Forecasting an Engineering Control or Institutional Control Breach, A Study of Third-party Excavation Activity

Excavation activity in one year period near Oakland California

Third party excavations by utility companies, developers and contractors are routine and frequent in urban areas. Excavations are shown intersecting cleanup sites that have institutional and engineering controls. The potential excavation extents are reported by utility protection systems, and the actual excavation limits are smaller. Shown are excavation notifications for a one-year period near Oakland, California. An overlay shows higher population densities associated with this urban area.

Excavations by third-parties are inevitable near any cleanup site. Of concern to cleanup site managers, these unexpected events can breach institutional or engineering controls relied upon in a site’s cleanup remedy. Third-party excavators include utility companies, developers and contractors whose work is not part of the site cleanup process. Terradex completed an analysis of excavation records for 313 sites in California from utility clearance centers during a one year period, July 1, 2012 through June 30, 2013. The 313 sites cover about 250,000 acres. The findings offer a formula for reasonable predictions of annual excavation frequency near cleanup sites.

Excavation activity near cleanup sites can unwittingly exacerbate residual contamination, violate the requirements of institutional or engineering controls, or expose workers to hazardous chemicals. Excavations range widely from small disturbances – such as for sidewalk construction and utility repair – to large projects for parking garages, basements, development site grading, water well development and many others.  The importance of controlling excavations and monitoring excavation activity has recently been reinforced in USEPA’s  Final Guide to Planning, Implementing, Maintaining and Enforcing (PIME) Institutional Controls at Contaminated Sites, a topic discussed earlier by Terradex.

How likely and how often will excavations actually occur? Site management or continuing obligation plans should estimate third-party excavation likelihood and arrange for risk mitigation methods. A broader goal within this posting is to introduce an empirical approach to stewardship. Too often, the planning and evaluations of stewardship are anecdotal.  While an empirical method is offered, Terradex recognizes numerous qualitative factors ultimately resolve the actual excavation occurrence.  With more empirical methods, stewardship duties can be better calibrated to the site setting and its unique risk factors.

Analysis Overview

The analysis reveals a general correlation between cleanup site area and population density; the correlation is the basis for a predictive formula.  For example, 22 excavations per year would be predicted at a suburban 3.5-acre cleanup site. As population density decreases, excavation activity decreases. Nonetheless, the analysis shows that even in under-populated regions, excavations at cleanup sites remains a risk, a risk that is accentuated by emerging development projects.

Excavation frequency increases as population density increases. The shading correlates to population density shown on the post's map posts.

Annual excavation frequency increases as population density as evaluated for 313 sites. The shading represents population density zones applied across figures.

This predictive model was based on LandWatch excavation activity at 313 unique sites across California over a twelve month period,  July 2012 through July 2013. The sites covered more than 250,000 total acres, with the smallest site being a mere 0.01 acre and the largest encompassing 65,000 acres (a median of 5.5 acres per site). The site characteristics vary considerably from rural military facilities to urban brownfield cleanups. Terradex has maintained its  LandWatch service at these sites for public and private clients, and through that effort assembled the information to permit this analysis.

The analysis of the data began with a hypothesis that excavation intensity near remedial sites is proportioned to both site area and site population.  Our analysis confirmed a general trend that helps site managers forecast excavation intensity.

Third-Party Excavation Forecasting Formula

The following formula, generated as a best fit to the data, provides a reasonable estimate of excavation frequency given a site area and population density.


A meaningful estimate of at-risk events sharpens design and evaluation of appropriate site stewardship. For example, consider a hypothetical site in Millbrae, California  near the San Francisco Airport. At our hypothetical site, the cleanup remedy relies on an engineered control – in this case a parking lot – to prevent surface water infiltration into residual contamination.  The area of the parking lot is 3.5 acres, and the population density in the census tract is 20 persons per acre (using the 2010 census). Applying the methodology, the estimated annual excavations near the site would be 22 excavation clearances per year.

While the yielded equation produces a numerical result, in reality and with emphasis, the resulting forecast should be used qualitatively and be incorporated in the overall review and assessment of the future third party excavation potential.

Once informed by the expected number of third-party excavations, a site manager can ask new questions to hone site management plans. For example, is an annual inspection adequate to assure protection of an engineering control? Can advance notice be provided to excavators of hazards associated with a cleanup? Should the interested parties – landowner, responsible party or regulatory agency – join and respond to notifications generated by the 811 system? If the forecasted rate of third party excavations seems too high, then what site circumstances would cause fewer excavations? Considering these questions sharpens any long-term site management planning.

Not all excavation notices would affect the cleanup site or affect the construction worker. As Terradex’s service is to qualify these incoming excavations notices against land use criteria for protecting an engineering or institutional control, Terradex has found that 10 percent of excavation notices upon review would breach the institutional or engineering control.  The excavation notice from a utility protection system show the maximum potential extent of an excavation, and are screened by Terradex to resolve the actual location, depth and nature of the planned excavation. While a cleanup site manager could take comfort that only one in ten of the excavations would likely breach the institutional or engineering control, the manager would not know which of the excavation notices would represent the event that could damage the cleanup remedy.

Sites with Density

The occurrence of 313 sites monitored for excavation varied across multiple population zones.


What was the methodology to generate this forecast of excavation frequency? There are three pieces of data that form this analysis: the excavation notices, cleanup site characteristics, and the population demographics.

Excavation Notices. The analysis was based on 10,700 excavation clearance records from July 1, 2012 through June 30, 2013.  Terradex receives excavation notices nationwide for cleanup sites monitored nationwide, and therefore not all of the 10,700 excavation records occurred near the 313 sites evaluated in this study.

Terradex processes excavation clearance requests as a participant in the 811 excavation clearance system.  An excavator has an obligation to clear each excavation to protect buried utilities, and the excavator accomplishes this notification by calling the state-sanctioned 811 centers. California has two systems, Underground Service Alert of Northern California covering the state generally north of Bakersfield, and Underground Service Alert of Southern California for the balance of the state south of Bakersfield. Terradex’s monitored coverages are maintained within each of the 811 centers. If an excavator contacts the 811 center, and the excavation is near the Terradex maintained coverage, then an email with the location and nature of the excavation is transmitted to Terradex. Of note, Terradex only has access to excavation notices associated with monitored sites, and therefore cannot do a state-wide analysis independent of cleanup sites.

For this analysis, excavation notifications were processed by Terradex, and then parsed to the individual cleanup sites. Terradex monitors cleanup sites  for numerous clients. The excavation records were stored as part of a geospatial database that facilitated this analysis.

Cleanup Site Characteristics.  Cleanup site boundaries typically begin as environmental covenants that are recorded into property records. The site boundaries are resolved by interpreting records of institutional and engineering controls often presented as legal property descriptions. These boundaries are resolved as polygons in Terradex’s geospatial database.  The polygons were analyzed to calculate  area of the cleanup site and the geographic centroid of the cleanup site.

Population Demographics. All the cleanup sites were characterized against the US Census 2010 to obtain the population density. Terradex calculated population density for each census tract by using the population and a land area provided within the census data attributes.

With the excavation notices, cleanup site boundaries and population density, the data analysis was completed. The centroid of the cleanup site polygon was queried against the census tract to determine the population density for each cleanup site. The excavations intersecting each cleanup site were counted (nearby excavation notices were not included).   The data was normalized to common units of area proportioned on the cleanup site. The findings were categorized into population density zones to allow grouping of the findings. The data analysis resolved 313 data pairs consistent with the two axis on the chart – annual excavation frequency at a given cleanup site, and population per acre at a given cleanup site.


Every cleanup site in this study had one or more excavation clearance requests over the one year period. However, there is considerable variance in the frequency of excavations versus population. As population increases, the variance decreases. Our experience shows that the greater variance is due to the sporadic nature of property development in underpopulated areas.

Third-party excavations still occur in lower populated regions.  While the frequency of excavation is low, there is greater scatter because excavation occurrence is episodic.  Excavation activity in less populated areas is often project-based. Project-based excavation is associated with a new development or land use change or a regional project (e.g. high-speed rail). Non-project-based development is associated with ongoing activities such as maintenance and repair activities. An example of non-project activity is municipal or private utility maintenance that commonly involves excavation. Consistent with the episodic nature of development, a project-based development will generate multiple excavations as the project proceeds in a region that, prior to development, would have been void of activity.

One reviewer commented that there are a multitude of factors that play into the potential of a third party excavation and often times it is not possible to make any type of prediction. The observer continued that the driving force behind the possibility of having a third party excavation event lies in the redevelopment potential for the area and whether or not the City or area’s infrastructure  needs repair. This observation coincides with our observations of project and non-project based excavations.  In remote regions the preponderance of excavations are project-based, and therefore less predicable.


These findings introduce an empirical method for site management discussions and offer the start of a calculated approach to cleanup site stewardship. Will these findings enter discussions about cleanup remedy stewardship? Many site managers rightfully want to be “done” once the remediation is complete, and they envision a future devoid of activities that could impact remedies or the residual contaminants. Lacking tools like this formula, site managers might draft site management plans in false comfort that no at-risk events would occur, or they might anticipate that if such events occur, their frequency is so low that a five-year inspection is protective of the remedy and the public. As shown by the frequency of third-party excavation events with our hypothetical site, site stewards should continue to plan for at-risk events well into the future. They must address questions about future vulnerability: “How does your site management plan consider the likelihood of 22 excavations at or near the site? Can the engineering control be protected? Are mechanisms in place to prevent excavator contact with shallow contaminated groundwater? Can construction workers be notified?” The applicability can move beyond institutional and engineering controls, and may offer forecasts to the likelihood of excavation within groundwater plumes that migrate from cleanup sites – a mode of pollution that is often difficult to manage through institutional or engineering controls.

Routinely monitoring notices of third party excavation activity avoids damage to a remedy, avoids disturbing residual contamination, avoids the exposure to the construction worker, and provides comfort that a site management program is protective. However, not all cleanups or the associated institutional controls may be breached by excavation activity, and thereby served by excavation monitoring.  For example, the enforcement of a prohibition of occupancy by childcare would not be served by monitoring excavation.  Rather, monitoring of day care licensing would be a proper land activity to monitor instead of excavation activity.  Additionally, clients value the use of zoning changes that can offer a longer term glimpse of a future development activity, and might opt to augment at a cleanup site excavation monitoring with zoning activity monitoring.

Site managers should be diligent in their overall assessment and include other qualitative methods such as location of the property, the location’s potential for redevelopment in a 0.5 mile radius, infrastructure conditions, future development plans of the actual site, potential for population growth, type of engineering control, type of contaminant, and economic status of the area. All of these factors would also be essential in assessing and refining the potential for third party excavations. It is important to emphasize that a more holistic view of all potential factors would be needed to make any type of determination – even though still, regardless of all these tools or factors, one can never be absolutely certain of the number of third party excavation activities.

Terradex’s LandWatch and Dig Clean systems are pioneering tools, increasingly used by our growing client base to responsibly manage cleanup remedies, institutional controls and engineering controls into the future. Our analysis here of third-party excavation activity shows the value of post-cleanup site monitoring. These monitoring approaches can meaningfully inform site management and continuing obligation planning.

Author’s Note.  I appreciate the patient proof reading and editing by Julie Jomo and Michael Sowinski.  I appreciated the review of Lenny Siegel who helped differentiate between third-party excavations and those associated with the remedy construction.  We are especially grateful for the review and permission by our clients to provide this analysis. We have carried their observations to further develop our initial review, and therefore the post hopefully carries greater value.



  1. Posted September 25, 2013 at 6:34 pm | Permalink

    Interesting Read…what successes have you experienced with daylighting services or softdig services?

  2. Posted September 26, 2013 at 9:24 am | Permalink

    Shannon, thanks for the comment. We don’t differentiate daylighting and soft digging.

    For those not familiar, “soft dig” is uses a suction excavator or vacuum excavator to remove earth from a hole on land. Soft dig would be less likely do damage an engineering control, and likely would reduce contractor direct exposure to residual contamination. However, notice to a soft dig operator of residuals in groundwater or soil that are contaminated would be important to help assure proper disposition.

    Daylighting is the uncovering and exposing of underground utilities and pipelines to the daylight hence the term daylighting. The probing associated with day lighting could offer the field worker greater risk of direct exposure, and therefore advance notice of excavation risk could allow selection of Personal Protective Equipment (PPE). Providing this notice is an effort we have undertaken with DigCLean with the states that have opted to sponsor the service.

  3. Peter Strauss
    Posted September 26, 2013 at 1:29 pm | Permalink

    As I understand this report, you have only included utility excavations. Could you define the universe of utility excavations that you considered? Do they account for hook-ups to new home construction?

  4. Posted September 26, 2013 at 2:35 pm | Permalink

    Thanks for the comment. Our study considered all notifications issued by utility clearance systems. This would have included hook-ups to new home construction if there was subsurface work performed.

    As background, any party – a utility or other contractor – is required to provide notice to a utility clearance center by dialing 811. They do this because it is a legal obligation, and also grants the caller liability protections in case they damage a buried utility. Therefore, the compliance is nearly 99 percent.

    The information that arrives is broad and represents an enormous variety of construction work. I made a cloud tag that I opted not to put in the post. The cloud tag shows the most common words in the work description, and allows an indication of what types of work comes through the notification system.

    Cloud Tag of common terms in 500 random excavation notices.

  5. Posted October 7, 2013 at 11:26 am | Permalink

    The post has enjoyed some discussion on EDR’s CommonGround. See the discussion at this link.

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  1. By The Monitor: A Terradex Blog on November 22, 2014 at 6:25 pm

    […] developed an analysis in California revealing a correlation between urbanization, the area of the impacted site and the […]

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