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General Glen E. Edgerton
2
September 19, 1949
along back of the abutment there has been a general subsidence which
has resulted in settlement of the abutment of about 2 in. at the
bridge seat and 18 in. at the rear ends of its side retaining walls.
Subsidence has not yet ceased.
All other foundations of the bridge are supported by long
steel piles driven to rock. We originally proposed long steel piles
under the north abutment. This was considered extravagant by the
then Bridge Engineer of the District who wanted concrete piles. Two
borings were made as a guide for foundation design. These seemed
to warrant the use of concrete piles for this abutment. They were
adopted, -subject to satisfactory showing by driving tests and to an
actual load test on a pile in place. Also the design of the super-
structure is such that a slight settlement of the abutment would have
no significance. Piles driven to a depth about 50 ft. below ground
showed satisfactory driving resistance. A load test of sübstantially
twice the desired pile capacity showed acceptable results.
When subsidence and settlement was noted, an additional
boring was made about mid way between the original borings. The
findings of the additional boring differ materially from the records
of the original borings. In areas where the first borings reported
"gray, silty clay" and "gray, silty clay with laminated layers of
clayey, silty sand, etc.", this last boring records at depth 72.5 ft.
some 4 ft. of "muck, no sample obtained"; at depth 78.5 ft. some 2.5 ft.
of "muck, sample lost"; and at depth 96.5 ft. some 4.5 ft. of "nucky
soil, unable to obtain samples.' There thus appears to be at least
11 ft. of mucky soil. The upper level of the muck as now reported
is more than 20 ft. below the bottom ends of the concrete piles.
This apparently justifies the conclusion that there is no evidence
of settlement by failure of the piles but rather there is a general
subsidence of the whole mass of soil which the piles penetrate,
resulting from the squeezing down of this soft material far below.
Arrangements are now under way for placing long steel piles under
the abutment as originally contemplated.
You will realize why this experience should raise the
question, are the borings which have been made in the White House
area completely reliable or are they as unreliable as the original
borings made for the above abutment? Is anyone, on the basis of
two small samples, each apparently representing only a couple of
feet of depth of material, justified in positive conclusions for
depths of 8 ft. or more and for an area of 25,000 sq. ft.?
We send separately a copy of a work sheet made in connection
with studies of these foundation loads. These studies confirm
ARCHIVES RECORDS SERVICE* AND
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"ocrText": "General Glen E. Edgerton\n2\nSeptember 19, 1949\nalong back of the abutment there has been a general subsidence which\nhas resulted in settlement of the abutment of about 2 in. at the\nbridge seat and 18 in. at the rear ends of its side retaining walls.\nSubsidence has not yet ceased.\nAll other foundations of the bridge are supported by long\nsteel piles driven to rock. We originally proposed long steel piles\nunder the north abutment. This was considered extravagant by the\nthen Bridge Engineer of the District who wanted concrete piles. Two\nborings were made as a guide for foundation design. These seemed\nto warrant the use of concrete piles for this abutment. They were\nadopted, -subject to satisfactory showing by driving tests and to an\nactual load test on a pile in place. Also the design of the super-\nstructure is such that a slight settlement of the abutment would have\nno significance. Piles driven to a depth about 50 ft. below ground\nshowed satisfactory driving resistance. A load test of sübstantially\ntwice the desired pile capacity showed acceptable results.\nWhen subsidence and settlement was noted, an additional\nboring was made about mid way between the original borings. The\nfindings of the additional boring differ materially from the records\nof the original borings. In areas where the first borings reported\n\"gray, silty clay\" and \"gray, silty clay with laminated layers of\nclayey, silty sand, etc.\", this last boring records at depth 72.5 ft.\nsome 4 ft. of \"muck, no sample obtained\"; at depth 78.5 ft. some 2.5 ft.\nof \"muck, sample lost\"; and at depth 96.5 ft. some 4.5 ft. of \"nucky\nsoil, unable to obtain samples.' There thus appears to be at least\n11 ft. of mucky soil. The upper level of the muck as now reported\nis more than 20 ft. below the bottom ends of the concrete piles.\nThis apparently justifies the conclusion that there is no evidence\nof settlement by failure of the piles but rather there is a general\nsubsidence of the whole mass of soil which the piles penetrate,\nresulting from the squeezing down of this soft material far below.\nArrangements are now under way for placing long steel piles under\nthe abutment as originally contemplated.\nYou will realize why this experience should raise the\nquestion, are the borings which have been made in the White House\narea completely reliable or are they as unreliable as the original\nborings made for the above abutment? Is anyone, on the basis of\ntwo small samples, each apparently representing only a couple of\nfeet of depth of material, justified in positive conclusions for\ndepths of 8 ft. or more and for an area of 25,000 sq. ft.?\nWe send separately a copy of a work sheet made in connection\nwith studies of these foundation loads. These studies confirm\nARCHIVES RECORDS SERVICE* AND"
}