25 Nov 2019

Micropiles, Rock Drilling Rigs and Hollow Bar Anchors

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Introduction

Micropiles, sometime are called as minipiles, anchor piles or root piles, are widely used in foundations constructions.

Especially for underpinning or emergency repairs, it is an effective method to provide reinforcement.

Micropiles normally consist of high-strength, small-diameter threaded steel bar. After the injection of cement, micropiles can supply 1,000 tons strength and more. In the same time, the rough, profiled surface of the grout body also transfers tension and/or compression loads to the deep ground.

Larger diameters of all thread solid rebar are most popular in the applications of micropile reinforcement. Meanwhile, hollow bars are also increasingly used since its rapid installation and adaptability of collapsing soil conditions.

ONTON hollow bar micropiles comply in Europe with DIN 1054:2010-12 (Subsoil-Verification of the safety of earthworks and foundations), Eurocode 2 and EN14199/10080 standards and in America with FHWA-NHI-14-007(2015). The steel material of the hollow bar, as well as the thread deformations comply with ASTM F432 and A615.

Basic Technique of Hollow Bar Micropiles

The piles are formed from a fully threaded steel bar with a hollow core which is drilled and simultaneously grouted into soils without a casing, typically employing rotary percussive skills.

How to Install Micropile

The installation procedure for mircropile is similar to bored piling it’s using drilling method.

  1. Drilling and Flushing
  2. Withdrawing Drilling String
  3. Inserting Steel Reinforcement and Grouting
  4. Completed Micropile
  5. Reinforcement Result Test

Advantages

  • Small lightweight equipment
  • Low noise and vibration
  • Suitable for tension loads
  • Minimal site preparation
  • Rapid & efficient installation, high output
  • High capacity capabilities in limited access areas
  • Works in all soil types, ideal for collapsible soils
  • Cased or uncased

Technical data

Specification From To
Drill bit diameter 42mm 300mm
Pile loads Up to 1500kN
Height of rig 2.5m 6.0m
Length of rig 1.8m 5.4m
Operating distance from face of wall to centre line of pile 450mm 500mm

Micropiles types

  • Cased
  • Uncased
  • Hollow Bar
Hollow Bar Micropiles

Hollow Bar Micropiles

Micropiles are available in multiple varieties, in order to meet special requirements, for example on public projects requiring certifications, ONTON also offers micropiles in specific certified grades including carbon steels and special alloys. Related test reports and certificates please contact us.

In the following, let’s see the tips of hollow bar micropiles.

The top 5 things you should know about hollow bar micropiles

Hollow Bar Micropile

Hollow Bar Micropile

What is a hollow bar micropile?

Simply defined, micropiles are heavily reinforced, in small diameter, drilled and installed with neat cement grout.

And hollow bar micropiles are self-drilled, no casing and no dewatering required.

This type of micropile is easily installed in low headroom or restricted access applications.

Onton provides this fully boned, drilled and grouted hollow bar micropiles which had been indicated that their hollow bars are able to meet the load and deflection requirements.

self-drilling-anchor-micropile

self-drilling-anchor-micropile

Where a micropile can be utilized?

There are two broad categories for micropile applications.

Structural Support and In-Situ Reinforcement.

As you know,

Structural Support can include foundations, underpinning applications, earth retaining.

And In-situ reinforcement includes applications such as slope stabilization and earth retention, settlement reduction.

What depth and capacity can a micropile achieve?

The micropile was preliminary called “root piles” by its inventor Dr. Pali Radici.(As the above image shows)

I was also impressed when onton showed the picture for the first time.

According to recent design, micropiles can penetrate in excess of 50 meters in depth, with a capacity of between 30 and 150 tons per pile.

Furthermore, same way as the tree roots, micropiles can be grouped together to replicate a root mass constructed of steel and grout. The installation of numerous micropiles can significantly increase loads and create a stable structure.

Are there different types of micropiles?

The other routine micropile technique is cased micropiles.

In fact, there are five categories of micropile type A, B, C, D and E characterized by different grouting methods.

Type A piles are done after the hole is filled and although can be used in soil, are more typically seen in piles socketed into rock.

In Types B, C, and D the grout is pressurized after initial filling.

As you know, hollow bar micropiles belong to Type E. The grout and drill are synchronous so the installation speed is accelerated.

Are hollow bar micropiles suitable for all geotechnical conditions?

Hollow Bar Micropiles are used in growing numbers each year.

It can be applied to more geotechnical conditions by choosing different drill bits.

You can contact onton to ask the menu of bit options for different soil conditions.

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