Archive for the ‘Structural Repair’ Category
Foundations and Swimming Pools December 31st, 2009
Introduction
Swimming pools, after they are constructed with RCC having thick floor slab and retaining walls and tiles fixed over the RCC spending huge amount, are often found to have problem of water leakage/seepage. As swimming pools are always in direct contact with water and the hydrostatic head is very high, the waterproofing of swimming pools has to be considered very seriously from beginning and adequate steps are to be taken to ensure their water tightness.
Usual Practice
Generally the way the concreting is done, is often not satisfactory. To ensure the strength of the concrete, water added is often insufficient to have a good workable mix. This makes the concrete difficult to be placed and vibrated. As a result honeycombs are formed inside the concrete. Some times in absence of supervision, excess quantity of water is added to make concrete more workable and easy for placing & spreading without compaction. This results in weak and highly permeable concrete.
Waterproofing compound used is either inadequate or not used properly. Construction joints are not treated properly. Only cement slurry is used as a bonding agent between old and new concrete. After the concreting, tiles are laid with ordinary sand/cement mortar and the joints are filled with cement paste.
Problems
- Concrete, not compacted properly and having honeycombs, allows water to seep through it. Weak and highly permeable concrete also allows easy passage of water.
- Water seeps more through the construction joints, as the bonding between old and new concrete is not proper.
- Due to shrinkage of cement, cracks are formed in the tile joints filled with cement paste and also in the tile laying mortar. Water seeps through these cracks.
- Leakage of water makes it difficult to maintain the water level of the pool.
- In overhead swimming pools seepage of water causes other problems for the area under it. In ground level pools, contaminated ground water from outside seeps into the pool and makes the water unhygienic.
- With water seeping through the concrete, the reinforcement gets corroded.
Suggestions
The concrete used should be made more workable (for heavily reinforced areas) by use of a suitable plasticizer to have a good cohesive mix that can be easily placed and properly compacted to avoid honeycombs.
- Integral waterproofing compound should be used for reducing the permeability of the concrete. But it should be added separately to the concrete mix, if plasticizer is also used.
- Any construction joints, prior to placing of new concrete, should be treated with a suitable water resistant bonding agent to ensure proper watertight bonding between the old and new concrete.
- After proper curing, all horizontal joints, vertical joints and construction joints are to be taken care of by coping with suitable waterproof mortar. Expansion joints, if any should be first filled with polysulphide sealant to permit movement of wall / slab of the joint.
- After necessary preparation of surface, concrete of all the walls (inside) and floor slab should be treated with a suitable surface applied waterproofing compound chemical. This chemical may be either hygroscopic crystalline reaction type or elastomeric polymer modified cementitious coating.
- Plastering over the treated concrete should be done with cement mortar admixed with a mortar plasticizer to avoid shrinkage cracks and to increase cohesiveness, adhesion and water tightness.
- Now, if tiling is to be done, tiles should be fixed with non-shrink, waterproof adhesives to ensure permanent bonding and water tightness. The joints should be filled with non-shrink, waterproof joint fillers available in various shades.
- The exterior of concrete retaining walls should for protected from aggressive chemicals by coal-tar based epoxy coating preferably in two coats.
Information provided by Trehun Associates (P) Ltd. http://www.tapl-in.com
Posted in Structural Repair | Comments (0)
Why Helical Piers?
December 28th, 2009
A comparison of alternative foundation supports
Gary Collins, P.E.
The simple answer is price and performance. In many cases helical piers are the easiest to install and this leads to lower cost. They also have the most predicable load carrying capacity. However, this is not always the case. Discussing these exceptions is the purpose of this article.
This article is aimed at installers and designers who are unfamiliar with helical piers and are trying to educate themselves to this increasingly popular form of foundation support. To do this, I will discuss the strength and weaknesses of all the varieties of foundation support. Then I will summarize them in a table for ease of comparison. The methods can roughly be divided into light and heavy structure supports.
A. Light to moderate structure supports
Helical Piers
Helical piers can be used almost anywhere traditional deep foundations can be used according to Don Bobbitt PE, an experienced helical pier engineer.[1] Typically, they are better suited to the lower capacity applications that make it less economical to use the larger install equipment required by the more conventional deep foundations. They also tend to be more economical in limited access sites or for vibration or noise free applications. However, the economics of each case generally controls the foundation selection.
Helical piers are installed where one has a torque driver machine that can screw them into the ground. Usually this is a hydraulic torque head mounted on anything from a portable torque frame that can fit into small spaces up to large backhoe mounted devices.
Helical piers screw themselves through the many layers and finally into bedrock. The layers are usually revealed by the varying driver torque. This is monitored by a torque pressure gauge read and recorded by the machine operator. The pier bearing capacity is roughly 10 times the “kips” indicated on the gauge. The operator is looking for a significant increase in torque indicating he has hit dense, firm load-bearing strata. For many locations, this is on average 20 to 30 feet below grade.
Sections are added as the pier is screwed into the ground. The final section is cut off at a level even with the other piers and capped with a load-bearing plate. It is immediately ready to receive a load. There is no cleanup. This process is quick if everything goes as planned and is a matter of hours for a multiple pier job. If difficult soil is encountered and pre-drilling is necessary to break into hard rock, it can take a matter of several days. A very good comparison with traditional drilled piers can be found at the Helical Pier World’s site: http://helicalpierworld.com/articles/taleof2part2.aspx This running account of two side-by-side jobs speaks volumes about many factors in pier installation.
An advantage of helical piers in expansive soil is that they resist upward forces. The helix is anchored in competent load-bearing soil or bedrock, and the frictional forces along the shaft are negligible compared to the end loading force. This means the helical pier is versatile with either upward or downward loads. This is not the case with other types of support without secondary operations or modifications such as filling them with grout.
There are different manufacturers of helical piers, and they are not all equal. Connection stiffness is an issue. It needs to be paid attention to since a weak joint under compressive forces will buckle. There are post installation techniques to stiffen the connections and shafts (see section on Helical Pull-Down Micropiles), but it is better to use piers that have stiff connections to begin with.
One advantage of helical piers is that if a rock is encountered that stops forward progress, the pier can be withdrawn and drilled several feet away. Don Bobbit has also written a very comprehensive paper on the many difficulties and non-technical factors involved in successful installation and use of helical piers. [2]
Hydraulic push piers
Hydraulic push piers are essentially a helical pier without the helix. These are steel rods driven down into the ground by a hydraulic jack which is pushing up against the foundation. To work, there needs to be something to push against, so these are not suitable for new construction where the foundation has not yet been poured. However they are suitable for remedial work and foundation lifting where the typical weight of the structure is in the neighborhood of one ton per lineal foot. However, one must be careful that the foundation that it is attached to is strong and can take a concentrated load.
The piers are typically driven one at a time so the full building weight is available to drive the pier. As each pier is driven, the friction between the soil and the pier accumulates until it exceeds the load being placed on the pier. This is called “driving to refusal” where the foundation just starts to lift and the pier refuses to go any deeper with the available foundation weight.
An advantage over helicals is that they 
can be installed without any torque equipment, generally closer to a wall, and can register the load capacity directly. The disadvantage is that since they are friction supported, expansive clay in the intermediate layers can lift them up unless they are deep enough into bedrock or other layers unaffected by moisture. This means that they must have enough force on them to drive them to bedrock, something that is not always done if the contractor is in a hurry or careless. Refusal should happen at bedrock but won’t if there is not enough reaction force from a lightly loaded foundation. In contrast, helical piers are end loaded, are not affected by a light foundation, and are generally unaffected by intermediate expansive soil.
The pier itself is cheaper than a helical pier because there is no helical blade.
Cable Lock piles
A cross between a concrete pile and a push pier is the Cable Lock pile. Developed by Olshan, it consists of segments of 6” diameter concrete cylinders each 3-4 ft long threaded onto a cable and lead cone called a cable anchor. See the illustration in
Micropiles
Micropiles are small diameter (5 inch to 12 inch) piles that can be installed in almost any type of ground where piles are required, with design loads as small as three tons and as high as 500 tons. They are termed “piles” because they involve driving a small diameter tube and forcing grout or concrete into it, as with large drilled piles.
Also known as minipiles, pin piles, needle piles or root piles, micropiles offer alternatives to conventional piling techniques, particularly in restricted access or low headroom situations. Hayward-Baker lists six types of piles. [3]They involve increasing degrees of soil compaction where the soil is too weak to carry the loads of a convention press pile.�
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Micro piles are drilled and grouted reinforced piles typically used for structural support where conventional deep foundation elements cannot be installed due to project constraints such as limited work space or where heavy machinery cannot be used.
Micropile installation causes minimal disturbance to structures, soil and the environment. Installation equipment usually consists of self-contained drill units, similar to those used for tieback anchor installation. Micro piles can be used in soil or bedrock. They can be installed for new construction or for existing foundation remediation.
Helical Pull-Down Micropiles (HPM)
A variation on helical piers is the helical micropile. This device uses a combination of grouted encased shaft and helical lead 
sections to form helical micropiles. This is especially useful in soft soils (N<5) which gives little lateral support to compressive columns. Chance who manufactures helical pull down Micropiles reports that a conventional helical pile of theirs failed at a compression load of 60 kips in soft silty clay whereas the Micropile resisted buckling at a cost of only 15 to 20 per cent more than the standard helical pier. [4]
As the helix borers into the ground, a plate attached to the column moves down with it. This plate pushes soil away and creates a void. The void is filled with grout and significantly stiffens the column. These micropiles have been tested to 200 tons, although the typical working load is 60 to 80 tons.[5]
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B. Heavy structure foundation supports
Driven piles
These are traditional driven steel piles. They require a pile driver, either hydraulic, explosive, gravity drop hammer, or steam driven. Because of the continuing blows, they are best used in remote areas such as highway and railroad bridges where the effect of the impacts cannot be felt in nearby structures or neighborhoods. This is important because damage can radiate hundreds of yards from where the pounding is done depending on the mechanics of the soil. It exposes the operator to liability from damaging nearby structures.
Driven piles can be various shapes and materials: H-piles or pipe piles or wood piles. Driven piles have no practical length restriction. When longer lengths are required, prefabricated splice plates are typically used. Structural capacity of the pile is easily calculated due to the consistent properties of modern rolled steel.
Auger cast piles
These are drilled piles. Auger cast piles are installed using a continuous flight auger (CFA), advanced into the soil by means of a hydraulic drill. 
The auger is drilled to the desired tip elevation or refusal where the grouting process begins. Grout is injected through the bottom port of the hollow stem auger, replacing the soil removed by the drilling operation. No casing is required.
The pile is then grouted to grade and set to the correct cut-off elevation. Reinforcing may be placed into the fluid grout. These piles range in diameter from 12 to 36 inches. Soil conditions and structural components of the pile dictate a capacity, usually from 50 to 100 tons. An advantage is that it is drilled and grouted with the same equipment. However, if a rebar cage is required, it is difficult if not impossible to insert it in a deep shaft. Therefore a rebar reinforced column must be shorter, reducing its integrity and the load it could handle.
A powerful torque head is required to rotate the column because of the torque friction. In addition, a large lifting capability is required because of the weight of the entire column of soil being lifted at once. An advantage of lifting the entire column is that the strata levels are visible immediately. However, there is a lot of cleanup.
A very informative give-and-take citing the advantages and disadvantages of auger cast piles compared to drilled piers is available on the chat page at http://www.eng-tips.com/viewthread.cfm?qid=164101&page=1 Also see www.augercastpiles.com
Concrete drilled pile caissons (Drilled shafts)
A drilled pier is a deep foundation system that is constructed by placing fresh concrete and reinforcing steel into a drilled shaft. This is the most traditional pile. It is done by drilling a large diameter hole several feet across, sleeving it to keep out water and debris, reinforcing it with a rebar cage dropped down the hole, and then pouring concrete in as the sleeve is withdrawn.
Drilled shafts can be used to sustain high axial and lateral loads. Typical shaft diameters range from 18 to 144 inches.
I recently spoke to a foundation engineer on the job who was drilling 60 caissons for a luxury home. As the crew was hitting water level and then blue clay bedrock, I asked why he didn’t use helical piers. He guessed that it was because he could see what was coming up out of the hole as they were drilling. In other words, he liked visual feedback. He said that if helicals hit a rock and went off at an angle, he couldn’t see it. However, he offered that he had never seen a helical fail. Moreover, he had never done a cost comparison.
Conclusions:
So why helical piers? In their load range, for remedial work I believe the choice narrows down to helical piers vs. push piers. For new construction it is helical piers vs. concrete caissons.
For helicals there are two things: the cost of blades and the torque head clearance hassle. Resistive piers don’t have those, but their weak point is friction in heaving soil. All the other methods have the disadvantages of large crews, large equipment, weather sensitive installation, and, except for driven piles, long waits for concrete to dry.
The choice also depends on economics. For example, if an operator is already set up to install push piers, then he should continue making sure there is proper force to push the pier into bedrock to make it insensitive to expansive soil upward forces. He also needs to make sure the reaction foundation is sound.
As always, all methods are subject to improper installation. One needs to choose the method least sensitive to operator error. There are stories of each method failing, dramatically. Usually these are the result of untrained installers or careless operators.
About the author
Mr. Collins is a graduate of Stanford University with a BS and MS in mechanical engineering. He has extensive experience in many aspects of mechanical and structural design. The owner of Collins Consulting, Mr. Collins lives in Boulder CO. A registered engineer in Colorado and California, he is available for consulting on helical pier applications. He may be reached at collins_consulting007@msn.com. (303) 530-4106.
[1]Bobbitt, Don, Helical Foundations & Tiebacks: An Introduction, Red Vector course at: http://www.redvector.com/LMS20/Account/Order/Shopcart/ItemDetail.aspx?id=0a778218-23b7-4eca-866b-c15baa190ff7
Posted in Deep Foundations, Foundation Retrofit and Repair, Structural Repair | Comments (0)