外堀の土塁 The Structure of the Outer Moat Ground 外堀土塁の構造 The Structure of the Base 牛込門から赤坂門までの江戸城外堀は、谷地形を利用し、広い堀幅の水面と、水面から高い土塁を持つ江戸城防御のための空間で構成されています。目の前に見える土手もその土塁の一部であり、2013（平成25）年から始まった石垣修復工事、その後のJR飯田橋駅西口駅舎等の工事に併せ調査が行われ、桝形石垣付近は江戸時代当時の規模を目安とした土塁の復元をしています。 外堀の土塁は、厚さ５～10cm程度ごとに土や粘土、砂など様々な種類の土を積み重ねて突き固められた版築と呼ばれる技法で作られています。地層断面をみると、土手の斜面方向とは逆に、水面の方に向かって地層が上がっています。これは、土砂崩れを防ぐためと考えられています。土塁の道路端には土留めの石垣が築かれていましたが、現在では土手に沿って通る道路の端に確認できる通り、コンクリートの小型擁壁に代わっています。
The section of Edo Castle’s outer moat between Ushigome and Akasaka Gates was constructed by utilizing the valley topography, which allowed for the development of a wide canal surface and tall earthwork fortifications. The embankment visible here formed a portion of the area’s earthen fortifications. In 2013, a series of excavations were carried out in conjunction with a project to repair the stone walls and redevelop the western side of JR Iidabashi Station. The project enabled the reconstruction of earthen fortifications on the same scale as those that existed in the Edo era. As a result, contemporary visitors to the site can experience the grand scale of the historical fortifications that existed in the area. The outer moat’s fortifications were constructed using a rammed-earth technique in which 5-to-10-centimeter layers of earth and clay were stacked on top of one another and tamped down. An examination of the fortifications geological profile reveals that the layers of earth and clay rise towards the towards the moat’s surface and away from the embankment slope, It is likely that this construction method was utilized in order to prevent landslides. Earth-reinforced stone walls were constructed along the roadside lining the earthen fortifications. However, as can be determined from the roadside currently located parallel to the embankment, the original earth-reinforced walls have been replaced by small-scale concrete retaining walls.
外堀土塁の水際部分（発見された水際石垣） Waterside Part of The Outer Moat Ground (Discovered Waterside Stone Wall 2015（平成27）年に行われた発掘調査の結果、外堀の水際には、土手が崩れないようにするための低い石垣が作られていたことが判明しました。発見された場所は、牛込橋から140m程度市谷方向に進む地点の線路の間で、石垣の基礎部分と考えられる 1〜2段の石積みが、外堀で初めて確認されました。この石垣は、明治期に甲武鉄道が敷設された際に、上段部分が撤去され、基礎部分はそのまま埋められたものと考えられます。
An archaeological excavation carried out in 2015 revealed the existence of low stone walls along the moat’s edge. They were likely constructed in order to prevent the embankment from collapsing into the moat. The site where the walls were discovered is located in a section of rail track approximately 140 meters away from Ushigome-bashi Bridge in the direction of Ichigaya. At that site, excavators found one- and two-tier layers of stone, which are thought to have comprised the stone walls’ structural foundation. When the Kōbu Railway was constructed in the Meiji period, it is thought that the upper portion of the stone walls were removed and the foundation was buried.
外堀土塁の植栽 Planting of The Outer Moat Earth Fence 1636（寛永13）年、外堀土塁が完成すると、幕府は翌年、堀方七組の東国大名に命じて、土塁上部から2.7m程下がった位置に、一間（約1.8m）間隔で大きめの松杉を、その内側には小さめの苗木を、二筋にわたって植樹させました。植木奉行（のち普請奉行）の管理下に置かれた牛込土橋～筋違橋の土手と堀は、近辺に屋敷を拝領する武家に割振り、植木の手入れと草刈りを担当させました。幕末に日本を訪れたオイレンブルクの『日本遠征記』によれば、「城壁天端の平坦部とその内側にはモミなどの針葉がぎっしりと並び、水面には何千という野鴨が住み着いている」と記されています。なお、真田濠（現上智大学グラウンド）の土塁上には、往時の姿を思い浮かべることができるような松の植生がみられます。
The outer moat’s earthenworks were completed in 1636. The following year, the Tokugawa shogunate order the group of eastern Japanese domainal lords previously mobilized to dig portions of the outer moat to plant a row of pine and cedar trees approximately 2.7 meters below the upper portion of those fortifications. The trees were to be planted 1.8 meters apart. In addition, they planted an inner row of smaller saplings parallel with the pine and cedar trees. This project was carried out under the direction of the Governor of Landscaping (later the Governor of Construction) and the maintenance of the newly planted fauna was entrusted to the warrior houses occupying estates along the outer moat and embankment between Ushigome-bashi Bridge and Kuichigai Gate. During the late-Edo period, the area was visited by Prussian diplomat Count Friedrich Albrecht zu Eulenburg. In his Record of the Eulenburg Expedition to Japan, he wrote, “Thick rows of fir trees and other conifers line the flat section crowning the castle walls and the walls’ interior section, and thousands of wild ducks inhabit the moats surface.” The rows of pine trees lining the embankment alongside Sanada-bori Moat (present-day Jōchi University Field) provide visitors with a sense of what the outer moat area was like in the Edo period.
Remnants of the stone walls that surrounded Ushigome-mon Gate can be found in their original form on the left and right sides of the Gate. Accordingly, they are important artifacts, which enable visitors to experience the scale and atmosphere of the stone walls that surrounded Edo Castle. During a 2013 archaeological survey, the stone wall on the northern side, which is visible here, was restored using original techniques and materials. The stone walls were constructed using a layering method in which stones on both ends were cut and chiseled in straight lines, and then laid in courses so that each stone fit tightly with the others. This was supported by a coursed masonry technique in which side joints passed through most sections of the wall. These methods gave the stone walls a precise, orderly appearance. In contrast, the stone walls’ lower portions and those touching the embankment were constructed using a comparatively imprecise layering technique in which small stones were pounded into gaps between the larger stones. The walls were constructed primarily using andesite from the Izu Peninsula. In addition, large granite stones transported from the Seto Inland Sea were used as cornerstones. Layering Techniques Used to Construct the Stone Walls The wall’s western side (Side B), which faced the outer moat, was constructed using a cutting-and-insertion method in which stones of different sizes were carefully chiseled to ensure that they fit tightly together. In contrast, the lower tier, which was buried under the embankment, was constructed using a comparatively rough pounding technique in which stones of different sizes and shapes were broken and beaten into the wall’s face. Continuing along the embankment, the protruding portion of the wall’s northern face (Side A) was constructed using the aforementioned cutting-and-insertion technique, whereas the section touching the embankment using the pounding technique. As a consequence, the northern face presents a less orderly appearance. In contrast, the wall’s south face (Side C), which was located inside the square enclosure surrounding the Gate, was comprised of square and rectangular stones and precisely constructed using the cutting-and-insertion technique. The stones used to construct the south face were carefully layered to ensure that side joints crossed the entire face of the wall. It is likely that this portion of the wall was so carefully constructed because it was located inside the enclosure and a space that those passing through would inevitably see. The Wall’s Internal Structure An examination of the wall’s internal structure reveals that cobblestones approximately the size of a fist, such as river stones and angular rocks, were embedded into it as backfill material. In the portion facing the road (Side C), each layer of the wall was embedded with a separate layer of backfill. The innermost layer (Layer IV) was precisely constructed using a technique in which flat stones were lined on their back. In contrast, the wall’s lower portion (Layer III) is filled with large cobble-stones, while the upper portion (Layer Il) is embedded with gemstone-shaped cobblestones the size of a fist and the outermost layer (Layer 1) is comprised of top soil. The existence of these distinct layers indicates that the rock walls have been repaired and refurbished several times in their history.
