Geotechnical engineering is one of the five main branches in civil engineering, which can further be categorized into soil mechanics, foundation engineering, retaining structures and excavations. Geotechnical engineering is a branch that interacts with all other branches, after all, every structure needs foundation system. Geotechnical engineering failures are usually among the costliest to fix, if the fix is possible at all in the first place.

Before we discuss failures, we should briefly describe how the process works in geotechnical engineering, which is something not many people may be familiar.

The process in geotechnical engineering starts with a site investigation, after the site is decided on by the owner of the project. The goal here is to come up with an adequate report that will inform everyone (stakeholders in the project) – and primarily the structural engineer – about the geotechnical site conditions with a detailed report, which is called a geotechnical report or commonly “soils report”. To make this site investigation a wide range of methods are employed as applicable, such as using previously made documents relevant to the area, visual site inspection, trenching, boreholes and field and laboratory tests. At the end, a detailed report is prepared by the geotechnical engineer that includes information about all relevant items such as the earth/rock material characteristics of the site and their relevant engineering properties that can be used in foundation system calculations, geotechnical engineer’s comments and recommendations. After this point, the structural engineer and geotechnical engineer can finalize the design of the foundation system. This very briefly summarized route in this paragraph is prone to errors or inaccuracies every step of the way, which is mainly a result of dealing with natural site and material conditions, unlike human-made materials and conditions which are far better controlled.

Below we will not distinguish the items by structure types, but will make a general list since most apply to a majority of the structures.

Some common failure items in geotechnical engineering include:

Excessive settlement of foundations may occur due to either overloading, inadequate foundation system details or dimensions or supporting earth material of inadequate strength. Note that we said “excessive”. This is because all structures settle even if it is a small amount. Our goal is to keep this within tolerable limits. The bearing strength of the supporting material is the key here. This is often the first thing that a structural engineer looks at in a geotechnical report.

Differential settlement of foundations is usually even worse, because in this case part of the structure settles more than another part of it, which causes undue stresses and deformations within the structure. For example in a building you may see this as cracked walls, ceilings. Doors and windows may not close. Parts of structure may even visibly tilt. Some members may start to show yielding signs…

Ground improvement is an important part of foundation engineering and soil mechanics. Through ground improvement, we improve mainly the strength and stiffness of the soil, and also reduce its permeability. Ground improvement basically makes it possible to use lighter, cheaper foundation system, and / or build heavier structures. Depending on type of soil and the estimated structural loads, a wide variety of ground improvement techniques are available. The decision about ground improvement and the foundation system are made simultaneously to find the optimum as far as cost and schedule. Any errors during this estimation or application of the proposed method during construction opens the door for multiple modes of failures.

For foundation material, sands are highly preferred over clays, due to their higher strength, stiffness and permeability. When we build on expansive soil such as clay, which is not preferred but due to cost reasons cannot always be avoided, soil will tend to undergo expansion, upon increasing moisture. This is another common failure that needs to be addressed during design phase. During construction phase, the compaction must be done per specifications such as 95% of max dry density, and this must be verified by tests. This compaction is not only to prevent expansion but actually mainly to prevent excessive compression after the structure is loaded. We commonly call clay as expansive soil but this also means easily compressible and also shrinking soil.

Sands also have a weakness though, because when they are shaken, they tend to settle. For example, if we have (water) saturated sand in an earthquake zone, which is also loose (not dense), upon cyclic shaking of an earthquake, it will start to settle, but before that it effectively turns into a liquid, which is called liquefaction which can cause entire structures to sink even if structurally they may remain intact (actually a “good” side of liquefaction is that it also serves as a seismic isolator at the very bottom of the building, by absorbing a huge amount of earthquake energy, before it even enters into the structure to cause damage, but of course we do not design for that! – plus this means considerable settlement of whole or part of structure even if the structure itself may remain undamaged). If a site with liquefaction potential cannot be avoided, we must perform ground improvement there, with one of the appropriate techniques to eliminate liquefaction risk.

Soil Structure Interaction (SSI) must be considered for earthquake prone sites especially when the structure is too stiff with respect to the ground. In other words, there is considerable, nonnegligible soil structure interaction with respect to a natural (free) ground surface without structure on it, when we have a structure that is heavy and stiff, on a relatively soft ground. Taking SSI into account during earthquake analysis of structures is a complex task, which is something only highly specialized engineers / teams can properly perform.

Another type of soil that exists between clays (very fine grained) and sands (coarse grained) are silts. Silts are prone to freeze-thaw cycles in cold climates. This is one of the main reasons that footings are buried at least a certain depth into the ground (other reasons are obtaining typically higher bearing strength and to protect the footing against damages). The colder the climate, the deeper minimum burial distance.

Another important part of geotechnical engineering is the design of retaining structures. Here it would be a good fit to give example from an anchored wall as it has quite a few, easily to visualize failure modes. For example an anchored retaining wall can fail as a result of: The anchors slipping off the ground, failure of anchor system in the ground, yielding of the wall material, global failure of the whole anchor/wall system together as in an overturning landslide, yielding of anchor caps or bearings, excessive settling of the wall due to poor support at its foundation, soil yielding in front of the wall at the lower side. As you see, listing the failures of a system helps us understand a structure much better.

The (lateral load) demand on a retaining wall will dramatically increase, when we have a structure on the retained side not far from the wall imposing load to the retained earth (called surcharge load). Similarly we must avoid water accumulation at the retained side, as it makes the retained soil considerably heavier, which translates into greater lateral load to be supported.

Piles (deep foundations) work by their bottom end (tip) and / or skin friction resistance through its side face. Failures may happen due to yield of pile material (not common), inadequate tip or skin resistance. During seismic events, top of piles, which are fitted into a pile cap often together with other piles, experience lateral force and moment. The piles must be designed by taking these into account, by modeling the soil around the piles (in the form of springs with certain stiffnesses, arranged to represent the ground conditions along the length of pile) with acceptable accuracy, which is not often an easy thing to do.

Lateral loads on piles can happen due to many reasons including seismic action, wave action in marine structures, lateral impacts on the structure such as a bridge, centrifugal force form vehicles on a curved viaduct, retained material’s lateral force at the foundation of a retaining structure… Inadequate lateral support can cause major failures of the structure entirely or partially. Pay attention that most loads we listed in this paragraph are also dynamic except the last one which is static. Dynamic nature of loads bring extra challenges.

For shallow foundations, base sliding can occur during lateral movements of earthquakes. There must be adequate friction between the footing and the soil, plus the passive resistance of soil in front of the vertical face of the footing.

In this post we tried to give a brief overview of geotechnical engineering related failures by listing some commonly encountered examples, which hopefully serves as a useful summary that lists many relevant items in one place, and a solid starting point for further exploration.

Reference:“How to Construct: Introduction to Civil Engineering, Structures and Construction” – A. Tuter, August 2024

Post By: Ahmet Tuter

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