Retaining Wall Installation Basics
Important Aspects to consider when constructing Retaining Walls
The strength of a wall comes from the design – not the blocks. It is therefore critical to consult an engineer when a wall is higher than 1.2m.
Bear in mind that a retaining wall must resist the lateral pressures generated by the soils behind it or, in some cases water pressure too. Every retaining wall supports a “wedge” of soil. As the setback of the wall increases, the size of the sliding “wedge” is reduced. This reduction lowers the pressure on the retaining wall.
The most important consideration in proper design and installation of retaining walls is to recognize and counteract the tendency of the retained soil to move downslope due to gravity. This creates lateral earth pressure behind the wall which depends on the angle of internal friction and the cohesive strength of the retained material.
Lateral earth pressures are zero at the top of the wall and – in homogenous soil – increase proportionally to a maximum value at the lowest depth. Earth pressures will push the wall forward or overturn it if not properly designed. Also, any groundwater behind the wall that is not dissipated by a drainage system causes further hydrostatic pressure on the wall.
It is critical to have proper drainage behind the wall in order to limit the pressure to the wall’s design. Drainage materials will reduce or eliminate the hydrostatic pressure and improve the stability of the material behind the wall.
When the weight of blocks alone is not enough to resist soil loads (walls higher than 1m), horizontal layers of geotextiles are used to reinforce soil behind walls. With proper soil reinforcement and design, retaining walls can be constructed to heights in excess of 20m.
Why retaining walls fail:
- Insufficient drainage
- Incorrect or non-use of Geotextiles
- Insufficient compaction of soil behind the wall
- Inadequate foundations.
- The NHBRC stipulation is that a wall of up to 1.2m doesn’t need an engineer’s approval if the soil is good compactible soil without excessive storm water and there is no load pressure on the wall such as vehicular traffic or buildings. Retaining walls which are higher than 1.2m need an engineer’s approval.
- You cannot build a retaining wall where clay and non-compactible sand conditions exist.
- For a wall which is higher than 3m the common rule is to use Geotextile in every third layer.
- Consider available space, drainage and water management
- All retaining walls should have a foundation, even if the wall isn’t high.
- After excavating and digging the foundation the soil must be levelled and compacted.
- Foundation dimensions are custom designed.
- For walls which are more than 6 layers high, construct a concrete foundation. If walls are less than 6 layers high the soil needs to be compacted and cement can be added.
- Wet your concrete foundation regularly for 24 hours for better curing and to prevent cracking.
- Inadequate soil compaction is the most common reason for retaining wall failures according to engineers.
- The wall blocks are merely the “skin”. Compaction is what determines structural integrity.
- Compaction needs to be done layer by layer behind the blocks. Lay one row of blocks and compact behind and inside the blocks using a mechanical compactor or roller and a hand tamper respectively before laying the next row.
- Every layer must be moist when compacted (neither dry nor muddy).
- Make sure to obtain the compaction density as per design. Normally 90-95% mod. ASSHTO.
- Make use of a string line or a dumpy level to pack straight walls.
- Alleviate creeping on higher rows around corners by using half blocks when required.
- Make sure blocks are level and not tilting or leaning in any way.
Use of Geotextile
Make sure Geotextile is laid smoothly and does not “concertina”.
- The Geotextile is placed over a block and then the next block which is placed on top of it anchors it in place.
- It is good practice to let the Geotextile protrude a few millimetres, making it visible. It is then cut off after completion of the wall.
- Geotextile can be used as a base to prevent soil from falling through the blocks in the open laying patterns.
Good drainage is essential to prevent water build up. Without sufficient drainage, a dam is created. Always be on the lookout for excessive storm water drainage flow and if necessary build V-Drains 1.5-2.5 metres from the edge to channel surface water away from the retaining wall.
- Drainage pipes must be laid in crushed stone and protected by Geotextile.
An offset measurement of 70 degrees is used by civil engineers to achieve the required construction angle.
|Degree||Tie Back (mm)|
|Side on||Side on||Front & back on||Front & back on|
|Closed orientation||Open orientation||Closed orientation||Open orientation|
|Blocks per m²||13.09||8.33||16.67||9.56|
|Weight per block (kgs)||28||28||28||28|
|Minimum wall angle from horizontal||50 degrees||50 degrees||46 degrees||46 degrees|
|Advantages||Easiest to install||Green wall||Most aesthetically pleasing||Green wall|
|Disadvantages||Possible creeping around corners||Most economical material cost||Most expensive material cost||Difficult to install|