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MICRO PILE REINFORCEMENT SYSTEMS

 MICRO PILE REINFORCEMENT SYSTEMS and CORROSION PROTECTION.
Horst Aschenbroich, Dipl. Ing. President and CEO CON-TECH SYSTEMS LTD, Delta BC, Canada

Introduction:

Since mankind started to design and build structures for different usages and environ­ments, foundation systems to support such structures had to be developed in order to match the architectural and structural needs. With the ever-increasing urban expansions, it is not always possible to find good supporting ground at or close to surface level. Therefore, foundations other than spread footings were designed to transfer compression loads down to a suitable load-bearing stratum.

Higher and slender structures subjected to wind and seismic loads need foundations capable to support compression as well as uplift and lateral forces. Instead of large, mass concrete foundations, which require large areas and mass excavations, smaller and deep­er drilled shaft or pile foundations became a more economical alternative, in which steel reinforcing systems embedded in concrete and cement grout are the major component.

Micro Piles belong in this category of foundation elements. They are very simple but unique in design and construction and are becoming more and more popular.

The evolution of Micro Piles

Since its original conception in the 1950’s by Dr. Fernando Lizzi, a number of micro pile sys­tems using steel-bar reinforcement / cement grout combinations with or without steel pipe casing, have been developed.

Lizzi’s idea was, to produce a foundation system consisting of small pile groups, which form a reinforced soil mass like the root system of a tree. He called these PALI RADICE or “ROOT PILES” (see Figure 1).

Further developments using different installation methods and reinforcing systems made it necessary to capture them all under a general heading, first “MINI-PILES”, which was later changed to “MICRO PILES”.

With the creation of the International Workshop for Micro piles (IWM), first in North America and later internationally, MICRO-PILE became a household name in the Geotechnical and foundation industry. They are mainly used as Friction Piles to take tension and / or com­pression loads.

Figure 1: Pali Radice or Root Pile foundation examples (after FHWM-SA-97-070, 2000)

What is a Micro Pile?

A generally up to 300mm diameter, drilled and grouted pile with a centrically placed steel reinforcing member consisting of single or multiple bars.

Why are Micro Piles such a unique foundation system?

They can be placed with relatively small drilling equipment, single or in groups, under lim­ited access and low headroom conditions. They can be installed, for instance as the Titan IBO system, using rotation boring only. This reduces or eliminates the risk of structural damages caused due to vibrations, by otherwise used heavy percussion and pile driving methods, especially inside or in close vicinities of buildings.

Figure 2: Typical micro pile sections, left with solid bar reinforcing, right with hollow bar reinforcing or casing (after FHWM-SA-97-070, 2000).

Figure 3: Threadbars, All-Thread Bars and hollow bars (left to right).

The reinforcing materials are simply single solid or hollow bars with continuous outside threads, which can easily be spliced and coupled to any required length.

The intent of this presentation is to introduce, to designers and specialized foundation-engineering contractors, the different types of reinforcing systems and corrosion protec­tion methods available for drilled and grouted Micro Piles.

Pile Types and Reinforcing Systems

During the evolution process of developing Micro-Piles over the past 40 years, besides different drilling equipment, a variety of continuously threaded reinforcing bars and grout­ing systems have been successfully introduced.

The “GEWI PILE” System

When I first introduced the Dywidag Threadbar System in North America (in 1967), I had the opportunity to propose this bar as a post-tensioned single bar reinforcement in Tension Piles for the Bonnybrook Sewage treatment plant extension in Calgary Alberta, Canada.

Approximately 1500 piles were required to support large sewage aeration tanks against uplift. This became the first application (worldwide) for Dywidag bars in Piles and in the geo­support industry. A drill-through Diesel Hammer was used driving casing through the over­burden, cleaning out the casing with an inner drill steel and advance-drilling the same into the underlying bedrock. In the free stressing length, the bars were corrosion protected by a shop applied heat-shrink sleeve with inner asphalt coating (Yellow Jacket). As an addition­al bond breaker, a metal sheath was placed over the coated bar. The drill hole and casing was tremie-grouted with cement grout. Each pile was stressed to a test load and locked off at a design load equivalent to the uplift force.

Soon after, in the early 1970’s,^, Dywidag started to market the grade 60 and grade 75 rein­forcing steel thread-bar system, called GEWI Bars, which lead to the development of the

Table 1: GEWI pile bar steel properties (courtesy DSI).

“GEWI PILE” by Dr. Thomas Herbst, who was at that time chief of the geotechnical devel­opment department. GEWI originates from the German word GEWINDE or THREAD. These piles are installed using open or cased hole drilling methods. The GEWI BAR forms the concentric reinforcing element. The drill hole is filled with cement grout. In order to increase the grout to soil bond capacity of the pile, especially in cohesive soils, post-grout tubes are installed at the outer perimeter of the grout body. Post-grouting can be repeated several times until the required pressure or skin-friction is achieved.

Figure 4: GEWI Pile (typical) with standard and double corrosion protected reinforcing bar (after FHWA-SA-97-070, 2000)

Figure 5: Typical post-grouting system (after FHWA-SA-97-070, 2000)

The “PIN PILE”System

The PIN PILE is a development in the 1970’s by the Nicholson Construction Company USA. This method uses an outer pipe casing to stabilize the drill hole and an inner drill rod for cleaning out the casing or drilling further into harder ground. After placing the centric rein­forcing element, single or multiple bars (see figures 6 and 7) and filling the casing with cement grout, the casing is slowly pulled under constant pressure grouting and partly left in the ground as additional reinforcement to increase bending moment and / or lateral load capacities and to prevent grout loss in grounds with large voids. Post-grout systems can be used with these piles as well.

The Threadbar or All-Thread Bar systems (tables 2 – to 5) are supplied by the ADSC Associate Members

CON-TECH SYSTEMS, (CTS)

DYWIDAG SYSTEMS INTERNATIONAL, (DSI)

WILLIAMS Form Engineering

Figure 6: Pin Pile installation sequence (FHWA-SA-97-070, 2000). Figure 7: Single (left) and multiple bar (right) micro pile reinforcing.

Table 2: Properties of cold-rolled grade 75 (yield) All-Thread bars.

Table 3: Properties of cold-rolled grade 150 (ultimate) All-Thread bars.

Table 4: Properties of hot-rolled grade 75 (yield) All-Thread bars.

All-Thread Bar Steel Properties Hot-Rolled Grade 96 (yield), ASTM-A615

Load Capacity

Nominal Major Thread Steel Area Weight Diameter Diameter D Ultimate

Yield Load

28 mm 616 mm² 490kN

410kN

32mm 4.83kg/m

1 1/8 in 0.95in² 110.2K

92.2K

1.26in 3.25lbs/lf

30 mm 720 mm² 575kN

480kN

34mm 5.65kg/m

1 1/4 in 1.12in² 129.3K

107.9K

1.34in 3.80lbs/lf

35 mm 962 mm² 770kN

640kN

39mm 7.55kg/m

1 3/8 in 1.49in² 173.1K

143.9K

1.54in 5.07lbs/lf

43 mm 1466 mm² 1170kN

980kN

47mm 11.51kg/m

1 5/8 in 2.27in² 263.0K

220.3K

1.85in 7.73lbs/lf

57.5 mm 2597 mm² 2080kN

1740kN

62mm 20.38kg/m

2 1/4 in 4.03in² 467.6K

391.2K

2.44in 13.69lbs/lf

63.5 mm 3167 mm² 2540kN

2120kN

68mm 24.38kg/m

2 1/2 in 4.91in² 571.0K

476.6K

2.68in 16.38lbs/lf

Table 5: Properties of hot-rolled grade 96 (yield) All-Thread bars.

The “TITAN / IBO – INJECTION-BORED MICRO PILE”

The successful construction of a Micro-Pile, involves a number of steps.

                Drilling,

                Placing of reinforcing steel.

                Grouting.

One of the latest developments is a system and method, which combines all in one single step installation.

This method uses hollow bars, sometimes in combination with inside solid bars or strand, which can also be post-tensioned (figure 10 and figure 14).

This Injection-Bored (IBO) pile is a joint development between the companies Friedrich Ischebeck Gmbh, Germany and Con-Tech Systems LTD, Canada. The goal was to pro­duce a drilled, grouted and reinforced Micro Pile following the original Root Pile idea by Lizzi. The pile totally integrates with the soil. It forms a foundation system of reinforced soil mass, in particular if placed in groups. The piles are drilled-grout-injected in one step, using the hollow bars as drill rods and grouting ducts with disposable special drill bits (fig­ure 12) and rotary drilling methods. The drill bits have jet openings allowing for pressure grout penetration while drilling. During drill advancement and grout injection through the hollow bars, with the aid of a flushing head, the drill cuttings are continuously flushed or

Figure 8: Exhumed TITAN Pile

tremied out by the cement grout. It is a clear advantage of this method that the drill hole is stabilized, and the ground cannot relax or cave, but to the contrary is grout penetrated and densified. Figures 8 and 13 show this on an exhumed pile.

The basic idea was, to produce a pile of very high capacity using small drilling equipment, which can operate in tight areas with limited overhead space inside buildings to underpin or seismic upgrade existing foundations (Figure 9).

Figure 9: Limited overhead installation of TITAN micro piles.

Figure 10: Typical TITAN/IBO micro pile details

Micro-Pile Reinforcement Systems and Corrosion Protection, Horst Aschenbroich, Con-Tech Systems Ltd.

Table 6: Properties of CTS-TITAN hollow bars

Table 7: Properties of MAI hollow bars

Figure 11: CTS-TITAN hollow bars

Figure 12: CTS-TITAN special disposable drill bits for various grounds

Figure 13: Section of exhumed TITAN / IBO micro pile

Densified Ground

Soil / Cement Mix Neat Cement Grout Hollow TITAN Bar

Two types of hollow bars are available (see tables 6 and 7)

The CTS-TITAN Hollow Bars (table 6, figure 11), supplied by CON-TECH SYSTEMS LTD, in sizes up to 130 mm, 5 1/8” diameter with tension design load capacities in excess of 400 Tons. These bars are rolled with special continuous TITAN Threads for excellent bar to grout bond development. The bond development and crack width distribution of TITAN bars in tension and embedded in cement grout, had been tested at the Technical University of Munich in Germany. The results show, that at 125% of the maximum allow­able design load, the maximum crack width in the grout is less than 0.1mm. This is still considered complete corrosion protection of the steel under the German Industry Norm (DIN). No additional corrosion protection is thus required.

For variable ground and load conditions, different drill bits (figure 12) are designed and available. For the TITAN IBO Micro Pile, venturi jet-grout holes in the drill bits allow the jet grouting pressure to over-ream and pressurize the drill hole. Because of the continuous tremie-cement grouting operation, 100% grout cover can be guaranteed (figure 13).

The MAI Hollow Bars (table 7), supplied by DSI. These bars are rolled with a standard continuous Rope Thread (R-Thread).

All hollow bars are generally supplied in 10 foot lengths, (a standard length of a drill rod for easier handling) spliced together with special couplers.

Figure 14: Internal post-tensioning of TITAN micro pile.

Another feature of the hollow bar is the possibility of adding an additional solid rebar inside the grout filled bar, or placing a strand tendon inside to apply an internal pre-stress force to control elastic movement of the hollow bar (figure 14).

A special type of pile is used in California by Caltrans to upgrade existing viaduct founda­tions for seismic events. This pile consists of a steel pipe casing drilled through the over­burden. DCP, Double Corrosion Protected Strand tendons are placed through the pipe and anchored into the bedrock below. The pile is then vertically post-tensioned and cast into the foundation (figure 15).

Figure 15: Post-tensioned piles for seismic upgrading of bridge foundations (CalTrans).

CORROSION PROTECTION

If, besides cement grout, additional corrosion protection is required, several methods are available:

1.) Hot Dip Galvanizing or Zinc-Metallizing

Steel bar components can be hot dip galvanized or metallized as per ASTM A­153(AASHTO M232). Zinc is a well known, common and relatively inexpensive coat­ing material for iron and steel. Zinc acts as a sacrificial anode, i.e. it corrodes in a corrosive environment and lets the steel play the role of the cathode. The high alka­linity of concrete and grout (pH > 12) dissolves the zinc to a certain extent, at pH < 12 a very low corrosion rate of zinc occurs due to the development of a passivation film on the zinc surface. This film will stabilize if atmospheric CO₂ reaches the sur­face of the zinc coating. This is the reason for the known durability of zinc coatings under open-air conditions.

Galvanizing requires tight control of coating thickness to assure threadability. In most cases the thread inside the nuts or couplers has to be oversized, which could cause a reduction in load capacity. We have found that by using the metallizing method, oversizing the threads is not necessary. Both methods, if properly applied to the ASTM Standard, provide a good protective coating. 

2.) Fusion bonded Epoxy Coating

Epoxy Coating shall conform to one of the following: ASTM A-934, ASTM A-775, or AASHTO No. M284. Applying this coating requires oversizing of the hardware threads. Care must also be taken not to damage the coating.

3.) DCP, Double Corrosion Protection System. (Not for hollow bars)

This method is mostly shop applied to solid Threadbars or All-Thread Bars. This Type

1)

2)

3) of additional corrosion protection was part of the original Dywidag GEWI PILE and ground anchor development and has found worldwide acceptance.

Figure 16: Corrosion protection: 1) Hot dip galvanizing, 2) Epoxy coating 3) DCP (left top to bottom) and DCP detail (right)

ADSC Micro-Pile Seminar, Charlotte NC, November 13, 2001

The bar is centered using spacers and is fully encapsulated inside a corrugated PVC or HDPE sheathing. The annular space between bar and sheathing must be a mini­mum of 5 mm (0.2 inch) thick and shop cement grouted.

The sheathing must have sufficient strength to prevent damage during construction operations, shall be watertight, chemically stable without embrittlement, softening, and nonreactive with concrete. The minimum sheathing wall thickness shall be 40 mils. The material must conform to ASTM D-3350 polyethylene, Index No. 335520 C, Table 1, ASTM D-1248, and AASHTO No. M252 for HDPE or ASTM D-1784 Class 13464-B for PVC.

The encapsulation shall be fabricated from material with the following properties:

i Capable of transferring stresses from the grout surrounding the tendon to the grout in bond length

i Able to accommodate movements during testing and after lock-off;

i Resistant to chemical attack form aggressive environments;

i Resistant to aging by ultra-violet light;

i Non-detrimental to the tendon;

i Capable of withstanding abrasion, impact and bending during handling and instal­lation and

i Capable of resisting internal grouting pressures.

If steel bar couplers are used, they will be field installed with a double or multiple cor­rosion protection (DCP or MCP) system as per manufacturer instructions.

The cement grout inside the annular space between steel and corrugated sheathing is the most efficient element of corrosion protection. It must provide a proper alkalin­ity, low permeability, high resistivity, minimum to no shrinkage in both plastic and hardened states, proper fluidity, little or no segregation and no bleeding.

4.) Sacrificial Steel design method (see table 8 )

Is used primarily for oversizing pipe casing but can also be used for the pile rein­forcing bars. The ISCHEBECK Hollow TITAN Bars are tested in various non-aggres­sive, mild-aggressive and aggressive soils for loss of steel area over a 60 to 120 year design life (see table 8). This method is extensively used in Europe and presently started to be accepted in North America.

Sacrificial Steel Method

The data and information can be used to determine the sacrificial steel thickness, if no additional corrosion protection (metallizing, galvanizing, stainless steel) is used on CTS/TITAN IBO Bar anchors.

Corrosion Of Buried Metal

Taken from: TRL Report 380/1993 Applied to: Ischebeck TITAN hollow groutable anchors

60 Years

120 Years

Table 8: Sacrificial steel method

References

Micro Pile Design and Construction Guidelines: Implementation Manual, US Department of Transportation – Federal Highway Administration, FHWA-SA-97-070, 2000.

Grouted Piles, DIN 4128 9.2, Deutsche Industrie Norm (German Industry Norm) Crack Width Distribution in TITAN Anchors, Technical University Munich, Institute of Civil Engineering, Prof. Dr. Ing. K. Zilch and H.H. Mueller.

Corrosion of Buried Metal, TRL Report 380/1983, Great Britain, 1983.

Posted in Deep Foundations, Foundation Retrofit and Repair, Ground Improvement | Comments (0)

Micropile: Geotechnical technologies stabilize effects of settlement

Building settlement and cracked concrete are simple facts of life in construction. However, though all structures experience settling in one fashion or another, the level or degree of the settlement determines whether or not problems will result. This occurrence also is a reality for sites slated for redevelopment if they are located in close proximity to other structures as the tight site may deem many geotechnical services unrealistic because of the harmful results of vibration or noise. However, one method that is growing in popularity as a means to reduce the effects of settlement for both existing and new construction is micropiles.

The Source of Settlement

In recent years, especially in urban areas experiencing a huge growth in redevelopment — either by tearing down an existing building and building something new in its place or by upgrading an existing structure — a means to cost-effectively stabilize structures has become paramount. Caused by poorly compacted fill, improper drainage, development of sinkholes, or even improper design, settlement can have drastic effects on a structure. If a structure experiences loads for which it was not designed, severe cracking in the floor or walls is likely and doors or windows may become difficult to operate. Further, drainage and plumbing may separate. In extreme cases, the building can actually collapse. As such, it is important to begin with a thorough examination of the subsoil, including its condition and compaction, as the subsoil greatly affects the long-term settlement of any structure. Once identified, measures can be taken to improve the stability of the ground conditions and thus stabilize the structure. However, geotechnical solutions must not only address a myriad of variables and complexities that make each project unique, but also solve underground challenges. An extensive knowledge of soils, structures and geotechnics is required. The right solution balances knowledge with specialized skills and equipment to improve ground conditions, control settlement, as well as stabilize and strengthen foundations and slopes. For many, micropiles are the ideal solution.

Micropiles Defined

Micropiles, also commonly referred to as minipiles and pin piles, are small diameter reinforced piles that are drilled and grouted to support structures in all ground conditions. These piles may reach working loads up to 300 tons, can be installed to depths of approximately 200 feet, and usually utilize some type of steel bar or bars and/or steel casing pipe. The bars are grouted into the ground and/or the casing pipe is filled with grout. While conventional pile — steel or reinforced concrete either driven or cast-in-place – is generally quite large and requires heavy equipment and large staging areas for installation, micropiles can be used in applications where conventional piling is not convenient or possible, such as for underpinning or retrofitting existing buildings or structures.

Micropiles have proven effective in many ground improvement applications by increasing the bearing capacity and reducing settlements, particularly in strengthening the existing foundations. The names minipile or micropile depict the respective size of the pile: minipiles are six- to 12-inches in diameter while micropiles measure two- to five-inches. Depending on pile diameter and soil conditions, micropiles can extend to depths of 200 feet and exceed design loads of 400 kips. The pipes used for micro or minipiles installations are in segments that feature threaded lengths (male and female ends that allow them to be fitted together).

Because the pipes are inserted one at a time in lengths of three to four feet, they are suited for low access and retrofit applications. Another benefit is that the drilled installation methods can be used for new construction applications where surrounding structures are sensitive to vibration. Micropiles using large-diameter threaded bars (75ksi) can also be used to attain specific design loads when traditional “H” piles (a common foundation structure) cannot be used because of overhead physical constraints.

Another method of overcoming the challenge of micropile installations in areas with restricted headroom is the use of the simultaneous micropile casing and drill rod method. The micropile process begins with drilling into the bedrock using a special rig capable of handling duplex drilling. Next, the piles are bonded to the wall of the rock socket and the pile casings are then advanced as micropiles by drilling into the bedrock. The micropile drill pipe is removed, leaving minipiles in the rock socket, and reinforcement bars are lowered into the micropile steel casings. Grout is pumped or pressure-fed into the casings and the piles are lifted to the mouth of the sockets to allow bonding to piles.

Finally, the micropile tops are cut to elevation and capped for foundation rebar. The last step involves load-testing the piles to prove the design.

Building A Better Foundation

After determining the cause of settlement through a geotechnical boring survey, a structural engineer and geotechnical engineer can evaluate if micropiles are the best solution for the specific project.

For projects requiring structural support, micropiles serve as an efficient means to underpin an existing or new structure, reduce/prevent settlement, upgrade foundation capacity, repair and/or replace deteriorated foundations and serve as a seismic retrofit. In contrast, for scenarios requiring in-situ reinforcement, micropiles provide settlement reduction, structural stability, slope stabilization and soil mass strengthening. In cases of strengthening an existing structure because of the effects of settlement and/or a desired change in use that requires an increase in loads, micropiles provide an excellent alternative because of their cost-effectiveness in the use of drilled shafts in which piles are installed down through the floor and the footer to give the building additional support. Such was the case for Michie Stadium in West Point, New York. Constructed in 1924, the football stadium for the United States Military Academy is one of the most beloved structures of its kind. At the dawn of the new millennium, many construction projects were undertaken to meet the new demands on this old structure. These included renovations, a new athletic center and press box, the Hall of Army Sports, the 150,000-square-foot addition of a new strength and development center, an auditorium, and other amenities.

These new structures were to be constructed outside and immediately adjacent to the existing stands. During the excavations for the new facilities, however, it was discovered that the footings of the columns and bearing walls of the stands were bearing on soils above the proposed bottom of excavation. This scenario caused great concern about the potential instability of the stadium structure. Underpinning of the footings was deemed the only option; however, there was also concern about significant impact to the construction schedule.

Working in concert with Schnabel Engineering, Structural Preservation Systems’ GeoStructural Division provided a turnkey solution to this problem that included utilization of micropiles for underpinning the footings prior to the start of the excavations. The micropiles were conceived as self-drilling, self-grouting bars embedded five feet into the rock. For the column footings, Schnabel Engineering designed a reinforced concrete cap attached to the existing pedestal. The bars were provided with a top plate that was embedded in the pile cap concrete. Then, for the wall footing, a bracket was used to transfer the load from the wall to eccentric micropiles.

This efficient design facilitated an accelerated micropile installation time with reduced or no impact to the project schedule. In fact, the entire underpinning system was in place in a mere week.

Being a Good Neighbor

Although micropiles are certainly a proven solution for existing structures, the method also is a growing solution for new construction because of its minimal impact to surrounding structures as compared to driven piles that typically cause extensive vibration. And, as compared to caissons or drilled shafts, micropiles are typically more cost-effective. Case in point was a building torn down between two other structures on 42nd Street in New York City.

The owner sought to construct a new 18-story condominium facility, but the site only provided inches between the two existing structures, requiring a solution with minimal vibration and noise effects. Driving piles using conventional methods was thought to be too risky because of the proximity to the existing buildings and drilled caissons were cost-prohibitive. Further, the soil conditions were very soft and the borings located rock 30-feet below the surface. Because of these factors, the micropile technique was selected by the geotechnical and structural engineers.

In January 2005, Structural Preservation Systems began installation of the 7-inch diameter, 35-foot-long piles. By February, the settling issues were resolved. A similar project recently constructed in a Harlem neighborhood in New York City involved the construction of a new 16-story condominium facility sandwiched between two existing structures. This site had no piling – just built-up boulders. Because of these conditions and the fact that other methods would create vibration harmful to the existing buildings, micropiles were chosen as the ideal solution.

Structural Preservation Systems installed the 9-5/8-inch diameter, 25-foot long micropiles at a rate of about four per day. Again, the micropile technique resulted in a high capacity pile in a short timeframe.

Pilling Up Success

In addition to its applications in commercial structures, the micropile method has gained interest from the public sector as a viable solution for bridges and roads. Regardless of the structure, a solution utilizing micropiles, developed by a team comprised of skilled geotechnical and structural engineers and an experienced geotechnical contractor, can meet underground challenges in a cost effective, efficient manner.

Reprint of article by Structural Preservation Systems, LLC – A Structural Group Company

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Foundations and Swimming Pools

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

1. Concrete, not compacted properly and having honeycombs, allows water to seep through it. Weak and highly permeable concrete also allows easy passage of water.
2. Water seeps more through the construction joints, as the bonding between old and new concrete is not proper.
3. 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.
4. Leakage of water makes it difficult to maintain the water level of the pool.
5. 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.
6. 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. 

1. 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.
2. 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.
3. 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.
4. 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.
5. 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.
6. 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.
7. 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

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Why Helical Piers?

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.

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