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Sinkholes: Causes, Identification and Remediation

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Sinkholes – The lurking underground menace. One day everything is fine and suddenly, with no warning, it’s not! OK, that may be a little sensational, but sinkholes sure do make the news when they occur. Just recently sinkholes in Florida caused the collapse of a large building and even a human death.

The areas in which sinkholes can occur are related to the geologic conditions, so the occurrence is not totally random. A knowledgeable consultant can assist in determining the relative risk of sinkhole development. As the title says, this article provides a short discussion of how to locate and treat sinkholes. Note the word ‘treat’ instead of ‘repair’. As with certain medical conditions, there is no cure for a site in sinkhole terrain-only treatment and application of suitable construction methods to reduce the risk of future sinkhole occurrences.

While the geologic conditions in which sinkholes can occur are known, the actual occurrence of a sinkhole is unpredictable. Subsurface conditions can stay in equilibrium and stable for many years, and suddenly change due to manmade changes such as pumping from wells or changes in surface runoff. Extreme weather variations such as heavy rainfall or drought can cause dropouts to suddenly occur at a site that was previously stable.

So, what is a sinkhole?
A geologist would say a sinkhole is a localized depression in the surface of the ground that was caused by the removal of underlying material. Naturally occurring sinkholes are mainly associated with limestone bedrock because limestone is susceptible to being dissolved by acidic groundwater. Other geologic formations conducive to sinkhole development include marble and dolomite.

Sinkhole Causes
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The dissolving (or solutioning) of carbonate bedrock occurs over thousands of years. As the limestone dissolves, soil erodes or ‘ravels’ into the open seams or caverns. The erosion progresses upward toward the surface until a surface collapse occurs. This is shown in the publication “Building on Sinkholes” by G. F. Sowers.

Consider the caves you may have visited. The cave networks are the underlying cause of surface sinkholes. When the roof of the cave collapses, or where cracks and fissures in the rock allow overlying soil to erode into the cave, the void can eventually reach the surface. The cave networks can be very extensive, such as Mammoth Cave in Kentucky where the network has over 50km of continuous subterranean passages.

When a large area underlain by such a cave network undergoes collapse and numerous sinkholes occur, the surface topography can change into a series of sinks and large closed depressions, the landscape is known karst topography. The term karst comes from the name of an area adjacent to the Adriatic Sea where natural sinkholes are numerous. The word ‘karst’ is used to describe areas where sinkholes, either natural or man-induced, occur.

The surface collapse can be slow or sudden, as seen in the Florida event. One of the largest sinkholes in Alabama occurred so suddenly that a local farmer felt his house shudder and then there was a sudden gust of wind within a few seconds. The sinkhole (referred to as the Golly hole) was later found to have a diameter of 300 feet.

Sinkholes may also be caused by man made factors as well. Broken storm water pipes are a common cause of soil erosion and subsequent surface dropouts. Underground mining activities have also been related to sinkholes.

Sinkhole Identification
Now that we’ve discussed the causes, how do you identify areas that are susceptible to sinkholes? Except for manmade features, geology is the major factor in sinkhole development. Most states and even some counties have publications that identify sinkhole prone areas. Aerial photographs (sometimes incorporating infrared cameras) can be used to locate existing dropouts or other karst features.sinkhole-4-300x225

After geological research of the site, a geotechnical investigation plan is developed and executed. The investigation may include:

Soil test borings to determine the consistency of the overburden soils. The soil test borings are usually drilled to auger refusal to check the variability of the bedrock surface. It is common to find soft soil conditions and an erratic rock surface in active sinkhole areas. It is important to determine the depth of the soft zone of soil above the rock relative to the thickness of undisturbed soil above.

Rock coring and/or air rotary drilling to assess the continuity of the rock. Clay filled seams, void spaces, or caverns in the rock increase the risk of future dropouts.

Geophysical investigations. Geophysical techniques may include electrical resistivity testing, ground penetrating radar, seismic soundings, or micro-gravity surveys. Electrical resistivity is usually used where clay overburden is present. Electrical resistivity imaging (ERI) is a technique used to map resistance to electrical flow in the ground at varying depths and lateral locations. Electrical current is introduced to the earth by two transmitting electrodes. The apparent resistivity of the subsurface is obtained by measuring the drop in potential between two receiving electrodes along the test alignment. The depth of investigation is proportional to the separations between transmitting and receiving electrodes. By varying this separation, as well as the separations between the pairs of transmitting the receiving electrodes, different depths and locations can be sampled to develop profiles of the subsurface apparent resistivity. These results can often distinguish more competent rock with higher resistivity values from water bearing, weathered or fractured rock having higher conductivity and lower resistivity values.

The results of the investigation are analyzed and the risk for future sinkhole development is assessed. Sites are typically assigned low, medium, or high risk depending on the findings of the investigation.

Sinkhole Remediation
The remediation or treatment options are based on the risk factors assigned to the site, the sensitivity and importance of the structure, and the risk tolerance of the client. There are several viable options to consider for sinkhole risk reduction. Some of the remediation options include:

Rock Supported Foundations. The use of rock supported foundations and a structural slab system is required to essentially avoid the potential complications related to future sinkhole development. Drilled piers, micropiles, or steel piles are considered for this application. However, there are limitations even for these systems and the risks should be evaluated carefully. This approach is not usually practical for most light to moderately loaded structures because of cost.

Grouting. Several grouting techniques are used for sinkhole remediation. Some of the methods include cap grouting, low mobility grouting, and dome filling. A typical grouting process is designed to fill the open voids in the soil and improve the consistency of the soil matrix. The grouting program is usually initiated on a 10 foot to 20 foot grid pattern. Depending on the grout take and pressures in the primary injection points, secondary or tertiary grout holes may be required.

Surface Stabilization Layer. The placement of a geogrid reinforcement layer at the subgrade level is sometimes used to reduce the risk of surface dropouts. The geogrid design is based on the ability of the geogrid to span over the certain size dropout. The major consideration with this method is the selection of the ‘design’ dropout opening. The geogrid manufacturer typically assists in the design services.

Excavation and Placement of Inverted Rock Filter. Repair of existing dropouts with excavation/rock replacement is probably the most common sinkhole treatment method. The excavation is extended to competent bedrock and durable rock is used to replace the removed soil. Large rock (rip-rap) is placed in the bottom of the excavation. Progressively smaller rock is used as the excavation is filled. The purpose is to allow flow of groundwater without causing soil erosion.

Conclusion
While sinkholes are and will continue to be a challenge for many project sites, a proper investigation program can provide the necessary insight to anticipate their presence and oftentimes mitigate their effect.

Old Man River Moving Faster

KY-Lock-257x300In 1935, the Tennessee Valley Authority built the Kentucky Lock and Dam. The site is located at the north end of the beautiful and historic Land Between the Lakes National Recreation Area, just off the Tennessee River in Grand Rivers, KY. Today, it’s operated by the United States Army Corps of Engineers‘ Nashville District and handles 50 million tons of barged cargo every year. To move more freight up and down the river, an initiative to build an Upstream Lock Monolith Addition was launched in 2009.

Building & Earth is providing construction quality control services on the new 110 ft x 1200 ft lock, which will increase capacity to handle larger barges and heavier traffic. The project required the excavation of 500,000 cubic yards of overburden, pre-splitting almost 40,000 square feet of rock, excavation of 100,000 yards of rock, and placement of 130,000 cubic yards of concrete, all with very tight tolerances. All QC testing was conducted with onsite USACE validated laboratory facilities.

lock2-300x192The new lock is adjacent to the existing lock channel which remained active during the construction process, increasing the difficulty of the construction. A series of challenges had to be overcome. The site is situated in a cavitated limestone geologic setting, and the project required pressure grouting of voids and cavities in the rocks before concrete placement could even begin.We worked in cold weather, with the wind from the body of water making conditions even worse. The extremely deep excavation required us to be innovative to transport equipment and samples from the base of the excavation to our laboratory facilities.

In the end, the Upstream Lock Monolith addition will allow for larger barge access and faster passage, reducing wait time for vessels. Building & Earth was proud to be involved with the construction of this $734.5 million project.

Setting the Pace for United Way

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The United Way, one of the nation’s largest and most well-respected charitable organizations, is a force within communities, identifying, funding and resolving a wide range of pressing issues. Each year, our local organization’s annual funding campaign kicks off in September.

In June, the United Way selects companies with a strong tradition of support to conduct their annual giving campaigns a little early, ‘setting the pace’ for the fall giving drive. Building & Earth was pleased to be selected for this Pacesetter Campaign.

The kickoff event was June 13th at the Birmingham Barons vs. Jackson General’s game at Regions Field. Click here to read more about our involvement with the structural steel, masonry and concrete at this downtown baseball park.

Our corporate office in Birmingham announced the Building & Earth campaign with a lunch and presentation by Karen Gandy and Ladye Franklin. Karen is a loaned executive for United Way of Central Alabama, and Ladye represented ‘Success by 6,‘ a preparedness program for pre-schoolers that the United Way sponsors. Branch offices also had representatives speak to employees and encourage generosity.

We are pleased to announce that when the campaign ended on July 19, our company raised more than $32,000, a nine percent increase from 2012.

Our involvement will continue with Controller Filmore Woodbery, who will be a Visiting Allocation Team member for the United Way. This fall, Fil will be part of a small group who visits with UW receiving agencies. They will be gathering information, then consulting and assisting the United Way in determining an appropriate allocation of funds.

Building & Earth couldn’t be more pleased with our team’s willingness to give both their resources and their time. We are looking forward setting the pace again next summer.

More for Moore

OK-for-WebLast month, our nation was struck with tragedy as the people of Oklahoma endured one of the most destructive tornados on record. The news was particularly hard for our team, as many of our Tulsa location employees witnessed the weather first hand.

Our own Tim Ellerbe, a technician based in Tulsa, has been volunteering his time outside of work to help with the recovery efforts in Moore, OK. Here is an excerpt from his heartfelt request for donations:

“As most people around the world now know, Moore, OK has been completely destroyed by an EF-5 tornado. After much speculation, it has been confirmed that the tornado caused 24 casualties in its path of destruction. The tornado was 1.3 miles wide, with winds in excess of 210 miles per hour and left a 17 mile trail of absolute devastation. What has been forgotten is that there was also a pair of F-4 tornados the day prior in Carney and Bethel Acres, OK, causing one casualty and destroying those in the wake.
The last total I heard on the news was 2,400 homes lost or damaged, 10,000 people immediately affected. In following with the compassion I have seen shown, I ask that you help in any way that you can…”

Our Birmingham/Corporate office was glad to aid Tim by collecting monetary donations to help in the recovery effort. Building & Earth Sciences generously matched employee giving. We are proud to have a leader like Tim in our midst, and our hearts are with those still recovering from the tornado’s aftermath.

On Our Radar: Birmingham Bond Work

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Last fall, voters overwhelmingly approved a bill allowing the city to issue $150 million of bonds for capital projects and improvements in Birmingham. Here’s what passed:

  • – $6.3 million for landfill improvements and expansion
  • – $20 million for public park and recreational facilities
  • – $48.7 million for street and sidewalk improvements
  • – $19 million for industrial park and economic development
  • – $45 million for public building facilities and equipment
  • – $11 million for storm sewers

The first phase of work will be $72 million, with the remaining funding on hold for future use.

We are pleased to be a part of a significant expansion at the New Georgia Landfill, a 15.5 acre Municipal Solid Waste (MSW) landfill unit just north of downtown. Work will include excavation, subgrade preparation, berm construction and installation of liners, collection systems and drainage structure. A protective layer and ditch system will also be put in place. Our Scope of work includes management of the project Quality Assurance/Quality Control (QA/QC) program for the project as well as all associated construction materials testing and inspections.

After a few months of debate, the Birmingham City Council still hasn’t given its final stamp of approval to all the capital projects, but the implications remain the same: There’s a lot of potential work ahead for local architectural, engineering and construction sectors.

In our view, the bond money will greatly influence the next few years of development in our industry, possibly marking the end of the Great Recession for Birmingham municipal construction.

From Railroad Park to Regions Field…

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While every project we touch is special in its own way, projects that help our hometowns hit extra close to home. Notable examples include ONEOK Field, home of the Tulsa Drillers minor league baseball team, and Auburn Arena, where the Auburn University basketball team plays.

When redevelopment began in Birmingham’s city center, we jumped at the chance to be a part of it. Just a few miles down the road from our corporate office, Railroad Park and the adjacent Regions Field were a large influence on what has become a downtown renaissance. Only three years old, the Park is used for frisbee games and symphony concerts alike, while Regions Field recently opened as home to our minor league baseball team.

Our work at Railroad Park started with the initial project planning in 2007. We provided environmental and geotechnical assessments of the site to aid in park design. This included evaluation of soil and groundwater, site earthwork, structure foundations and groundwater depth. As the park progressed, we provided geotechnical evaluation for elevated walkways, the planned pond and additional structures. During construction, we monitored the earthwork, tested soil, sampled and tested concrete and grout, and inspected the structural steel.

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At Regions Field, our responsibilities included special inspection of structural steel and sampling and testing of above-grade masonry and concrete.

The structural steel inspection was a challenge due to the fast track schedule required for the steel erection. We had as many as four Certified Welding Inspectors on site at any given time. Our steel inspectors performed visual inspection and non-destructive weld inspection using Ultrasonic Testing methods. Construction inspections continued right up through opening day and beyond.

We were proud to be part of both projects, which are considered integral to downtown redevelopment and revitalization in Birmingham. They are a rallying point for our hometown and for the Building & Earth team.

Running For a Cause

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In January of 2011, Building & Earth founding Principal Deepa Bhate lost a very close family friend, Sundeep Caplash. At only 38 years old, he was hit by a car while training for the Mercedes Half Marathon.

Not willing to rest on their laurels, the Caplash family and friends decided to participate in the Mercedes Marathon to honor Sundeep’s memory. The Bhate family is a part of the ‘Caplash Crew.’ Every February, Deepa and her husband gather with a group of friends to walk the Mercedes Half Marathon in his honor. They have now participated three times, and plan to continue the tradition in 2014.

Filmore Woodbery Joins the Building & Earth Team

Fil-2-250x300We are pleased to announce the hire of Filmore Woodbery as Controller. He is a Certified Public Accountant (CPA) with 27 years of experience in the accounting and financial realm, most recently at Brice Building Company. Fil has recognized strengths in financial and regulatory reporting, administration and tax, and also holds a Masters of Business Administration. He will be based at Building & Earth’s Birmingham, AL corporate headquarters.

Fil and his wife Deb have three children and seven grandchildren. He is an avid reader with a personal library of more 1,500 books and enjoys bowling, fishing and gardening. Fil is a traveler and has even kissed the Blarney Stone in Ireland. Welcome to the team, Fil!

Read more about Fil in the Birmingham Business Journal.

Welcome, Srjd

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Our Huntsville office is pleased to announce the hire of Srdan Boskovic as a staff engineer.

Srdj (pronounced like ‘surge’) received both bachelor and master of sciences degrees in civil engineering from the University of Alabama at Huntsville. His thesis was on analytical and numerical analysis of laterally loaded pile group-cap systems. As a UAH basketball letterman and team captain, Boskovic maintained an athletic scholarship and academic honors. Srdj speaks five languages.

Click here to read his new hire announcement in the Birmingham Business Journal. Welcome, Srdj!