Scheme of Titan micropile with tubular reinforcement. Source: http://www.ischebeck.com/ - access 20.06.2014. A great benefit of this technology is the speed, ...
Chapter 15 Kamil Dubała1, Jacek Selejdak2
MICROPILES – APPLICATION AND METHODS OF CALCULATION Abstract: Micropiles are a modern method of execution of works of the foundation, reinforcement of the construction and securing slopes, embankments. The article describes and presents the use of micropiles in the modern constructions. Eurocode 7 and PN-83/B02482 do not specify the ways and methods of calculation of the micropiles. Following the approach in accordance with Eurocode 7 and by applying calculation methods provided in PN-83/B-02482, the way of calculating the load capacity of push-in micropiles has been described, supplying the safety level required by Eurocode 7. Key words: micropiles, small dimensional piles, pile foundation, injection micropiles, projection, application of micropiles
15.1. Characteristics and application In the 50s Dr. Fernando Lizzi has developed an innovation technique, based on the observation of nature, “root piles” (pali radice) – small diameter piles, which made in the ground form along with the substrate a tree root system-like, capable to transmit the vertical forces as well as breakout and indentation transverse forces. With the development of technology, the system of self-drilling injection micropiles has become a modern method of executing foundation work wherever ground conditions do not allow to execute traditional leveling of foundation or when there is a need to strengthen or repair existing foundation. Micropiles are also used to secure slopes and walls of excavations, strengthening of embankments, stabilizing landslides, retaining constructions, tunneling. Micropile is defined as a slender structural element transferring the load on substrate mostly by friction on the side. In connection with a small diameter in relation to the length of the pile, the loads that can be transferred on the substrate through the vibration poker (base) of the pail are small, so micropiles are typically designed as floated elements. In accordance with classification of micropiles determined in [1], the distinguished micropiles are: − drilled with a shank diameter of up to 300 mm, − displacement (ramming, indentation, vibration or screwing) with a diameter of up to 150 mm. The load element in accordance with standard [2] in the micropile can be screwed steel pipe, rebar (reinforcing bar), cluster of bars or steel profile (Fig. 15.1). The 1
2
mgr inż., Faculty of Civil Engineering, Czestochowa University of Technology dr hab. inż. prof. PCz, Faculty of Civil Engineering, Czestochowa University of Technology
load of these element can be increased by injecting the base (by creating vibration poker) and side (ground tie bar).
Fig. 15.1. Load elements of micropiles Source: Kłosiński B., Mikropale – stan techniki i perspektywy, „Nowoczesne Budownictwo Inżynieryjne”, Maj – Czerwiec 2011, page 72 - 76
Micropiles are used as elements (Fig. 15.2): −
−
load-bearing, that operate as press-in and pull-out elements (they transfer axial forces), used as intermediate fundaments attached to the main construction through a head beam or to strengthen existing structures, barrier, which transfer bending moments and transverse forces, used to stabilize landslides, securing slopes and excavations. a)
b)
Fig. 15.2. Application of micropiles: a) strengthening of fundament,, b) stabilization of landslides Source: FHWA NHI-05-039, Micropile Design and Construction, Reference Manual, December 2005
15.2. Injection micropiles Technology of executing injection micropiles consist in drilling into the ground with a steel rod edged with a drill bit and then pressure injection of the borehole. Forming of the poker of load-bearing micropiles is carried out by zone injection at a pressure of from 1 to 6 MPa or by concreting or by single injection at a pressure
of minimum 0.5 MPa. The poker should have dimension of at least 3 meters in the ground and from 0.5 to 1 meter in the rock. Micropiles can be formed of reinforced concrete with reinforced frame made of rods of dimension equal or greater than 12 mm or complex in which the load-bearing element can be a steel rod, thick-walled pipe or steel section (i.e. I-section). In micropiles operating on tension, reinforced steel should be used with higher strength such as RB500W. Popular method of making drilled micropiles are self-drilling technologies developed by Titan or SAS (Fig. 15.3). In these systems, the pole (usually tubular rod with diameter of from 30/16 to 130/60 mm with cold-rolled thread) with connectors, stand offs and drill bit is a complete set which is both a drill and a load bearing structure of micropile. Poles are treated as stay-in-place formwork of the rod and used as injection pipe.
.
Fig. 15.3. Scheme of Titan micropile with tubular reinforcement Source: http://www.ischebeck.com/ - access 20.06.2014
A great benefit of this technology is the speed, because the entire process of carrying out an element is in one stage with the usage of standard rotary-hammer drilling rigs. It also allows to micropile in hard to reach places such as basements and making piles of large diagonal tilt, formed so to produce trestle system and to transfer horizontal forces on the ground center. For injection, the paste from Portland’s cement is used with strength class of minimum 32.5 N/mm2 and the water-cement ratio of 0.7, pumped through the core of the rod at a pressure of from 1 to 2 MPa. The paste acts also as scrubber making cement film on the walls of the formed hole, ensuring their stability which gives the opportunity to resign from the use of casing pipes. Scrubber penetrates into the ground and solidify the zone formed around the hole, so that the load-bearing of that micropile can be higher by approximately 30% from micro pales formed in the traditional way. The appropriate injection is carried out after the borehole and consists in filling the hole with cement scrubber of water-cement ratio – 0.4 (Fig. 15.4).
Fig. 15.4. Scheme of micropile Source: System TITAN, http://www.titan.com.pl/pobierz/78 - access 20.06.2014
Micropiles made in the technology mentioned above are used as anchoring elements and loaded periodically. As oppose to ground tie bars (anchors) they do
not require pre-overloading or loose (free) section of tie rod. Usually the edge of the pile heading consist of standard steel resistive plate with cast-iron nut screwed on the thread of the reinforced pole. The flaw of self-drilling technology is the limited amount associated with the value of possible, obtainable injection grout pressure and the difficulty of execution of anti-corrosion protection other than through enfolding the reinforcement with the poker of micropile.
15.3. Micropile projecting methods Projecting the pile foundation requires thorough knowledge of the design of the entire structure, functional requirements that must be fulfilled by the fundament (load-bearing of the structure, permissible settlement, maximal permissible unevenness of the settlement, vibration-sensitivity) and taking into account the neighborhood of other objects as well as the amount of available space for drilling equipment. The basis of each piles project are: − examination of the ground foundation to the depth required for safety setting of structures, − choosing the right type of pile for the projected structure and taking into account the ground conditions and technological capabilities, − calculating the forces both in piles and fundaments, calculating the loadbearing and settlement of the fundament, − determining the technology and conditions of forming of the piles, − examination of load-bearing and quality of the executed pile fundament by carrying out a test of static and dynamic loads in accordance with [1], − executing a measurement of as-build settlement and displacement of the entire structure. According to the standards record [1] and [3] due to the still imperfect models describing the ground foundation, the only reliable method of assessing the actual bearing capacity and settlement of the piles are static load test. To determine the theoretical calculation of the load-bearing of micropile, the following should be taken into account: − side resistance and the base of the pile (external –geotechnical load), − strength of the reinforcement of the pile (internal load), − protection against bulging (press-in micropiles), − strength of connection between pile and construction, − borderline condition of utilization, − durability of the element.
Cooperation between pile and ground is showed in a generalized way on Fig. 15.5.
Fig. 15.5. Cooperation between pile and ground (Q = Qs + Qb – ground resistance, Qs– side resistance ,Qb– base of the pile resistance, t – individual resistance on the side, q – individual resistance on the base of the pile, D – diameter of the pile, h – cavity of the pile in the ground) Source: Gwizdała K., Projektowanie fundamentów na palach, WPPK 2005, Wisła – Ustroń 2005
The standard [1] point 2.1 (4) introduces four methods of projection (or combination of these methods): − perform calculations by using analytical model, the semi-empirical model, the numerical model, − application of the required provisions determined for the country, − experimental models in real or smaller scale or load tests, − methods of observation, in which the solution of project is continuously adjusted during construction. [1] requires examination of construction by using borderline load condition methods (ULS) and utilization (SLS). In accordance with point 6.2 (1), the following borderline load conditions are determined: − EQU – loss of overall stability (loss of balance of the structure or modeled ground as a rigid block), − STR – internal damage or excessive deformation of the construction or construction’s elements, including fundaments footing, piles, walls of the underground, − GEO – damage or excessive deformation of the substrate, when the importance for the capacity of construction has the strength of substrate or rocks,
−
UPL – loss of balance of the construction or ground caused by water uplift forces, − HYD – water bulking, inner erosion and hydraulic rupture. Depending on the subsurface and hydrological conditions and the localization of the fundament, there is considered one or more state of borderline , however usually it is sufficient to demonstrate the fulfillment of load capacity requirements in the states STR and GEO ([1], point 2.4.7.3.): ,
. (5)
∑KIL CAI JI GAI 8 = 8? + 8A = B C? D(5) G? + (6) (:) where: q - calculated resistance of the ground under the base of the pile, (:) t 1 - calculated resistance of the ground along the side of the pile present within the i-layer, S= , S>1 - technological factors that depend on the type of pile, state and type of ground, A= - transversal section area of base of the pile, depends on the technology of the pile and the type of the ground, A>1 - area of the side in the I-layer of the ground, m - reduction factors taking into account the work of the piles in the group, γ. – material factor of the ground, in accordance with [3] shall be 0.9. Micropiles transfer the load mainly through friction t generated on the side of the pile, that is by touching on the surface of injection poker and the ground center. As oppose to traditional piles, the ground resistance q under the base of the pile is omitted in the load capacity calculations (except for micropile settlement on the rocks). The standard [1] does not specify the ways and methods for determining the value of q and t, so the methods of determining these values presented in [3] can be accepted as reliable. The values of factors of friction under the base of the pile and side are determined in accordance with [3] in relation:
D (5) = B D (5) DI = B JI where: γ. – material factor, γ. ≤ 0.9, q – ground resistance under the base of the pile, t - ground resistance along the side of the pile.
(7)
While using the calculation methods given in [3] and in accordance with the process described in [6] for case of a single fundament, the calculation is made for one examined ground profile, therefore R +,+,-+ = R +,+,-+ ./,0 = R +,+,-+ .10 oraz ξ = ξ( = ξ) = 1,40 according to Table 15.1.
Table 1. Correlation factors for determining the characteristic values on the basis of the results of ground examination (n – number of given profiles)
ξ for n= ξ3 ξ4
1 1.40 1.40
2 1.35 1.24
3 1.33 1.23
4 1.31 1.20
5 1.29 1.15
7 1.27 1.12
10 1.25 1.08
Source: PN-EN 1997-1:2005 Eurokod 7, table A.10
Using the patterns (4),(5),(6) it can be written: ,
=
,
∙ ∙
=
V
B C? D
(5)
G? +
∑KIL CAI JI(5) GAI W
(8)
Ergo, the load capacity of push-in pile in the borderline state of the load calculated according to the requirements provided in [1] following the process described in [3] and [6] can be written: , XY L
∙
∙Z∙
V
B C? D
(5)
G? +
∑KIL CAI JI(5) GAI W
(9)
In the above mentioned relation there are technological factors Sp and Ss, which shall be in accordance with table 4 of [3] point 2.2.7. The standard [3] does not specify which technological factors should be chosen for micropiles, however as it is known, micropiles have much better capacity for transferring loads along the side than traditional piles, thus taking into account the technological factors as for classic piles, the provided results should be on the safe side, lowering the real load capacity of the micropiles. The technological factors for micropiles are listed in the table 2. It appears advisable to accept the technological factors in accordance with point 4c of Table 15.2, because of the lack of accurate factors for micropiles.
Table 15.2. Technological factors
Source: PN-83/B-02482 Fundamenty budowlane. Nośność pali i fundamentów palowych. Table 4
Internal load capacity (load capacity of the reinforcement of the micropile) is determined by the pattern: [ < \, (10) where: [ – calculative load affecting the micropile, – calculative strength of the reinforcement of the micropile \, \,
=V
],
]
W.
Partial safety factors are chosen in accordance with [5] same as for reinforcing steel rod in sustainable calculation situation: γ^ = 1.15 According to [1] examining the bulging of push-in micropile is not required, when the strength on the wall of the ground in conditions without backflow `a, exceeds 10 kN/m2. When calculation of bulging of the pile is required, should be applied at `a < 10 kN/m2 m2 without the side ground reinforcement layer, at 10 kN/m2 < `a < 30 kN/m2 with a layer of side bedding.
15.4. Conclusion Micropiles are widely used in civil engineering. They are used in both the repair, reinforcements as well as in industrial and bridge constructions. The advantage of micropiles is the speed of execution, high load capacity and capability of execution, even with limited amount of space (i.e. basements, retaining walls). The regulations of executing micropiles is provided in the standard [2], however this standard is a little of use upon projecting. Both the standards [1] as well as [3] do not specify detailed requirements and calculative processes for micropiles. The process of calculation of load capacity of push-in micropiles, described in detail in [6], is an attempt of connection of the requirements between the standard [1] to widely known processes and calculation methods of the piles in accordance with [3]. The described method provides by omitting the capacity under the base of the micropile, as well as through the use of technological factors for traditional piles to obtain the calculative load capacity of micropile of a lowered value in relation to the actual load capacity. References 1. 2. 3. 4. 5.
PN-EN 1997-1:2005 Eurokod 7: Projektowanie geotechniczne - Część 1: Zasady ogólne PN-EN 14199:2009 Mikropale. PN-83/B-02482. Fundamenty budowlane. Nośność pali i fundamentów palowych. Gwizdała K., Projektowanie fundamentów na palach, WPPK 2005, Wisła – Ustroń 2005. PN-EN 1992-1-1:2008 Projektowanie konstrukcji z betonu -- Część 1-1: Reguły ogólne i reguły dla budynków 6. Sobala D., Wyznaczanie nośności geotechnicznej pali wciskanych według EC7 I metody opisanej w PN-83/B-02482, „Mosty”, 6, 2011, page 54 - 56 7. Wiłun Z., Zarys geotechniki, WKiŁ, Warszawa 1987 8. www.ischebeck.com/ - access 20.06.2014 9. Kłosiński B., Mikropale – stan techniki i perspektywy, „Nowoczesne Budownictwo Inżynieryjne”, Maj – Czerwiec 2011, page 72 – 76 10. System TITAN - Przewodnik Projektowy, www.titan.com.pl/ - - access 20.06.2014
