Retaining Wall –What Are My Options?

Retaining wall are walls which retain soil or soil banks and are used to create usable space which cannot be done by sloping the ground surface because it will fail.

There is a wide variety of ways that retaining walls are constructed. The most common ones which are used are in general order of cost are reinforced Soil Sloped Walls, Steel Sheet Pile Walls, Concrete Modular Unit Gravity Walls, Soldier Pile, and Lagging Walls, Mechanically Stabilized Earth Walls, and Cast in-Place Reinforced Concrete Wall.

From the least to the most expensive wall type there is over a 5-fold difference in the installation costs. In addition to costs, the other factors that are considered include the wall systems workability with the site and the appearance of the wall. May be considered in the selection of the wall.

Example photos of these different types of walls. In the selection of the most appropriate retaining wall type for a project, a geotechnical engineer with sufficient experience in this area should be consulted. For more information on reinforced soil slope walls see Engineering Updates 39 and 42.

Causes for Building Damage

There are a host of potential causes of building damage, however, they can be broken up into three primary categories. Those primary causes are from: environmental conditions, material/structural defects, and ground movements.

The environmental causes can be broken into mainly two subcategories: climatic and seismic (earthquakes). Climatic conditions which can cause damage to the structure are primarily in the form of temperature, wind, and precipitation. More typical examples of building damage from climatic conditions consist of temperature straining, material freeze and thaw, wind damage, and catastrophic weather (e.g. hurricanes, tornadoes, flooding).

Environmental

The second category of material or structural defects consists of flawed constructed materials or elements which result in unintended damage. The two main subcategories of this cause of damage involve improper design and defective installation. Examples of damage from improper design could involve underestimating building loads, missed load or deformation concentrations, inappropriate building elements, poor run-off drainage, and selection of improper materials such as wrong concrete type. Defective construction could involve, for example, poor honeycomb or weak concrete, missing or faulty welds, missing reinforcing steel bars, curing cracks due to improperly poured concrete, etc.

Material Defects

(Photo Credit: Gopal Mishra)

https://theconstructor.org/practical-guide/honeycombs-in-concrete-and-remedies/6889/

(Photo Credit: Caitlin McCabe)

https://www.inquirer.com/news/water-damage-new-home-construction-regulation-bill-pennsylvania-suburbs-philadelphia-20190329.html

The other primary cause is related to ground movements. The different subcategories of ground movement which most typically result in building damage include: soil settlement, soil/rock heave, landsliding, land subsidence, and earthquake shaking and associated ground failure. Land subsidence damage mainly involves sinkholes and surface depressions in karst terrain, from underground mining, and settlement from soil collapse from water saturation. Although earthquake shaking alone can result in building damage, foundation failure can also result from ground failure in the form of settlement, soil liquefaction, and landsliding (including lateral spreading). Please refer to the following blogs for additional explanation: “Causes for Building Settlement”, “Landsliding: What to Do”, “What is Karst Subsidence”, “What is Mine Subsidence”, and “Causes for Building Uplift”.

Ground Movements

If confronted with building damage, it is important to contact the appropriate experienced forensic engineer. In most cases, a qualified structural engineer would be the most appropriate initial investigator to assess the nature of the damage and provide the proper direction to determine the cause.

If MEA can assist you with your building damage problems, please contact us at 314-833-3189

Causes for Building Uplift

The most common sources of building uplift and resulting damage are expansive clay soil or rock. These soils and rocks have the ability to lift buildings when the swell pressure exhibited by these clayey materials exceeds the foundation loading. This is why flatwork, like on grade concrete slabs which are lightly loaded, can be more susceptible to uplift.
What makes the clay soil or rock have significant swell potential is its water absorption capability. The greater the absorption capability, the greater the amount of potential building uplift. The absorption strength depends on the clay mineralogy content, the density and the amount of moisture in the soil or rock. For example, a relatively dry, densely packed clay soil or rock which contained a significant amount of expansive clay particles, would have a very high swell potential.
Therefore, wetter materials such as from a possibly wet climate or surface drainage and higher groundwater tables can limit building uplift. Conversely, under drier conditions uplift can occur in such materials where access to water increased. More common examples are where surface drainage has been rerouted to, and/or landscaping has been removed (removing roots and allowing more ground absorption) from the foundation area.
In colder climates, another source of building uplift would be from freezing soils. This results when the building foundation is placed above the frost depth in the soil and subsequently the soil moisture freezes beneath the foundation, resulting in heave during cold weather.
Building heave can be mistaken for building settlements or possibly other causes. If an investigation is merited, it is recommended that a qualified geotechnical forensic engineer be consulted.
If MEA can assist you with your building uplift problems, please contact us at 314-833-3189.

Causes for Building Settlement

The most common causes for building settlement are from underlying deposits of compressible fill or native soils. Compressible soils which are under unchanged building foundation loading cause settlement to start immediately and taper off over time. Therefore, if the settlement is not noticed until much later in time, the presence of compressible foundation soils is not likely the culprit. One cause, which can result in building settlement at any time, would be the shrinkage of plastic clay soils. These clay soils will shrink when they “dry out” and are problematic where they are subjacent to the foundation and have significant initial moisture. Shrinkage of foundation clay soils is typically associated with added landscaping which causes water to be “sucked out” of the soils.

Another fairly common source of settlement are foundation soils that can collapse when exposed to moisture. Therefore, settlement of the structure would be noticeable after significant precipitation and is likely to occur early after and even during construction. Soils which would exhibit this behavior are loose, drier fine sands to silts. More common in colder climates, another typically early post-construction source is thawing soil. More specifically, building settlement results from thawing of frozen soils left below the foundation.

Two other more typical causes are less time dependent but are location dependent. These are building settlement from land subsidence in karst terrain and underground mining. In other words, there are only certain regions where either karst conditions and/or underground mines are present. These karst and mine subsidence events may occur at any time. These land subsidence events are discussed in blogs entitled “What is Karst Subsidence” and “What is Mine Subsidence”.

There are some causes of building settlement which are more directly identifiable. These include from underground tunneling, structures next to temporary or permanent yielding retaining walls, earthquake shaking of mainly loose fine sands which can contain some silt, and high extraction underground mining which causes immediate ground collapse.

Red herrings of building settlement, even to the professionals, can be building foundation heave, and from subtle landsliding. Landsliding is discussed in “Landsliding What to Do” and building heave will be discussed in an upcoming blog. Where the building damage is apparently from settlement but requires proper investigation a qualified geotechnical engineer expert in forensic analysis is recommended.

If MEA can assist you with your building settlement problems, please contact us at 314-833-3189.

FIGURE 1: SETTLEMENT DAMAGE FROM MINE SUBSIDENCE

FIGURE 2: SETTLEMENT DAMAGE FROM KARST SUBSIDENCE

FIGURE 3: FOUNDATION SETTLEMENT OF A BUILDING IN MEXICO CITY
(Photo Credits: Tim Leffel) https://www.perceptivetravel.com/issues/0409/mexico_city.html

What is Karst Subsidence?

Karst subsidence is land subsidence that is caused by cavities or voids in the underlying bedrock which collapse or from soil filling them in from above resulting in surface subsidence. Under normal circumstances, the voids or cavities were created by the flow of groundwater in fractures in soluble bedrock over a great deal of time. The most significant land subsidence effects occur over voids which have been solutioned in limestone bedrock but also result in other soluble rocks such as dolomite, gypsum, and halite. The most typical land subsidence results from groundwater draining downward into these solution voids carrying soil particles with it. This results in the ground settlement in the form of a sinkhole to a more gradual depression on the ground surface. Therefore, when downward drainage of groundwater is caused into open bedrock voids, the potential for subsidence results. Some more common triggers are: unlined surfaced drainage trenches, pumping of water wells, quarry pit dewatering and retention/detention ponds.

Figures 1 and 2 are examples of this.

FIGURE 1: SINKHOLE CAUSED BY DOWNWARD DRAINAGE FROM DEWATERING OF NEARBY QUARRY PIT

FIGURE 2: IRREGULAR DEPRESSION WHICH FORMED FROM DOWNWARD SEEPAGE OF WATER STORED IN A RETENTION POND

For more information see: Risk Investigation of Karst on Sinkhole/Subsidence Prone Land.

What to Look for when Selecting a Geotechnical Engineering Company

If you have a project which requires geotechnical engineering how can you determine which company best suits your needs? The most common means used to find the most appropriate company for your work is to ask an associate, colleague, or even a friend who is some how connected to your area of inquiry. If a company is found by some other means, or even such a company(s) is suggested, that company should be further vetted. Some of the more relevant company information to obtain when determining the geotechnical engineering company to select are:

  • How long has the geotechnical engineering company been in service?
  • Does the company specifically do the geotechnical engineering work you need done?
  • Given their expertise, are they reasonably local, especially if there is significant site work involved?
  • Experience in serving your industry

The last point can be over looked. Companies that have experience servicing your industry generally understand your needs and wants and therefore can more intuitively anticipate critical site conditions which may otherwise not been properly emphasis, or even been ignored.

Geotechnical engineering companies service different entities. This includes contractors, commercial property developers, government agencies, mining companies, homeowners, petroleum related industries, power industry, and co-professional (architects, civil engineers, structural engineers, and environmental engineers). These different entities have different needs and wants.

In addition to having an appreciation for the industry it is more important, however, to have sufficient expertise in the specific areas of need and circumvent industry service.

However, when problematic, unexpected geotechnical conditions has resulted at the site, one is looking to perform a Geoforensics investigation and remediate the unwanted conditions. Examples of this are unexpected earthwork difficulties, excessive structure settlement or heave, earth retaining structure excessive movement, dam failures, pavement damage, and land sliding. If there is potential, this situation may result in some form of dispute resolution. Then this work becomes more individual dependent having the unique abilities of an expert witness. See finding such an expert please refer to What to Look for in a Geotechnical Engineering Expert and Traits to Dig for In An Engineering Expert.