What you need to know about granite retaining walls
Last week, we took a look at a beautiful and large granite stone building. That building was historic and today we’re looking at a granite retaining wall that’s also historic, built many generations ago. So much of the historic masonry construction, in Washington DC, was designed and built around common and pressed brick construction. At the time, about 150-100 years ago the material was prevalent. Also, much of the discussions we have with our clients and the industry revolve around the upkeep, maintenance, and repointing or tuck pointing of those historic brick building facades. Stone masonry construction is very similar to brick masonry construction. Also the elements of upkeep and tuckpointing or repointing also overlap significantly between these different types of construction, nonetheless there are some key differences between historic brick masonry and stone masonry.
A link to last week’s article follows for reference – Ashlar Sandstone and Granite Facade
Retaining Wall Vs A Building Partition
The retaining wall shown in the picture below is a massive stone structure. This retaining wall is not only built with a significant length, over 75′ down the side of a parcel, but also very tall compared to the average or typical retaining wall. This particular wall is holding back an immense amount of earth and has been standing for many decades. In comparison to the stone built structure we looked at last week, there are many similarities, but there are also some key differences in the form and function of a retaining wall versus a building partition.
This wall happens to be over 7′ tall and we estimate that the minimum thickness at the mid height area of the wall is over 24″ in depth. This is similar to a octuple whythe brick wall, but even thicker, in this case. Since it is retaining and holding back so much earth it’s required to have a greater mass and overall strength. Retaining walls, without modern implements of steel or concrete reinforcement have very little native or intrinsic tensile strength or structural resistance in a tensile direction. If you look closely, you can see there is a slight lean in the wall, leaning back towards the elevated land which the wall is retaining. That’s not accidental or coincidental, nor a matter of deterioration or disabilization. This particular lean happens to be entirely purposeful, as the wall was built both thicker at the bottom and slightly leveraged against the soil being retained.
The soils being retained by the wall experience hydration cycles coincident or corresponding to weather patterns. Effectively, there’s no way to limit or reduce the amount of exposure to rainfall or limit the amount of hydration that the retained land experiences (purposeful swales and berms in the upper topography not discounted). As that land area becomes wet and hydrated, the water percolates and moves through the earth to the area of the retaining wall. The amount of weight or load that the wall experiences is largely a factor of the hydrostatic pressure as a result of rainfall. An area of relatively dry soil has a much higher amount of force or applied load to a retaining wall when it becomes hydrated. The difference in retained load is multiples higher when the Earth or retained land above the retaining wall is wet. The water builds up and becomes it’s own force against the wall and the soils also experience a degrading friction resistance factor, resulting in an increased load or pressure against the wall.
Why Masonry Bond And Condition of the Bonding Mortar Is Important
The cohesive and interlaid nature of the individual elements of the wall become critically important when the soil or earth is hydrated. Over time though, in the case of this particular wall and all historic retaining walls, the mortar between the joints of the individual masonry units or stones in this particular case, is critically important. You can see in the photos below that there is significant deterioration, shown or seen in omissions in the mortar between the stones. This wall needs tuck pointing or to be repointed and restored. The restoration process of tuck pointing or repointing removes deteriorated and / or defunct mortar at the outer edges, where specifically already omitted or missing mortar at the outer edge of joints, the stonework now requires new mortar. When you think about the cohesive or interlaid nature of the individual stones, that pattern or bond applied to both brick and stone masonry, as well. There’s a common television commercial that shows two cars chained to two phone books. The vehicles are not strong enough to separate the books from one another yet the books are not really attached, instead each page is just laid between pages of the other book creating a bond made from friction between the individual pages. Brick and stone masonry is similar because it relies on the bond between bricks to create an exponentially stronger resistance, based on the friction between them. This one of the many reasons the masonry bond and the condition of the bonding mortar is so important.
At the mortar joints, where the mortar meets the edges of the adjacent stones, where pointing or repointing type restoration has been completed, the gray color of the newer mortar is distinct and although newer it’s not new. Pieces of the most recently repaired mortar are loose and missing in many areas.
You can also see signs of efflorescence in a few spots of the mortar. There are areas that look like bubbly cement , but those spots are actually salt deposits on the surface of the masonry. Efflorescence and these types of salt deposits are a form of crystals, which translate to signs that the mortar has experienced significant hydration. Essentially, as water moves across or through a masonry assembly, portions of the cement or mortar become dissolved and Those particular chemicals which are dissolved common normally in the form of salts, then dry up and leave these deposits on the face of the brick or stone work, in this case. Although the particular masonry assembly is much larger, as we explained above, there are many similarities between this type of masonry assembly and traditional or typical historic brickwork.
These types of pressures and forces work against all types of retaining walls. Particularly when they are retaining or supporting loads of soil which are exposed to the elements. We’ve also talked and or discussed, in other blogs, the issue of efflorescence as it affects brickworks in building partitions or exterior walls of brick buildings. The issue is similar in both cases, but in the case of a retaining wall, lack of waterproofing in histotic times allows the non-exposed the side of the wall to face the pressures of much higher exposure to moisture.
Weep Holes In Modern Retaining Walls
Weep holes are required to be installed in modern retaining walls of almost all types which are supporting or retaining loads of earth or soils. Even in this wall, although it’s very old, it still happens to have a terracotta draintile type weep installed at every 8′ along the length of the wall. Some of the main components of hydrostatic pressure relief that are commonly used today we’re not available many decades ago.
In modern times though, these types of hydrostatic pressure relief systems are built very different though. Generally the backbone or main component of a hydrosatic pressure relief system, whether a brick or stone retaining wall, would be a aggregate embedded perforated pipe. That perforated pipe, ideally with a corrugated pipe wall, runs paralell to the footing of the wall and because it’s surrounded by crushed stone, water easily finds a pathway to the pipe itself. Since the pipe is perforated, water then pours from the stone into the pipe and then that water is carried through the wall through the perpendicular pipes under the footing of the wall itself or through the base of the wall and then deposited or emitted on the open side of the wall. That essentially relieves the pressure from the wall and channels the water away from the loaded side of the wall.
The picture below shows the exposed terracotta te drain pipe as it comes through the wall. You can see the orange color, round shape pipe edge at the base of the wall in the picture below.
When looking closely at the pipe, as shown in the next picture below, you can see that that terracotta tile is just a simple clay pipe that has been in use in building construction for over 100 years. Over a 100 years ago, at the time this was terrific technology because it allowed for rudimentary or basic drainage of the wall. However, compared to modern construction elements, there are many deficiencies in the historic methodology and many advantages in the modern methodology.
How To Maintain A Historic Building
To properly maintain, repair, and care for these historic buildings, a knowledge, interest and understanding of historic building principles is required. Here in Washington DC, historic masonry buildings are extremely expensive and the amount of financial loss caused by improper repointing and low quality construction is staggering. However, in addition to the direct financial value of the property, there is also a cultural loss when historic buildings are damaged. By comparison, consider neighboring poor cities, when historic buildings are damaged, it’s not just the loss of value to the property owner, there’s also a loss to all inhabitants and visitors of a city, present and future, who care about architecture, history, and culture.
We encourage all of our clients, and all readers of this article and to our blog in general, to prioritize the historic built environment of Washington DC and neighborhoods such as Capitol Hill, Dupont Circle, and Georgetown and become educated on on the difference between proper historic preservation versus improper work which leads to significant damage to the historic fabric of a building.
From a conservation and preservation perspective, several approaches can be taken to improve conditions related to deteriorated historic brick masonry. Primarily, lime mortar brick joints and low temperature fired soft red clay bricks should be inspected and checked on a routine maintenance schedule, either seasonally or at least annually. If brick masonry is kept in good condition, the life of embedded wood elements can be significantly extended. Hire a professional contractor which specializes, understands and appreciates historic construction elements and buildings.
In this article we talked about the terminology and concepts of historic masonry restoration, follow the links below for more related information from the IDS website:
- Binders in mortars and concrete
- Brick burns
- Butter joint
- Capillary action
- Cantilever
- Cementitious siding
- Cheek wall, masonry — Draft
- Chemical testing
- Code, building — Draft
- Cold joint
- Cold weather masonry work — Draft
- Damp proof course
- Downspout
- Electrical distribution panel — Draft
- Fenestration
- Ferrous metals
- Great Chicago Fire
- Green bricks
- Gutter, roof
- Lime mortar
- Lintel
- Load path
- Oriel window
- Oxidation
- Parapet coping
- Plug, clay
- Pressed bricks
- Raking, of mortar joints
- Raggle, aka reglet
- Rectilinear
- Roman bricks
- Roman arches
- Roof eave
- Roof termination
- Row buildings and row homes
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- Rubbed bricks
- Rubble stone masonry
- Sand, Builder’s
- Sedimentary rock
- Scratch coat
- Sprung arch
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- Squint bricks,
- Strike, or striking of mortar
- Tapestry bricks
- Tooth-in, interlocking masonry connections
- Vitreous
- Water diversion systems
- Zipper-joint
These concepts are part of the fundamentals of historic masonry restoration, tuckpointing, and brick repair.
The links in the list above will take you to other articles with more information on defects, failures, preservation and repair of historic masonry. You can learn a lot more on our blog. Feel free to check it out. If you have questions about the historic masonry of your building in Washington DC, fill out the webform below and drop us a line. We will be in touch if we can help.