Masonry Team

Masonry/Monument Team Mission
The Masonry team is dedicated to preserving the structures and monuments of our National Park sites as well as other federal agencies and local governments. We provide construction services, consulting, documentation and staff training delivered by highly skilled, knowledgeable and detail-oriented masons and crew-members utilizing both modern technology and time honored techniques.

The large retaining wall section was hand dismantled over a two week period.  The work crew is near completion of wall dismantle and approaching footing level.

HPTC/NPS

Featured Project: Harpers Ferry National Historical Park
Harpers Ferry, WV.
Project: Stabilize the High Street Retaining Wall

Significance of the Resource:
The High Street Retaining Wall is a prominent feature in the historic landscape of Lower Town, Harpers Ferry National Historical Park. The rough masonry structure is 215 feet long, 3 feet wide, ranging between 10 and 18 feet tall, and constructed of Harpers Ferry Phylite Stone. It is the remnant of 19th century stone buildings that once stone on the site. What appear to be buttresses are actually the locations of intersecting foundation walls. You can also see various evolutions of construction and repairs within the wall.

Historic image of the High Street Retaining Wall

Harper's Ferry National Historical Park

Why Are We Doing the Project:

During the last 40 years there has been no structural surveys or and little maintenance of the High Street Retaining Wall. In the last five years, some serious water problems have emerged-- water was building up behind the semi-dry laid stone, increasing in hydrostatic pressure. The water poured through in some areas, but caused a wall collapse of one wall section, prompting an emergency repair. Excessive water leakage and stone movement might be signs of more serious wall failures in the near future.

In order to avoid further collapses and to save the resource, HPTC’s Masonry Team was brought in to inspect the wall, work with contractors to document the feature and develop a methodology for dealing with drainage, and complete the needed repairs.

Harpers Ferry - High Street Retaining Wall Dismantle and Rebuild - August 2018

HPTC/NPS

How Is This Done?

Simply looking at the wall identifies the major problem areas: water seepage, stone displacement, bulging section, and deteriorated stone. But good historic preservation work involves a lot of documentation--what is there, what condition it’s in, and what work is completed on it.

We partnered with the Historic American Landscape Survey (HALS) team who digitally documented wall conditions, details, and features through photogrammetry. This technique combines digital images and laser distance meters to capture a site in three dimensions. We used those images to document inspection locations for wall coring tasks, inspection probing locations and incorporating measurements for all. Those images can be seen on the Library of Congress website.

There’s a lot of information that can’t be seen from the face of a retaining wall:

• Wall thickness,

• How the back of the wall is constructed,

• If there’s voids between the built wall and bedrock,

• How much earth is between built wall and depth to bedrock

• How the wall relates to the bedrock

To answer these questions, the HPTC team consulted with an engineering firm, and completed 16 core drill-outs at various wall locations where we had visually identified problems. Under the guidance of NPS archeologists, we dug five shovel test pits to inspect the backside of the wall. We also conducted vertical rod probes driven into the ground to locate the depth of bedrock.

From the inspections, we determined that the built wall is only a “true” retaining wall for approximately the upper 30% - 50% of its height. Most of the lower built wall sections are facing bedrock that is at or close to the backside of the wall. The upper built wall sections are indeed retaining the upper grade as the area has been infilled with soil, stone and other materials.

With the existing conditions and construction techniques documented, we could get to work. Based on the information we found, we determined we needed to rebuild the middle 100’ of wall. Section by section, we dismantled the wall, salvaging the stone, and staging it on site. To rebuild the wall, we had to construct a footing below grade, then start laying up the stone, mechanically pinning the face to bedrock. As the wall climbed higher, we infilled behind it with gravel and a drainage system. At the top we installed coping stones--larger, flat stones that help keep water from flowing down inside the stone wall.

As we built, we replicated the character defining features of a stone wall--the coursing and bonding patterns, wall height, and wall alignment. We “stitched” the new wall face to the back of the wall by laying a long stone perpendicular to the face every few feet along the wall length. We set the stones in a mortar bed kept back from the stone face, giving the appearance of a dry laid wall. The mortar was composed of 1 part Natural Hydraulic Lime (NHL-5) to 2 parts clean coarse concrete sand.

Duration:

3 minutes, 19 seconds

Sara Stratte talks about her work to repair and preserve the stucco walls of the Pink House at Grand Teton National Park.