  

      \begin{figure}[!ht]
        \setlength{\unitlength}{0.8cm}
        \centering 
        \begin{picture}(18.89,11)
        \put(0,0){\includegraphics[width=\textwidth]{figures/mixedarrrayb}} 
        \put(0,10){(a)}
        \put(0,4.6){(b)}
        \end{picture}
            \caption{Test shaft \#4 with a combined 0.5 m and 1 m electrode spacing:  (a) inverted resistivity section; (b) inverted IP section}
            \label{fig:mixed}
            \end{figure}
            
The first array is spaced 0.5 m and uses the dipole-dipole array.  The second array is spaced 1 m and uses the combination Wenner-Schlumberger array.  The smaller spacing and dipole-dipole array is used because of the clear inversions from the shallow foundations.  The Wenner-Schlumberger array with wider electrode spacing is used because of it's ability to gather deeper data.  Both files converge to acceptable RMS values (4.54\% and 3.26 mV/V), and using the known dimensions and location of the pile a strong chargeability response is seen along a section of the pile to approximately 9 m below the surface.  This response, however, is spread much larger than the foundation and does not penetrate to the full depth of the foundation.  The inverted resistivity section has zones of low resistivity around the pile, but these zones are spread through out the inversion.

The results of Fig.~\ref{fig:mixed} do not provide more details of the pile to justify the extra time spent for this experiment.  Not only is it time consuming to run two different arrays for the same pile, but the data processing afterwards is more cumbersome requiring the electrodes with smaller spacing to be shifted to align with the longer array line.  Again, the IP results are not as clear regarding the shape of the foundation when only SS electrodes are used instead of the combination of SS and NP electrodes.

\subsection{Steel H-Piles}
\noindent All foundations tested so far have been reinforced concrete.  But bridge foundations are also commonly supported by steel H-piles.  Adjacent to the NGES sand site are 17 driven steel H-pile anchor piles, 12 vibratory driven steel pipe piles, and 12 vibratory driven steel H-piles, shown in Fig.~\ref{fig:hpiles}.  Three of the steel H-piles are tested to expand the materials database.  The soil profile surrounding the driven piles is stiff red sandy clay to 2.1 m, firm red-brown sand from 2.1 to 6.7 m, stiff red sandy clay from 6.7 to 8.2 m, firm tan sand from 8.2 to 11.3 m, and hard dark gray clay to the terminated bore hole depth of 12.2 m \citep{boss}.  The three piles that are shown herein are labeled on Fig.~\ref{fig:pilelayout} as ``N'', ``6--20--A--P'', and ``Q''.

\begin{figure}[!ht]
\centering
\includegraphics[width=\textwidth]{figures/hpilelayout}
\caption{Final layout of piles \citep{boss}}
\label{fig:pilelayout}
\end{figure}

The first test of a steel pile is pile ``N'', shown in Fig.~\ref{fig:octh} , shows the issues with testing in soil with a high contact resistance.  The top soil near the driven piles is a very stiff sandy clay and when the electrodes are driven into this, the soil becomes very loose around the stake.  This causes the resistance at the electrode--soil contact to be very high (called the contact resistance).  Because the IP signature is so small for foundations, in order to obtain a measurable IP response as much current as possible must go into the subsurface.  Ideally, maximum current comes from contact resistance of less than 300$\Omega$.  The contact resistances for this test are 700--1,500 $\Omega$, with several from 4,000--8,000 $\Omega$.

\begin{figure}[!ht]
\centering
\begin{subfigure}[Test site]
{\includegraphics[width=70mm, height = 60 mm]{figures/hpiles}\label{fig:hpiles}}
\end{subfigure}
~~
\begin{subfigure}[Experimental setup]
{\includegraphics[width=70mm, height = 60 mm]{figures/field}\label{fig:octh}}
\end{subfigure}
\caption{Steel H-piles near the NGES}
\end{figure}
\newpage

In Fig.~\ref{fig:hp1b} a very low resistivity, or alternatively high conductivity response is seen along the length of the pile.  But this response is also seen to the left of the pile on the image.  This could be the response from the nearby steel tube, ``6--30--B--P''.  The RMS for the resistivity section is also very high (9.75\%) so the wide spread highly conductive zone could be due to the inability of the model to correlate with the measured data.  Note the zone of highly resistive soil at the top of the profile, again related to the high contact resistance.  The IP section shows no decipherable response of the pile, and the RMS is extremely high, 20.87 mV/V.  The response in this model should not be considered accurate.

 \begin{figure}[!ht]
 \setlength{\unitlength}{0.8cm}
 \centering 
 \begin{picture}(18.89,11)
 \put(0,0){\includegraphics[width=\textwidth]{figures/hp1b}} 
 \put(0,10){(a)}
 \put(0,4.6){(b)}
 \end{picture}
 \caption{13.8 m long steel anchor pile with 1 m spacing:  (a) inverted resistivity section; (b) inverted IP section}
 \label{fig:hp1b}
 \end{figure}
\newpage
Following the Fig.~\ref{fig:hp1b} test, an array is set up along the three piles ``N'', ``6-20-A-P'', and ``Q'' with the 15th electrode array centered on ``Q''.  In Fig.~\ref{fig:h5}, pile ``Q" is the farthest pile on the right.  Fig.~\ref{fig:h5} shows the first test of the three piles in a line with the electrodes spaced 0.5 m and Fig..~\ref{fig:h1} shows the test with the electrodes spaced every meter. 
\begin{figure}[!ht]
\centering
\begin{subfigure}[Test site]
{\includegraphics[width=70mm, height = 60 mm]{figures/20121219}\label{fig:h1}}
\end{subfigure}
~~
\begin{subfigure}[Experimental setup]
{\includegraphics[width=70mm, height = 60 mm]{figures/20121218}\label{fig:h5}}
\end{subfigure}
\caption{Three pile test}
\end{figure}

The first test run along these three piles is the array shown in Fig.~\ref{fig:h1}.  Similar to the test from Fig.~\ref{fig:hp1b}, this test has very high contact resistance (averaging 3,000 $\Omega$ with maximums near 7,500 $\Omega$) so the inversion quality is very poor with high RMS values.  The two exterior piles are 13.8 m long and the interior pile is 6.3 m long.  The exterior piles are embedded 11.5 m and the central pile is embedded 6 m.

Fig.~\ref{fig:oneone} shows the results from the first test.  Little information is gathered from this experiment because of the high contact resistance and the number of negative apparent resistivity data measured at the surface (112 or 29\%).  This low data quality is addressed by using a bentonite slurry around both the NP and SS electrodes.  The bentonite slurry successfully reduces the contact resistances in half, with the average now around 1,600 $\Omega$ and the maximum down to 3,300 $\Omega$.  Despite this, the negative apparent resistivity data are not resolved.  Only the 1 m spaced test is shown, Fig.~\ref{fig:cat}.

\begin{figure}[!ht]
\setlength{\unitlength}{0.8cm}
\centering 
\begin{picture}(18.89,11)
\put(0,0){\includegraphics[width=\textwidth]{figures/oneonehpile}} 
\put(0,10){(a)}
\put(0,4.6){(b)}
\end{picture}
\caption{1 m spacing along ``N'', ``6-30-B-P'', and ``Q'':  (a) inverted resistivity section; (b) inverted IP section}
\label{fig:oneone}
\end{figure}

The inversion quality of Fig.~\ref{fig:cat} is even lower than Fig.~\ref{fig:oneone} even though the contact resistances are greatly reduced.  The low data quality surrounding the H-piles is most likely due to the soil.  The red-brown sand that begins at 2 m until 6.7 m is classified as a poorly graded sand (SP) according to the Unified Soil Classification System with a coefficient of uniformity C$_u$ of 1.5 and coefficient of curvature of C$_c$ of 1.0 \citep{boss}.  The gradation curve is shown in Fig.~\ref{fig:grain}.

\begin{figure}[!ht]
\setlength{\unitlength}{0.8cm}
\centering 
\begin{picture}(18.89,11)
\put(0,0){\includegraphics[width=\textwidth]{figures/cat}} 
\put(0,10){(a)}
\put(0,4.6){(b)}
\end{picture}
\caption{One meter spacing along ``N'', ``6-30-B-P'', and ``Q'' with bentonite surrounding the electrodes:  (a) inverted resistivity section; (b) inverted IP section}
\label{fig:cat}
\end{figure}
\newpage

\begin{figure}[!ht]
\setlength{\unitlength}{0.8cm}
\centering
\begin{picture}(18.89,7)
\put(4.72,1){\includegraphics[width=0.5\textwidth]{figures/hpilegradation}} 
\put(5,0.75){10}
\put(7.9,0.75){1}
\put(10.7,0.75){0.1}
\put(13.1,0.75){0.01}
\put(4.25,6.8){100}
\put(4.45,6.35){90}
\put(4.45,5.8){80}
\put(4.45,5.2){70}
\put(4.45,4.6){60}
\put(4.45,4.0){50}
\put(4.45,3.45){40}
\put(4.45,2.9){30}
\put(4.45,2.3){20}
\put(4.45,1.7){10}
\put(4.7,1.2){0}
\put(7.9,0){Grain size, mm}
\put(3.75,2.2){\rotatebox{90}{Percent Passing (\%)}}
\end{picture}
\caption{Grain size distribution for firm red sand \citep{boss}}
\label{fig:grain}
\end{figure}

This highly uniform soil has more connectivity of pore space than a well graded sand or a sandy clay soil.  This highly connected pore space greatly reduces the apparent resistivity of the ground and it also allows the current to dissipate more easily resulting in lower quality IP curves.  Also, despite the high RMS values, the chargeability in Fig.~\ref{fig:oneone} and Fig.~\ref{fig:cat} are extremely high.  Recall that chargeability is the ratio of the secondary to primary voltage potentials measured at the surface (Eq.~\ref{Charge}).  Typical values for chargeability in this research using a two second delay time and a maximum current of 2,000 mA are from 100--300 mV/V.  In Fig.~\ref{fig:cat}, the chargability is 700 mV/V.  Because the primary voltage should not vary greatly, this means that the secondary voltage is much higher.  A higher secondary voltage indicates that the current does drop initially from the primary voltage when the current is turned off.  In other words, the current is being stored in the ground instead of dissipating.  Most likely, for these tests, the current is stored in the highly chargeable steel which leads to a very high IP response and may also reduce the quality of the calculated IP data, thus increasing the RMS.  While digging holes for the bentonite slurry, several pieces of steel near the surface were found.  This scattered steel debris also affects the output, forcing current to be stored in many places in the subsurface.  Because the soil and site conditions of this site cannot be easily overcome at the surface no more testing is conducted at around the H-piles.
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

\section{Full Scale Bridge Testing}
\noindent The testing method developed at the NGES has been used to plan surveys at four bridges in the Bryan TxDOT district.  Each bridge is tested using 28 SS and 28 NP electrodes utilizing the dipole-dipole array.  The electrode spacing is either the width of the exposed foundation or 1 m, depending on the necessary depth of investigation.  The results from each bridge are used for the reliability estimation in Chapter~\ref{chap:Stats} and are used to identify the decision process for estimating the depth of an unknown foundation.  The locations of the four bridges discussed in this section along with the unknown bridge used for validation are shown on Fig.~\ref{fig:map}.
\begin{figure}[!ht]
\centering
\includegraphics[width=\textwidth]{figures/map}
\caption{Tested bridges in the Bryan TxDOT district}
\label{fig:map}
\end{figure}
\clearpage
\subsection{Beason Creek Bridge}
\noindent The bridge on FM 362 over Beason Creek is 1.77 km south of FM 2 in Grimes County, Texas.  It was constructed in 1956 and is a minor collector rural road.  The total length of the bridge is 27.4 m and it is 7 m from curb to curb.  The piles are 6.10 m long H 10x42 steel piles with 4.88 m embedment.  Beason Creek is currently completely dry with very loose sand at the surface by visual classification (Fig~\ref{fig:sandb}).
\begin{figure}[!ht]
\centering
\begin{subfigure}[Test site]
{\includegraphics[width=70mm, height = 90 mm]{figures/sandbeason}\label{fig:sandb}}
\end{subfigure}
~~
\begin{subfigure}[Experimental setup]
{\includegraphics[width=70mm, height = 90 mm]{figures/beasonarray}\label{fig:barray}}
\end{subfigure}
\caption{Beason Creek Bridge}
\end{figure}
This very loose sand causes the contact resistance to be very high, thus leading to poor data quality.  The average contact resistance in this loose sand is 2,140 $\Omega$ with a maximum resistance of 8,320 $\Omega$. 
\begin{figure}[!ht]
\setlength{\unitlength}{0.8cm}
\centering 
\begin{picture}(18.89,11.5)
\put(0,0){\includegraphics[width=\textwidth]{figures/navasotab}} 
\put(0,10){(a)}
\put(0,4.6){(b)}
\end{picture}
\caption{Beason Creek Bridge with 1 m spacing:  (a) inverted resistivity section; (b) inverted IP section}
\label{fig:navasota}
\end{figure}
Due to fence line encroachments and in order to isolate one pile, the foundation is located between the 10th and 11th electrodes and it is side scanned instead of placed directly in the array.  Note that the first electrode is only 0.5 m from the second electrode.  Again this is due to the property fence line near the first electrode.  Low resistivity is seen around the pile as well as high chargeability but several other mathematical anomalies are also present.  This is likely due to the loose soil causing a high contact resistance resulting in low current injection.  Both RMS values are above the acceptable range for these data to be interpreted with confidence.  The predicted depth is 5.12 m using the post processing algorithm.
\clearpage
\subsection{Clear Creek Bridge}
\noindent The Clear Creek Bridge is located in Leon County, Texas on FM 977, 1.85 km southeast of FM 3.  It is a minor collector road that was built in 1988 and is 55.5 m long and 10.5 m wide from curb to curb.  It crosses the Clear Creek; however, the creek is so low that only one pile group goes through it.  The dry piles along with the severe scour can be seen in Fig.~\ref{fig:bbent}.  

\begin{figure}[!ht]
\centering
\begin{subfigure}[Test site]
{\includegraphics[width=70mm, height = 60 mm]{figures/borrisbent}\label{fig:bbent}}
\end{subfigure}
~~
\begin{subfigure}[Experimental setup]
{\includegraphics[width=70mm, height = 60 mm]{figures/bor2}\label{fig:bor2}}
\end{subfigure}
\caption{Clear Creek Bridge}
\end{figure}
The line is set up with two different electrode spacings using the dipole-dipole array.  First, the electrode spacing is 1 m in order to gather data deeper than the 4.6 m piles.   The inverted resistivity section converges to an RMS of 3.85\% in eight iterations.  A zone of high resistivity is noted at the top 0.25 m of the pile, which is typical of resistivity inversions of long slender piles.
\begin{figure}[!ht]
\setlength{\unitlength}{0.8cm}
\centering 
\begin{picture}(18.89,11)
\put(0,0){\includegraphics[width=\textwidth]{figures/borrisb}} 
\put(0,10){(a)}
\put(0,4.6){(b)}
\end{picture}
\caption{Clear Creek Bridge with 1 m spacing:  (a) inverted resistivity section; (b) inverted IP section}
\label{fig:borrisb}
\end{figure}

Data are gathered to the full depth of the pile; however, no zone of high chargeability is seen at the top of the pile.  This may be due to the fact that the current is not transmitted in the 2D plane; instead, it spreads outward in a circular 3D plane that may not capture the upper portion of the foundation.  Despite this, a strong chargeability is shown at the bottom half of the foundation with a chargeability much higher than that of natural objects within the ground.  The chargeability of the foundation, appears to cover a much larger area than the actual foundation and it does not directly line up with the location of the foundation.  The slight skewness of the results may be attributed to the nature in which the array is set up.  In order to isolate the exterior pile only a corner of the pile is placed within the array.  This corner is less than 0.3 m wide, so much of opening for the foundation is free space.  The predicted depth is 2.96 m.  In response to this, the pile is also tested with the electrode spacing equal to the width of the foundation.

\begin{figure}[!ht]
\setlength{\unitlength}{0.8cm}
\centering 
\begin{picture}(18.89,11)
\put(0,0){\includegraphics[width=\textwidth]{figures/borrishalfb}} 
\put(0,10){(a)}
\put(0,4.6){(b)}
\end{picture}
\caption{Clear Creek Bridge with 0.57 m spacing:  (a) inverted resistivity section; (b) inverted IP section}
\label{fig:borrishalfb}
\end{figure}

Fig.~\ref{fig:borrishalfb} is the same location with the electrode spacing reduced to the width of the pile, 0.57 m; no topographic data are available for this test.  Again the inverted resistivity section is marked by a zone of high resistivity at the top of the pile.  Note that the model requires extra iterations (ten) to converge to an acceptable RMS of 5.35\%.  The inverted IP response shows a zone of high chargeability in the middle of the pile that is contained to approximately the width of the foundation.  This zone fades from 219 mV/V to 165 mV/V which penetrates to the full depth of the inversion.  Because of the smaller electrode spacing data are not gathered to the depth of pile so it is not possible to identify the depth of the foundation with this test.  Although the IP inversion shows a strong chargeable response on the foundation the RMS is 16.89 mV/V which is above the confidence level of these inversions.  The predicted depth is 1.59 m.
\clearpage
\subsection{Davidson Creek Bridge}
\noindent The SH 21 over Davidson Creek Bridge is located in Burleson County on SH 21, 0.32 km north east of SH 36 in Caldwell, Texas.  It is a principal arterial road that was built in 1968 and reconstructed in 1988.  The bridge is 97.5 m long and 19.9 m wide from curb to curb.  The piles are a 0.4 x 0.4 m$^2$ straight shaft that extend approximately 6 m into the subsurface (the current depth of embedment is slightly different for each pile). The current embedment depth is approximated by measuring the length from the bridge bent to the ground surface and using the bridge drawings.  This is necessary because of the scour that has occurred, as shown in Figure~\ref{fig:c}.  28 NP electrodes and 28 SS electrodes are spaced at 1 m as shown in Figure~\ref{fig:cald} with the foundation 13 m from the first electrode for each of the six piles tested.
 \begin{figure}[!ht]
     \centering
           \begin{subfigure}[Test site]
             {\includegraphics[width=70mm, height = 60 mm]{figures/cpic} \label{fig:c}}
       \end{subfigure}
       ~~
     \begin{subfigure}[Experimental setup]
       {\includegraphics[width=70mm, height = 60 mm]{figures/cald} \label{fig:cald}}
 \end{subfigure}
 \caption{SH 21 over Davidson Creek Bridge}
 \end{figure}

Fig.~\ref{fig:caldlayout} shows the layout of the piles of the Davidson Creek Bridge.  The six tested piles are hatched to highlight them and number as the order they are presented herein.
\begin{figure}[!ht]
\setlength{\unitlength}{0.8cm}
\centering 
\begin{picture}(18.89,15)
\centering
\put(3,0){\includegraphics[width=0.75\textwidth]{figures/caldwell}}
\put(7.35,3.54){5}
\put(7.35,2.66){6}
\put(9.5,3.54){2}
\put(9.5,2.65){3}
\put(9.5,10.55){4}
\put(9.5,9.65){1}
\put(8,13.1){Caldwell, Tx}
\put(7.75,1){Bryan, Tx}
\end{picture}
\caption{Pile layout of Davidson Creek Bridge}
\label{fig:caldlayout}
\end{figure}

The first pile tested is the exception to the rest, the electrode spacing is set equal to the width of the pile, or 0.4 m.  This only allows for data collection to 2.07 m using the dipole-dipole array.  The inverted resistivity section converges to an RMS of 4.19\% in ten iterations but the inverted IP section is only able to achieve an RMS of 25.95 mV/V.
\begin{figure}[!ht]
\setlength{\unitlength}{0.8cm}
\centering 
\begin{picture}(18.89,11)
\put(0,0){\includegraphics[width=\textwidth]{figures/caldwell1b}} 
\put(0,10){(a)}
\put(0,4.6){(b)}
\end{picture}
\caption{Davidson Creek Bridge pile one with 0.4 m spacing:  (a) inverted resistivity section; (b) inverted IP section}
\label{fig:caldwell1b}
\end{figure}

The results in Fig.~\ref{fig:caldwell1b} are similar to Fig.~\ref{fig:borrishalfb}, in that the zone of high chargeability is confined to the area of the foundation and the IP RMS is above the accepted level.  The IP inversion also does not image below 2 m of the pile, which is less than half of the 5.75 m pile embedment.  The predicted depth is 1.59 m.
\newpage
Pile two is tested with a 27 m long array so data is collected to 5.2 m beneath the surface.  After accounting for scour, this pile extends 6.08 m into the subsurface.  A very strong, localized zone of high chargeability relative to the background chargeability is seen beginning halfway down the pile to the ultimate depth of the data gathered.   
\begin{figure}[!ht]
\setlength{\unitlength}{0.8cm}
\centering 
\begin{picture}(18.89,11)
\put(0,0){\includegraphics[width=\textwidth]{figures/caldwell2b}} 
\put(0,10){(a)}
\put(0,4.6){(b)}
\end{picture}
\caption{Davidson Creek Bridge pile two with 1 m spacing:  (a) inverted resistivity section; (b) inverted IP section}
\label{fig:caldwell2b}
\end{figure}
\noindent Again, the results in Fig.~\ref{fig:caldwell2b} is useful for updating the National Bridge Inventory (NBI) by noting that the foundation is at least 5.2 m deep.  Unlike Fig.~\ref{fig:caldwell1b}, the inverted resistivity section and the inverted IP section both converge to acceptable RMS values (2.47\% and 1.94 mV/V, respectively) so there is much more confidence in this model than from previous data.  The predicted depth is 5.87 m.
\newpage
The third pile is embedded 6.19 m after accounting for scour.  The depth of penetration in Fig.~\ref{fig:caldwell3b} is 5.9 m so the pile depth is now less than the depth of the data.  Again, the RMS for both tests indicates that the data quality is very high.  The inverted resistivity section shows no details in reference to the pile but nearly 2/3 of the pile is imaged in the IP section.  Note that similar to Fig.~\ref{fig:caldwell2b} the foundation appears much wider than it actually is.  This is a trade off for gathering deeper data in order to image the entire pile.  
\begin{figure}[!ht]
\setlength{\unitlength}{0.8cm}
\centering 
\begin{picture}(18.89,11)
\put(0,0){\includegraphics[width=\textwidth]{figures/caldwell3b}} 
\put(0,10){(a)}
\put(0,4.6){(b)}
\end{picture}
\caption{Davidson Creek Bridge pile three with 1 m spacing:  (a) inverted resistivity section; (b) inverted IP section}
\label{fig:caldwell3b}
\end{figure}
When the electrode spacing is wider than the foundation, the IP response appears wider than the foundation in the subsurface.  The predicted depth is 6.15 m.
\newpage
Pile four is embedded 5.99 m and the depth of investigation is 6.3 m for the 27 m long array.  The inverted resistivity section in Fig.~\ref{fig:caldwell4b} converges to an RMS of 2.55\% in only seven iterations and the inverted IP section converges to an RMS of 1.90 mV/V.
\begin{figure}[!ht]
\setlength{\unitlength}{0.8cm}
\centering 
\begin{picture}(18.89,11)
\put(0,0){\includegraphics[width=\textwidth]{figures/caldwell4b}} 
\put(0,10){(a)}
\put(0,4.6){(b)}
\end{picture}
\caption{Davidson Creek Bridge pile four with 1 m spacing:  (a) inverted resistivity section; (b) inverted IP section}
\label{fig:caldwell4b}
\end{figure}
Looking at the inverted IP section, the localized zone of high chargeability nearly directly correlates to the depth of the foundation.  This inversion highlights the importance of the calculated decision for the foundation depth so engineers are not guessing at random as to the depth of the foundation.  The calculated decision algorithm  is introduced in Chapter~\ref{chap:Stats}.  The predicted depth is 5.2 m.
\newpage
Pile five, shown in Fig.~\ref{fig:caldwell5b}, has 2.72 m of the pile above the ground surface leaving 6.12 m below after accounting for scour.  The depth of investigation for Fig.~\ref{fig:caldwell5b} is 6 m, nearly the depth of the pile embedment.  The RMS for the inverted resistivity section is 3.58\% after ten iterations; with the exception of Fig.~\ref{fig:caldwell1b} all of the other piles on the Davidson Creek Bridge converge to an RMS of less than 3\% in 8 or less iterations. This high ERI section generates a high RMS IP section as well; RMS of 8.95 mV/V.  Looking at the IP section, the zone of high chargeability is only seen from -1.5 m to approximately -3 m.  Though the zone of high chargeability stops around -3 m using visual inspection, the predicted embedment is 6.14 m using the prediction criteria described in Chapter~\ref{chap:Stats}.  The pile is over predicted by 0.02 m.
\begin{figure}[!ht]
\setlength{\unitlength}{0.8cm}
\centering 
\begin{picture}(18.89,11)
\put(0,0){\includegraphics[width=\textwidth]{figures/caldwell5b}} 
\put(0,10){(a)}
\put(0,4.6){(b)}
\end{picture}
\caption{Davidson Creek Bridge pile five with 1 m spacing:  (a) inverted resistivity section; (b) inverted IP section}
\label{fig:caldwell5b}
\end{figure}
\newpage
Pile six is the exterior pile on the third bent.  The pile in Fig.~\ref{fig:caldwell6b} has 2.57 m above the ground surface and 6.27 m below.  The depth of investigation for the 27 m long array is 5.9 m.  A zone of high resistivity is noted at the top of the pile and high chargeability from 2 m down to the depth of penetration.  The predicted depth is 5.89 m.
\begin{figure}[!ht]
\setlength{\unitlength}{0.8cm}
\centering 
\begin{picture}(18.89,11)
\put(0,0){\includegraphics[width=\textwidth]{figures/caldwell6b}} 
\put(0,10){(a)}
\put(0,4.6){(b)}
\end{picture}
\caption{Davidson Creek Bridge pile six with 1 m spacing:  (a) inverted resistivity section; (b) inverted IP section}
\label{fig:caldwell6b}
\end{figure}
\newpage

\subsection{Gum Creek Bridge}
\noindent The final bridge is located in Grimes County, Texas on SH 90, 6.04 km north of SH 30 outside of Roans Prairie, Texas.  The bridge is on a minor arterial road.  The Gum Creek bridge was built in 1927 and reconstructed in 1960; it is 36.6 m long and 13 m wide from curb to curb.  Gum Creek is flowing around two pile groups and there is also stagnant water in a small channel in the traffic direction of the bridge.  The array is set up in the traffic direction so the banks created by scour can be used to keep the cables dry (Fig.~\ref{fig:water}). 
 \begin{figure}[!ht]
     \centering
           \begin{subfigure}[Test site]
             {\includegraphics[width=70mm, height = 90 mm]{figures/DSC04227} \label{fig:water}}
       \end{subfigure}
       ~~
     \begin{subfigure}[Experimental setup]
       {\includegraphics[width=70mm, height = 90 mm]{figures/DSC04233} \label{fig:waterb}}
 \end{subfigure}
 \caption{Gum Creek Bridge}
 \end{figure}
 \newpage
 Electrodes are spaced every 1 m again using 28 NP and 28 SS electrodes.  As shown in Fig.~\ref{fig:waterb} two piles are tested directly in the array path.  Additionally two more are side scanned because of their close proximity to the array.  Also, the first electrode is within 1 m of the bridge abutment due to limited space.  The piles are located between the 6th and 7th electrodes and the 13th and 14th electrodes.  Each pile is embedded approximately 3 m,
\begin{figure}[!ht]
\setlength{\unitlength}{0.8cm}
\centering 
\begin{picture}(18.89,11)
\put(0,0){\includegraphics[width=\textwidth]{figures/gumcreekb}} 
\put(0,10){(a)}
\put(0,4.6){(b)}
\end{picture}
\caption{Gum Creek Bridge with 1 m spacing:  (a) inverted resistivity section; (b) inverted IP section}
\label{fig:gumcreekb}
\end{figure}
\noindent The inverted resistivity section in Fig.~\ref{fig:gumcreekb} converges to an RMS of 3.81\% in ten iterations, but again not clear foundation is seen.  In the inverted resistivity section, a zone of high chargeability (over 400 mV/V) is seen around both of the piles.  The response of the first pile, between the 6th and 7th electrodes, has this high chargeability along the pile that terminates at the approximate pile depth.  The predicted depth is 2.83 m.

The second pile, between the 13th and 14th electrodes, has the same zone of high chargeability but the pile is only imaged to 1.5 m beneath the surface.  Only the approximate depth of the foundations is available for this bridge, not the bridge plans, so the lengths of the piles are not measured and adjusted for scour.  After adjusting for scour the second pile may indeed terminate approximately 1.5 m into the subsurface.  The RMS for the IP inversion is well over the acceptable bounds for a reliable model, so the pile may be deeper than the inversion shows.  The high RMS in Fig.~\ref{fig:gumcreekb} indicates that the pile should not be assumed to terminate at 1.5 m without further testing.  The predicted foundation depth is 4.09 m.  

Despite the high IP RMS this test shows that piles can be imaged through water, which is important because this expands the realm of applications for this method to include both bridges with dry conditions and wet.  Only one case is tested through water because the cables at TAMU are not submersible so extra care must be taken to keep them above the water line.  Commercially designed submersible cables are available for bridges with piles through water.

\section{Experimentation Summary}
\noindent In this chapter the optimal experimental setup has been identified by testing the different parameters possible at the NGES on reinforced concrete foundations.  The optimal setup is listed below.

\begin{enumerate}
\item Arrange the array so that the foundation is in the middle of the electrode line
\item The central electrodes (14th and 15th in a 28 electrode line) should be placed on either side of the foundation
\item SS electrodes should be next to the foundation and NP electrodes offset by less than 0.3 m for a side scan survey
\item The IP cycle time should be two seconds
\item The dipole-dipole array should be used
\item For concrete structures, the foundation must contain reinforcement
\end{enumerate}

Three steel H-piles were also tested.  While the optimal setup was used, it was not successful on the steel H-piles.  The use of a bentonite slurry for lowing the contact resistance at the surface is showed at the steel H-pile site.  No discernible foundation is seen at the steel H-pile site, but the highly chargeable response indicates that the current is stored at the site and not dissipating at a typical rate for foundations.  More testing is needed  on steel foundations at this time.

Four full scale bridges have been tested using the experimentally determined optimal setup for imaging foundations.  It is shown that bridges can successfully be imaged using ERI and IP from which foundation depths can be estimated.  In these tests again the dependence of the soil type is shown as well as the importance of gathering data deeper than the foundation itself.  Between these four bridges it is seen that ERI and IP can be used to successfully image bridges in different soil types, that are both shallow and deep, with and without water present.  The results of these bridge tests are used in Chapter~\ref{chap:Stats} where a post processing bridge foundation depth estimator is developed.  

