Green Roof and Landscape Development

Utilising Leca® Lightweight Expanded Clay Aggregate to create iconic Landscapes

Lightweight Fill for Green Roofs


Leca® Lightweight Expanded Clay Aggregate has been an important ingredient for civil engineers throughout the world in providing the perfect material to solve a variety of engineering issues when developing Green Roofs within a city landscape.

In general, Leca® Lightweight Expanded Clay Aggregate is used to reduce, compensate or prevent settlements, to improve stability of a fill structure or landscape or to decrease soil pressure on a retaining structure produced by backfill. In the same applications, Leca® Lightweight Expanded Clay Aggregate can be used both as drainage intensifying and as moisture retaining for vegetation and also as a frost protection layer.

Liverpool One Civil Engineering Development

Liverpool One Civil Engineering Development

Settlement Management for Landscapes

Fill Structure Related Load Reduction

A fill structure produces additional loads when it is placed and compacted on a landscape environment; the effects of the additional load are typically most pronounced in the locations of the landscape containing a soft soil subgrade. The load magnitude depends on the fill material density and thickness, and the effect of the additional load produces long term settlement of the landscape embankment or structure. Magnitude of the load affects settlement sensitive soil in proportion of the softness of the soil, with impacts ranging from high total settlement to unacceptably high differential settlement or tilting of the structure. 

Blowing Facility for simple and effective installation onto site
Leca® Lightweight Expanded Clay Aggregate can be pneumatically blown onto site thanks to its lightweight properties

By using Leca® Lightweight Expanded Clay Aggregate as a fill material within landscapes, the impact of additional loads produced by the soil layers on the subgrade can be reduced significantly and long term settlements decreased. Differential settlements created by loads or soil conditions can also be compensated with the help of a transitional lightweight structure.

It is usual to take in to account geotechnical evaluation during lightweight structure or fill design in which soil parameters and load are accounted for each construction project individually. The lightweight designed structure or fill can entirely or partly compensate additional loads on the subgrade. Partial compensation can be used where settlements have largely occurred, the settlement period after construction is relatively short, or controlled settlements can be accepted. Entire compensation eliminates settlement periods altogether.

When the groundwater or surface water level is high, it can be good practice to partially reduce the weight of the fill structure to ease the pressure on the surrounding area. The structure shall be designed with consideration to buoyancy connected with rising water levels within the structure, a particularly relevant factor for Leca® Lightweight Expanded Clay Aggregate structures located in vicinity of a water body or flood area.

Utilising Leca Aggregate when it is next to water

When Leca® Lightweight Expanded Clay Aggregate is applied to reduce settlements for landscapes, the following design aspects shall be taken into account:

  • Properties of the underlying subgrade and its loading history
  • Allowed rate of settlement and differential settlements
  • Designed life time
  • Groundwater and/or perched water level and possible open water level
  • Distance from ground surface to top surface of structure (height of fill layer)
  • Adjacent buildings.

The dimensioning principles of Leca® Lightweight Expanded Clay Aggregate structures for load reduction and buoyancy are pre-sented in Annexes 1 and 2.

Principles of settlement reduction and management by using Leca® Lightweight Expanded Clay Aggregate are shown in Figure 4.1. Loads produced by structural layers of road and railway embankments and related long term settlements are presented in different colors. The red line indicates the loads of standard soil material; the loads of Leca® Lightweight Expanded Clay Aggregate are marked with the green line. Dashed lines in same colors present the settlement of subgrade impacted by the loads from different fill materials.

Landscape Settlement Settlement Period for Leca Settlement Period for Leca

Leca graph when installing foundations

Annex 1. Calculations of lightweight fill structure for road embankments

Leca® Lightweight Expanded Clay Aggregate fill structures are designed to compensate additional loads produced by a fill structure on a subgrade either entirely or partially. Partial compensation can be used when settlements have mostly happened, the settlement period after construction is relatively short, or controlled settlements can be accepted. Additionally, when groundwater or surface water level is high, it is helpful to partially reduce the weight of the fill structure (limited buoyancy).

Calculations of Leca® Lightweight Expanded Clay Aggregate embankment located above the groundwater level shall be performed using Equation 1.1. Loads produced by soil layer to be excavated, lightweight aggregate and structural layers shall be applied in the calculations. If part of lightweight aggregate is placed below the groundwater level Equation 1.2 shall be used. Symbols used in the calculations and explained below are presented in Figures 1.1 and 1.2.

(1.1)

q_(kaiv.maa)≥q_rak+q_kev

(1.2)

q_(kaiv.maa)≥q_rak+q_kev+q_kev^'+q_w

qrak           = load of structural layers above groundwater level (γrak x hrak)

qkev           = load of lightweight material above groundwater level (γkev x hkev)+(γkev x h)

qkev'          = load of lightweight material below groundwater level (γkev' x hrak')

qkaiv.maa     = load of excavated soil (γmaa x hkev)+(γmaa' x hkev')

qw             = load related to drop of groundwater level caused by construction (γmaa - γmaa') x hΔW

hΔW               = reduction of groundwater level

Load Diagram

Pressure Graph

 

Figure 1.1. Dimension principle of Leca® Lightweight Expanded Clay Aggregate fill structure above groundwater level.

equation for Leca

Figure 1.2. Symbols used in Equations 1.1 and 1.2.

Annex 2. Calculations of buoyancy in road structures

If a Leca® LWA fill structure is located in a vicinity of a water body or in a flood area, and water can rise into the Leca® Lightweight Expanded Clay Aggregate layer, a buoyancy impact shall be taken into account. The fill structure shall be dimensioned to the highest possible groundwater level. Safety against buoyancy shall be calculated using Equations 2.1, 2.2 and 2.3. Corresponding calculation symbols are presented in Figures 1.2 and 2.1. For the purposes of calculations, a unit weight value of Leca® Lightweight Expanded Clay Aggregate is estimated to be the same above and below the groundwater level and equals to 3 kN/m3.

where:

qrak       load of structural layers above groundwater level (γrak x hrak)

qkev      lightweight aggregate load in whole structure (γkev x hkev)+(γkev x h)

Leca Lightweight Aggregate Calculations

Figure 2.1 Symbols used in calculation of fill structure safety against buoyancy

Calculation example:

Calculations of safety against buoyancy are performed for a Leca® Lightweight Expanded Clay Aggregate fill structure which top level is+1.5 m from the ground surface. During the flood the groundwater level may rise to 0.8 m above the ground surface. Thickness of the Leca® Lightweight Expanded Clay Aggregate fill structure is 1.5 m; part of the structure is located below the normal groundwater level. Geometry of the structure is presented in Figure 2.2.

Normally, groundwater level GWnorm = -0.4 m below ground surface:

In flooding conditions, groundwater level GWnorm = +0.8 m above ground surface:

Figure 2.2. Geometry of buoyancy calculations.

We have a team of representatives in the UK with extensive experience and knowledge on utilising Leca® Lightweight Expanded Clay Aggregate on a variety of Green Roof and Landscaping projects.

For more information and a free consultation on Leca® Lightweight Expanded Clay Aggregate contact Leca® UK on:

TEL: 0844 3351770 

EMAIL: enquiries@leca.co.uk

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