Drainage Failure: Cause and Repair

Posted on 02/09/2010 by Jeff Lineberger, P. E.

Residential ground surface storm drain systems must transport excess moisture away from designated areas where excess water would otherwise prevent the normal use of a residential property.   If rainfall is allowed to pond or collect adjacent to a structure built on expansive soil, the structure may be subjected to unscheduled distress caused by swelling bearing soils due to increased soil moisture content.  Lot surfaces must be graded to drain away from the structure in accord with the International Residential Code R401.3. Likewise, perpetually moist ground surfaces limit recreational use of residential yard areas, and can become a breeding ground for disease carrying insect infestations. Water must freely exit the property to assure unrestricted, healthy, and environmentally safe use of land areas within residential property boundaries.

Drainage Improvement Guidelines and Criteria.

Section 7.3 of the ASCE provides the following non-structural guidelines and criteria for improving or modifying residential storm water drainage systems:

  • Install a Roof Rain Gutter System (ASCE, section 7.34). Uncontrolled roof rainfall runoff can erode the ground surface along the foundation perimeter and provide a source of excessive and non-uniform water input to the foundation perimeter beam bearing soils.  Variances in bearing soil moisture content distribution along the foundation perimeter can result in unscheduled foundation system vertical displacement and rotational movement.  Rain gutters and downspouts should be placed along the entire house perimeter eave lines where the sloping roofline discharges rainfall runoff.  The gutters will capture and convey roof rainfall runoff into downspouts. The runoff is then discharges onto the ground surface swale, or into a subsurface solid pipe drain system.  This type of gutter system will help to eliminate ground surface erosion and prevent excess water accumulations near the foundation system.
  • Improve or Modify Existing Ground Surface System (ASCE, section 7.35). The following criteria may be used to improve or modify the existing ground surface drainage system:
  • Surface Grading along the Foundation perimeter.  A minimum slope of 5% (6” fall per 10’) away from the foundation perimeter should be provided for adjacent ground areas.
  • Ground surface swales parallel to the house walls (rear yard and both side yards) shall have longitudinal slopes of at least 2% (6” per 25′) if practical, and 1% (3” per 25’) minimum.
  • Eroded surfaces should be replaced with vegetated surfaces.
  • Gaps between concrete surfaces along the foundation system perimeter that allow surface water to infiltrate into the foundation bearing soils should be eliminated.
  • Concrete surfaces that may allow water to flow towards the foundation system perimeter should be modified to direct water away from the foundation perimeter.
  • Erosion Control.  Ground cover should be placed in areas where ground surface erosion currently exists.
  • Surface Water Drainage Option A. The ground surface should be graded to slope to one or more subsurface solid drainpipe (plastic or PVC) single collector inlets or continuous grate type rectangular inlets.  The drain inlets should be located to drain excess water from the side and rear yards and discharge to daylight in the front yard. The street serves as the drain outfall where storm water is directed into street gutter inlets. Cleanouts should be provided at 50 feet intervals for proper maintenance.  Roof rainfall gutter downspouts may be connected to the subsurface solid pipe system provided the pipe has sufficient capacity to prevent a backwater condition.  The pipe should have a minimum slope of 1% to the daylight discharge.  In any case, the ground surface slope along the foundation perimeter must comply with local building code requirements.
  • Subsurface Water Drainage Option B. Subsurface water drains are appropriate to control surface water runoff.  They may consist of a perforated pipe placed in an aggregate filled trench (“French Drain”) along with an optional filter fabric to prevent pipe stoppages.  The pipe should have a minimum slope of 1% to the surface outfall.  Cleanouts should be provided at 50 feet intervals for maintenance.  In any case, the ground surface slope along the foundation perimeter must comply with local code requirements.  Gutter downspouts should not be connected to a perforated pipe system.
  • Monitor foundation performance after completing drainage improvement measures to assure their satisfactory implementation.

Recommended drainage improvements, including Option A and Option B above respond to the requirements of section 5.8 “Remediation Criteria” of the ASCE Guidelines and does not constitute an engineering or construction design document.  The above guidelines and criteria are for planning and pricing purposes and require an on-site civil survey and geotechnical evaluation at the time the work is done.

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Slab on Grade Foundation Stabilization: Structural vs. Non-Structural

Posted on 02/03/2010 by Jeff Lineberger, P. E.

Slab on Grade Foundation Stabilization: Structural vs. Non-Structural

SLAB ON GRADE FOUNDATION SYSTEM

SLAB ON GRADE FOUNDATION SYSTEM

Stabilizing a slab on grade foundation system requires a design approach that mirrors its original design principle. Since shallow bearing foundation systems are designed as single units, they must be repaired as single units. At a minimum, the remedial structural design should uniformly stabilize the entire foundation system so it can resume resisting the soil, dead and live loads imposed upon it. A reliable, durable, and time tested method for uniformly stabilizing slab-on-grade foundation systems includes underpinning, where slab on grade foundation interior and exterior grade beams bear upon and span underpins (drilled and steel reinforced concrete piers, or DRCP’s). This is commonly known as “Full Piering”. DRCP’s convert the slab on grade foundation system from a shallow bearing structural element to one that bears on deep, yet stable soils. DTCP’s allow the engineer to specify and limit the vertical movement of the foundation system over its life, improve slab on grade foundation performance, and minimize foundation superstructure brittle material cracking.
“Partial Piering”, as it is commonly known, limits the number and placement location of underpins beneath a slab on grade foundation system. Partial piering restrains vertical movement near the part of the foundation system where the underpins are placed, and leaves the remaining portion of the foundation devoid of underpins to move unpredictably with the seasonal changes in soil moisture content. Over time, stress builds up between the restrained and unrestrained areas of the foundation system creating a hinge, or weakened area where the concrete can fracture and structurally fail.
The initial cost to underpin slab on grade foundation systems using DRCP’s can appear disproportionally high when compared to other non-engineered structural repair methods such as “partial piering”. Where soils are highly expansive, or when soils shear strengths are unusually low, the owner may decide that slab stabilization using DRCP’s is simply too costly. Cheaper structural repair alternatives to DRCP’s may have a lower initial cost and may appear to initially stabilize the foundation system, but over time, their life cycle costs often exceed typical DRCP life cycle costs. And since “partial piering” fails to uniformly stabilize a slab on grade foundation system over its entire expanse, this repair method is less reliable than DRCP’s and can cause further damage to an already weakened slab on grade foundation system.
Option 1: DRCP’s.
When properly engineered, DRCP’s are reliable, yet costly. For residential foundation systems, these underpins are strategically placed beneath the expanse of the foundation system, usually to depths exceeding eight feet to provide maximum support. Unlike the typical (and inexpensive) multiple segmented concrete pile components stacked atop one another like unstable kinder blocks, DRCP’s are poured in place, reinforced with vertical steel, and placed to bear on deep, yet stable soils, free from seasonal moisture variations. More importantly, and unlike segmented piles (which are often used in “Partial Piering”), DRCP’s provide superlative resistance to soil friction that tends to push the underpin up (float) or push the underpins from side-to-side (lateral). As such, DRCP’s provide a stable bearing surface for the damaged foundation system by limiting its ability to move vertically or horizontally. For a residence of like kind, quality, age and size, 30-40 reinforced concrete underpins drilled to depths exceeding ten feet and placed strategically under the interior and exterior perimeter beams of the foundation system may be expected. The depth, spacing, and number of DRCP’s are determined jointly by competent and experienced Geotechnical and Structural engineers.
Option 2: Non-Structural Repairs to Minimize Foundation Movement and Improve Performance.
Slab on grade foundation systems incorporate design criteria relating to climate, soil, and structure. Reinforced concrete foundation performance can be impacted by post-construction activities unrelated to its core design criteria.  If rainfall is allowed to pond or collect adjacent to a structure built on expansive soil, the structure may be subjected to unscheduled distress caused by swelling bearing soils due to increased soil moisture content.  Lot surfaces must be graded to drain away from the structure in accord with the International Residential Code R401.3. In accord with 304.100(a)(2) and section 7.3 of the ASCE Guidelines, the following summarizes the recommended non-structural remedial measures:
Roof Rain Gutter System (ASCE, section 7.34).  Uncontrolled roof rainfall runoff can erode the ground surface along the foundation perimeter and provide a source of excessive and non-uniform water input to the foundation perimeter beam bearing soils.  Variances in bearing soil moisture content distribution along the foundation perimeter can result in unscheduled foundation system vertical displacement and rotational movement.  Rain gutters and downspouts should be placed along the entire house perimeter eave lines where the sloping roofline discharges rainfall runoff.  The gutters will capture and convey roof rainfall runoff.  The runoff is then discharged via downspouts into a ground surface swale, or into a subsurface solid pipe drain system.  This type of gutter system will help to eliminate ground surface erosion, and prevent excess water accumulations near the foundation system.
Drainage Improvements (ASCE, section 7.35).
  • Surface Grading along the Foundation perimeter.  For adjacent ground areas, a minimum slope of 5% (6” fall per 10’) away from the foundation should be provided for the first five feet all the way around.  Swales shall have longitudinal slopes of at least 2% (6” per 25 ‘) if practical, and 1% (3” per 25’) minimum.  Eroded surfaces should be replaced with vegetated surfaces.  Gaps between concrete surfaces along the foundation system perimeter that allow surface water to infiltrate into the foundation bearing soils should be eliminated.  Concrete surfaces that may allow water to flow towards the foundation system perimeter should be modified to direct water away from the foundation perimeter.  Erosion Control.  Ground cover should be placed in areas where ground surface erosion currently exists.
  • Surface Water Drainage Option A.  The ground surface should be graded to slope to one or more subsurface solid drainpipe inlets.  Cleanouts should be provided at 50 feet intervals for proper maintenance.  Roof rainfall gutter downspouts may be connected to the subsurface solid pipe system provided the pipe has sufficient capacity to prevent a backwater condition.  The pipe should have a minimum slope of 0.5 percent to the surface outfall.  In any case, the ground surface slope along the foundation perimeter must comply with local code requirements.
  • Subsurface Water Drainage Option B.  Subsurface water drains are appropriate to control surface water runoff.  They may consist of a perforated pipe placed in an aggregate filled trench along with an optional filter fabric to prevent pipe stoppages.  The pipe should have a minimum slope of 0.5 percent to the surface outfall.  Cleanouts should be provided at 50 feet intervals for maintenance.  In any case, the ground surface slope along the foundation perimeter must comply with local code requirements.  Gutter downspouts should not be connected to a perforated pipe system.
  • Monitor foundation performance after completing all non-structural repair measures to assure their satisfactory implementation.
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Homes Evacuated in San Antonio as Hill Crumbles

Posted on 01/26/2010 by Jeff Lineberger, P. E.

Associated Press/SyFi.com/2010-01-26 08:00:00

Construction crews moved dirt to shore up a group of houses precariously perched on a crumbling hill in San Antonio on Monday as engineers tried to determine why the land below was shifting, causing dozens of homes to evacuate.

Gaping crevices, some 15 feet deep, cut across several yards as dirt cascaded into a towering stone retaining wall that nearly split in half. Fences crumpled like accordions as crews packed dirt under one home and around its exterior after part of its foundation was exposed.

One soil expert said the cause of the landslide appeared to be the result of poor retaining wall design, and a city official said the nearly 1,000-foot-long wall in the upper-middle class neighborhood of sprawling two-story homes was built without a permit.

No one has been injured, but about 80 homes were evacuated on Sunday after a resident in the northwest side subdivision reported that his backyard was sliding down hill. By Monday afternoon, residents in about 55 of those homes were allowed to return after inspections and soil monitoring found them to be safe, said Valerie Dolenga, a spokeswoman for Pulte Homes Inc., the parent company of the neighborhood’s builder, Centex Homes.

One neighbor who was among the first homebuyers in the subdivision set among rolling hills on the outskirts of San Antonio said he was initially told no homes would be built on the crumbling ridge because it was too steep.

Romeo Peart, 32, said one retaining wall failed several years ago before the current one was built and homes were constructed above it.

“They can keep the view now,” Peart said, shaking his head as heavy equipment stuffed dirt beneath an exposed foundation. “And they paid an extra $10,000 for those lots.”

The development, which was started in 2004, has nearly 750 homes with others under construction. The neighborhood, with houses selling for $250,000, is one dozens that have sprung up on hilly former ranch land as San Antonio grew to be the nation’s seventh largest city.

The near-vertical retaining wall likely failed under the weight of the area’s clay soil that readily expands when drenched with heavy precipitation as it was last week, said Sazzad Bin-Shafique, an assistant engineering professor and soil expert at the University of Texas-San Antonio who went out to the site on Monday. Steep, tall retaining walls can hold up if built correctly, he said.

“It’s safe, honestly. We have engineering solutions, but sometimes we do something because we want to reduce costs,” Bin-Shafique said. “Many times, it will be OK, but sometimes, it will not.”

Roderick Sanchez, the city’s planning and development director, said the builder built the retaining wall without a permit. The city was still waiting for verification that the wall was designed by a certified engineer and built to specifications, Sanchez said.

Dolenga said the city approved construction plans for the subdivision including the retaining wall, though she said the company was investigating the permit allegation. She said she didn’t know if the street with the now-jeopardized homes was added later to the subdivision’s plans, though developments are usually built in phases.

“We’ve been building out there a long time. This is an unusual circumstance,” she said.

Engineers spent Monday assessing each of the structures in the evacuated area, while fire officials escorted some families to retrieve belongings from the neighborhood. At least seven homes would likely remain vacant for an extended period, said Fire District Chief Nim Kidd, who is also the head of the city’s emergency management office.

Kenny Crawford, 32, asked fire officials to be allowed to retrieve his car and some belongings on Monday, but because his home is directly below the disintegrating wall, he and his girlfriend were told it was too dangerous.

“They really haven’t given us any info,” Crawford said. “We don’t know what’s going to happen. Of course, property values are going to fall.”

Dolenga said geologists and engineers were looking for a cause of the slide and monitoring for any additional movement of the dirt that was sliding at a rate of 4-inches-an-hour on Sunday. She did not know if there was additional movement on Monday.

Utilities were cut off in the area, and construction crews were moving dirt to shore up the homes on the hill and to protect those below the retaining wall.

Resident Lakeika James, 41, said she had noticed odd noises over the three years she has lived in her house.

“I would hear, laying in my bed at night, grumbling and vibrations. A few nails popped out lately,” she said.

She said she hadn’t planned on staying in the house long-term, and now after the landslide, the mother of a 5-year-old girl wants out.

“I’m just going to be uncomfortable and worried for my family,” she said.

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