Part 3 – Demolition and retaining wall construction process
This post is the third in a series covering the construction of an elegant but practical front entry. In this post, we look at retaining wall construction and how, once again, the quality and durability is in the preparation and foundation. Let’s take a look at the original wall on the left to compare with what was created on the right.
The original wall had a strange slope down to very narrow steps to the walkway. The stairs entered onto the driveway pretty close to the foundation of the house. The new wall was made level increasing the number of steps to the walkway to provide a flat walking surface.
The first thing we did was call Dig Safe to mark the utilities. Once we had an idea of where the lines were, we removed the existing wall and cut back the asphalt driveway to allow for excavation for the new wall footing. Excavating the old wall had to be a careful process so that we could find all underground utilities that may be in area of excavation.
As we did with the landing, we also needed to dig and remove all organic soils at base of wall and then remove large landscape plants as needed for later transplanting back into the new redesigned garden area. We also needed to excavate 4 to 5 feet behind wall to allow for a geogrid soil reinforcement and drainage system behind wall. Geogrid reinforcements used in conjunction with the soil, enable the soil to be more stabilized than it would in its unreinforced state. Walls that are 4 feet or taller require permitting and the city requires an approved engineered plan.
Now that we have the ground prepared, next we take field measurements and layout wall to site. It’s important to check plan measurements and verify that what we think we’re building matches the plans. We then locate where we want to put the staircase and excavate soils to the proper setbacks to allow for construction. Excavation for the wall footing may vary in depth depending on soil types and slopes of land.
Our plan shows two Brussels Block Stones to start below grade with a crushed stone compacted footing below that. Once again, the foundation needs to be set on non-organic soils with a minimum of 6 to 8 inches. This means the excavation is around 18 to 24 inches below finish grade. We use a laser level to shoot grades and verify these measurements.
Since the the driveway area where the wall is being constructed is not level, we must step the wall to allow for the grade change. Walls should be built level with stepping rather than building a sloping wall.
Once all the excavation is complete, we install 1.5 inches of crushed angular stone for the footing base to a depth of 6 to 8 inches. The stone must be compacted.
Next we build a form with flexible PVC board. This form will be set level on top of the crushed stone base and secured with pins. The top of the form represents the level pad and bottom of our first stones. This is a tedious and time consuming process, but will make substantial forward progress later.
Once the forms are set and secure, we install three quarter inches of gravel to bring footing up to grade. Water and plate compaction ensures footing is fully compacted and there will be no further settling. A final level of stone dust of less than one inch is used to level the base. This is then hand tamped and screeded to leave a perfectly level base.
Retaining wall construction can now begin
On any modular retaining wall system, the prep work, first coarse, drainage and soil reinforcement systems are the most important aspect of the entire project. These retaining walls should last a lifetime if constructed properly, but many fail prematurely due to poor workmanship. Remember not all work is created equal.
Now that our excavation and footing installation is complete the first layer of stones can be set. Since our base is already level, stones can be set with ease. We use a paint or string line to follow the shape of our layout.
As the base layer of stones are set, we vary the size of the stones as this particular block has three different sizes to avoid joints lining up with each other as wall height builds. Multiple base stones are set and then checked with a four foot level to make sure they are perfect. Any flaws in this base course will be multiplied as the wall height increases. A 3 foot 4 by 4 block of wood is used along with sledge hammer to smash the blocks into submission as needed. These blocks must be dead level side to side and front to back!
After the first course is installed consecutive layers can be stacked, there is a modular wall adhesive that is used to secure the next layers, alignment is key before adhesive sets up. We must pay attention to the plan as the structural integrity of the wall requires it. There are multiple structural elements going on behind the wall too.
- Geogrid – A soil reinforcement system for walls must be installed according to plans. Multiple layers are installed as wall height increases.
- Drainage – 4″ perforated pipe with crushed stone and siltation cloth ( to prevent clogging) drainage is pitched and outlet at pre-determined area. Note: the number one reason retaining walls fail is because of turgor water pressure. This is why the drainage system behind the wall is so important.
- The crushed stone backfill and soil separation fabric – This keeps the the native soils from contaminating the free flowing drainage stone behind wall and eliminates turgor or hydrostatic water pressure build up. This is also the structure of the retaining wall.
Many times retaining walls are constructed without all of the details described above at a fraction of the cost of a professional installation. Buyer beware of of low bids. Make sure you know what’s behind your wall, as its longevity is at stake.
Once all the structure elements are in place, the wall can continue to be constructed. Every 8″ of height, the wall requires compaction of the stone backfill. As the wall is still fragile, hand tapping directly behind the wall may be required while a plate compactor can be used further away from the back of wall.
This particular project also included a staircase with 6 steps through the wall. This is achieved by creating returns in the wall with a 4′ opening to form the staircase. One return is built partially first then bluestone tread risers will be set as the stairs. This process must be constructed in conjunction with the wall returns. Each bluestone tread must be set square inside the return structure and must have a slight pitch forward to shed water. The stone treads are heavy and must be set by machine on top of 1.5 inch crushed compacted stone. Care must be used to avoid damage or scuff blues stones. The returns are built as the wall height continues and stairs continue. After each step is set, crushed stone is installed and compacted to support the next step.
Eventually the wall, stair returns and staircase come together and are complete. At the same time, we complete the soil reinforcement system, drainage system and backfill except for the 12 to 18 inches of loam that will be installed behind the top of wall to supply soil for plant bed.
There is a siltation fabric that separates the drainage stone layer from the native soil and loam layer. This is folded over the stone before the screened loam is installed to prevent siltation contamination.
Once the plant bed is complete, graded and lightly compacted, we remove and reorganize all the existing plant materials. The new hard scapes have altered the landscape and therefore reorganizing the plant material is necessary to complement the transformed front yard.
Landscaping is done before the final piece of the project – the walkway – so we can move and place the plants using our heavy equipment.
Once again, solid foundation preparation and attention to detail is critical in the construction of a retaining wall. If you are planning to have a retaining wall built, make sure your contractor has these critical components included in their plans.