1.     Bottom liner and final cover of landfills
There are 3 types of landfills based on the type of waste dumped in them:

•  Hazardous waste landfills
•  Municipal waste landfills
•  Inert waste landfills
In hazardous and municipal solid waste landfills it is obligatory to use an impermeable bottom liner and the specification of this layer(s) is determined by social, political, economical conditions of each country and also the climate of each region. These minimum specifications are devised by local authorities.    
Why we should contain wastes in landfills:
To stop toxic leachate emission to the surrounding soil, groundwater and surface water, we should build an engineering system which realizes this goal. Using different geosynthetic sealing element, this engineering configuration stops leachate to come in contact with proximate environment.  An engineered landfill also have mechanisms to control the entrance of surface water to landfill, collect the gas produced in waste body, and leachate management and treatment.

The main components of an engineered landfill
Different arrangement of products are used to form landfill bottom liner. In the following figures you can see some of these arrangements:
USEPA Minimum Technology Guidance for hazardous waste landfill final cover

USEPA Minimum Technology Guidance for municipal waste landfill final cover

USEPA Minimum Technology Guidance for hazardous waste landfill bottom liner

USEPA Minimum Technology Guidance for municipal waste landfill bottom liner

Bottom liner arrangement -European approach (Germany)

In all proposed arrangement of bottom liner system compacted clay layer (CCL) is a main component. Considering the fact that Geosynthetic Clay Liner (GCL) is a cheaper and also more efficient solution, the application of GCLs in bottom liners is growing rapidly in landfill industry. A clay liner is hard to be built and it requires a great deal of time and effort to line a large area with it. On the other hand GCL is fast and easy to install, therefore many people prefer the GCL to CCL.  The below chart shows the Clay standards:
CCL layer specification
A brief comparison between GCL and CCL

It is worth to mention that construction of clay layers in slopes is a difficult and time consuming task and after CCL is constructed, it is also hard to maintain the moisture in the layer especially in hot climate.
Method for construction of CCL layer in the slopes
Some landfills including Tehran municipal waste landfill, Tabriz municipal waste landfill and Qazvin municipal waste are constructed using GCL material. From 13 landfills that have been designed in the country, seven landfills have benefited from geosynthetic material and in 2 landfills GCL is used as the sole liner.