Editing Washington Monument (section)

===Foundation===
[[File:Washington Monument foundation.png|thumb|upright=1|Cross section of foundation, both old and reinforced, showing dimensions]]
The first phase began with the excavation of about {{convert|7|ft|8|in|m|1}} of topsoil down to a level of [[loam]], consisting of equal parts of sand and clay, hard enough to require [[pickaxe|pick]]s to break it up. On this "bed of the foundation" the [[#Cornerstone|cornerstone]] was laid at the northeast corner of the proposed foundation. The rest of the foundation was then constructed of bluestone gneiss rubble and spalls, with every crevice filled with [[lime mortar]].<ref name=Torres/>{{rp|23, 68}} The dimensions of this old foundation were {{convert|23|ft|4|in|m|1}} high, {{convert|80|ft|m|1}} square at the base, and {{convert|58|ft|6|in|m|1}} square at the top, laid down in eight steps, similar to a truncated [[step pyramid]].<ref name=Torres/>{{rp|18–19, 23, 47}} At the center of the foundation a brick-lined {{convert|2|ft|cm|-1|adj=on}} square well was dug to a depth of {{convert|20|ft|m|0}} below the bed of the foundation to keep it dry and to supply water during construction.<ref name=Torres/>{{rp|19}}

During the second phase, after determining that the proposed weight of the monument was too great for the old foundation to safely bear, the thickness of the walls atop the unfinished stump was reduced and the foundation was strengthened by adding a large unreinforced concrete slab below the perimeter of the old foundation to increase the monument's load bearing area two and one half times. The slab was {{convert|13|ft|6|in|m|1}} thick, with an outer perimeter {{convert|126|ft|5+1/2|in|m|2}} square, an inner perimeter {{convert|44|ft|m|1}} square, with undisturbed loam inside the inner perimeter except for the water well. The area at the base of the second phase foundation is {{convert|15992|sqft|m2|1}}. The strengthened foundation (old foundation and concrete slab) has a total depth of {{convert|36|ft|10|in|1}} below the bottom of the lowest course of marble blocks (now below ground), and {{convert|38|ft|m|1}} below the entry lobby floor. Casey reported that nowhere did the load exceed {{convert|9|LT/sqft|psi kPa}} and did not exceed {{convert|3|LT/sqft|psi kPa}} near the outer perimeter.<ref name=Casey1884/> To properly distribute the load from the shaft to slab, about half by volume of the outer periphery of the old rubble foundation below its top step was removed. A continuous sloping unreinforced concrete buttress encircles what remains. The buttress is {{convert|100|ft|4|in|m|1}} square at its base, {{convert|64|ft|6|in|m|1}} square at its top, and {{convert|20|ft|5|in|m|1}} high. The perimeter of the original top step of the old rubble foundation rests on the larger top of the concrete buttress. Its slope (lower external angle from the vertical) is 49°. This buttress rests in a depression (triangular cross-section) on the top surface of the concrete slab. The slab was constructed by digging pairs of {{convert|4|ft|m|1|adj=on}} wide [[drift mining|drift]]s on opposite sides of the monument's center line to keep the monument properly balanced. The drifts were filled with unreinforced concrete with depressions or dowel stones on their sides to interlock the sections.<ref name=HSR/>{{rp|3{{hyphen}}3{{ndash}}3{{hyphen}}5, figs 3.1{{ndash}}3.6, 3.9, 3.13, 4.11}}<ref name=Torres/>{{rp|39, 47{{ndash}}48, 67{{ndash}}73}} An earthen terrace {{convert|60|ft|m|0}} wide with its top at the base of the walls and steep sides was constructed in 1880–81 over the reinforced foundation while the rest of the monument was being constructed. During 1887–88, a knoll was constructed around the terrace tapering out roughly {{convert|300|ft|m|-1}} onto the surrounding terrain. This earthen terrace and knoll serves as an additional buttress for the foundation. The weight of the foundation is {{convert|36912|LT|ST t|0|abbr=off}},<ref name=Casey1884/> including earth and gneiss rubble above the concrete foundation that is within its outer perimeter.