Ask an Engineer: Geotechnical Engineering Inspection – Worth the Cost?

Helping our clients make wise investments is part of what we do.  Often homeowners and contractors want to know, “Is a soil engineering inspection really necessary?” and “Can I just look at it myself?”.  One builder asked us if he could bring us the soil from the bottom of the footing trench so that he could save on the cost for the geotechnical engineering footing inspection.  In the long run, it is actually most cost-effective to invest in a geotechnical engineering inspection.  This case illustrates why.

The “Great Wall”

A builder of a mini storage wanted to build a keystone retaining wall to enhance the aesthetic appeal of the project along a highway.  However, he chose to not invest in an soil engineering inspection.

He hired a soil engineer to perform a limited study with minimum scope to determine bearing capacity and soil friction angle.  Then, the information was given to the retaining wall system design engineer.  The soil engineer recommended inspection of the footing subgrade and installation of geogrid.  Contrary to the recommendation, the builder opted out of the engineering inspection.  By skipping the geotechnical engineering inspection, he appeared to save about $1000.

Within a few years, the wall started to fail due to settlement of the foundation on loose soils.  The settlement caused the face blocks of the wall to fall apart.  As time passed, the whole retaining wall sagged due to insufficient pull-out resistance, which was likely caused by improper installation of the geogrid.  Eventually, the entire retaining wall had to be completely removed because of unsafe conditions.  The cost of removal was reportedly 1 million dollars.

A geotechnical engineering inspection completed at a fraction of the cost enables our clients to avoid these problems which regretfully culminated in a removal cost of 1 million dollars for the builder.

Ask a Geotechnical Engineer: Compaction Testing – Just the Top?

Sometimes we are asked if we can “just test the top one foot of pad subgrade” and nothing more.  The reasoning we hear is that “it will bridge over the soft soils, and it should be fine.”  However, achieving an accurate assessment of site conditions necessitates that we test to the full depth of the fill, and this case demonstrates why.

A homeowner in Fresno purchased a finished lot and had a custom home builder construct a house for him.  During construction, when the house framing, roof, and stucco were completed, a large portion of the house started to settle.  The amount of settlement was up to four inches, with major cracking of slabs and footings.

Compaction tests were taken on the subgrade soil in the area of the cracked slab.  The subgrade compaction was above 90 percent and met the CBC code requirements for engineered fill.

Further investigation was performed with auger borings and deeper compaction tests.  The findings revealed that the site had two feet of compacted engineered fill.  Below two feet, a layer of loose, uncompacted, dry soils was present with thickness ranging from four to six feet and relative compaction ranging from 66 to 84 percent.  These loose soils settled significantly by hydrocompaction through surface water infiltration.  To mitigate the problem, the house had to be supported by underpinning or mini-piles.  A repair cost of around a million dollars was estimated.

This problem and subsequent repair cost could have been avoided if the compaction tests were taken to the full depth of the fill instead of just the top two feet.  Furthermore, the conditions of loose fill could have been discovered if a soil investigation was performed prior to building construction.