Should You Perform a Geotechnical Investigation Before Building?

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(Frost heave damage | Photo courtesy of The Wallace Group, Inc.)

It is a typical day in the office. As I am performing a plan review for a new commercial building, my phone rings. On the phone is a gentleman that explains he is purchasing land in Bend and would like to build a new home. I request the address and begin an initial review of the area’s geology as we speak. Then comes the frequent questions: “What is your opinion? Do I need to have a geotechnical study performed? I heard that Bend is all rock and I can have my structural engineer design my house using just code minimums.”

My answer? “Yes!” Not only can performing a geotechnical investigation save you money in construction (i.e. higher design values mean using less concrete and steel), but it can help identify geotechnical and geologic hazards which can result in a safer building. Again, saving you money on potential future repairs and fixes.

“But, wait! I though Central Oregon was all rock? Plus, the groundwater is hundreds of feet deep, right?”

Central Oregon Geologic Hazard #1: Differential Settlement

While many areas of Bend have shallow basalt and/or welded tuff bedrock, the surface of the bedrock is highly irregular and unpredictable. The bedrock may be at the ground surface in one location, but could be 15 to 20 feet deep very close by. The ground may appear flat because the underlying erratic basalt surface is infilled with loose sand.

Settlement, or vertical movement of a building, is a function of the strength or density of the underlying soil or rock and the building loads. Settlement of basalt bedrock under typical building loads is negligible because no movement is expected. On the other hand, the settlement of the portions of the building supported on the loose infilled sand can be significant. Buildings need to be designed to accommodate zero movement on rock and the differential settlement caused by adjacent foundation elements bearing on sand. Geotechnical exploration reports will provide estimates of settlement that should be accounted for in design.

Central Oregon Geologic Hazard #2: Undocumented Fill

Don’t forget that Bend was once a former mill town. What goes hand in hand with mill operations? Wood waste. In addition, some portions of Bend were made level by loosely placing soil fill, debris and large boulders.

What are the former uses of the site? A geotechnical exploration will help identify areas of old, undocumented fill by excavating test pits and/or drilling borings and reviewing historical air photographs.

Wood waste decays and compresses over time. It is often covered by soil and not visible from the surface. When wood waste is left below building foundations, the settlement can be significant, causing structural distress. Geotechnical studies in the area have shown that wood-waste related building settlement of two feet can occur within the lifespan of the building.

Other types of hazards associated with uncompacted or rubble fill include differential settlement (cracking within buildings, unlevel floors and difficulty opening doors and windows) and erosion or piping through rubble and boulder fill.

Large boulders, poorly placed and covered with soil, can result in erosion of the cover soil. The soil cover will migrate, with water, to fill the rubble and boulder voids. Often times, old fill needs to be removed and replaced to have acceptable building performance. A geotechnical exploration will identify the depth and location of old fill.

Central Oregon Geologic Hazard #3: Lava Tubes

Do you know what a poor support for building foundations is? Air! While often not visible from the ground surface, large lava tubes (or underground caves) are found in many areas of central Oregon.

A lava tube is a type of lava cave formed when a lava flow develops a hard crust, which thickens and forms a roof above the still-flowing lava stream. The lava river cave near Sunriver is a mile and a half long. During our geotechnical explorations, we have encountered lava tubes from Sunriver all the way to Redmond. A geotechnical exploration can identify near surface voids, and if needed, provide recommendation to mitigate hazards, such as rock bolting, void filling or deep foundations.

Central Oregon Geologic Hazard #4: Groundwater

Just recently, I received three calls within one week from home and business owners because their buildings have water flowing in through basement walls or into crawl spaces. Although groundwater is several hundred feet deep in Bend and Redmond, perched groundwater is often found on the surface of the bedrock where it cannot easily infiltrate. As higher density building occurs, more problems from perched water is expected. The water has nowhere to percolate when the surface is covered by buildings, asphalt and concrete.

Groundwater levels in Sunriver, La Pine and Prineville can be near the ground surface. Construction methods and design values used in Bend and Redmond are not applicable for these areas. Higher frost heave (vertical movement of the soil during freezing and thawing) is also expected with higher groundwater.

A geotechnical exploration report will provide recommendations for waterproofing, drainage and expected frost heave which can prevent costly future repairs.

Central Oregon Geologic Hazard #5: Soft and Weak Soil

While much of Bend and Redmond is covered with relatively shallow bedrock, parts of Sunriver, Prineville and La Pine are former ancient lake beds (think of the soft soil you sink into at the edge of Mirror Pond). Not only are the soils soft and weak, former algae skeletons (diatoms) can be found throughout. Design of buildings supported on the diatoms and soft soil cannot be performed using code minimums because the standard values will be unconservative. Diatoms can be crushed and settlement needs to be evaluated by a geotechnical engineer on a case-by-case basis.

Lisa Splitter, P.E., G.E. is a professional geotechnical engineer with licenses in Oregon, Washington and California. She works for Wallace Group, Inc. in Bend (wallacegroup-inc.com) and can be reached at 541-382-4707 or lsplitter@wallacegroup-inc.com

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