Forensic Engineers and Consultants

Archive: Industrial

When the Walls Come Tumbling Down… Retaining Wall Basics

A wall is really boring until it fails. A retaining wall is supposed to hold back soil to either support a structure or keep a space clear. When it fails, both of those roles are compromised. A retaining wall does not have to collapse to fail. In fact, a failure is perhaps better defined as when the wall does not perform as expected.

Retaining Wall Photo Carlos defect

A retaining wall is a structure that is designed to provide lateral support for soil or rock and vertical loads. For example, a basement wall supports both lateral soil loads and the vertical loads of the walls, floors and the roof. Conventional retaining walls are gravity walls, counterfort walls, cantilevered walls and crib walls. Gravity retaining walls are typically built of plain concrete or stone, and the self-weight is a mechanism to resist failure from overturning and sliding. Counterfort walls include a footing, wall stem and vertical ribs known as counterforts that tie the footing and wall stem together. Crib walls are made of interlocking concrete members that form cells that are then filled with compacted soil. There are also proprietary systems such as prefabricated concrete T-walls.

There are several types of cantilevered walls, with common features being a footing that supports a vertical wall stem. Cantilever wall shapes can be T-shaped, L-shaped, or reverse L-shaped. To prevent the buildup of hydrostatic water pressure on the retaining wall, clean granular material is good practice for backfill and a geofabric wrapped pervious drainage system is included at the heel of the wall.

Retaining Wall Photo CarlosCOMMON CAUSES OF FAILURE

There are many different reasons why retaining walls fail, such as soil bearing, sliding, overturning, inadequate design, improper construction, or unexpected loadings. Other causes of failure include the following:

  1. A typical cause of failure is when the wall is backfilled with fine grained soils such as silt or clay that have a low shear strength. They also have a low permeability. As a result, they can become saturated with heavy rainfalls and overload the wall. The clay backfill can also exert swelling pressures on the wall. The swelling pressures develop when water infiltrates the clay backfill that was originally compacted to a high dry density at a low moisture content.
  2. To reduce construction costs, soil available on site is sometimes used for backfill. These soils have a lower strength and permeability than clean granular material. Using in-situ available soil, rather than importing granular material, is perhaps the most common reason for retaining wall failures.
  3. Detailing errors, perhaps of the steel reinforcement, can lead to misinterpretation by the contractor.
  4. Foundation problems can be prevented with a geotechnical investigation. A soils report will provide criteria for design such as, the allowable soil bearing capacity, friction factor for sliding resistance, seismic, expansive soil or potential liquefaction. If a soils report is not provided, building codes provide minimum recommend values for bearing capacity, friction resistance, and allowable passive pressure.

Whether the retaining wall is tilting out of plumb, cracking, or collapsing all together, the reasons for these types of failures may be the result of a combination of causes. When in need of a retaining wall expert to provide a failure analysis in the event of a construction defect, contact Warren and let us help you get to the facts.

Carlos Zarraga has more than 8 years of engineering experience in the structural field specializing in building design, building components and foundation design.  Carlos has designed and analyzed structures, supervised designers and drafters, prepared construction documents and provided on-site duties for field supervision and inspection of construction projects. Certified in RISA 3D, RISA Foundation and RISA Connection, he is well versed in the analysis of foundation failures.   He often determines the root cause of failure and the resulting scope of damage.  He has designed retrofits to existing structures in addition to repairing construction defects.  He also has experience in the industrial and petrochemical industry designing structures for materials handling facilities and industrial buildings.  Carlos holds a Bachelor of Science in Civil Engineering from the University of New Orleans. 

Machine guard for blog

Machine Guarding and Risk Assessment

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The Occupational Health and Safety Administration (OSHA) “Top 10 for 2018” violations once again have Machine Safeguarding earning a position on the list. Machine safeguarding was the 9th most cited standard as noted in the list below:

  1. Fall protection, construction (29 CFR 1926.501)
  2. Hazard communication standard, general industry (29 CFR 1910.1200)
  3. Scaffolding, general requirements, construction (29 CFR 1926.451)
  4. Respiratory protection, general industry (29 CFR 1910.134)
  5. Control of hazardous energy (lockout/tagout), general industry (29 CFR 1910.147)
  6. Ladders, construction (29 CFR 1926.1053)]
  7. Powered industrial trucks, general industry (29 CFR 1910.178)
  8. Fall Protection–Training Requirements (29 CFR 1926.503
  9. Machinery and Machine Guarding, general requirements (29 CFR 1910.212)
  10. Eye and Face Protection (29 CFR 1926.102)

(Source: www.osha.gov/Top_Ten_Standards.html)

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Unguarded Shear point 1 white

Unguarded Shear Point on Force Tester Amputates Worker’s Finger

A worker was injured while testing gas springs similar to the type that hold the hatchback of an SUV open. The hazard that injured the worker was an unguarded shear point. The tester contained a mounting plate that was raised and lowered by a pneumatic cylinder.

The pneumatic cylinder lowered the mounting plate while the worker’s fingers were in the hazardous, unguarded shear point. Read More

Electric Hazard Guardrail photo

Hazards Can Lurk Anywhere … Watch Your Step …

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While on a lunch stop during a recent vacation trip through Tennessee, I happened across a safety hazard that required immediate attention.  The establishment had a raised concrete patio at the front with a steel railing around the perimeter.  At one edge of the patio was a set of stairs with a continuation of the steel railing used as a handrail.  The top edge of the patio had light strings wrapping the top metal bar as accent lighting for the perimeter.  The light string continued down the stair handrail wrapped in the same manner as the rest of the patio. Read More

Orange mini excavator

Case Study: Fatality Servicing Unsupported Excavator Boom

A mini-excavator at a job site developed a leak at a hydraulic fitting at the base of the cylinder that raises and lowers the boom. A subcontractor foreman at the site raised the boom to search for the leak. The foreman found and attempted to tighten the leaking fitting. When he did, the fitting separated from the base of the cylinder, releasing the hydraulic pressure that held the boom aloft. The boom fell and the bucket struck a nearby superintendent for the general contractor.

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Gas Appliance testing lab

Testing As Part of Gas Appliance Incident Investigation

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Equipment and appliances supplied with fuel gases like natural gas, propane and butane are a common and convenient part of most of our lives.  Such devices as gas grills and ranges, ovens, furnaces, space heaters and water heaters usually perform without incident.  However, when they malfunction the potential for incidents such as fires and explosions, carbon monoxide (CO) poisoning and burn injuries may occur. These incidents may be due to design and manufacturing defects in the product, or improper installation or operation of the device.

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box bailer

Defective Vertical Baler Causes Serious Crush Injury to Operator’s Arm

I recently worked on an interesting case involving a box baler. An employee of a butcher shop put some empty cardboard boxes in a vertical box baler and pushed the control switch to compact the boxes. After the 30 by 60 inch platen weighing 851 pounds returned to its raised position, the employee reached into the open space above the bottom door on the baler and began to clear cardboard from the bale tie slots in the bottom of the raised platen. Suddenly, and without warning, the steel pin attaching the platen to the raised hydraulic cylinder rod failed. The heavy steel platen fell and crushed his arm which was outstretched over the baler door into the compaction space.

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An example of a process flow diagram

OSHA’s Process Safety Management – Is This Process Covered?

In February of 1992, the OSHA Process Safety Management (PSM) standard was issued. The official title is: ‘Process Safety Management of Highly Hazardous Chemicals.’ As its title implies, not every facility is covered by this rigorous standard. A process must contain highly hazardous, as defined by OSHA, chemicals above a certain weight threshold, again defined by OSHA. Notice that this is a process by process determination, so there could be certain processes at a manufacturing facility that are not covered by this standard situated beside other processes that are.
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A corroded boiler tube

Boiler Blowdown – It’s Not a Dance Move

When thinking about the safe operation of boilers (and don’t we all?), several systems can readily be named; flame control, fuel/air ratio; steam pressure control, levels in the vessel, etc. What about the water? It seems so passive, as long as there is enough for level control, what’s the big deal? Well, it turns out, that as the steam produced by a boiler is used in the process, the condensate from that steam is returned to the boiler as feedwater. However, since 100% of the condensate is not returned, whatever solids had been in that water before it evaporated to form steam are left in the remaining water.  Fresh feedwater is added to maintain levels, but even fresh water contains some dissolved solids. So over time, the water in the boiler system gets saturated with all sorts of dissolved minerals.
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Figure 1: A typical Ground Fault Circuit Interrupter (GFCI) Receptacle.

Testing…testing… Is this thing on?

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Many people just take for granted that something is just going to work, and in many cases assume that it will work forever.  One such device that does not get enough attention is the Ground Fault Circuit Interrupter (GFCI).   Simply put, a GFCI is a protective device that compares the current flowing on the hot and neutral wires of the circuit and will “trip” to disconnect power to the circuit if a small imbalance of current is detected.  The imbalance of current is an indication of a dangerous alternate path for the current to flow from a damaged line cord or a fault inside an appliance and constitutes a shock hazard to a person. Read More

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