. A p p r o r r m o l el o c o l r o no f
seplrc tonks
I Approrimole locotion of
deloil ond oeriol pholos
. o C o n d e m n e dh o u s e s
- Seismic lines
^Communriy woler lonk
- Arroyos
r G e o t e c h n i c o lg r o u n d -
--
slobilizotionsludy oreos
Approximote roule of
unsuccessful ocequio
25
FIGURE 1-Location map of study area in and around El Llano, near Espafrola,Rio
Arriba and SantaFe Counties. New Mexico.
Ground
nearEspafrola,
subsidence
NewMexico
Engineering
Geologist,
byDeborah
Assistant
Editor,
andGaryJohnpeer,
Shary,
NewMexico
Bureau
of Mines
andMineral
Resources,
Socorro,
NM
FIGURE 2-Crack in exterior wall of
adobe house that the owner has repaired without lasting success
FIGURE 3-Wall crack in rwostory stone and wood house
FIGURE 4-Concrete-block
wall that was cemented flush
originally. The hole in the ground is called a s6tano, which
literally means basement or cellar but, in this case, signifies
an unexplained hole that, if found in a field, could drain
much of the surface water directlv to the subsurface.
May 1985 Nau MexicoGeology
FIGURE S.-Cracks
backyard.
and circular depression in an E1 Llano
Engineeringand environmentalgeologistsfrom New Mexico Bu_
reau of Mines and Mineral Resouries(NMBMMR) were called late
last fall to investigateground subsidencein and around the small
area.
In early December,NMBMMR geologistsand El Llano homeowners met in Santa Fe with representativesfrom the offices of the
Governor,Civil EmergencyPreparedness,Red Cross,and Construction Industries Division (CID). The CID concludedthat it would be
potentially hazardous for some of the people to remain in their
homes,and they condemnednine houses.Ttie familieswere housed
M o i o r l o e s sd e p o s i t s
R e p o r l e d c o l l o p s ei n
other type deposifs
FIGURE6-Map of the U S showing extentof loessdeposits,soils
known to be collapsible,and other areaswhere soil collapsehas
been reported; modified from Arman and Thornton, 1972.
suchlosses.
Geology
Surficialdepositsin the El Llano area are as much as 100 ft thick
FIGURE 7-Aerial photograph of part of El Llano taken in 7949. Area
shown includes, but extends farther west than, detail shown in Fig. 1.
The scaleis in feet.
Possiblecauses
FIGURE 8-Aerial photograph of part of El Llano taken January 15, 1985.
Note the concentration of structures east of the acequia near the highway.
Area shown includes, but extends farther west than, the detail shown in
Fig. 1. The scale is in feet.
New Mexico Geology May 1,985
magnitudesup to VII on the modified Mercalli scalehave been re-
damage.
although several cracks extended deeper than the 12-ft-deep trenches;
other cracks died out a few feet below the surface. In several trenches
animal burrows (krotovina) appear in close association with the cracks.
The geotechnical ground-stabilization study is intended to demonstrate the relationship between ground wetting and subsidence
and may provide data that can be used to prepare sites before construction. Data from the study also are being analyzed to determine
the feasibility of saturating an area so that the ground (and any
structures on it) will subside uniformly. For the first phase of this
study three areas were chosen that had not been wetted previously
(Fig. 1). The ground is being saturated at each site by injecting water
continuously into holes lined with perforated PVC casing. The areas
are being monitored carefully to measure the amount of wetting and
the uniformity of subsidence. A larger-scale experiment is anticipated to begin soon. The ground beneath an existing or specially
built structure will be wetted and the subsidence monitored. Although never attempted before in a developed area, this procedure
could be applied to ground beneath already cracked structures in
order to stabilize them
Summary
Collapsingsoilsare a natural geologicprocessthat causeproblems
when structuresare built on them before subsidenceis complete.
However, adequatepreparation of these soils before construction
can minimize subsequentdamage. Unfortunately, remedial measures made on existing utilities or other structureshave not been
very successful.In New Mexico, ground subsidencefrom collapsing
soilsas well as from other causesis a common occurrence.Increases
in population and building necessitatepractical and expedient soc_onnected
to the Espanolasewagesystem, septictanks are utilized. lutions becausethe damagecausedby soil collapseis not only costly
Other sourcesof increasedmoisture include leakv water lines, sur- in dollarsbut alsoin emotional stressfor thosedirectly affected.Data
facerunoff (unusually high rainfall was recordedin October 1984), generatedduring this study will help mitigate the problem of ground
subsidencein El Llano and in other parts of New Mexico.
disruption of natural runoff, and outdoor watering.
ACKNowLEDGMENTS-Appreciation
is extended to John Hawley,
Dave Love, Danny Bobrow, Mark Hemingway, Felipe Valdez, and
Geotechnicaltests
Fritz Reimersfor providing information and useful suggestionsand
Numerous tests have been set up for continuous monitoring. to JaneCalvert Love for editing the manuscript.John Hawley wrote
Ground-vibration monitors were installed in two condemnedhousei. the geologic summary. The study, being conducted in association
Theseinstruments should reveal whether or not vehicular traffic or with the New Mexico Environmental Improvement Division and the
New MexicoHighway Department,is funded through the stateCivil
EmergencyPreparednessDivision. GeotechnicalconsultantsRobert
McNeill, Randy Holt, and Charles Reynolds have worked closely
with NMBMMR geologists.Drilling was done by Fox and Associates.
References
convertedto ground-water monitoring wells so that fluctuations in
the water tablecan be documented.Other drill holes are being used
to monitor changesin surfaceelevation;settlementmonumentswere
placed vertically in six drill holes to detect surfacesubsidence,and
Arman, Ara, and Thornton, S I,1972, Collapsiblesoils in Louisiana:LouisianaState
University, Engineering ResearchBulletin 111, 131 pp
Hawley, J W. (compiler), 1978, Guidebook to Rio Grande rift in New Mexico and
Colorado:New Mexico Bureau of Mines and Mineral Resources,Circular 1,63,241,pp
Kelley,V C , 1,978,Geology of EspanolaBasin, New Mexico: New Mexico Bureau of
Mines and Mineral Resources,GeologicMap 48, scale1:125,000
Stover,C W., Reagor,B. G., and Algermissen,S T, 1983,Seismicitymap of the state
of New Mexico: U.S Geological Suruey, MiscellaneousField Studies Map M'F-L65O,
scale1:1,000,000
!
areasaway from suspectedsubsidence.One control areawas located
west of El Llano between the two acequias,and others were located
away from any possiblemoisture souicessuch as surfacedrainage,
acequias,or septic tanks.
In addition, 12 explorationtrencheswere excavatedwhere surface
cracksoccurredto determinedepth of the cracksand if faulting was
evident. After the trencheswere excavatedthe details of the tiench
walls were logged. Offset indicative of faulting was not apparent,
May 1985
Neu Mexico Ceology
FIGURE 9-Drilling operation in front of a house whose owners
have patched the numerous cracksthat developed while they were
on vacation in the summer of 7984