Basement Waterproofing Repair
Regardless of your level of knowledge and experience on the subject of basement waterproofing repair, my goal on this page is to share with you some useful things you do not know. I will start with some basic principles along with common terms and current technology. I will also be discussing the construction procedures required to complete a basement waterproofing repair job. I will present some unique challenges which must be addressed together with options to consider in producing the most cost-effective basement waterproofing repair project.
In this discussion, the term “basement waterproofing” is intended to encompass the topic of moisture protection-and protection from all undesirable liquids and gases- as applied to a variety of existing below-grade structures to include basements, underground houses, split-level buildings, sunken living areas, underground storage areas, root cellars and underground vaults. All of these structures are subject to foundation failure, distress, and moisture-related problems.
The knowledge and ability to provide basement waterproofing repair on existing structures is a vitally important service and to be expected from a competent foundation repair contractor. An understanding of the how moisture behaves as it interacts with soil and building materials is fundamental in this regard. My discussions of Yard Drainage and the French Drain, published on this website, contain some of this background and should be reviewed to get the full picture.
In my experience, past basement waterproofing practices and techniques in new construction have been poorly developed and building codes have provided little oversight. There was a time not too long ago when the general building contractor would instruct his laborers to quickly slap on anything black in color just as the excavating contractor was starting to push dirt and rock backfill up against the newly placed basement walls.
In recent years, expensive lawsuits have raised the level of awareness. Manufacturers have responded with an explosion of new basement waterproofing products. Architects and engineers are providing more rigorous plans and specifications. Nevertheless, a huge percentage of existing underground structures remain in serious need of improved basement waterproofing.
As the cost and availability of real estate increases in our urban areas, basements are becoming more and more common in new construction. Basement waterproofing installers are highly specialized in new construction and normally do not deal with hardscape, demolition, excavation, underground utilities, backfill, compaction and a variety of other tasks which come into play during a basement waterproofing repair job. It is important for the visitor to this page to understand that our subject is basement waterproofing repair and that basement waterproofing repair jobs are best handled by a well-qualified foundation repair contractor.
Concrete Porosity
Concrete is heavy and dense and some of us have trouble understanding why waterproofing of concrete surfaces is even necessary. The best way to explain the problem is to remember that basic concrete is made by mixing together three components: 1. aggregate (sand & gravel), 2. cement and 3. water.
In the chemical reaction which takes place (hydration), the water joins together with the cement to form a solid binder which holds the aggregate together in a rock-like mass. The amount of water required to form the binder is determined by the amount of cement in the mixture. To facilitate mixing and workability, we always put in more water than is necessary to make the binder.
Once the mixture has been placed and finished, the excess water bleeds out and most and the remaining moisture is eventually removed from the concrete mass by evaporation. The final concrete mass now contains tiny voids or capillaries which were left by the evaporating moisture. Those capillaries give the concrete a certain amount of “porosity”. The result- microscopic liquid and vapor molecules will easily pass through the finished product.
Water/Cement Ratio
Now we can see why the water/cement ratio is important in determining the porosity of concrete. The minimum water/cement ratio (by weight) required to complete the hydration reaction is about 0.25. The maximum water/cement ratio normally recommended for concrete is 0.50.
Where there are sulfates in the soil, the Uniform Building Code (UBC) requires that the water/cement ratio be kept below 0.45. This is to prevent corrosive moisture from seeping in and destroying the concrete. As a consultant, I recently prepared repair estimates for a large California homeowner’s Association suing the developer for failure to use a 0.45 water/cement ratio on a high-sulfate jobsite. The recommended repairs included complete replacement of the slab-on-grade foundations for many of the affected homes. In addition to the 0.45 maximum water/cement ratio, type 5 Portland cement (high sulfate resistance) should be used on all foundation repair applications where sulfate damage is suspected.
For most basement waterproofing repair jobs, we should assume a relatively high water/cement ratio was used during original construction of the existing concrete floors and walls. Most concrete will soak up water like a sponge because of the microscopic capillaries which I have described. However, our concerns should not be limited to water.
Environmental Hazards
All liquids and gases which are present in the underlying and surrounding soil have the potential of penetrating the below-grade concrete and entering the structure. Soil vapors, like all expanding gases, will move from high-pressure environments to lower pressure environments. Since vapor pressures in soil are generally higher than inside structures, the soil vapors will be drawn into the building through the capillaries in the concrete which separate the two environments.
A competent basement waterproofing contractor will seek to protect not only the structure but also the people who occupy the structure. Groundwater and soil in developed areas are subject to contamination by toxic liquids and gases. Decomposing organic matter releases methane as do leaks in sewer and septic systems.
Underground natural gas lines and buried fuel tanks leak petroleum products into the groundwater. Insecticides used to treat termites and garden pests (later banned as carcinogens) have been sprayed into the soil. Radioactive radon gas from natural sources and certain building products is also a serious problem in some areas.
These environmental hazards can cause serious disease in humans and should not be tolerated anywhere in our environment- especially in our homes and businesses. Recent energy-saving measures have included “tighter” building construction without improved ventilation. This trend has the unfortunate side effect of reducing fresh air circulation. The result is that environmental toxins tend to accumulate in the form of “indoor pollution”- also known as “sick building syndrome”. Sound basement waterproofing practices must seek to prevent these toxins from getting into the underground spaces where people live and work.
Ground Water and Hydrostatic Pressure
Returning to my discussion of the permeability of concrete as it relates to moisture, I should mention a couple of things about liquid water. In the ground, water behaves under the influence of gravity. Water will “percolate” down through the soil until it encounters an “impermeable” layer such as a tight bedrock or clay. Certain clays, such as bentonite, are particularly resistant to water penetration and are being used very effectively in basement waterproofing applications as I will discuss later.
Water accumulating above an impervious below-grade horizon will saturate the overlying soils. This water is called “ground water” and the upper limit of the zone of saturation is called the ground water “table”. If enough ground water is present, it will accumulate above grade as surface water. The ground water table is not necessarily flat like a table as one would assume.
Besides gravity, ground water is influenced by its close interaction with earth materials. For example, ground water can move laterally or upward by “capillary action” or “wicking”. Ground water will move in the direction of the most permeable soils. Often the ground water table will slope in some approximation of the topography at the ground surface.
A permeable soil layer which holds and conducts ground water is called an “aquifer”. Ground water moving through an aquifer, such as buried “alluvium”, can become confined by an overlying clay layer which is less permeable. If ground water or surface water upslope is “charging” the confined aquifer, the ground water downslope will act under the influence of “hydrostatic pressure”. Wells drilled into such “confined aquifers” may become “artesian” or “flowing” wells and, depending upon the “hydrostatic head”, the unconfined water coming up may actually spout many feet into the air!
Highly permeable backfill soils surrounding a buried structure provide an excellent environment for the accumulation of ground water. Basement waterproofing systems must be designed and installed to resist the effects of water acting under pressure. Ground water saturating the backfill will exert hydrostatic pressure against the floor and walls of the structure with the greatest pressure being at the greatest depth. If the saturated backfill is confined and being charged by an uphill source, the hydrostatic pressure on the structure is increased in proportion to the hydrostatic head.
The relief of hydrostatic pressure is paramount in all basement waterproofing jobs and is accomplished by creating an envelope of highly permeable material around the structure. Coarse gravel, protected from the infiltration of fine material by a “geotextile fabric”, will act to capture and divert groundwater. Water percolates down the gravel envelope and enters a perforated pipe which is placed at the base of the gravel. The pipe then conducts the moisture to a safe point of discharge. This type system is commonly called a French Drain. There is a separate page on this site devoted entirely to the subject of the French drain.
Liquid Water and Water Vapor
In the design and installation of a basement waterproofing system, it is important to understand that hydrostatic pressure is not required for basement walls to become saturated and create serious problems. Capillary action works in concrete just like it works in natural earth materials. The tiny capillaries in concrete will easily become conduits for any moisture which comes in contact with the concrete. Moisture from the air, storm water and irrigation will all contribute to provide soil moisture and to replace moisture which is absorbed by the concrete floors and walls of basements and other underground structures.
Water seeping through basement walls may pond on the floors and even lead to flooding after a heavy rainstorm. More often, the moisture enters the building as vapor which circulates in the air of the interior spaces. Warm air holds more moisture than cold air so that when the air cools, some of the moisture evaporates. The moisture is usually deposited on cold surfaces such as window panes exposed to the cool night temperatures.
Moisture may also condense on cool surfaces under carpeting or behind wall coverings which are in contact with the surrounding earth. These moist micro-environments provide a habitat for unwanted indoor guests such as spores, molds, fungi, and bacteria. In addition to the chemicals hazards already discussed, these biological hazards add to the mix of indoor pollutants which must be excluded by a proper basement waterproofing program.
Moisture seeping through concrete floors, footings and walls will lead to damage to the concrete itself. I have already mentioned sulfates in soil. These and other minerals dissolved in ground water, aided by the presence of oxygen from the atmosphere, will react with the concrete and the reinforcing steel which has been embedded in the concrete. The result is corrosion and ultimately destruction of the structural integrity of the concrete.
Types of Basement Waterproofing
The more common of the first basement waterproofing systems were petroleum-based or “bituminous”. Asphalt, a common roofing material which is highly resistant to moisture penetration, remains an important component of some but not all waterproofing membranes which are currently being applied. Asphalitc or bitumen-based systems, usually applied as a liquid emulsion, are sticky and can be applied to vertical concrete surfaces to form a monolithic membrane. Along with the asphalt emulsion, reinforcing fabric such as “fiberglass webbing” is normally layered between applications.
Other bituminous systems which form a monolithic membrane include hot or cold liquid-applied “rubberized asphalt”. With modifications in viscosity, the asphalt-based coatings may be applied with a trowel, a roller or by spray. Also available are “polymer-modified” bitumen sheets which are adhered at the seams.
A major criticism of bituminous basement waterproofing systems is that over time, the membrane can dry out and becomes brittle. With slight movements of the building materials, which may be caused by thermal expansion and contraction or soil-related movement, the cured coatings will crack, providing soil-borne liquids and vapors a way into the structure. This is an unacceptable result in a system which is so difficult and expensive to replace.
When I first started in the foundation repair business, we installed the asphalt systems on our basement waterproofing repair jobs. Surface preparation was brutal. After cleaning up the walls, we would hire a sandblasting subcontractor to come to make the wall cleaner than a hounds tooth. In the confined space of the trench next to the wall, the sandblaster had to wear a respirator and the dust went everywhere. All cracks and joints had to be “parged” or filled with asphalt caulking. This was particularly important at the base of the wall where hydrostatic pressure is at the maximum.
Once the wall had been cleaned and parged, we would spread on the liquid asphalt using either a trowel or a roller. Then we would put on the webbing followed by more coats of mastic. Finally we would get four-by-eight foot sheets of one-half-inch-thick “sound board” and press them against the surface as “protection board”. The sticky asphalt got all over everything and was impossible to get off. Usually we would have it on our hands for days until it wore off.
Elastomeric Waterproofing
One day I landed a large commercial basement waterproofing and drainage job. The existing waterproofing- asphalt and webbing- had failed. I was looking for a new approach. I found a supplier who introduced me to a new product which he called an “elastomeric”. I decided to try the new product.
The term elastomeric simply means that the material- whatever material is being discussed- is flexible throughout its lifetime. For the basement waterproofing application, a synthetic material called “urethane” or “polyurethane” (a colorless, odorless, crystalline compound) has been formulated into a liquid which can be applied with a roller to form a monolithic basement waterproofing membrane. The polyurethane basement waterproofing membrane is equally impermeable and has a much longer useful lifetime than the asphalt-based material which we had been using in earlier jobs. Interestingly, some urethane waterproofing products are described as “bitumen-modified” so that the bituminous component is still out there.
The application process for the elastomeric basement waterproofing system is similar to the asphalt-based system. One or two coats are usually sufficient. Reinforcing material is not normally required. A good subdrain system with perforated pipe placed entirely below the top of the footing is essential and protection board must be installed against the new membrane prior to the placement of any backfill.
The elastomeric system worked great on my commercial job. I have used the polyurethane elastomeric basement waterproofing system on numerous basement waterproofing jobs over the years and I have never experienced a call-back because of failure of the waterproofing membrane. The cost is competitive with the asphalt-based system but the durability is much improved.
Cement-based Systems
Another type of material which I have used frequently on foundation repair jobs is the “cement-based” or “cementitious” coating. However, I have never used this type material on basement waterproofing jobs where hydrostatic pressure may be a factor. I have seen certain cement-based coatings promoted for use in basement waterproofing applications but I have also seen data which does not support this application due to a significantly higher permeability to moisture.
I have had very good success using cementitious coatings to patch and resurface existing above-grade foundation stem-walls. These coatings are relatively inexpensive and easy to apply. Where damp conditions are causing corrosion and destruction of the concrete, I usually will recommend replacement of the foundation. However, when the customer does not have the budget for foundation replacement, I will suggest one or two coats of a cementitious coating to retard the moisture-related destruction.
Bentonite and Blind-side Waterproofing
In my earlier discussion about ground water, I mentioned a natural clay called bentonite. Bentonite is very effective as a basement waterproofing material because it swells up when brought in contact with moisture. The first time I used bentonite as a sealer on a job, we smeared it on the wall with a trowel. This was a crude for-runner of current bentonite-based systems. On the most recent several jobs where I have used bentonite, it was supplied in rolls. The bentonite comes impregnated into a geotextile mesh. One can also find it adhered to polyethylene sheets.
Bentonite rolls or sheets, like the other basement waterproofing materials I have discussed so far, are designed for a “positive-side” application. When you do a positive-side basement waterproofing repair job, the new material is placed on the outside surface of the wall- between the wall and the earth. It is a simple concept- put the waterproofing between the soil and the material you are trying to protect.
However, there are also other approaches to basement waterproofing. “Negative-side” waterproofing systems, which I will discuss in a bit, have the waterproofing material applied to the inside surface of the wall. A modified version of the negative-side system has the material injected from the negative side but applied to the positive side. Then there are the so-called “blind-side” waterproofing methods.
Blind-side basement waterproofing is where you put the waterproofing up first and then place a new basement wall up against it. For example, I recently installed two new stair tower foundations for an existing three-story hotel building. The original wood towers had been destroyed by dry rot and had to be demolished. One of the new foundation designs incorporated a new basement wall which had to be poured up against a vertical cut of earth beneath the foundation of the existing three-story building.
Before completing our excavation, we had to “underpin” the existing building so that it would not settle as we made the vertical cut. Then we placed and secured sheets of bentonite up against the face of the cut. The bentonite in this case was attached to plastic “drain sheets” with “filter fabric”. In blind-side basement waterproofing, everything is done in the reverse order of normal new foundation construction:
The filter fabric side goes in direct contact with the soil of the vertical cut. The drain sheets came next, providing free drainage to relieve any hydrostatic pressure. The dry bentonite clay goes away from the vertical cut where it will be in direct contact with the freshly poured concrete. At the bottom, we ran perforated pipe and wrapped it in more filter fabric. After installing the reinforcing steel, we set forms to create the inside face of the new basement wall and then we poured the new concrete.
Blind-side basement waterproofing is often required for new construction and retrofit jobs where property lines and adjacent structures prevent excavation and access which would allow positive side waterproofing to be installed after completion of the basement walls.
In addition to poured walls as I have described, bentonite sheets are used behind “shotcrete” walls and “soldier-beam” walls with wood “lagging”. “Tiebacks” and other penetrations of the bentonite membrane present challenges to the installer. Bentonite mastic is needed to work around penetrations and irregular surfaces. On contact with moisture, the bentonite expands into small irregularities and aids in the sealing process.
Bentonite rolls or sheets are also used on conventional positive-side basement waterproofing jobs. The natural clay material would not be expected to degrade and the swelling characteristics give bentonite an added component of performance. I should note that the bentonite sheets are heavy which makes handling and installation more difficult than the liquid-applied elastomerics. While bentonite can be an outstanding basement waterproofing product, one must consider the material and installation cost. In my experience, bentonite is significantly more expensive than the polyurethane membrane systems which seem to be well accepted for most positive-side applications.
Negative-side Basement Waterproofing
Negative-side basement waterproofing systems on existing structures have one huge advantage: the removal and replacement of the perimeter soil and other improvements is not required. However I have yet to see a negative-side system which offers the level of protection you get from a positive-side application.
The “crystalline” basement waterproofing systems with which I am familiar consist of a liquid solution which is applied to an existing concrete surface. Also called a “capillary” basement waterproofing system, the solution penetrates existing concrete and seals the capillaries within the concrete mass. Proprietary chemicals within the solution react with lime in the concrete to form crystals which fill the capillaries and reduce the permeability of the concrete. Moisture must be present in the concrete in order for the chemical reaction to take place.
The crystalline basement waterproofing system gives the contractor an option when the property owner does not have the budget to do a positive-side job. It is more of a maintenance program than a repair. I would never give any type of guarantee on this type of system. The general idea is to apply the material to the concrete and then wait for the moisture to seep in from the outside. The chemicals dissolve and follow the water, sealing up the cracks and capillaries in those zones of moisture. The next month or the next year, it may be necessary to apply more solution as the chemicals seep into the zones of moisture and are used up.
Injected Basement Waterproofing
Another type of basement waterproofing system which I have investigated is installed from the negative side but applied to the positive side. “Injected bentonite is being used to create waterproof barriers beneath floors or behind walls. “Injectable polyurethane” is pumped into holes drilled in floors or walls to form a waterproof barrier.
Before using bentonite beneath a floor, I would want to satisfy myself that changes in moisture levels in the surrounding soil would not result in heaving or settlement of the floor or foundation due to expansion or contraction of the bentonite material.
The injected polyurethane material is “hydrophilic” or attracted to water. The drilled holes are used as injection points and also to gauge and monitor the penetration of the material. Crack sealing can also be accomplished by injecting “hydrophobic” polyurethane into the cracks.
The urethane and bentonite injection systems provide an alternative to the capillary system for basement waterproofing of existing underground structures. Though significantly more expensive than the capillary system, bentonite or urethane injection offers a positive side application. When used on concrete floors- whether above or below grade- bentonite or urethane injection is a less-expensive option than membrane applications which require the removal and replacement of the concrete. When used on below-grade walls, bentonite or urethane injection saves the expense of removal and replacement of the exterior soil and other perimeter improvements.
However, for reasons I have already expressed, I do not favor a waterproofing system on below-grade walls which does not include a drain system to eliminate hydrostatic pressure. I would never assume that an existing French drain system was working properly. Therefore for most basement waterproofing repair jobs, my first recommendation will be a positive-side treatment with a liquid-applied elastomeric membrane together with a carefully installed perimeter French drain. With the relief of hydrostatic pressure, I would then be comfortable injecting a below-slab hydrophilic polyurethane barrier to complete the job.
Below-Slab Vapor Barriers
In my experience, below-slab waterproofing is not very often recommended for basement waterproofing jobs. People seem to have the idea that it is not needed or that an existing “vapor barrier” will take care of any under-slab moisture. This point of view needs to be carefully examined.
The 6 mil polyethylene vapor barriers, which are sometimes placed under slabs during new construction, do not prevent but only retard water and other soil vapors which I have discussed. These large sheets of plastic are often installed improperly at seams and penetrations while damage during concrete placement is inevitable. What is more, over time they become degraded and destroyed from contact with chemicals in the soil and concrete.
I believe that a discussion of the “vapor barrier” can lend insight to our subject of basement waterproofing. Some people argue that the vapor barrier causes more problems than it solves, especially in hot, arid climates. In California, vapor barriers are used extensively and as long as you put at least a couple inches of sand on top of the plastic, it seems to work OK.
In Arizona and New Mexico, you are smart if you don’t put in a vapor barrier at all. The difference is humidity. My feeling is that use of a vapor barrier on a hot dry day will result in too much of the excess water coming up through the new concrete during and shortly after placement. The result is more capillaries, more permeability and a weaker finished product.
When I first started doing concrete jobs in Arizona, I used the vapor barrier and I had cracking and “crazing” in my finished product. Later I learned to pour my mud on a nice bed of cool wet sand with no plastic. By keeping the new concrete surface wet and allowing excess moisture to freely drain downward, the curing process was improved and the cracking and crazing was eliminated. In the final analysis, I believe that the reduction in permeability of the concrete by proper curing will make a more important contribution to vapor control than would be achieved by using a plastic sheet.
This argument about vapor barriers goes to highlight my earlier comments about the importance of using a 0.45 water/cement ratio (or less) to minimize excess water (and capillaries) in the concrete mix. The use of admixtures can allow that ratio to be reduced without sacrificing workability. While I have not used them, I have seen interesting discussions about so-called “waterproof” concrete mixes. For below-grade and basement waterproofing applications, these new mixes should be considered.
More Basement Waterproofing Systems
There are a number of other kinds of basement waterproofing elements and systems which I will mention briefly. The sub-drain systems and drainage sheets I cover on another page on this website- see French Drain. Epoxies are available to seal concrete cracks, but I have not used or researched any epoxy-based basement waterproofing membrane. This is not to say that they are not out there.
Other basement waterproofing materials include coal tar, butyl rubber, metal oxides, neoprene membranes, thermoplastics (polyvinyl chloride), and latex coatings. Protection boards help prevent damage to the waterproof membrane during backfill and can be in the form of sound board, asphalt board, drain sheets, or foam sheets the latter having the added function of providing insulation to the basement waterproofing job. Other accessories to basement waterproofing jobs would include flashings and fasteners, waterstops for control and construction joints, and a wide variety of adhesives, sealers and mastics.
By now it should be evident that there are many types of basement waterproofing products and techniques which are currently being used. Many basement waterproofing materials are proprietary. In some cases, the manufacturer may require that the installer be trained or certified for a particular basement waterproofing system. For a warrantee to be valid, the manufacturer may require that basement waterproofing systems include an integrated package of products for surface preparation, membrane installation, protection board, and drainage system.
Lessons from a Basement Waterproofing Repair Job
As a foundation repair contractor, I have encountered basement waterproofing problems on many jobs. Often, as was the case with the hotel building mentioned earlier, basement waterproofing can be just a minor (albeit important) aspect of a much larger job. On a positive-side basement waterproofing replacement project, the task of removing and replacing the perimeter soil and all the improvements can greatly overshadow the scope of the actual installation of the waterproofing materials. A waterproofing specialist may not have the background to be at the top of the food chain.
To illustrate this point, I recall a basement waterproofing project on a large two-story apartment building with many leaking below-grade units. It was a case of the tail wagging the dog. There were many layers of “authority” on the project. The homeowners were represented by a board of directors, the board hired an architect, and the architect hired an engineer. When the plans were ready, the architect appointed one of his staff to be the project manager. The project manager hired a waterproofing specialist to be prime contractor, and the waterproofing specialist hired my company to do all the site work including excavation and backfill.
After demolishing perimeter improvements, my company started excavation on our new basement waterproofing repair job. Unfortunately, we soon discovered that significant settlement of the backfill soils had compromised unknown footings which were supporting heavy building loads. There were gaps of three to four inches between the bottom of the footings and the soil. We also discovered that extensive below-grade concrete demolition would be required to access the below-grade wall surface for waterproofing. Working quickly with the engineer, we designed a shoring system to support the footings and prepared “plan B”.
All of these unknowns could have been discovered ahead of time if the engineer had undertaken to explore the subsurface prior to submitting his basement waterproofing design. We submitted a change order to the “prime” contractor who, by the way, did not understand anything about what was going on. Because of the jackhammer work required around the shored footings, I added an indemnity clause to protect us from liability should foundation cracks develop during the course of our work- a consequence which I fully expected based upon past experience.
When the change order hit the board of directors, the job came to a halt. Lots of their money had been spent on architect and project management fees and now they had no money in the budget for extras. The board’s contract was with the basement waterproofing “specialist” and his contract made no allowance for unforeseen below-grade conditions. Fortunately, my contract with the waterproofer was very strong and I had received a deposit which more than covered my costs on the job. I refused to go forward without a signed change order. The board hired a lawyer.
In the foundation repair business, just like in the Kenny Rogers song, you have to know when to walk away. In my career, I have walked away from three jobs and this was one of them. I used the extra deposit money as leverage so when I returned it, I received a written indemnity agreement for any and all problems on the project. There are several important lessons for any would-be basement waterproofing repair contractor to be drawn from this project:
1. Know and understand all aspects of the job.2. Have an excellent contract.3. Always get a substantial deposit. 4. Deal directly with the property owner whenever possible.5. Be the prime contractor whenever possible.6. Stay away from lawyers.
Excavation Equipment for Basement Waterproofing Repair
Before any excavation takes place on a basement waterproofing repair job, the contractor should contact the underground locating service for his area. Underground utility locations should always be verified with hand-dug potholes. The contractor should notify the owner as soon as possible of any unanticipated underground conditions which will require extra charges on the contract.
An important decision on any positive-side basement waterproofing repair job involves the method of excavation to expose the failed below-grade walls. My first consideration in making this decision is safety. I have discussed safety elsewhere on this site but my words on the subject bear repeating:
Trench safety must be paramount during construction. People can die in a heartbeat! Get an OSHA permit where-ever required and follow all rules and regulations. Never permit workers to go into un-shored trenches. Expect wet weather which will weaken the trench walls. Keep heavy trucks and equipment away from the sides of trenches. Keep trenches covered. To reduce exposure, backfill trenches as soon as possible. Fence off the job and post keep-out signs. Use common sense.
In general, the economics of moving earth out of a hole or trench favor using the largest piece of equipment you can manage. However, when working around existing improvements on a basement waterproofing repair job, room to maneuver is an important limiting factor. Also, one must consider the demolition and damage which might otherwise be avoided by using smaller equipment or hand excavation methods. Does the cost of damage repair justify the savings?
These kinds of decisions are usually a matter of judgment on the part of the basement waterproofing repair contractor. If the choice of excavation methods is a close call, the original estimate and repair bid should be conservative and assume hand excavation methods. Once the preliminary underground location and hand pothole work has been completed, the decision about equipment and what size equipment can be made. Any savings in cost can be put into the contingency for unanticipated damage repairs, etc.
Equipment which I have used to excavate on basement waterproofing projects includes hydraulic excavators, grade-alls, backhoe-loaders, skid-steers, and mini-excavators. When earth is to be hauled off-site, the grade-all and larger hydraulic excavators may be able to excavate material and deposit the bucket loads into the back of a dump truck as a single motion, saving the cost of moving the material twice. When this is not possible, a wheel loader will be required to move the material from piles into the trucks.
The versatile backhoe-loader should be used where space permits and offers the advantage of digging, “walking” the material, loading trucks and rough grading where needed. On tight-access jobs, a mini (rubber track) excavator may work well for digging with a small skid steer on the job for moving off the spoils. It is important to use a skid-steer which can reach high enough to load the haul trucks to be used!
Compaction Equipment for Basement Waterproofing projects
When installing a basement waterproofing system to the underground walls of an existing basement, the excavation must usually be shored or sloped back for safety reasons. Although I have put in shoring on several basement waterproofing jobs where hand excavation was the only option, shoring is expensive and may be impractical. If the excavation must be sloped back, a large amount of spoil must be stockpiled and then replaced in the excavation as compacted fill. In this case, the basement waterproofing installer must make important decisions about compaction:
The choice of compaction equipment on a basement waterproofing backfill job depends upon jobsite access. For very large jobs where access is excellent, track rolling with a track loader or dozer may work. However, large track equipment is seldom practical when working closely around existing buildings. Good compaction requires that the soil be layered in “lifts” with the “optimum” moisture content. Soil can be brought in and spread with a backhoe or wheel loader. An excellent vehicle for spreading backfill on a basement waterproofing job is the skid steer. For compaction of large volumes of earth around existing building, either the ride-on or the walk-behind vibratory “sheep foot” roller should be considered.
For smaller basement waterproofing repair jobs with limited access, there are other compaction methods to consider. The “whacker-tamper” (or jumping jack) consists of a steel foot or plate which bounces along under the power of a gas engine sitting on top. The operator holds on for dear life as he attempts to steer the device along the desired path.
There is also a “pneumatic” or air-powered compactor called a “powder puff” which consists of a round metal foot on a sliding rod inside a cylinder. The powder puff is less productive than the whacker but lighter and much easier to use, especially on hillside basement waterproofing jobs. The powder puff requires an air compressor and hose. The operator moves the tool around as the rod oscillates up and down. Of course there is also a purely manual compaction rod and foot which is guaranteed to wear out the operator well before lunchtime.
When moving large volumes of dry soil on a basement waterproofing job, it may be necessary to use a water truck or hook up to a fire hydrant in order to get enough water on the fill. On the other hand, soil which is “over optimum” must be dried out prior to placement. This is done by spreading the soil out and praying that the sun shines and the wind blows. Be sure to stand by with plastic sheeting to cover the spoils in case of rain.
Soils Engineering for Fill Control
During a basement waterproofing job where significant volumes of fill are to be placed and compacted, it will be necessary to employ a soils engineering firm to perform compaction testing. As part of a “soils investigation”, the soils engineer will take a representative sample of the fill material and determine the optimum density by a laboratory procedure commonly called a “Proctor”. By mechanical compaction of the same soil under controlled conditions, while varying only the moisture content of the sample, the engineer can determine the moisture level which will produce the highest dry density (maximum density).
That moisture level which corresponds to the highest dry density becomes the optimum moisture level which the basement waterproofing contractor will then try to achieve in the field. The soils engineer will perform in-place density testing during the course of the fill operation to verify that the soil is being compacted to some pre-determined percentage (usually ninety percent) of maximum laboratory density.
Alternatives to Compaction
Because of the high cost of hauling and disposing of spoils in urban areas, the contractor may wish to stockpile spoils on the job if possible. If the volume of backfill is large, spoils are of good quality, and there is room on the job for stockpiles, then earth backfill and compaction may be the best option on a basement waterproofing project. In this case, both the gravel and the earth backfill will be brought up in lifts. Handwork will be necessary to manage the geotextile fabric which goes between the gravel and the earth backfill.
However, if there is no space to put the spoils, they must be hauled off. It is obvious from the discussion above that a basement waterproofing job can become more complicated when compaction is required. When hauling spoils away, we now have the option of stockpiling our spoils off site and returning them later to the jobsite. Alternatively, we can dump the spoils and import new material. With new material we have new options to consider:
Gravel costs more than common fill but, unlike soil and fill, gravel does not have to be compacted. This is because the large individual rock pieces fall quickly into a “closest packed” configuration upon placement. When you must import material for a basement waterproofing project, it may be more cost-effective to buy more gravel. The added cost of the gravel will probably be more than offset by the cost savings on compaction and compaction testing.
Plastic Drainage Sheets
When working with a large excavation and backfilling against a fresh application of basement waterproofing material, the contractor should consider alternatives to the conventional French drain design. I have mentioned plastic drainage sheets in connection with Bentonite waterproofing. Plastic drainage sheets may also be useful as a substitute for gravel to reduce hydrostatic pressure on a basement waterproofing job.
Drainage sheets are installed continuously and placed directly against the vertical wall. These sheets are manufactured with a network of channels which capture ground water from the adjacent soil. Filter fabric is attached to the outer face of each sheet at the factory so that only the water gets into the channels. Water flows down the network of channels by gravity and is collected by pipe or tubing running along the bottom.
I have used this type of system on a number of occasions. When doing so, I have always placed gravel at the base of the wall with the usual perforated pipe sloped to drain. But the sheeting saves me from placing gravel all the way up the face of the wall. The sheeting also serves as protection board so that a new waterproof membrane on the wall is protected from damage during the backfill process.
Closing Comments
It is important to remember that the topic of this website is foundation repair. I have argued on this page that basement waterproofing repair should be undertaken by a foundation repair contractor and not a new construction waterproofing specialist. In this discussion of basement waterproofing repair, I have talked about soil moisture and hydrostatic pressure. I have discussed how liquids and gases in the soil can easily penetrate normal concrete which is permeable because of the tiny capillaries generated during hydration. I have talked about a variety of waterproofing materials and made a distinction between positive and negative-side applications. I have also discussed some of the important tasks of a basement waterproofing repair job, the majority of which require knowledge and skills which are not directly related to the application of basement waterproofing materials. A successful foundation repair contractor will possess knowledge and ability in broad variety of trades, and basement waterproofing is among the most important. #tyj
