Do We Really Need a Soils Report?
The quick answer is… Yes!
Occasionally we will have a client who wants to build a small professional or commercial building. Sometimes this is a one-time client like an accountant, doctor or dentist with little or no experience with construction. We always act as guides for our clients, but in this type of situation it is critical that we advise them correctly concerning everything about the design and construction process. One critical item is a properly designed foundation for the building. Everyone knows that an insufficient foundation can lead to major problems with even a small building. No one wants to have to deal with a failed foundation on a building. To accomplish this proper foundation design, a soils report is critical information.
What is a Soils Report?
A soils report or geotechnical report is an engineering document that explains the type of materials found under the surface of the natural grade of a proposed building site. The steps to create, process and utilize this type of information are very simple. A geotechnical laboratory will send a large drilling rig to the site and obtain drilled core samples of the soil conditions underneath the surface. Depending on several factors, the soil samples can be representative of 10 feet to 100 feet below the surface. The number of multiple boring locations varies depending on the size of the building. The laboratory then conducts tests on the soil samples to determine the characteristics including moisture content, composition and load bearing strength. The written report is then made available to the project’s owners and any others involved.
Why is the Soils Report so Important?
I once heard a story about a very famous real estate developer who owned numerous large projects in the North Texas region. As a young man his father had asked him to build a very long fence on their ranch. The father had agreed to pay him a set amount to drill the fencepost holes, set the posts and hang the barbed wire for about 1000 feet or more. The young man ventured to get this done. After a few fenceposts he realized that the ground contained a substantial number of rocks and boulders. This slowed him down considerably and tripled the time he had estimated to complete the project. After he completed the fence, he complained to the father about the subsurface conditions. The father told him, “Let this be a lesson that you always need a soils report before you bid on a project”.
The story about the young real estate developer above is a simple illustration but it exposes some hard facts. To understand the needs of a project the architects, engineers and contractors all need to have detailed knowledge of the conditions of the subsurface soils. There are also some large liability issues for the professionals as well as the general contractors. To limit their potential liability, they need to have the geotechnical laboratory share in the investigation and analysis of the subsurface conditions. The professionals also need the soil’s characteristics to determine the best design of the foundation.
How is the Proper Foundation Design determined?
It all starts with the quality of the dirt. The types of soil vary considerably over the surface of the earth. They can vary widely even on a single building site. Therefore, it is imperative to fully understand the characteristics of the soil upon which a building foundation will be constructed. Some soil types are very stable, like certain sandy materials and rock. Other soils that have a large percentage of clay materials are very problematic because they expand and shrink with the amount of moisture in them. Soils that vary in volume can cause problems for certain types of foundations. A report on the soil will state in detail the types of soils found and their characteristics. The report will also make recommendations regarding any soil treatments needed to make the existing soils more stable. The architect and engineer will then decide which type of foundation design will work best for this building on a specific site.
A Tale of Two Towers
Two examples of failed foundations will serve as examples as to the need for a comprehensive soils report from a geotechnical laboratory. The first tower will be very familiar to you. It is the Campanile for the Pisa Duomo or better known as the Leaning Tower of Pisa. The height of the tower on the high side is 185 feet 11 inches. It currently is inclined at about 3.97 degrees from true vertical. Construction of the tower began in 1172. Once construction reached the second floor in 1178 it began to sink. This continued until the tower was completed in 1372. In 1990 the tower was closed and significant structural remedies were made to ensure the tower would not collapse; it reopened to the public in 2008. The cause of the original sinking was due to soft soil and an inadequate foundation that was only 9 feet deep. Of course, no soils report was available at that time. However, an experienced competent architect should have anticipated the problem and provided a deeper, more robust foundation.
The second tower example is a modern high rise in San Francisco, California. Millenium Tower at 301 Mission Street is a 58-story, 645-foot-tall luxury residential building. The tower was completed in 2009. It was determined in mid-2016 that the building was sinking on one corner resulting in a significant tilt. The latest measurements indicate the building is out of true vertical by 28 inches. This is only about 0.25 degrees; however, residents can sense that their floors are not level. Balls placed on the upper floors will “roll” without anyone pushing them. Although many accusations and lawsuits are still active, the consensus is that inadequate foundation piles were used in the tower’s foundation design. The soils in this portion of San Francisco are soft mud with the bedrock over 200 feet below the surface. In 2020, some remedial underpinning of the foundation was accomplished to stop the settlement. However, the building is still at a 0.25-degree tilt. So, in this case even though there were soil reports available the “Proper Foundation Design “fell short.
The Soils Report is a Vital Component of a Building Design.
It does not take a lot of review and analysis to understand that an accurate and thorough soils report is a vital component of a building design process. We have had clients tell us, “Just give us a heavy slab on grade foundation”. In many instances that may work just fine. However, it may not be adequate in numerous locations. The fact is you just do not know until borings are made and analyzed. The problem is not always just “soft” or “expansive” soil. In some locations there may be a high-water table which means you will have trouble with ground water if you need to install drilled piers to bedrock. So, we will always recommend to the client that a competent geotechnical laboratory perform and complete an accurate soils report before a foundation design is completed. We do not want any surprises like our young real estate developer who had to dig the post holes into rock.
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