What should I consider when designing Interbloc for use in refuse and recycling infrastructure?

This article outlines key considerations when designing Interbloc for use in refuse and recycling infrastructure.

Introduction

This article is intended to provide valuable insights into designing and implementing effective resource recovery storage infrastructure. 

A well-thought-out design can save both time and money in the future, and it should consider all aspects of the anticipated build and use of the bins while allowing for sufficient redundancy for volume growth and site reconfiguration. 

Safety is of paramount importance in any design, particularly for sites where the general public interacts with the infrastructure, and the council or operating entity has PCUB responsibilities. Unfortunately, many council transfer sites are ticking time bombs with aging, unsafe infrastructure built using low cost and non-structural systems.  

At Interbloc, we have 18 years of experience in designing and supplying resource recovery infrastructure. This document compiles our learnings over those 18 years, and we hope it provides you with useful guidance for your resource recovery projects.

NB: The following information has been influenced by an engineering report into existing storage infrastructure across a territorial authority area which had a mixture of construction systems used without undertaking a proper engineered design process. The report was produced as the basis for improving the safety and reliability of the infrastructure using the Interbloc construction system. 

Compliance Pathway for Storage Infrastructure:

For the purposes of the NZ building code, modular concrete blocks walls used in resource recovery infrastructure can be classified as Ancillary. According to Building Regulations 1992 ? Schedule 1 > Clause A1 > 8.0 Ancillary applies to buildings or use not for human habitation and which may be exempted from some amenity provided, but which are required to comply with structure and safety related aspects of the building code. 

The following design standards are used for the design of Interbloc storage infrastructure;;

AS/NZS 1170.0	Structural Design Actions Part 0: General Principles
AS/NZS 1170.1	Structural Design Actions Part 1: Permanent, Imposed and Other Actions
NZS 1170.5	Structural Design Actions Part 5: Earth Actions - New Zealand
NZS 3101	Concrete Structures Standard
*NZS 3404	Steel Structures Standard
*SNS TS 3404	Durability Requirements for steel structures and components
*AS/NZS 2313	Guide to the production of structural steel against atmospheric corrosion by the use of protective coatings

* Only applicable for vertical reinforcing of corner shear key systems.

Material Properties: the table below lists the material properties used for design.

Material	Grade
Concrete	fc; : 30 MPa (28 day strength)
Hot Rolled Structural Steel	Fy: 300 MPa
Steel Plates	Fy: 250 MPa
Structural Bolts	Grade 8.8

In all cases, Interbloc storage infrastructure can achieve a design life of 50 years. 

Quantity and Size, and Height of Storage Bins

The quantity of individual bins is generally determined by the sorting requirements for the different glass, other recyclable materials, and refuse being stored. The size of the bins is generally determined by the volume of material which needs to be stored. There is a practical limit on the width of the bin which will inevitably need to facilitate the use of a front end loader operating in the bin - typically this requires the width of the bin to be at least 3.6m wide, if not wider. 

Another consideration when deciding the dimensions of the bin is future capacity increases. A growing population, and an increased understanding of the importance of recycling will inevitably lead to greater volumes of materials flowing through the facility. Anticipating this early on may lead to a slightly larger footprint in the width and depth of the bin, but built to a lower height. This means that when additional storage capacity is required, it simply requires another layer of blocks placed on the existing footprint, rather than a wholesale rebuild of the bins. 

The ideal maximum height of a bin is 2.4m high. Taking into account the note above about expanding future capacity, the ideal design height would be between 1.2m and 1.8m high. Walls up to 2.4m high offer the most efficient design approach. This is not the absolute maximum height of the bin - this can be whatever you require. For bins higher than 2.4m high the use of Interbloc base blocks at the bottom of the wall and  capper blocks at the top of the wall may be an option to balance the need for extra storage without significantly increasing the loading on the blocks. 

Impact of Forces and Location of Bins

Resource recovery storage infrastructure is generally used to store glass, cans, general refuse, or green waste. In most cases a simple gravity wall design approach can be undertaken, meaning the combined weight of the blocks with a shear key interlocking to form a homogeneous wall is sufficient to take the load of the stored material. 

Analysis from the report mentioned above looked into the ability of a Interbloc gravity wall 1.8m high to withstand these forces, using glass (the heaviest material stored) to resist overturning and horizontal sliding forces. The results of that analysis are outlined below:

Global Stability Analysis:

Resisting Moment (kNm/m)	Overturning Moment (kNm/m)	Safety Factor	Overturning Stability
9.94	4.04	2.46	Satisfactory

Resisting Moment (kNm/m)	Overturning Moment (kNm/m)	Safety Factor	Horizontal Sliding/Shear Stability
16.56	5.06	3.27	Satisfactory

Local Layer Stability:
This analysis looks at the stability of each individual layer of blocks.

Layer	Resisting Moment (kNm/m)	Overturning Moment (kNm/m)	Safety Factor	Overturning Stability
1	2.48	0.06	41.33	Satisfactory
2	4.96	0.50	9.92	Satisfactory
3	7.42	1.70	4.37	Satisfactory

Layer	Resisting Moment (kNm/m)	Overturning Moment (kNm/m)	Safety Factor	Horizontal Sliding/Shear Stability
1	4.14	0.31	13.35	Satisfactory
2	8.28	1.26	6.57	Satisfactory
3	12.42	2.84	4.37	Satisfactory

NB: The above analysis is intended to be illustrative of the capacity of Interbloc storage bins in a real world environment. It is not supposed to be a design guide, and should not be used as a substitute for proper engineering design processes. 

However there are three additional forces which the design process needs to take into account as illustrated in the table below. 

Loading	Illustration	Description	Notes
Earthquake		This is the impact of earthquake forces on the wall as outlined by NZS1170.5 and MBIE NZGS Module 6 based on importance level 1.	In the assessment activity mentioned above it was found that with good ground conditions this was a low risk to the overall design with the gravity wall expected to be capable of withstanding ULS and SLS action.
Backfill		This is when a wall needs a permanent load of backfill behind the wall.	Most resource recovery bins do not carry a permanent backfill.
Accidental		This is when accidental forces are exerted onto the wall during the handling of the stored material.	This is a major design factor as all equipment operating in or near the bin will likely come into contact with the bin.

Accidental Loading

The primary design method for resisting accidental loading is the use of vertical reinforcement in the wall. Interbloc blocks incorporate cast in 60mm diameter reinforcing ducts at 600mm centres. Vertical reinforcing ties the individual blocks together, and into the underlying foundation providing additional protection against accidental impact loads. 

Where corrosion is an issue, galvanised vertical reinforcing can be used to ensure the structure meets standards. There are additional techniques described in Interbloc’s vertical reinforcing literature for corrosion protection of the steel.  

Interbloc supplies the correct ‘fixing kits’ for your specific project. Fixing kit is a term used by Envirocon to describe the components required to vertically reinforce the wall. This is a straightforward process, backed by well established design principles and credible construction systems.

Foundation Requirements

Foundation design is heavily dependent on the local ground conditions. As a general rule a geotechnical report is needed to confirm ground conditions before a concrete foundation can be designed. 

Other Features of Bins 

Storing Food Waste

Storing food waste can result in acid leaching and attacking concrete. Combined with the action of a front loading scraping against the face of the wall, this can result in loss of thickness of the concrete wall. A simple and effective method for protecting against this is to incorporate a ‘sacrificial’ layer of blocks at the back of the bin in front of the permanent wall. This low cost solution protects the structure, and eliminates the need for more expensive remediation work on the permanent structure. 

Broken Glass

Visy Recycling [the largest receiving of glass recycling in NZ] allows for glass to be broken, providing it is not mechanically broken. Glass breaking when discharged into the bin is OK however. 

Additional safety measures may be implemented to prevent broken glass from spilling out of the bin and creating a hazard for staff and public users of the bin. Different design approaches can be incorporated to achieve this - for example disposal from behind, or the top of the bin. 

Creating Physical Barriers

The Interbloc construction system incorporates 12 different block configurations, each designed to solve a specific issue which may arise from safety or environmental considerations. Key block types include:

Flat Bottom Blocks - provide a seamless connection to the foundation, and allow for a sealant to be added during installation to prevent leachate from the bin.

Angle Blocks - provide a physical barrier to staff and public from climbing up the wall of a tapered bin, which still delivers the cost benefits of tapering the front of a bin.

Flat Top Blocks - provide a platform for roof structures to be fixed to the top of the bin.