The final crit in Seoul: The Gift

Critics: Peter Ferretto, Robert Mull, Stef Rhodes, Haewon Shin, Ryul Song

Overview of comments to consider before January's crit in London:

- Fascinating topic – the walls are visible to us and you make them visible. But what do you think they are?
- Drawings similar to Appia's – stage set designer
- Consider accidental qualities of walls – drainage, flower pots
- Ideas interesting but Gift proposal is a bit simplistic. Revisit some of the ideas that have been explored in Seoul
- Models are missing!
- The way of representing ideas is rich. Walls are a colliding and layering of space
- Think about meaning of drawings (things you may not be aware of yet)
- Topography – when you look at retaining walls, you are participating in a larger scale site
- Consider continuous element of the walls. The enjoyable element is that you walk along them
- Linearity of Victorian railway tracks – an English comparison. Could final site be in England?
- Traditional, practical, poetic aspects of walls

Reflection of crit and time in Korea
- After ‘intervening’ physically in a long retaining wall, it is important also to reflect more on the wall itself, rather than simply cutting through it. Rather than think about the object, think about the theme
- Close analysis of this part of Seoul has been useful in order to engage with a specific site. However, there is also the subject of the wider scale – how retaining walls offer a re-reading of Seoul’s landscape
- A reference brought up in the crit was Victorian railways in London and how these shape the city, with similarities to retaining walls in Seoul. The walls navigate, guide, divide, and link spaces in the city. Equally, so do London’s railway routes.
- Although perhaps not the most obvious step, this seems particularly relevant to the project because of the connection I have between retaining walls and transport i.e. before Seoul, my view of retaining walls was always associated with transport systems as they are often found next to motorways, rivers and so on.

Sketch model: peeling back the contours revealing a lost landscript

Quick sketch models begin to suggest how revealing the natural contours can begin to shape the paths across the wall. The front of the site (mid-rise residential) whereas the back is rationalised and planned (apatu complex). The retaining wall is the meeting of these two patterns.

Plan diagrams: steps

A landscape strata for Hongjie

Like Paju Book City, the landscape strata begin to influence the levels of the spaces for the journey traversing the wall. This sets out a set of constraints on the vertical plane.

Plan diagrams: steps

Plan diagrams (bottom to top): street, half landing, courtyard

Spaces dictate location of steps

Defining spaces begin to dictate layout of steps and routes through wall. Consideration of different paces: direct routes, ramped access, slow staircase

A dialogue: View from existing 'half-landing' to under-used public space on opposite side of the road

View from top is marred by 1400 mm high balustrade in front of car park

Thinking about adjacent spaces

Notes:
- Conversation / dialogue across street
- Set back entrance
- From top, a view
- Pedestrian connection from playground, pedestrian right of way over vehicles

Section across street

Elevation looking from street

Retaining wall elevation (existing)

Elevation: retaining wall

Elevation close-up: site existing

Rather than a series of interventions along the wall, I have decided to use this location as a test bed which will act as a catalyst for activity along the rest of the wall. Perhaps surprisingly, I have decided to use a part of the wall which already has a pedestrian route through. The reasons for this are as follows:

- working with existing ideas of ways to traverse the wall. What works, what doesn't?

- developers have attempted to introduce a pedestrian route through but it is hardly used and priority remains on vehicular circulation

- this is a key point near two main routes up to the mountain - hierarchy of routes (referencing village on Inwangsan)

Site plan

Steps: perspectives

Starting with the steps was the wrong way round!

Sketching..

Above top: Existing
Above below: ideas.. steps parallel to street, exaggerated in scale like retaining wall and Apatu?

Welcomm City comparison: street, half-landing, courtyard?

L-r: Hongjie pedestrian access, Welcomm City

Two points of focus

Two key openings in the retaining wall: pedestrian and vehicular

Sketch elevations of routes leading retaining wall

Elevations from street (l) to retaining wall (r)

The two main roads up from the main main road. I had previously thought there were no access points through the retaining wall at the end of the street. However, during my third visit, I noticed a small pedestrian route up to Apatu and mountain beyond.

Access

Moving through the landscape

Since becoming engaged with the way landscape is dealt with in Seoul, retaining walls have remained an elemental idea of this project. What goes hand-in-hand with these walls is movement across them. The gift project has therefore begun to consider ways of traversing the walls as a way of bringing two fractured pieces of land together.

Moments of the staircase, looking towards the small bookhall through the staircase

I was particularly taken by the way the architecture leads visitors around the YoulHwaDang buildings. The staircase which links the two rooms of the bookhall and leads up to the mezzanine level, is incredibly refined and simple, yet seems to add so much to the space. When I first visited, it felt like a religious space, yet also domestic. It’s very homely, but special, it offers a little something extra than other comparable spaces. Also notable is that the vertical levels of each space are higher or lower depending on the importance of the space. This results in a shifting of vertical floor heights across each storey, and several sequences of steps. It makes the building feel bigger and more experiential.

Level changes with stepping retaining walls and steps

Paju Book City – with Florian Beigel and Philip Christou

It was such an experience revisiting a place with architects who have been involved with the aspirations of Paju Book City for so long. Hearing such informed opinions of the whole project, allowed me to fully appreciate the influence of the landscape on the new city. We concentrated on the three buildings that Beigel and Christou designed; Phase 1 and 2 of YoulHwadang, and Positive Thinking.

We were asked to study the buildings concentrating on three scales: landscape infrastructure, envelope and detail which presented two days later. I looked with a critical eye, thinking especially about how the buildings are relevant for my project in Seoul. Firstly though, it is important to put the idea of Paju Book City in context. It is called a ‘city’ but zoning policies in Korea are different to the UK. Housing and industrial areas are generally separated, due mainly to land values. Paju was originally only allowed to include industrial buildings (for publishing). However, the government later agreed to allow apartments on the two upper levels of buildings, a small but useful concession meaning more integration of workplace and living was made possible.

1 Landscape infrastructure

Socio-political context: publishers who came immediately after military rule brought with them a purposeful move away from the prior regime. Some of them had previously published books or magazines that were banned during the military rule, and some we imprisoned for it. The whole philosophy of Paju looked forward in a new way.

Site section East-West: landscape informs city strata

Geographical context: the land was originally a flood plain but a motorway was built along the wetland in the 1980s, now the main road which leads north towards the South-North Korean border. The motorway is on a raised embankment, eight metres above the river level protecting land to the east from flooding. Next to the motoway is Paju Book City. Due to Paju’s close proximity to the border, military observation of the River Han (which runs through both countries) from Simhak mountain (on the opposite side of Paju) is crucial. The landscape influences the form of buildings creating a maximum height (14m) to allow a view of the river from Simhaksan. There is a second strata which is the same level of the motorway thus allowing a view of the river to the east from second storey roof terraces. Paju is divided into the following strata: below embankment, embankment (and fill/ground) level, above embankment. Roads patterns also follow the contours of Simhak mountain. Roads take up 20% of the overall scheme, arguably too much resulting in a lack of pedestrian spaces.

Street view: a lack of pedestrians, a similar comparison to my site where there is little human activity, with parking taking up most of the street

2 Envelope (structure)

Youlhwadang phase 1 – 2003
- The form of the building began as cube which then had chunks cut out of it. The cut outs are clad in glass, and the rest in black timber. This reminds me of excavating the earth to create a new landscape via retaining walls.
- Construction: concrete frame with timber rainscreen cladding and translucent glass facades (multi-layered polycarbonate provide transparent insulation). Concrete block infill and steel lightweight studwork, waterproof membrane on external wall behind cladding.

Concept diagram, detail showing transparent and opaque facades

Positive Thinking – 2007
- Construction: Originally a solid brick wall but this changed due to new Korean regulations which follow Japanese earthquake construction. This states that buildings over two storeys cannot be built with load-bearing brick construction. Therefore, there are concrete walls with external brick cladding. ARU wanted to ensure tectonic clarity, that the building was read as cladding, and not pretend to be load bearing. Steel rail bolted to concrete wall in grid, and bricks are laid into grid.
- Windows all stand on the floor, like French doors – figurative. Internal balustrades become like balconies when doors open. Similar ideas to Youl Hwa Dang phase 2.

YoulHwaDang phase 2 2009

'Miracle at San Lio' by Mansueti (1494) compared to the blackness of a retaining wall

- Façade: this was approached very much as a city building, quite different from phase 1. Therefore the building tries to have a relationship with the city. It offers ‘gift to the city’ with the public space. ‘Miracle at San’ Lio (1494) by Mansueti shows people in almost every window looking out to religious ceremony taking place in the square. All windows in phase 3 come to the floor, with French doors and balustrades allowing the room extend into the city. Street-like lighting and the ‘elephant’ teahouse, autonomous from the main building create a city scenario. It is interesting to compare the ‘blankness’ of a retaining wall to both phase 1 and 2 of YoulHwaDang – could there be a similar approach in order to improve the  adjacent ground conditions which is often public space?

Section, YouHwaDang 2: interior to public space, a gift to the city. What can my site offer to the city?

- Construction: In-situ concrete walls. “Phenolic’ (plastic) layer on plywood formwork enables a smooth concrete finish. External stain on concrete exterior is insulating. It is sprayed on and penetrates the façade. Slabs need additional waterproofing in this scenario but it is adequate for walls as they are vertical. In principle, it is best to have insulation on the outside because it keeps the building warm and the structure doesn’t alter or contract with temperature changes.

3 Detail

YoulHwaDang phase 3
- Facades the overall façade is broken down into four smaller facades, giving the impression of a series of small houses. This is similar to Summerson’s concept of aedicules being stacked up to form a city. The façade is broken up by a series of reliefs which divide up the façade according to importance of the interior spaces

I will continue to study and explore certain aspects as my project continues..

Two European precedents

Chatting to Beni a while ago about my ideas of traversing the retaining wall and perhaps considering steps as part of my gift, she kindly pointed me towards some European examples that also deal with movement across steep slopes:

Adept and LIWPlanning, Måløv Axis
Ballerup, Denmark, 2010

Stepped seating (adeptarchitects)

This project connects the new district Søndergård with the old part of the suburb Måløv across two large traffic barriers, Frederikssundsvej and the S-train line. It is an extensive landscape project but I am interested in the part pictured above. The stepped seating creates a new condition on the slope. This has similarities to the retaining wall study at Seonyudo Ecological Park which takes elements in the landscape and reconsiders their function. For example, at Seonyudo, a wall becomes a seat, and in Ballerup, a seat becomes a step.

La Granja, Escalator
Toledo, Spain, 2000

Cutting and folding the landscape (Jose Antonio Martinez Lapena and Elias Torres)

The historic city of Toledo has struggled to make vehicular and pedestrian routes compatible. The escalator and staircase transport pedestrians from a car park at the lower part of the city, to the upper part. The staircase is divided into six tracts which zigzag across the landscape. A retaining wall folds over to provide shelter and ‘give continuity to the landscaped slope of the hillside’1. At night the staircase and escalator transform into a fissure of light cut into the vertical face of the city.

A stair-wrapped church

Photo and plan diagram

After visiting Welcomm City, we stopped to see this incredible church which is surrounded entirealy by a series of brick steps - an interesting way of moving across sloping land.

Welcomm City - ARU and IROJE Architects

Having wanted to visit this building for so long, I had high expectations. On approach, the four little houses perched atop the concrete base subtly adjust their orientation along the curving street, as if nodding to passers-by. However, the monolithic rusty corten facades are somewhat stark and harsh. It was interesting to find out later from Florian Beigel that it was Seung’s insistence that corten was used!

Moving closer, the consideration and care apparent in so much of IROJE and ARU’s work reveals itself strongly. Walking up the series of entrance steps into the central courtyard, I began to appreciate the usefulness of empty or ‘purposeless’ space1. The building could have been designed so that doors next to the street opened up straight into an internal foyer, allowing more square footage for exhibition space. However, as Beigel might describe it, Welcomm City offers a ‘gift’ to the city with its semi-open courtyard. From the courtyard, glimpses through the various levels of the building become visible, unfolding a series of routes across the vertical landscape.

L-r: Looking in, looking out, looking between

The changes between levels are so far removed from the situation in Hongie; from street level to upper level. The study below concentrate on the stepped circulation.

Plan diagram illustrating level changes (lighter-darker, lower-higher)

Hongjie currently

Drawing the existing: A high retaining wall divides two communities; the exclusive Apatu development cuts off routes to the mountain. How can the retaining wall become a focus of activity; how can routes through be re-established?

Existing: conceptual stekch view

More to follow..

DMZ: a barrier between two nations

Looking towards the central demarcation line

L-r: wall of ribbons, freedom bridge, barbed wire fence

The Demilitarised Zone, on 38th parallel marks the barrier between North and South Korea, once a united country. It can be considered as a wall, living up to the negative connotations in the dictionary definition below. An extrapolation perhaps of a retaining wall dividing two communities?

Considering the permeability of walls in Hongjie, the DMZ wall tries to prevent movement in or out of either country. North Korea has managed to dig several tunnels along the border and recent violence between the nations has once again threatened its protective control.

Gift project: a potential site?

Plan diagrams l-r: building heights, current routes, main circulation through (retaining wall in yellow)

A typical example of city meeting landscape, Hongjie is not dissimilar to many neighbourhoods in Seoul. As mentioned in the post when I first visited area, as one emerges from the tube station to street level, a bustling shopping street is revealed with mountains lurking on the horizon behind.

I am interested primarily in the long retaining wall which now acts as a barrier between neighbourhoods just off the main street. Currently, retaining walls guide us around the city but are there alternative ways of moving around them? This returns to the idea of ‘one-sidedness’. At Hongjie the retaining wall divides an older low/mid-rise residential area from a new Apatu development which cuts off the mountain from the city.

Site section: main road, retail, mid-rise, apatu, mountain

How is the wall traversed? How have circulation routes changed since its construction? How does city meet landscape?

Landscript by H-Sang Seung

I was first referred to Seung by Haewon Shin, after I described my interest in the Korean approach to landscape Most recently, I read Landscript which has confirmed some previous ideas and widened my horizons in terms of understanding Seoul’s landscape. Some notes and comments:

‘If there was a mountain, it had to be leveled; if there was a valley, it had to be filled in; if there was a river, it had to be redirected’.1

Seung places much of the blame of Seoul’s skyline today on a greedy Western influence. Towers – which now dominate much of Seoul’s skyline – originally emerged as an architectural form of compensation. Multiple stories distinguish from other buildings, but with so many now, they have become almost indistinguishable from one another. Seung argues that Seoul doesn’t need to emulate landmarks of Paris’ Eiffel tower or Dubai’s Burj Khalifa because the landscape is a landmark in itself. It gives Seoul its identity. Perhaps this is an unrealistic suggestion for a world city but Seung argues that older buildings were built at a scale so as not to damage the natural landmarks, thereby respecting the landscape.

From Inwangsan towards the city (Ian Cooper)

“Seoul was not based on a diagram. Among the 20 cities in the world with a population of 10 million people, Seoul is the only metropolis located within a mountainous region. This is the element that distinguishes Seoul from all other metropolitan cities… Its mountainous geography is the most important element in recording the 600-year urban history of Seoul, a period beginning with its designation as the capital of the Joseon dynasty… There are four inner mountains – Bugaksan, Naksan, Namsan, and Inwangsan – and four outer mountains – Bukhansan, Yongmasan, Gwanaksan, Deokyangsan… The mountainous landscape is in itself the landmark of Seoul”.2

L-r: Map of Seoul with fortress wall 1765, Map of Palmanova - venetian star fort 17th century

Of many examples, Seung notably mentions Palmanova, a radial, hierarchical city where the land has been altered to make a diagrammatic city work. It is a city severed from the land, where surroundings were considered the enemy. A moat was dug and a high castle walls built to ward off evaders. Comparatively, the old city of Seoul has a peripheral fortress wall, not dissimilar to Palmanova. However, Seoul was chosen as Korea’s capital due to its location within mountains, a natural form of defense. Seung claims Palmanova illustrates that the ‘ethics of the land’3 was an unfamiliar concept in Europe, and that this has resulted in western ideas trampling the Korean landscape. Whereas old cities sustained the logic of the land, development simply became an accumulation of money-hungry towers.

Korean hillside neighborhoods are a typical feature of Korean urban landscapes. Seung mentions ‘Daldonge’ which is located on a dramatic topography, not dissimilar to some areas I’ve been exploring in Seoul. The road layout is not only a circular network but they include communal courtyards, a meeting place, a playground… ‘It is architecture molded from the land.’4 Such communities, along with the Shamanist village on Inwangsan have been torn apart by pockets of redevelopment all over Seoul. Daldonge as remembered by Seung no longer exists.

Richard Sennett’s Wallenberg Lecture in1998 entitled “The Spaces for Democracy” spoke of a decentralised democracy having not only political, but visual dimensions too. Many prefer the jumbled, polyglot architecture of neighborhoods to the symbolic statements made by big central buildings. This is an issue I hope to address this year, thinking of older neighborhoods verses large complexes that dominate much of the city today.

“Teomuni” is a pattern that is inscribed on the ground, almost like a pattern of retaining walls revealing the nature of the landscape in Seoul. Like the palimpsest of a continuously re-written piece of parchment, landscape – an  infrastructure within it – reveals of what once was.

Hongjie: A revisit

A long retaining wall, dividing two communities

A typical scenario in Seoul, Hongjie reveals the layers of the edges of the city. As we begin to consider a site for the gift project, I have decided to return to the place where I first starting looking for retaining walls.

Plan diagram drawn during visit

The apatu development behind the long retaining wall disconnects the older mid-rise buildings from the mountain. Routes through to the landscape are almost non-existent.

Apatu meets mountain

Re-thinking the language

Retaining walls are a fundamental structural element of Seoul's infrastructure. Up to this point, I haven't thought much about the two words and their meanings. I have been encouraged to think about what the term ‘retaining wall’ means, and possibly propose a more appropriate term for them for this project.

Definitions from Oxford dictionary online

Above is a breakdown of the dictionary definitions. I was struck mostly by the repeated negative words used to describe ‘wall’. Instead of thinking of retaining walls as preventative barriers, we should think of them as something that introduces a new ground condition. They provide a new way of reading the city and defining the landscape. A series of verbs reinforces their potential: linking, cutting, folding, threading, guiding, layering...

Understanding the ground conditions at Inwangsan

Axonometrics: Retaining walls in APT development, retaining walls in Inwangsan village - representing materiality, integration and hierarchy of circulation routes

As mentioned in previous blogposts, Inwangsan presents two clear distinctions of types of retaining walls. The first can found around the APT development, and the second in the village. It is important to consider the positive aspects of the village in order to understand why the retaining walls at the APT’s have such a negative impact on the streetscape. Three observations:

1 Materiality and construction of retaining walls

Apatu: Tall and long concrete walls with textured concrete surfaces. Monogamous and blank vertical screens cut of new developments from existing urban fabric. From discussions, this is a problem for developers who often employ artists to try and improve the appearance, but is this enough?

Village: Variety of materials and construction give clues about the process and order of development of the village. They give a sense of time and richness here. Materials include stone bare-faced concrete, and rendered concrete

2 Integration of retaining walls into buildings

Apatu: The buildings and walls tend to be entirely separate entities, creating a series of barriers – from road, over wall, to stark apatu facades.

Village: The walls are almost always a part of the village buildings, the use of materials give clues to where building begin and wall ends but they are very much a part of each other, as if one couldn’t exist without the other.

3 Hierarchy of circulation routes

Apatu: Main vehicular routes around the edges of the apartments are wide with narrow pathways, not inviting for pedestrians. The private streets within the residential developments are cut off from the public routes. The retaining walls cause a ’one-sidedness’ – public verses private are disconnected domains.

Village: The streets weave around following the contours of the steep hillside slope, widening and narrowing a various points. The main route up to the mountain beyond is wide with steps and a slope, secondary  smaller routes lead of this main path. 

An English historical comparison: The Ha-Ha Wall

In a very different context, a type of retaining wall is used to form an invisible barrier. Found on many historic estates, they are a devices designed to avoid obtrusive fencing so when the gentry used to look out to the horizon, they couldn’t see where their land ended and where the countryside began. Furthermore, they keep livestock out of private grounds. In 1712 Architect John James translates from Dezallier d'Argenville's La theorie et la pratique du jardinage (1709):

"Grills of iron are very necessary ornaments in the lines of walks, to extend the view, and to show the country to advantage. At present we frequently make thorough views, called Ah, Ah, which are openings in the walls, without grills, to the very level of the walks, with a large and deep ditch at the foot of them, lined on both sides to sustain the earth, and prevent the getting over; which surprises the eye upon coming near it, and makes one laugh, Ha! Ha! from where it takes its name. This sort of opening is haha, on some occasions, to be preferred, for that it does not at all interrupt the prospect, as the bars of a grill do."

There are hundreds of examples of these walls in England, mostly built in 17th and 18th centuries. They are rumored to have been introduced from France but also link strongly to Chinese gardening ideas of concealing barriers with nature.

Long sectional diagram: view from house over garden and beyond to the countryside

In comparison, a more contemporary example can be found at the Washington Monument. This ha-ha wall though is designed to minimize the visual impact of security measures after 9-11. The one-sided ha-ha is made of granite that reach deep enough into the ground and overlap at just the right points to stop an explosive-laden Humvee. The wall doubles as a seating bench and incorporates lighting. The secondary functions have similarities to the new cor-ten walls at Seonyudo Park.

Although the ha-ha walls are set in different context e.g. topography, density of construction and scale, it is useful to understand how retaining walls are used to control views. It highlights the importance of a visual connection with the landscape.

Workshop 1: project swap

Model-making in studio at KNUA, models from workshop

This workshop gave us the opportunity to discuss and make models of each of our projects. We chose names out of a hat and made a model which we felt expressed the chosen project. We swapped twice an had discussions at the end. I tried to work with an attitude of learning through making. I jotted down only a few notes to begin with and started making, knowing that my models were likely to alter and change as I made.

Clockwise from top left: My model of Tomi's project, my model of Beni's project,
Alpa's model of my project, Beni's model of my project

Firstly, I got Tommy’s project:
This model attempts to convey the peripheral conditions of the city. Urban mountain ranges overlap with natural mountain ranges, pushing and pulling the edges of the city. If looking here to place death (and celebrate and remember it), careful consideration of this situation is integral. Is a proposition placed next to a building, at the foot of the mountain, between city and landscape? How does it connect to the existing urban fabric?

Secondly, I got Beni’s:
Beni has mentioned that in Korean public spaces, there are often pockets of domesticity, which these models have tried to express. The first sectional model looks at how residential apartments and markets feel very disconnected in the city, and yet small areas of domestic living appear in the markets. Somehow, these pockets reveal more about life within the home than the Apatu buildings themselves. Secondly, the dark market alley shows a sequence of doors as if looking into life behind the façade of high-rise apartment blocks.

Models of my project:

Beni’s description of her model of my project:
‘The model tries to think about retaining walls spatially. Traditionally, piles of stones were used to form terraced landscapes: the rocks where placed on top of each other following the horizontal layering of the land. In contrast, modern walls are vertical elements that disrupt the landscape and hide the different strata of the ground. I tried to express this by covering with white paper the corrugated cardboard layers stacked on top of each other.
The direction of the circulation in relation to the wall can also be explored spatially. The ladders leaning on the walls try to illustrate the impossibility of climbing up the mountains due to the scale of retaining walls. In contrast, traditional villages, like Inwangsan, allow a more permeable circulation.’

Alpa’s description of her model of my project:
'Mountains can be considered as modulations in the horizontal plane - contour lines (a planar expression of a 3D phenomena) reinforce this reading.  The retaining walls are a strong vertical presence that impose a re-reading of the landscape.  Initially the mountains determined the position of the walls but in aggregate, the walls themselves begin to define the landscape.'

This has been a useful exercise, both in modeling other peoples’ projects and seeing what people make of my project. Points to take away:
- Traditional verses contemporary walls,: differences in construction and scales
- Integration verses separation of walls to buildings
- Impact on circulation from the location and orientation of the walls, permeability
- Horizontal verses vertical planes defining the landscape
- A new way of reading the landscape through city infrastructure

For images of all the models, see the most Seoul Satellite blogpost.

Inwangsan: Drawing the structure

Long sketch section showing changes in level (retaining walls in thick black)

After visiting Inwangsan for a second time, I have drawn a quick sketch plan remembering the structural objects along the journey i.e. retaining walls, steps and buildings.

Plan Diagram: Drawing the structure

It has been useful to visit this area but it is important now to identify positive aspects of the village in order to draw comparisons to the apatu street conditions. A few initial ideas:

- Materiality: there is more variation and similar materials to buildings are used for the wall construction e.g. such as stone, render, brickwork

- Integration into buildings: walls seem to be part of the village whereas in the new developments the walls appear as separate entities to the buildings

- Hierarchy of routes: wider and narrow paths open and close gradually through the village. On the other hand, the new developments tend to have very wide main roads with narrow paths and then private streets within the developments. There is less fluidity in the latter so the walls seem to create a barrier.