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Sinkholes 101: How They Happen & How to Treat Them

Sinkholes are the lurking underground menace no one wants to think about and definitely don’t want to deal with when they happen. One day everything is fine and suddenly, with no warning, the ground caves in! With spring comes rainy, cold weather—a perfect setup for a sinkhole to form.

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.

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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.

What Is a Sinkhole?
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.

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Causes of Sinkholes
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 50 kilometers 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 manmade factors as well. Broken stormwater pipes are a common cause of soil erosion and subsequent surface dropouts. Underground mining activities have also been related to sinkholes.

Identification of Sinkholes
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 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—Used 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’s 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—This is used to assess the continuity of the rock. Clay filled seams, void spaces, or caverns in the rock increase the risk of future dropouts.
  • Geophysical investigationsGeophysical 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.

Treatment of Sinkholes
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 due to 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- 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.

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.

Your Guide to Pile Driving Analyzer (PDA) Tests

Pile Driving Analysis (PDA)
Building & Earth performing pile driving analysis testing.

 

Have you ever wondered what we do day in and day out on our jobsites? We’d love to tell you about it! Building & Earth provides a full range of services to our clients, including geotechnical engineering services, construction materials testing (CMT), Special Inspections per IBC and environmental consulting.

Some of the specialized studies include seismic site classification utilizing the Refraction Microtremor device (RE-Mi), non-invasive studies using StructureScan and UtilityScan Ground Penetrating Radar (GPR), thermal imaging studies using the FLIR thermal image camera and floor flatness profiling with the FACE Dipstick devices.

We’ll be breaking down some of the services we provide and how we do it on our blog. First up: Pile Driving Analyzer (PDA) tests. Read more below.

What is pile?

If you’ve been around a construction site, you are probably familiar with what a pile is, but just in case, here’s a refresher. A pile is a long structural element (usually concrete, steel or wood) that is used as a support for structures that will be built on top of it.

Piles are often used when the soil at the surface of a construction site is weak and cannot support the weight of the structure that will be built on top of it. Piles are driven into the ground to carry the weight of the building that can’t be supported by that first layer of soil. The other common situation where piles are used for support of structures that will have heavy, concentrated weight in certain places. These structures include water towers or tanks, bridges, high-rise buildings, and more.

The support for the piles is normally developed from skin friction, end bearing, or a combination of the two. These support mechanisms are characterized as follows:

  1. Skin Friction—The load on the pile is supported across the entire surface of the pile by using friction.
  2. End-bearing—As the name would suggest, end-bearing piles transfer the weight of the load onto a layer of strong soil or rock below the end of the foundation.

How do we determine the need for piles?

We start by performing a geotechnical site assessment using soil test borings, rock coring and geophysical methods, such as seismic refraction. When our site assessment shows that the existing site soils cannot support the structure loads on the near surface soils, we evaluate the potential for supporting the structure on piling.

Based on the conditions we encounter in our geotechnical site assessment, we then provide estimates for the load carrying capacities of the recommended piles. The estimation of the pile support capacity can be very complicated due to the variability of the soils encountered on site.

When the construction begins, it is necessary to verify that the recommended pile capacity exists. The capacity verification is normally done with static load tests performed on one or more test piles. The biggest drawbacks to the performance of static load tests are the cost and the very limited amount of information that they generate.

A static load test normally verifies the estimated pile capacity at a single location, but it does not provide the ultimate pile capacity or the individual support capacities attributable to skin friction or end bearing. As an alternative to, or in conjunction with, static load testing, we have equipment that can determine the ultimate pile capacity and details of the support mechanism for driven piles. The equipment is called a Pile Driving Analyzer (PDA).

What is the Pile Driving Analyzer system?

PDA tests, also known as high-strain dynamic load tests, can be used to determine the load-bearing capability of the piles installed for any given structure. PDA tests are also helpful in evaluating the shaft integrity of the pile, and measuring the stresses generated in the pile during driving. This gives assurance that the pile driving equipment is not damaging the piles during driving.

The PDA test is performed on driven piles at the time of driving. Instrumentation is installed near the top of the pile that allows us to measure the stresses in the pile as well as the pile movement. The information that is collected is evaluated to determine the ultimate pile capacity, the portion of the capacity attributable to skin friction, the location of the frictional resistance, and the portion of the capacity attributable to end bearing.

What Is the advantage of PDA testing?

The PDA test generates significantly more information than Static Load Tests, but at a fraction of the cost. This allows us to perform several tests across the project site and based on the results, potential modify the structural design to lower the cost of the structure foundations. Using PDA testing, we have been able to justify very high pile capacities that has resulted in the elimination of hundreds of piles on a single project.

What do the tests do?

The main goal of performing PDA tests is to provide accurate determination of the capacity of driven piles to help our clients get the most efficient foundation system. Two prominent projects on which Building & Earth has performed PDA tests include the AC Marriott Hotel in Huntsville, Alabama, and the Vesta Apartments, a 17-story residential tower we are currently working on in Birmingham, Alabama.

 

Stay tuned for new blog posts explaining some of our other services. Interested in discussing how we could help you on your next project? Give us a call at 1-800-775-2468 or fill out our user-friendly Request a Proposal form. We look forward to working with you!

 

Building & Earth is Named an Engineering Excellence Award Winner at ACEC National and State Ceremonies

Building & Earth is excited to announce it was recently honored at two American Council of Engineering Companies (ACEC) ceremonies for its geotechnical consulting work on the Vesta high-rise apartment development in downtown Birmingham, Alabama.

Building & Earth was first awarded an Engineering Excellence Honor at the Alabama ACEC chapter’s 2019 ceremony. President Jeff Cowen then traveled to ACEC’s National Convention and Legislative Summit in Washington, D.C. where the company was awarded a National Recognition 2019 Engineering Excellence award at the Engineering Excellence Awards Gala. “There were many excellent projects represented in this year’s ACEC Engineering Excellence Awards, and we are very honored to have been included” said Cowen.

Owned by Harbert Realty and Cortland Partners, the Vesta project is an approximately 390,000-square-foot, 18-story, high-rise and 5-story, mid-rise apartment development in the Five Points area of Birmingham. The project is the tallest concrete structure built in Birmingham in 30 years.

When completed, the building will house 318 luxury units, a rooftop pool, exercise facility, yoga and Pilates center and a club lounge. BL Harbert is the general contractor on the project. Building and Earth provided a full range of geotechnical services on the Vesta project, including geotechnical exploration, value engineering consultation, materials testing, and Special Inspections per IBC.

The purpose of ACEC’s Engineering Excellence awards is to recognize engineering firms for projects that demonstrate a high degree of innovation, achievement, value and engineering innovation. Building & Earth was recognized for the Geotechnical Engineering portion of the project, which included exploration, analysis and consultation during the foundation installation phase of the project.

Get to Know the Team: Joe McFalls

Have you ever wondered what it’s like to work at Building & Earth? It’s important to us that we place a focus on empowering the members of our team through technology and knowledge. Our leadership team encourages continued education and professional development, because we are nothing without the knowledge and expertise of our team members.

We foster an open-door policy at every level, and we make it our goal to continue to be a diverse and inclusive workforce. We would love to talk to you about how you may be a great fit for our team! Visit our Careers page to check out our current openings.

It’s truly the people who are hard at work in the office, on the jobsite, and connecting with our clients every day who make our company culture great. That’s why, over the next few months, we will be catching up with several of our employees at some of our 16 locations across the South and Midwest. First up, learn what daily work life looks like for Joe McFalls, who works out of our Raleigh-Durham location.

Joe has been a part of the Building & Earth team for 7 years. He is currently the CMT Department Manager for the company’s Raleigh-Durham branch in North Carolina. This role includes working in and managing satellite offices in Jacksonville, North Carolina; Virginia Beach, Virginia; and NS Guantanamo Bay, Cuba.

“My goal is to provide our clients with professional and valuable service,” McFalls said. “The path to this is quite simply having well-trained individuals providing industry-leading service to our clients.”

Joe has been married for 22 years to his wife, Michele. They have three boys—a freshman in college, a senior in high school, and a freshman in high school. He previously served in the United States Navy Submarine force.

Learn more about Joe, his role at Building & Earth, and why he loves going to work every day below.

Why do you love what you currently do?

I love my current position because it affords me the opportunity to be involved in the professional growth of the great group I work with every day. It is very satisfying to see our team provide top-level testing and inspections to our clients.

What does your day look like at Building & Earth?

My day typically starts with a list of priority items to accomplish. Inevitably, though, the phone starts ringing, and I go into multitask mode, making every effort to resolve emergent issues in addition to completing my scheduled tasks. I love the fast-paced work environment and ever-changing landscape.

What makes an optimal working environment? How does Building & Earth help facilitate that?

I believe that a work environment that fosters continued growth and a genuine care for one another is what most individuals desire from their work. Building & Earth strives to meet the need for growth through training and certification opportunities, as well as providing professional development tools to aid leaders to this end. I genuinely feel that Building & Earth cares for each individual that works with it. I consider the team I work with to be my family, and I want to see each and every person in my work family succeed in his/her career and life.

What has been one of the biggest achievements—or wins—you or your team has had in your time at Building & Earth?

The team’s biggest win is the fact that we continually work to grow our bonds with one another. This creates a team environment, where no one is left to do it alone. That is the military in me—never watch your teammate (family) struggle.

What makes you proud to work at Building & Earth?

I am very proud of our commitment to be the best in our industry and continuing to explore new market sectors. Building & Earth does not just make empty statements; they back it up with support and training.

 

Click here to watch a short video and learn more about what we do and why we do it.

 

Building & Earth Opens New Raleigh Area Location

Building & Earth is excited to announce the opening of its eighteenth location in Garner, North Carolina, just outside of Raleigh, North Carolina. The new office, located at 1027 U.S. Highway 70 West, Suite 102, strategically positions Building & Earth to take on new projects in the Raleigh area.

“Our goal with the new location is to better serve our current and future clients in Wake County and the surrounding region,” said Building & Earth Business Development Director Joel Tucker.

All projects and client relationships will fall under the purview of the Raleigh-Dunn Building & Earth location. The 1,280-square-foot office building will hold six employees at capacity.

The location is currently working with North Carolina State University on the Centennial Campus, providing Special Inspections per IBC services, a general geotechnical investigation and soils testing.

 

Building & Earth Partners with UAB on Gardendale Medical Center Project

We are currently providing services for the new $33.9 million University of Alabama at Birmingham (UAB) Gardendale medical center project, located in Gardendale, Alabama. Brasfield & Gorrie is the general contractor for the project, which broke ground in November.

Building & Earth completed a geotechnical investigation for the project in 2016 in support of the design phase and is providing a full range of materials testing and Special Inspections per IBC services during the construction phase of the project.

The 6.2-acre site will encompass a freestanding emergency department as well as a medical office building housing a broad range of primary and specialty care services.

The 26,700-square-foot emergency department will offer a heliport, advanced MRI/CT/X-ray imaging, 12 exam rooms, laboratory services, a bariatric lift and a pharmacy. The 24-hour staff at the department will have the capability to perform trauma care. In addition, the facility will also provide space for decontamination and isolation.

The 38,400-square-foot medical office building will provide primary care, obstetrics, gastrointestinal care, orthopedics, cardiology, neurology, neurosurgery, ophthalmology and specialty care programs.

In addition to Brasfield & Gorrie, other key players are Sims Architectural Studio, architect for the emergency department, and Gresham Smith & Partners, architect for the office building. The facility, which will be located at 960 Mount Olive Parkway, has an expected completion date of spring 2019.

For more information on the project, click here.

Taking to the Field for a Good Cause

For our team, dedication to both our work and the communities we live in are of the utmost importance. This year, we are proud to continue our support of Children’s Harbor and Magic Moments.

Children’s Harbor is a local organization that provides children with serious illnesses and their families with two locations in the Birmingham, Alabama, area where they can find strength, comfort and support during and after hospitalization.

Magic Moments is a wish-granting organization devoted to children in Alabama with chronic, life-threatening diseases. According to the organization’s website, since 1984, it has fulfilled over 4,500 moments for children.

The Children’s Harbor/Magic Moments 13th annual Sporting Clay Shoot, held at Selwood Farm in Sylacauga, Alabama, took place on September 29.

Teams of four spent a half day shooting on Selwood Farm’s multi-station course.

The main mission for both organizations is to provide support for families with chronically ill children. Each year, the proceeds from the sporting clay shoot are split equally between the two organizations.

Next year’s event will be held on September 28, 2018, at Selwood Farm in Sylacauga.

To make a donation or find out more about Children’s Harbor, click here. To make a donation or find more information about Magic Moments, click here.

Building & Earth Names Regional Vice President

We are pleased to announce Marco Vicente Silvestre, P.E., has been promoted to Regional Vice President.

Marco joined the team as branch manager for the Tulsa, Oklahoma, branch more than nine years ago. Over time, He has expanded his role and responsibilities as the Tulsa branch operations have expanded throughout Oklahoma. The branch now has satellite offices in Oklahoma City and Durant. Prior to his appointment as Regional Vice President, Marco served as Regional Manager for the Oklahoma and Arkansas branches. Marco holds a professional engineer (P.E.) license in both Oklahoma and Arkansas. Originally from The Netherlands, Marco attended the Delft University of Technology, majoring in engineering geology.

In his new role, Marco will oversee geotechnical, environmental and construction testing services for the Oklahoma and Arkansas branch offices. He will work with branch managers to develop and implement strategic plans that will guide the direction of each branch. His day-to-day responsibilities will involve leading and directing branch managers toward implementing corporate safety policies, providing professional engineering oversight, mentoring and developing leadership and implementing the company’s vision, mission and overall direction.

“Marco has proven himself to be an effective leader and strong planner,” said Mark Nolen, P.E., Building & Earth Senior Vice President, Branch Operations. “He sets clear, meaningful, challenging goals and expectations that are aligned with those of the overall organization. He also creates a positive work environment where branch personnel are motivated to do their best.”

Building & Earth Announces Arkansas Branch Promotions

Building & Earth is pleased to announce the promotions of Curtis Osier to Branch Manager in the company’s Little Rock, Arkansas, branch, and Michael Evans to CMT Department Manager in the company’s Springdale, Arkansas, branch.

Curtis has been a leading team member in the Little Rock branch for over 3 years. He earned his bachelor’s degree in agriculture, food and life sciences and a master’s degree in soil classification from the University of Arkansas in Fayetteville. He has over 20 years of industry experience. In his new role, Curtis will be responsible for leading a team of engineers, geologists and engineering technicians to provide reliable construction materials testing and special inspection, and innovative and practical geotechnical consultation services for our clients throughout central and south Arkansas.

“Curtis’s broad experience in the construction industry, strong communication skills and passion for the profession is instrumental in client and market sector diversification and the continued growth of the branch office,” said Regional Vice President Marco Silvestre, P.E.

Michael joined Building & Earth as an assistant project manager. He has over 20 years of combined experience in construction materials testing (CMT) and construction management. In his new role, Michael will be tasked with all scheduling for technicians and lab assignments. He will also have direct, regular communication with Building & Earth’s clients.

“We’re glad to have the benefit of Michael’s expertise and talent on our team,” said Springdale Branch Manager Kevin Hodges, P.E. “We look forward to seeing the progress we make in Arkansas with Michael in this role.”

Building & Earth Completes a Project Milestone with Kansas Wind Farm

Building & Earth recently completed work providing quality control materials testing and inspections on the Cimarron Bend wind farm project, located in Clark County, Kansas.

The project is Building & Earth’s nineteenth wind project overall and our largest project to date.

Project plans included the construction of about 200 wind turbines, associated access roads and underground collection lines, substations and a 13.5-mile-long overhead transmission line.

Electricity generated by Cimarron Bend will be sold to Google and the Kansas City Board of Public Utilities under two, 200MW bundled, long-term power purchase agreements (PPAs).

Learn more about the project by clicking on the links below.