A status report

1. INTRODUCTION

Asphalt concrete is an excellent waterproofing material and well established in hydraulic engineering for many decades.

In Germany the essential requirements for the waterproofing materials and the water proofing constructions are published by the Deutsche Gesellschaft für Erd- und Grundbau e.V. (German Association for Soil- and Groundworks) and is titled "Empfehlungen für die Ausführungen von Asphaltarbeiten im Wasserbau - EAAW" (Recommendations for Asphalt Works in Hydraulic Engineering).

As these recommendations were recently published and give a good overview on the application of the asphalt product in this area, the German experience is hereafter given as an illustrative example on how to apply asphalt in this area.[1]

Whereas the mix design for asphalt concrete for road constructions is focused on durability, stability and surface quality, the main point in hydraulic engineering is impermeability. In a fundamental study the water permeability of asphalt concrete under variable to hydraulic pressure has been tested as a function of the voids content. It could be shown that asphalt with a void content below 3 % by volume is impermeable even at high water pressures. This investigation is the basic of the EAAW in which is specified that construction methods with low or no voids have proved to be impermeable when the water absorbtion in the structures is =< 2.0 % by volume or the calculated void content is =< 3.0 by volume. The values must be found in the structure itself, not the specimen. For the mix design it is necessary to pre-select a low marshall void content which accounts the special features of the site.

It must be noted that for dense asphalt concrete there is no water permeability k-value. Asphalt concrete with a calculated void content =< 3.0 % by volume is impermeable and the k-value could be compared to mineral sealing in hydraulic engineering, with a value of throughout 10^-00.

The application of asphalt linings is not exclusively done in Germany, but also in other Europen countires. Severeal examples have been added for reference in the tables in the end.

2. LANDFILL CONSTRUCTIONS

Because of the good experiences in i.e. hydraulic engineering the next step was to use asphalt concrete for lining the bottom of landfill deposits for solid waste.

The German technical specifications for disposal of industrial wastes (TA Abfall) and domestic wastes (TA Siedlungsabfall) permit alternatives to the standard mineral and plastic lining system if an equivalence will be proved.

The standard is

The questions of the equivalence of asphalt concrete linings had been discussed for some time by German technicians. In 1996 an Information Sheet on Asphalt Landfill Linings has been published by the German Association for Water and Agricultural Industries (DVWK) as a part of a series of Information Sheets on Hydraulic Works. Later during 1996 a general type approval was given for asphalt landfill lining systems by the German Institut for Construction Technology. This general type approval documents the above mentioned equivalence of asphalt landfill lining systems and explains all necessary measures.

2.1 Waterproof lining

The difference between asphalt in hydraulic engineering and in landfill lining is that in the former a small degree of permeability is allowable, whereas this cannot be tolerated in the latter, which needs to protect our ground water for centuries to come.

For this reason the Information Sheet envisages the laying of the lining in two layers, with staggered joints, intended to provide additional protection should one joint fail. This procedure is actually a retrograde step compared with present laying techniques for asphalt in hydraulic works. The former practice to lay the linings in several layers, gave rise to the possibility of blister formation between two dense layers, when laid in damp water. Nowadays it is possible to get a higher degree of compaction when laying thick layers with modern screed, even in difficult areas like slopes. However the danger of blister formation is very low or appears to nil when a minimum layer thickness of 60 mm will be built.

Landfill lining mixes have to meet more stringent requirements relating to properties and quality assurance than asphalt road mixes. As far as linings for landfill slopes are concerned, the requirements will not differ much from those in hydraulic engineering, where they are already stringent. In addition to testing the density of the completed linings by means of the vacuum test, the non- destructive testing of density and thickness is envisaged with an isotope probe. The resistance to chemical attacks was already documented.

2.2 Lining substrate (base course)

Normally linings in hydraulic works are applied on a drainage layer, capable of conducting any possible seepage in a manner that does not endanger the structure. Such layers need to remain serviceable. In landfill constructions such a drainage system under the lining is undesirable because any localised cracks could be transformed into major failures through the cross-distribution of aggressive substances.

For this reason the asphalt base course (void content between 7 and 15 % by volume) was replaced by a more or less dense asphalt base with a void content of =< 5 % by volume (measured in the laid asphalt).

2.3 Support for the asphalt base

The requirement for the asphalt base support depends of the value of the deformation modulus E_v2. With E_v2 =< 45 MN/m² an additional unbound base is required with the goal to get a deformations modulus E_v2 >= 45 MN/m². This deformation modulus is required to ensure a satisfied compaction of the asphalt base. The German Institut for Construction Technology has given a construction permit for a combination of an asphalt and mineral lining.

This is intended to be equivalent to the standard combination lining of the specifications TA Abfall und TA Siedlungsabfall. Since asphalt layers are much thicker than the standard plastic membranes, the thickness of the mineral layer can be reduced.

2.4 Requirements

1. Structural design:

2. Asphalt concrete:

3. WATERPROOFING CONSTRUCTIONS

Normally the theme of waterproofing construction should have been described at first because it is the basic for the asphalt landfill lining systems. That means basicly if a void of an asphalt concrete layer measured in the structure is lower than 3.0 % by volume, the layer is impermeable. In chapter 1 this context has been explained.

The requirements for dense asphalt concrete are written down in the EAAW

The maximum particle size can be selected as a function of layer thickness. Though no traffic loading needs to be considered, it is nevertheless essential that joints are watertight. For this reason mixes with lower maximum particle size in proportion to layer thickness are to be preferred. The mortar ratio should not to be too low and the ratio of voids filled with binder should be more than 90 %.

3.1 Mix design

The Marshall specimen are the mix design basic of asphalt waterproof layers. In order to take account of varying site conditions, particularly the differing gradients of the slopes, the number of blows and the temperature of the asphalt are varied in the laboratory mix design procedure. In practice when rolling steep slopes only a part of the roller weight bears on the slope when rolling steep slopes.

In addition the stability on the slope and the impermeability in the event of deformation and settlement need to be determined. These tests are made with the mix which has been selected. For testing the stability on the slope a specimen, 200 x 300 mm, is prepared in laboratory. The specimen thickness is the same like the material that will be laid. The density of the specimen should correspond to the Marshall specimen density. The stability is tested in a chamber heated to 60 °C or 70 °C, with the specimen placed on a slope having the required gradient. The flow of the asphalt is measured.

The working life of an asphalt lining must be guaranteed also when settlement occurs in the subsoil or on the dam base. The asphalt lining must be able to take up such settlement without loss of impermeability. A special test according to van Asbeck verifies this important property.

Besides the impermeability requirements the immunity to chemical or solvent attack is demanded especially for landfill linings for both domestic and special waste.

3.2 Test methods

In the Netherlands a European wide inventory was made to determine what test method was availble to determine wether an asphalt is impermeable. The following criteria were used:

• the test should be applicable both in the laboratory prepared specimen as well as on cores drilled from the landfill,
• the test should be applicable on specimen of different height,
• during the test, the pressure should be raised to speed up the testing process.

It was decided that ISO-DIN 7031, originally designed to test the penetration of water into cement concrete is the most appropriate test. The procedure was slightly modified. The water pressure was raised to 1 mbar over 72 hours. In the Netherlands it was also found that mixtures with 3% voids could be classified as water impermeable.

[1] "Asphalt für Deponieabdichtungen". Deutsches Asphalt Institute, Berlin 1996.

Tab. 1 ASPHALT WASTE DISPOSAL SITES IN GERMANY (BUILT)

Tab. 2

ASPHALT WASTE DISPOSAL SITES IN GERMANY (TO BE BUILD)

Tab. 3

ASPHALT WASTE DISPOSAL SITES IN SWITZERLAND (BUILT)

Tab. 4

ASPHALT WASTE DISPOSAL SITES IN THE NETHERLANDS

60 mm mastic asphalt
60 mm dense asphalt concrete
250 mm secondary raw material: unbound base
600 mm sand with controll drains
2 mm HDPE membrane

Hydraulic applications of hot mix asphalt in the Netherlands have a history that goes back to round 1930, when in first experiments revetments of canals were protected with impervious asphalt. Later, after World War II, asphalt has been widely applied in coastal protection.

The first application of bottom protection dates from 1957 when an industrial landfill was lined with 20.000 m² of asphaltic concrete. In general the application of asphalt in linings is more or less restricted to reservoirs for industrial waste. Several examples of constructions for temporary storage are known for hazardous waste. In these type of landfill pits hazardous waste is stored before being processed to be cleaned or handled in an other way. Constructions may consist of dense asphaltic concrete, in some cases in combination with a second impervious geo-membrane.

In the period '80 - '90 several landfills for domestic and industrial waste have been lined with reinforced bituminous membranes. This type of membranes consists of a prefabricated membrane of modified bitumen on a fabric or non woven. The membranes are 3 - 5 mm thick and produced in 4 - 5 m width. In situ the strips are overlapped and sealed with hot bitumen.

A special application of mastic as a lining for a landfill is the so called C2 Deponie near Rotterdam harbour. This facility has been constructed as a permanent storage for extremely hazardous waste. The total construction has the shape of a hall, approximately 350 m long, 60 m wide and 40 m high. The walls are made of concrete and lined with polymer membranes. The bottom of the hall is lined with a multiple impervious layer of asphalt mastic. The top of the building has a movable roof.

According to official regulations of the Ministry of Environment for bottom and top lining of landfills for domestic waste still only polymer membranes in combination with sand/bentonite are accepted.

Tab. 5

ASPHALT WASTE DISPOSAL SITES IN THE UK

200 mm granular drainage layer
70 mm stabilizing binder
85 mm dense asphalt concrete

Tab. 6

ASPHALT WASTE DISPOSAL SITES IN FINLAND