Retirement dreams require a firm foundation — Ground Truth
266
post-template-default,single,single-post,postid-266,single-format-standard,ajax_fade,page_not_loaded,,qode-title-hidden,qode_grid_1300,footer_responsive_adv,hide_top_bar_on_mobile_header,qode-content-sidebar-responsive,qode-child-theme-ver-1.0.0,qode-theme-ver-16.7,qode-theme-bridge,disabled_footer_bottom,wpb-js-composer js-comp-ver-5.5.2,vc_responsive

Retirement dreams require a firm foundation

“Tilted Dreams; Lakeside home settles half a metre”

Jerry and Emma had plans for a dream home on a lakeshore property in cottage country. Emma’s family already had a place on the lake and she spent her summers there as a child. When a virgin parcel of land went up for sale, she and Jerry jumped at the opportunity to purchase it.

The lot was not ideal, as it had a marshy shore, but the opportunity for a spot on Emma’s childhood lake trumped other considerations.

They designed a beautiful three-story stone and log home.

The builder believed that pile foundations might be needed, and suggested that they seek the opinion of Down to Earth Inc., a local geotechnical engineering firm.

Down to Earth examined the site and suggested that test pits be excavated to determine the depth down to good ground for building support.

Jerry and Emma were excited at the start of work. They showed up for the first ground-breaking work with the engineer.

The test pit excavation was messy and smelly – dark black muck with a strong odour was found below the chosen spot for construction.

Down to Earth had a simple and practical solution for Jerry and Emma’s dream home foundation – remove the peat and black muck, and replace it with sand and gravel, placed and compacted up to the level of the ground floor. On top of the compacted granular fill, a reinforced concrete slab-on-grade would be built and the dream home would rise from there.

The dream home was built, and Emma and Jerry were thrilled. It was beautiful. The builders were fast and they began to enjoy their winter in front of a grand stone fireplace that had a hearth on both the ground floor and second story.

However, within a few months, the house seemed to be tilting and the stone-clad ground floor walls had started to crack. Within a year, their dream home was tilting by 6 inches!

Geotechnical engineers, now with Ground Truth Engineering, were called in to explore the problem. They discovered that though all of the peat had been removed, as Down to Earth recommended, there was a deeper problem that would require that the cottage be temporarily moved and piles be placed. The cost of this remediation would be more than half the of the original cost of construction.

Jerry and Emma’s retirement dream was spiraling into a nightmare.

Down to Earth deflected calls to their lawyer. Jerry and Emma’s lawyer initiated a claim against Down to Earth, but they quickly learned the legal progress would be measured in years, not weeks. Fortunately, they did not lose use of their home, as there was no immediate structural danger.

A program of periodic surveys and inspections provided assurance that the structure was safe, but also showed that it continued to settle. While they were not at physical risk, the inconvenience of living with the tilt, the mounting legal, engineering, and repair costs, and the uncertain outcome of a dream turned sour took an emotional toll. Even the emotional calm from a relaxing hot bath was diminished when it was necessary to bail out the puddle at the low end of the tub created by the tilt.

Several years after observing that first crack in the stone, Jerry and Emma had their “day in court.” The result was bittersweet – they were awarded the full amount under Down to Earth’s liability insurance policy, but it was not sufficient to cover the full repair cost. With the costs of a growing family, Jerry and Emma could not sink more of their own money into the property, so they invested the damages award and tolerated the leaning cottage until they could afford the re-levelling work.

Two decades passed, during which Jerry and Emma came to accept their imperfect house. When they were grandparents and ready to retire, they started to think about how nice it would be not to have to continue bail out the bath tub once the grandkids were clean. They decided to act and use the saved funds to repair their twenty-year-old dream home at long last.

The settlement had continued, but at a progressively slower rate.  The latest survey showed that the cottage had settled half a metre (about 20 inches) on one side, and a quarter of a metre (about 10 inches) on the other.

Repairing the building meant lifting and moving the two upper log stories. The concrete foundation and stone ground level was completely demolished. The site was leveled and built up using polystyrene foam blocks – a solution that did not add any additional weight, so avoided the original problem.

Then the stone and concrete ground floor was re-built, and the upper two log stories were placed back on top.

At long last, Jerry and Emma have the home of their dreams, better than new!

 

So What Happened?

Sampled boreholes were drilled on each side of the house after the settlement was first observed.

The boreholes demonstrated that underneath the pad of compacted granular fill that the builders had used to support the slab-on-grade house foundation, there were deposits of light brown marl (a soft, shell-rich silt) and soft grey clay. These deposits were in turn underlain by bedrock that was 15 m to 20 m below ground surface.

Laboratory tests demonstrated that both the marl and the clay were highly compressible and that the weight of the house, and more significantly, the weight of the granular fill placed at the site, could induce a settlement of 300 mm to 500 mm over time.

The range in settlement magnitude from one side of the home to the other is due to differences in the thickness of compressible material under each side of the house (the thicker the compressible material, the greater the amount of settlement) and differences in the compressibility properties from one side of the house to the other (the soils were slightly more compressible on the lake side).

Ground Truths

Earth Fill is Heavy

The granular fill used to support Jerry and Emma’s dream home weighed more than 2000 kg/cubic metre – about twice the weight of the compressible peat that was removed from below the building footprint. In fact, the total fill that was placed also far exceeded the weight of the home itself.

Thus, the settlement of the home was primarily due to the weight of the granular fill that was placed to support the home’s slab-on-grade foundation. The weight of that fill caused compression of the underlying marl and clay deposits.

Soil Classification is a Tactile Process  

Down to Earth made a critical error during their investigation. They used a backhoe to excavate a test pit at the site and identified problematic peat soil immediately below ground surface. That was a good first step.

Their error occurred when they got below the peat. Down to Earth observed a light brown soil that looked like silty sand; however, they did not take a sample of that light brown material, touch it, or remould it.

Had they done so, it would have been apparent that the light brown material below the peat was not silty sand, but instead a marl deposit, composed of compressible silt, fine ground shells and organic matter – material that was unsuitable to support the compacted granular fill and the home.

Site Investigation in an Iterative Process  

Down to Earth’s choice of a test pit investigation was appropriate for the initial site investigation. Elsewhere on the site there were bedrock exposures, and test pits are usually an excellent, cost-effective means to delineate variations in shallow bedrock elevation across a site. However, for Jerry and Emma’s dream home, the bedrock was too deep to be properly delineated with test pits.

Had the marl deposit been accurately identified during the test pit investigation and efforts made to advance below it, it would have soon become apparent that competent found material could not be reached using a small backhoe at the site.

Down to Earth’s next step should have been to recommend that the test pit investigation be followed by drilling of one or two boreholes down to a competent bearing stratum; in the case of Jerry and Emma’s home the competent stratum would have been encountered at depths of 15 m to 20 m below ground surface.

This extra work would have been initially more expensive, and would have indicated that a more expensive foundation was necessary for the home. However, the added investigation and foundation construction investments would have paid for themselves 20 times over by avoided the expensive repair and the 20 years of anxiety and frustration from living with a crooked dream home.

No Comments

Post A Comment