No Fines Concrete Gravity Walls

The use of “No Fines Concrete” is an effective alternative to that of traditional geogrid reinforced structures. This solution is also advantageous when walls between 1.6m to 3m in height are required close to property boundaries or adjacent to rock cuttings. This solution reduces excavation costs and is fast to erect compared to the geogrid solution. As with every retaining solution all stability checks need to be carried out consistent with a gravity wall, i.e sliding, overturning, bearing and global stability.

Please consult with your local council for design regulations prior to the construction of your wall. Generally, councils require walls over 0.5m in height; walls where loading such as a car or building is nearby and walls in a steep or unstable terrain, to be designed and certified by a suitably qualified engineer. Retaining Solutions provides this service in house.

“No Fines Concrete” typically consists of cement, water and coarse aggregate. Cement will comply with the definitions for cement per AS3972-1991 – ‘Portland and Blended Cements’. The quantity of cement is specified as 210kg/m3 with a total water/cement ratio between 0.45 and 0.55, to ensure the cement paste coats the aggregates and does not run off.

The particle size distribution of the aggregate shall comply with the limitations for the nominal single sized between 10mm to 20mm as specified in AS2758.1.

Refer to your local concrete supplier for manufacturer specifications.

STEP 1:BULK & DETAILED FOOTING EXCAVATION

Suitable excavation equipment will be required subject to site access to excavate to lines, levels as per construction drawings and most importantly to the widths of “No Fines Concrete” required by design. Excavate a footing trench 600mm wide by 250mm deep. This allows for depth of leveling pad plus minimum embedment requirements for the base course. Typically a minimum of 100mm embedment is required for walls up to 2m in height (H/20). Walls in excess of this height will be subject to review by certifying engineer.

STEP 2:PREPARATION OF LEVELING PAD

For walls less than 900mm high, spread coarse sand or 12-20mm gravel for the leveling pad and compact. For higher walls, a concrete “kerb & gutter mix” footing may be required. All footing types to be approved by certifying geotechnical/structural engineer.

STEP 3:BASE COURSE PLACEMENT

Lay the base course of segmental blocks side to side over the prepared base with a string to the rear of the units to ensure alignment. Check blocks for level “side to side” and “front to back” with a 600mm or 1200mm level. Base course installation to line and level is the most important and time consuming part of any segmental wall installation. With proper care all acceptable tolerances will be achieved whether the wall is 2m in height or 10m in height.

Depending on type of segmental block chosen, concrete nibs or pins will ensure approximate alignment of subsequent courses. Once each course is laid it is recommended a string line and levels be used at all times to maintain vertical and horizontal alignment.

STEP 4:“NO FINES CONCRETE” BACKFILL

Backfill the wall with “No Fines concrete” to the designed depth. All voids inside and between the units must also be filled. The vertical height of any pour of “No Fines concrete” is limited to 800mm. For walls greater in height the backfill may be placed in consecutive pours each 800mm in height. Each pour must be allowed to harden prior to pouring the next lift.

STEP 5:SUBSEQUENT COURSES

Sweep the top of the previous course of segmental units clean of any loose aggregate. It is recommended that this is done with a course brush and with the aid of water to ensure cement past does not dry on the block facing. Place the next course of blocks and level “side to side” and “front to back” as per Step 3. Repeat steps 3, 4 & 5 until the finished wall height is reached.

STEP 6:CAPPING UNITS

Lay capping units as for full units. If top 300mm of the wall is to be landscaped ensure a non woven geotextile is placed over the top of the previously placed “No Fines Concrete” zone and backfill and compact with onsite soil to the required grade. If the capping is accessible to public vandalism, it is recommended that the capping units be secured using construction adhesive or epoxy cement (e.g liquid nails).

Notes: if fences are to be located to the rear of the segmental units (within “No Fines Concrete Zone”), the wall will require design verification for additional wind loading. Also it is recommended PVC sleeves be incorporated during “No Fines concrete placement” so “concrete coring” costs are eliminated.