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Archive for August, 2009

Soils and Settlement

Thursday, August 27th, 2009

By Kenton Shepard, InterNACHI’s Director of Green Building

 Soil is a naturally-occurring mixture of mineral and organic ingredients with a definite form, structure, and composition. It’s composed primarily of minerals which are produced from parent material which is broken into small pieces by weathering. Larger pieces are stones, gravel, and other rock debris. Smaller particles are sand, silt, or clay. Since the original materials vary from place to place, the exact composition of soil varies according to location. A common example of soil composition by volume might be: settlement

Mineral particles give soil texture. Sand particles range in diameter from 2 mm to 0.05 mm, feel gritty and can be easily seen with the unaided eye. Silt particles are between 0.05 mm and 0.002 mm and feel like flour. Clay particles are smaller than 0.002 mm and cannot be seen with the unaided eye. Because of the small particle size, clay soils can sometimes experience large amounts of expansion and contraction in volume with changes in moisture content.
Water and air occupy the pore spaces—the area between soil particles. The final ingredient of a soil is organic matter. Organic matter consists of dead plant and animal material and the billions of living organisms that inhabit soil.
The concern with soil in respect to building is the ability of soil to bear the load of the structure while remaining stable. Ensuring long-term stability requires proper compaction and consolidation of soil before a permanent load is placed upon it. Examples of a permanent load would be foundation footings and walls or a concrete floor or driveway slab.
The excavation process disturbs soil, loosening it and causing spaces between soil particles to become much larger. For this reason, engineering specifications often require that foundations be placed on undisturbed soil.

In areas at which a home is built partially or completely on fill, such as homes built on hillsides, that fill must be made as solid as possible before a permanent load is placed on it. This is done by mechanical compaction of the soil. Soil is placed in layers (called “lifts”). Each layer is mechanically compacted by impact and sometimes by vibration.

When larger areas such as a hillside lot are compacted, heavy equipment is used. For smaller areas like backfill around basement foundation walls, a jumping jack tamper is used which is operated by one person.

Compaction is the process of forcing air from the spaces between soil particles. Compaction with a jumping jack tamper is somewhat inexact. In determining the point at which soil is adequately compacted, the operator listens to the tone of the tamper impacting the soil. When soil is adequately compacted, the tone will have a ringing quality which will not change. A change in tone indicates that compaction is still taking place.

Compaction increases the density of the soil and improves its ability to bear a load. Compaction is affected by a number of factors:settlement2

Consolidation is the process of forcing water from the spaces between soil particles. Soil is more permeable to air than to water. This means that the compaction process may remove from the soil a large percentage of air, but a significant percentage of water may remain.

Soil undergoes both primary and secondary consolidation.
Primary consolidation is short-term and takes place during the mechanical compacting process. Secondary consolidation is long-term and takes place after the compaction process is complete and the permanent loads are in place.

During secondary consolidation, the weight placed on soil slowly forces water out of the spaces between soil particles. As this happens, soil particles will move close together and settling will occur. The source of the weight would be both the structure and the overlying soil.

The amount of secondary consolidation which can be expected increases with the depth of the affected area. An excavation with backfill 15 feet deep would experience more secondary consolidation than an excavation with backfill 8 feet deep.

A common scenario is when a structure is built partially on undisturbed soil and partially on compacted fill. Soil in these two areas will consolidate at different rates as the weight of the newly-built structure forces water from between soil particles. This is called “differential settlement”.

Settling will be reflected in any part of structure bearing upon the settled soil. In adequately-compacted soil, settling will be so minor that evidence won’t be visible. Extreme differential settlement will create stresses which are relieved by cracking.

Which materials crack depends on the properties of the material and the rate of settling. More brittle materials will crack first. The effects of soil movement are most often seen as cracks in interior and exterior wall coverings like drywall and plaster and in masonry foundation walls.

 

Even concrete, which most people think of as brittle, can bend if pressure is applied slowly over a long time period. If pressure is applied over a shorter time period, concrete will crack.
 
Compaction and consolidation are affected by the composition of the soil. Fine-grained soils have more interior surface area and can hold more air and water than course-grained soils.
 
Here’s an example. Drywall is made of much courser particles than cement. An ounce of drywall dust contains about 5,000 square feet of interior surface area. An ounce of cement dust contains about 50,000 square feet of interior surface area.
 
This means that fine-grained soils like clays have more interior surface area which can contain water. In order to force water out of the spaces between particles, surface tension must be overcome. “Surface tension” is the tendency of water to cling to a surface. When you fill a glass with water, it’s surface tension that makes the water level slightly higher around the edges where water comes into contact with the glass surface. Water is clinging to the glass.
 
The greater interior surface area of fine-grained soils results in greater surface tension. Fine-grained are also typically low-permeability soils, meaning that water moves through them slowly. These conditions increase the amount of time and pressure required for soil tosettlement3 consolidate. Soils will continue to consolidate until the resistance to pressure of the materials of which the soil is composed reach equilibrium with pressure from the weight of soil and structure above.

The rate of consolidation is affected by the soil composition, levels of moisture saturation, the amount and nature of the load on the soil and state of consolidation of the soil.

Another moisture-related problem is the addition of excessive moisture to the soil. This can create a condition in which water is absorbed into spaces between soil particles. Soil becomes less dense, which reduces its ability to support a load.

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Clearing a Dishwasher Air Gap

Wednesday, August 19th, 2009

Proper plumbing should take health and safety into consideration. One of the key safety concepts to consider with plumbing is the prevention of contaminating your house water supply. Almost all of the water that enters your house leaves through the drains. It is essential to keep these systems apart to prevent bacteria from contaminating your drinking water.

The water enters the house clean, and leaves dirty. You must keep the water supply and the waste system separated. You have to prevent dirty water from coming into contact with the supply side of the plumbing. This is often accomplished through the use of air gaps. An air gap is simply a separation of two plumbing components. An air gap breaks any siphon that could be created, which would allow water to flow from the drain system to the water supply.

Air gaps are located throughout your house. Air gaps are built into your toilet fill valves, clothes washers, and dishwasher. The following video was created by a colleage Greg Chick, owner of Ramona’s Plumber. In this video, he talks about how to clear a clogged dishwasher drain line.




Greg and his website www.ramonasplumber.com are a great resource for your plumbing questions. He even has a pocast to which you can subscribe to receive regular tips and tricks about plumbing. Check it out!

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Water Heater Inspections – Part 2

Friday, August 14th, 2009

After you give your water heater a good visual inspection, there are some other very important safety items you need to check. The first is the Seismic Strapping. These are metal belts that strap the water heater to a wall. The seismic strapping holds the water heater upright in the event of an earthquake. The straps are not intended to protect the plumbing, or avoid flooding. Instead they are specifically required to protect the gas lines. During an earthquake, the water heater can move substantially. Even if it does not tip, the somewhat fragile flexible gas line could easily be broken, which could leak gas and cause numerous fires.

When the requirement for strapping was first introduced, plumbers used what is known as “plumber’s tape” to secure the water heater. Plumber’s tape is that thin metal strapping that they use to support pipes under a house or in an attic. It is about 3/4 inch wide with holes running down the middle. But due to all the holes, the plumber’s tape easily breaks. Remember that a full water heater can weigh several hundred pounds. Plumber’s tape is no longer considered adequate for strapping a water heater (see picture at left). Instead you

Home inspection should check for proper strapping

Home inspection should check for proper strapping

should use a water heater strapping kit that is available at any home improvement store. These kits contain two metal bands with mounting hardware, and when properly installed will reduce the likelihood of your water heater jumping around during an earthquake.

The proper way to install the straps is to use two straps. One at the top 1/3 of the tank, and one at the bottom 1/3 of the tank. A general rule of thumps is to install the straps 9 inches from the top of the tank, and 4 inches above the contols at the bottom of the tank. The straps should be secured to a stud in the wall with a 3 inch lag bolt. Do not use plastic anchors in the drywall. The straps should be looped around the tank. And finally, if the water heater stands more than three inches away from a wall at the back, a supporting brace or block should be used.

If your water heater is in the garage, it should be placed on a platform that is 18 inches above the ground.

Proper water heater strapping.

Proper water heater strapping.

This too is a safety precaution against fires. Many of us store items in the garage that give off fumes. These could include gas for a lawn mower, paints and varnishes, and even some cleaning products.  There could also be a leak from a gas line from a clothes dryer or even the water heater. Most of these fumes will sink to the ground in a similar way that oil and water separate. The vents in the exterior walls of your garage should allow the fumes to escape. If your water heater was sitting on the floor and enough fumes collected, a fire could easily start when the water heater ignites to heat the water. It happens all too often. If your water heater is in the garage and on the floor, you need to have it raised.

To determine the age of your water heater, you will need to check the serial number on the tank. Newer water heaters will print the date of manufacture, but older units used a date code included in the serial number. Some are easy to desipher; the year and month or year and week are the first four digits of the serial number. For example a tank manufactured in March of 1999 will have 9903 as the first four digits (or 0399 depending on the manufacturer). Other companies use a date code using letters. If you cannot determine the age of your water heater, call a plumber or a Home Inspector to help you determine the age of your water heater.  

We recommend replacing older water heaters. Water heaters become less efficient as they age due to sediment that builds in the tank at the bottom. This sediment acts like an insulation layer which causes the water to heat slowly, and requires the burner to stay on longer using more gas. This can also cause over-heating of the surrounding area which is a fire hazard. The other reason to change an older tank is to avoid leaks. In a perfect world, water heaters would fail with only a little drip. Unfortunately water heaters can fail with catastrophic results. Since your plumbing system – and the water heater – are under pressure, failure can send excessive amounts of water pouring into your house. Even if your water heater is in the garage, the force of the water can quickly disolve the drywall, and end up in your house ruining flooring, drywall, and personal items.   

And finally, don’t store items up against your water heater. It is particularly important not to store combustible items such as wood, cloth, cleaning supplies, etc. But even non-combustible items can reduce air flow and cause the water heater to over-heat.

For more information about home inspections in San Diego, please visit www.sdinspections.com

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Water Heater Inspections – Part 1

Sunday, August 9th, 2009

Water heaters are often taken for granted. They sit in our garage or utility closet and perform their assigned duty day in and day out. If installed correctly they will operate with little fuss for many years. Most of us rarely give the water heater a second thought, but they have a life cycle just like every other component or appliance in your house.

At the very minimum you should take a careful look at your water heater at least once a year. One of the most common points of failure is at the top of your water heater where it connects to the house plumbing. If the water heater was not installed with proper dielectric connectors, you will get corrosion where the copper plumbing meets the steel tank. This area is often difficult to see. A tank can be five

Corrosion at the top of the water heater can indicate pending failure

Corrosion at the top of the water heater can indicate pending failure

 feet tall and when properly installed on an 18 inch platform, the top of the tank is 6-1/2 feet off the ground. This area can easily be over looked.

If you see heavy rust in this area, your tank is ready to leak. remember that your plumbing system is under pressure 24 hours a day. It doesn’t take much corrosion to eat through the plumbing. Even a small leak can flood your house. Even if your water heater is in your garage, a leak can find it’s way to the interior of your house. Water often sprays out of a small breach in the plumbing with such force that it quickly destroys the drywall, and get’s inside the walls, and into your house. Just a small amount of water can destroy wood flooring, baseboards, carpeting, furninture, or personal items that are sitting on the floor.

In addition to looking for corrosion, look for evidence of soot at the exterior of the tank. This can be found around the burner compartment cover at the bottom of your water heater. If you are comfortable doing so, open the cover and look inside the compartment. Be careful of hot parts. If the water heater is on the inner cover will be hot. Once you remove the inner cover look for scale and rust inside the burner compartment. Old, damaged parts in the burner compartment can cause the water heater to remain on and over heat. This is a major cause of house fires.

Proper venting is critical to the safe operation of a water heater. Check the flue. It should be properly aligned with the top of the tank. And the sections of the flue should be well seated. Do not use duct tape on the flue. It can catch fire. An improperly installed flue can allow Carbon Monoxide to leak into your living space.

A general rule of thumb is to proactively replace your water heater when it has reached the end of its warranty period. That usually means that if your water heater is more than 7 years old, it should be replaced before you have problems with it.  The price ofa water heater is usually less than your insurance deductable. When in doubt, have your water heater inspected by a licensed plumber. It does not cost much and could save your family from injury, and your home from damage.

Part two of this blog will include information about proper seismic strapping, recalled gas flex lines, and determining the age of your water heater. For additional information about home inspections, please visit www.sdinspections.com

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