Subsurface properties and settlement potential at MHM 8/5/16

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Elkhorn Slough TMRP: surface and subsurface properties and settlement potential at Minhoto and Hester Marsh 

Ivano W. Aiello

Summary 

During the initial phase of the Elkhorn Slough Tidal Marsh Restoration Plan, about 45 acres of subsided marshes, intertidal mudflats and tidal channels at Minhoto and Hester Marsh (SA) will be restored to a ‘healthy’ elevation for marsh growth by sediment addition (the MHHW datum (1.76m or 5.8 ft NADV88). For the success of this restoration project it is essential that the final elevation (10+ years later) of the new marsh settle very close to the targeted elevation. However, adding sediments to an existing soil has mechanical and sedimentological implications that are dependent upon the thickness, extent and weight of sediment added, the relief of the area, the hydrology, and the geotechnical/physical characteristics and the thickness of compressible sediments that will be subjected to the increase in lithostatic stress caused by the overburden.

Through a combination of field, analytical, and modeling exercises, this study addresses four main questions, which are critical to the success of the restoration project. Note that the values provided by the model are for the M2, M3 and H1 subareas (~27 acres fill area; ESA, 2014) of the MHM (~36.4 acres) and that the phase I restoration also includes M1 (an additional 9.5 acres):

1) What is the subsurface lithology/stratigraphy of MHM?

Using a combination of sedimentological/physical properties analyses of core samples, field surveys using a modified dynamic cone penetrometer and available geotechnical reports and science literature, the subsurface of MHM can be described as a 4 layer system laying on a ‘basement’ which forms the bottom of the paleo-valley. From top to bottom: 1) a ‘slurry’ layer at the very top (~0-0.6m thick) that has high water content (>>~55%) and is above the Liquid Limit; this layer is thicker in lower relief habitats; 2) a ‘plastic’ layer (~0-0.6m thick) which is more cohesive and thicker in higher relief habitats; 3) a ‘stiff’ layer (~2m calculated thickness) which is the remnant of the previously dried marsh; 4) a normal consolidated layer which thickness is not known but was predicted using different modeling exercises.

2) What is the accommodation space currently occupied by the slurry?

The MATLAB model created for this project calculates the volume of sediments needed to fill the elevation between the uppermost cohesive surface (top of the plastic layer) and the target elevation (MHHW) based on 136 sites surveyed with a modified penetrometer and a Terrestrial Laser Scanner and extrapolating the results to the SA. The total volume of accommodation space in SA, if the slurry layer was to be completely removed, is 23,375 m3 (33,967 cy) (Table 1).

Subsurface properties and settlement potential at MHM 8/5/16

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3) How much settlement will occur after the predicted volume of sediments is added?

The oedometric empirical calculations for the total settlement that will occur after the overburden layer is added depend on the geotechnical characteristics (e.g. Compressibility Index Cc, and the Initial void ratio e0) of the subsurface layers and their thickness. Note that for all the calculation done by the model the thickness of the overburden layer is calculated as the distance between the top of the plastic layer and the MHHW datum.

The geotechnical data available to calculate the settlement were limited and were based on the reports by ENGEO (2013) and Kleinfelder (2002) which analyzed boreholes at the nearby Parsons Bridge area. The Cc values reported by ENGEO were only for the uppermost layers and relative low, so the calculations for settlement presented in this memo are conservative (underestimates). Since the cores collected in the area are very shallow (max 4 m) the total thickness of compressible sediments above the ‘basement’ was modeled with MATLAB using and final target elevation of MHHW datum (1.76 m or 5.8 ft NADV88) and 3 different scenarios that describe different geometries: 1) ‘paleo-valley’, max thickness ~11m below the axis of the main channel; 2) ‘shallow bowl’, max thickness ~7.5 m below the axis of the main channel; 3) ‘shallow flat’, essentially a flat basement surface at a constant depth of 3m (Tables 1, 3 and 4). The average settlement of the 3 models is 21.15 cm and the average loss of volume for the SA is 15,170 m3 (19,720 cy).

What is the total volume of sediment that needs to be added to the surface to attain the target elevation.

This is an iterative process, as the weight of additional overburden will cause additional consolidation of the underlying sediments. See Table 1 for the volume of sediment needed to reach a final target elevation of 1.76 m (5.8 ft) NAVD88 (including slurry and consolidation of underlying soils).

Adding the average settlement of the 3 models for the SA (15,170 m3 or 19,720 cy) to the volume of the slurry layer (23,375 m3) and the accommodation space between MHHW and the top of the slurry layer (73,093 m3) extrapolated from the TLS surveys we obtain a total of 111,638 m3 (145,129 cy), with values of 107,782 m3 (140,117 cy) and 117,238 m3 (152,410 cy) for the lowest and highest settlement estimate, respectively (Table 1).

The bottom of Table 1 also shows how the previous figures would change if the slurry layer is not removed and partially dried from ~70% to ~40% water content. Under this scenario, the slurry would turn into a more plastic layer with a volume of 10,264 m3 (13,424 cy) which can be then subtracted to the previous estimates.

Subsu

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Subsurface properties and settlement potential at MHM 8/5/16

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layer 4 is normal-consolidated and the thickness has been modeled using 3 different scenarios.

Study goals 

There are many challenges associated with this type of restoration approach, some of which are addressed by this study:

1) This study describes the physical and geometrical characteristics of the ‘slurry’ and ‘plastic’ layers using field measurements with a cone penetrometer, and a MATLAB script that calculates the volume to account for the additional sediment addition needed if the slurry layer is entirely removed and/or dried during restoration. The thickness of the overconsolidated layer (‘stiff’ layer) beneath the plastic layer is estimated based on the amount of pore water loss proportional to subsidence.

2) The addition of a sediment layer (herein referred to as the ‘overburden layer’) will create an excess lithostatic load. According to Terzaghi’s theory of sediment consolidation, in fine soils (silts and clays) with low permeabilities the soil is undrained as the load is applied; slow seepage occurs and the excess pore pressures dissipate slowly, while consolidation settlement occurs. Different MATLAB scripts were created to calculate: a) the depth at which the sediment overburden will produce an excess stress using the Boussinesq equation; b) the amount of sediment settlement (compaction) for each of the layers shown in Figure 2 (using the oedometric formula and the available geotechnical measurements); c) the extrapolation of the amount of settlement and the volume loss caused by the sediment overburden to the entire MHM area based on 3 different model scenarios describing the thickness of sediments above a non-compressible (basement).

Methods and Results 

Field surveys with the Terrestrial Laser Scanner (TLS) and data post‐processing 

In order to measure the thickness of the slurry layer and the plastic layer underneath it we used a Terrestrial Laser Scanner (TLS) and a survey rod with a 360º laser prism modified to be attached to the head assembly of a standard Dynamic Cone Penetrometer without the 8kg drop-weight. A total of 136 survey sites (approximately 1m2) were occupied in the zones M2, M3 and H1 covering different habitats of the Sample Area (SA) area (Figure 1).

At each of the 136 sites three surfaces were surveyed during low (<1m) tides including (Figure 3): 1) the ‘surface’ elevation (top of the slurry layer); 2) the bottom of the slurry layer using the free-fall penetration of the rod+penetrometer, assuming that the slurry layer is frictionless and non cohesive; 3) the bottom of the plastic layer using 'push' penetration, which is how far the operator was able to push the rod+penetrometer assembly before refusal.

A MATLAB code was created to transform the topographic datasets and to identify the 3 surfaces, for each 1m2 window. The MATLAB code uses an algorithm to detect and separate each surface type (all measurements were done continuously) and then places depth and thickness information in three different matrixes with the standard format (NAVD88, m): northing, easting, depth/thickness. Finally, the code calculates the thickness of the slurry and the plastic layer, and calculates various statistical parameters (means, SDs) and plots the measured parameters in histograms and bivariate plots (see results).

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8/5/16

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analyses.

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8

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mbination of tation/habitahat has been‘slurry’ laye mixing and

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8/5/16

form the pla

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8/5/16

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10

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8/5/16

100% of thediment additd a sedimentrple line in

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Subsurface properties and settlement potential at MHM 8/5/16

11

The volume of the slurry layer calculated by the model for the 136 sites is 29.09 m3, which extrapolated to the SA covered by M2, M3 and H1 is 23,375m3 (33,967 cy) (Table 3). If the slurry layer is not removed and partially dewatered from ~70% to ~40% water content (which would turn the slurry into a more plastic sediment) the volume would become 10,264 m3 (13,343 cy).

‘Plastic’ layer: In the ENGEO (2013) report, this layer is described either: 1) as ‘very soft highly organic clay’ (sample S1@1’-2’ at 1.75 feet), with a moisture content 94.4%, e0=2.636, porosity=~72%; 2) as ‘very soft with visible water’ (sample S3@3’-4.5’ at 4. 5 feet, which is the deepest sample that they have analyzed), with a moisture content 82.7%, e0=2.242, porosity=~69%. If this layer of plastic clay is correlated to Kleinfelder’s (2002) shallowest sample (B-1, 13.0 feet) the compression ratio would be Cc/1+e0 = 0.235. Hence, if we calculate the Compressibility Index Cc for sample B-1 using the value of e0 from the two ENGEO samples we would obtain values for Cc between 0.615 and 0.761 which would result in almost three times more compressible than Cc=2.5 reported by ENGEO. Note that using Kleinfelder’s rather than ENGEO’s Cc values would significantly affect the settlement and volume loss calculations (up to ~3 times larger).

‘Stiff’ layer: Although not clearly explained in the ENGEO’s report, sample S2@0-24” at 1.67 feet (silty, clayey SAND and that has a e0=0.667, and 22.4% moisture), could have been collected from the ‘stiff’ layer since it is much drier than any other sample analyzed. Since the compressibility of this layer is 1 order of magnitude smaller than normal consolidated sediments and to simplify the calculations, the model used here approximates this layer to incompressible.

Normal consolidated layer: this layer, which likely represents the bulk of the sediment between the ‘basement’ and the bottom of the stiff layer, is the least known amongst the layers that make up the sediment column at MHM. Although there is no direct geotechnical information from this depth, the model considers this layer as normal-consolidated and based on the analogy with the ENGEO samples, the value chosen is Cc=2.5 as for the plastic layer.

In conclusion, given the lack of information on the Compressibility Indexes for the different layers at MHM, the model assumes the most conservative and lowest values (i.e. the settlement could be much higher) for both the plastic and normal consolidated layers (Cc=2.5) and no compression for the stiff layer.

To calculate the stress (kN/m2) generated by the overburden layer the model uses the density value for dry clay or=1600 (kg/m3);

Calculation of the thickness of the ‘slurry’ and the plastic layers 

‘Slurry’ layer The ‘slurry’ layer is the layer of non-consolidated material ~>55% water content and above

the LL. The thickness of this layer was obtained by measuring the vertical distance of the free-fall of the rod+penetrometer assembly from the surface. A prominent linear correlation between slurry thickness and surface elevation (top of the ‘slurry) is observed for the western side (M2 and M3) of Minhoto (~39%; Figure 9). However the correlation is less significant for the eastern side (H1; Figure 10; ~25%). In other words, at H1 there are low relief areas where the slurry is relatively thin and highs were is actually thicker.

Subsu

Potentbetween use practland was

 ‘PlastThe ‘p

and ~<55present aassemblybetween

groups. Tthicknesslayers (>high elevlayer has

Calcu

To calsubsidenthe soil (hseismic eprecursorone can cof the sti

Let's abased onthe marshover-concorrespon

Figure 9 – c‘slurry’ laye

urface proper

tial explanatM2&M3 vs

tice: M2 and used for agr

tic’ layer plastic’ layer5% water coat the surfacey through thethe thicknes

The group wses which co0.20m) corr

vations. As ss two main m

ulation of th

lculate the thce experienchowever it m

event could hr of the stiff calculate howff layer.

assume that the differenh was at MH

nsolidated samnds to a poro

orrelation betweer and topograph

rties and sett

tions for the . H1 include

d the North ericultural pu

r is the layerntent occurr

e. The thickne sediment uss of the ‘slu

with the thinnoincide with esponds to rhown in the

modes, and th

he thicknes

hickness of tced by the mmust be notehave contriblayer, the to

w much of th

the total subnce in elevatiHHW). The pmple S2 (~2osity of ~40%

een thickness of hy at M2 and M3

tlement pote

different rele different ineast area of Hurposes.

r of partially ring below thness of this launtil refusal. urry’ and the

ner plastic layrelatively lo

relatively thiplots of Fig

he mean thic

ss of the ov

the layer of omarsh prior toed that other uted to subs

otal amount ohe initial soi

bsidence has ion before anporosity of th2 feet) report%.

f the 3.

ential at MHM

12

lationships bnfluence fromHesters mars

consolidatehe ‘slurry’, aayer was obtThe plot in ‘plastic’ lay

yer (<0.20mow elevationn ‘slurry’ th

gure 11 and ickness of the

erconsolida

overconsolido diking wasfactors such

sidence): byof subsidencil has been o

been 1m (thnd after desihe stiff layerted by ENGE

Fig‘sl

M

between elevm agriculturash were duck

ed material walthough in stained by puFigure 11 sh

yers which ar

m) correspondns; a second ghicknesses win Table 3, the slurry is 0.2

ated (stiff) 

dated sedimes entirely cauh as oxidationknowing the

ce, and the poverconsolida

his value, proiccation assur is based onEO (2013) w

gure 10 – correllurry’ layer and

vation and slal runoff andk ponds whil

with plastic csome higher ushing the rohows the comre clearly arr

ds to relativegroup with t

which coincidhe thickness21m (M2, M

layer 

ent we assumused by the dn of organice initial poroorosity of thated and hen

obably an ovuming that bn the initial vwhich is 0.66

lation between thtopography at H

8/5/16

lurry thickned different lale the rest of

characteristicrelief areas

od+penetrommparison ranged into

ely thick ‘sluthicker plastide with relatis of the ‘slurrM3 and H1).

me that the desiccation o

c matter and osity of the she stiff layer,nce the thick

verestimate,efore desicc

void ratio for67, which

hickness of the H1.

ess and f the

cs is

meter

two

urry’ ic ively ry’

of

soil , then

kness

is cation r the

Subsu

Then the porosis 2.636, before an

Hencemeters, wwill requbecome a

In consedimentrealistic n

Tota

As shoEq. 1) is Figure 6)in the oedthicknesshypothetexample

For eawere calc

Figure ‘slurrydistribu(right).

urface proper

let's assume sity measureand porosity

nd after is a l

e, 1 cubic mewith a reductuire a 3.3m tha 2.1m thick

nclusion, bast and the subnumber, and

al settlem

own previoua very impo

) and is moredometric fors of sedimenical basemenof the MAT

ach of these culated first

11 – Multipanel’ and ‘plastic’ laution of the thick

rties and sett

that the initd by ENGEOy is ~72% (aloss of ~29%

eter of normtion in thicknhick layer of

k stiff layer a

sed on the prbsidence histd this is the th

ment calcu

usly, the thicortant paramee influential rmula, Eq. 1)nts relative tont scenarios:

TLAB model

scenarios thfor the 136 s

l figure from the ayers. Middle rowkness of the over

tlement pote

ial porosity O (2-13) foralmost twice% volume du

mal consolidaness of ~0.3mf sediment beafter desiccat

revious geotetory of the arhickness tha

ulated fo

ckness of cometer in determthan the thic). MATLABo the underly: 1) v-shapedl (model run

e total compsurvey sites

Matlab calculaw is similar to fiburden layer (lef

ential at MHM

13

of the sedimr the shallow as much as

uring desicca

ated sedimenm. Consequeefore it is drtion (becaus

echnical worrea a ~2m that was used i

r differen

mpressible smining the fckness of the

B models weying incompd valley, 2) s2_4) is repo

paction and tand then ext

tions: top row shigures 7 and 8. Beft) and the total

M

ment before dwest ‘soft’ sam

S2). Thus, tation and por

nt after desicently, to accried (1m/0.3me of the ~29

rk, the knowhickness for tin the model

nt ‘basem

ediments (Hfinal compace overburden

ere developedpressible ‘basshallow boworted in App

the total volutrapolated to

hows histogramsBottom row showsettlement after

desiccation wample S1; whthe differencre collapsing

ccation becomcount for 1mm). The 3.3m% loss of po

wn physical pthe stiff laye

ls.

ment’ mod

H in the oedoction of the sn load (propd to calculatsement’ base

wl, 3) flat baspendix I.

ume loss afteo the areas M

s of thickness disws histograms ofr the overburden

8/5/16

was the samehich void ratce in porosityg.

mes 0.7 cubim subsidence

m layer will orosity).

properties ofer seems a

dels 

ometric formsediments (sportional to ‘te the total ed on three sement. A sc

er compactioM2, M3 and H

stribution of the f frequency layer is applied

e as tion y

ic we then

f the

mula, ee dvi’

cript

on H1.

d

Subsu

Restoraticalculatio

To extH1) we uRestorati3), whichthat the ~accommoslightly hto the ent

Our causing thethe MATrepresentthus are rincludes M1 comp

Calcu

As expincreasedand the sadding ththe thickn0.89m (n

urface proper

ion area M1 ons of slurry

trapolate theused the infoion Project”.h, according ~103,000 cy odation voluhigher than ttire area.

alculations ae survey rod+TLAB algoritt a 1m2 area.representativM1 (~9.5 acpared to M2

ulation of th

plained befod stress at dehallowest m

he accommoness of the s

n=136). The

rties and sett

was not incly volume or c

e settlement ormation refe. The total fito the ESA figure repor

ume betweenthe one that w

are based on +penetrometthm for each. In this calcuve of the enticres fill area,, M3 and H1

hicknesses 

ore the thicknepth) was ob

most cohesivedation space

slurry. As shtotal volume

tlement pote

luded in the compaction

calculationserenced in Tll area of M2(2014) will rrted by ESA n the surface we have calc

an extrapolater and the ch of the threeulation we aire SA. An e, ESA 2014)1), as shown

and volum

ness of the otained by cae surface, i.ee (differenceown in Table obtained fo

ential at MHM

14

penetrometrfor this area

from the 13able 1 of the2, M3 and Hrequire ~103is the LiDA(top of slurr

culated base

ation of the ronsolidatione basement sassume that textrapolation) was done bin the exam

es used for

overburden lalculating thee. the top of te between Mle 3, the overor the 136 sit

M

ric surveys aa.

36 surveyed e report ''Elk

H1 is 27 acre3,000 cy of s

AR-based calry) and the Md on the extr

results obtain following tscenarios. Eathe 136 sitesn to the entirby adding 35mples of Tabl

r the conso

ayer (the sede difference the plastic la

MHHW and thrburden layetes (which c

and thus ther

sites to the Skhorn Sloughes (or ~101,2sediment (~7lculation of tMHHW daturapolation o

ined from 13the overburdach site is cos are randomre restoration5.32% (the reles 1, 3 and 4

lidation ca

diment load between the

ayer. This is he surface oer has a meancorresponds t

8/5/16

re are not dir

SA (M2, M3h Tidal Mars265 m2) (Tab78,750 m3).the um. This valf the 136 sur

36 survey sitden calculateonsidered to

mly chosen ann area that elative exten4.

lculations

that creates e MHHW da

calculated bf the slurry) n thickness oto an area of

rect

3 and sh ble Note

lue is rveys

tes ed by

nd

nt of

and atum by and

of f 136

Subsu

m2) is ~1each volu803. A fuadding 3

The catwo phasdistance that accocreated bwith the

The ‘p

This sv-shapedapproximthe deepeelsewher

The ‘s

This sv-shapedModel 1_

The ‘s

This sshallow d

urface proper

19 m3. The ume calculaturther extrap5.32%.

alculation ofses: 1) a firstbetween the

ounts for the by phase 1. A>10 cm of th

paleo‐valle

scenario descd as for a dromately to the est point is ~re in Elkhorn

shallow bow

scenario descd as for a dro_4. The deep

shallow flat

scenario descdepths (3 m)

rties and sett

ratio betweetion done forpolation to th

f the volumet phase that a top of the pextra burden

As Table 4 shhe first one.

ey’ scenario

cribes the ‘bowned stream

location of ~12m (basedn Slough (Ga

wl’ baseme

cribes the ‘bowned streampest location

t’ basemen

cribes the ‘b).

tlement pote

en surveyed r the 136 sitehe whole rest

e loss do to saccounts for plastic layer an caused by hows, the se

o (Model 2_

asement’ unm valley andthe present d

d on the maxarcia-Garcia

ent scenario

asement’ unm valley but n is ~7.5m.

nt scenario 

asement’ un

ential at MHM

15

area and SAes can be exttoration area

settlement usthe thicknes

and the targethe sedimencond total se

_4) 

nderneath thed the deepest deepest tidalimum acouset al. 2013)

o (Model 2

nderneath thewith a much

(Model 2_6

nderneath the

M

A is ~109,000xtrapolated toa that includ

sing the oedoss of the sedet MHHW e

nt needed to ettlement is

e compressib location of l channel. Thstic basemen, the minimu

2_5) 

e compressibh shallower,

6) 

e as flat surf

0 m2/136 m2

o the SA by des M1 can b

ometric formdiment needeelevation; 2) account for only of few

ble sedimentthe basemenhe depth of t

nt depth calcuum sediment

ble sedimentbowl-shape

face which at

8/5/16

2 = ~803. Thmultiplying

be done by

mula was doned to fill the

a second phthe settlemecm compare

ts at Minohont corresponthe basemenulations dont thickness i

ts at Minhotoed profile, th

ttains very

hus by

ne in

hase ent ed

oto as ds

nt at ne s 3m.

o as an in

Subsurface properties and settlement potential at MHM 8/5/16

16

References Cited 

ENGEO, 2013. Elkhorn Slough Tidal Marsh Restoration Project Watsonville, California: FINDINGS OF LIMITED SUBSURFACE EXPLORATION. 23 pp.

ESA, 2014, Final Elkhorn Slough Tidal Marsh Restoration Project Restoration Plan, July 1, 2014.

Garcia-Garcia, A., Levey M.D, and Watson, E.B., 2013, High resolution seismic study of the Holocene infill of the Elkhorn Slough, central California. Continental Shelf Research, V 55, 108-118.

Kleinfelder, 2002. GEOTECHNICAL ENGINEERING INVESTIGA TION�FOR UNION PACIFIC RAILROAD BRIDGE (103.27 COAST) REPLACEMENT PROJECT AT PARSONS SLOUGH IN MONTEREY COUNTY, CALIFORNIA. I0-3012-84/GEO (1011 R877)/jb. 233 pp.